Audio Signal Generator with Specific Requirements - Page 1
Forum section: electronics / beginners

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 12, 2020, 11:16:06 pm
content: Greetings,My desire is to build an audio signal generator for testing amplifier circuits. I did find quite a few circuits online, but I am unable to find a circuit that meets some of the requirements that I have in mind. So, I would like to know if I should keep looking or if my requirements are not technically possible or practical.1 - I don't quite like the idea of having a switch that selects one range at a time. I would much more prefer to have a multi turn pot that could be dialed in from about 20Hz to about 20KHz.2 - I don't like the idea of having a ganged pot so I am hoping that there's a way have a circuit that has a single gang pot.3 - I would prefer if the sine wave was not a converted square wave or converted triangular wave. And I really like to idea of using a light bulb in the circuit.So far I have not been able to find a schematic that meets these requirements.Is what I am looking for technically possible, or practical.Thanks...

author: gbaddeley
author_rank: Regular Contributor
timestamp: May 13, 2020, 01:56:45 pm
content: You probably can’t find a design that meets all your requirements because a practical and good performance design like this does not exist. Can you think why most (non synthesized / analogue) designs do use a ganged pot (or variable cap) and 4 switched ranges to cover 20-20K with reasonably low distortion?

author: Manul
author_rank: Super Contributor
timestamp: May 13, 2020, 03:01:18 pm
content: For what kind of testing? One method for noise and distortion measurement is to subtract input from output. I guess in that case it does not matter so much, how good is the quality of input signal, but I'm not an expert about that.

author: fcb
author_rank: Super Contributor
timestamp: May 13, 2020, 03:17:57 pm
content: ICL8038/MAX038 IC's were always the staple of homebrew 20-20K signal generators. You can set these up to sweep the full band with one pot. The sine-converter isn't half bad either.

author: TimFox
author_rank: Super Contributor
timestamp: May 13, 2020, 05:40:15 pm
content: I am aware of two single-range/one knob R-C tuned analog audio generators from the good old days.1. HP 207A (vacuum tubes) 20 - 20kHz in one range. Seehttp://hparchive.com/Manuals/HP-207A-Manual.pdf2. General Radio 1313A (transistor), 10 - 50 kHz in one range. Seehttps://www.ietlabs.com/pdf/Manuals/GR/1313%20A.pdf(General Radio seemed to prefer beat-frequency generators for wide-single-range devices.)You may find the manuals instructive, but good luck finding one of these very rare items for sale.The wide-range R-C networks are extremely interesting in themselves.If you literally want a single control, you might look at the G-R beat-frequency generators for ideas.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 14, 2020, 02:18:36 am
content: Thank you all for your replies.I actually breadboarded a few circuits that I found online but the results were never good. I did manage to produce sine waves, but there was always something not working right with each circuit.For example, last one I built was this one:http://www.next.gr/oscillators/sine-wave/Audio-sine-wave-generator-l12331.htmlI did get a sine wave but when I adjusted the output level pot, R16, the sine wave would disappear towards both ends of the pot rotation. I got similar problems with other circuits and got frustrated.At one point I would like to build a function generator and for that project I would like to use one of the usual choices of IC's but for the audio generator project I kind of like the idea of building a circuit that doesn't use a function generator IC. Using a light bulb kind of appeal to me, but I don't know why. I guess it feels more organic.I recently built a dual adjustable power supply. It started as a circuit that would use a ganged pot and then I discovered that you can use a single pot that have an OP Amp track the pot setting for the other rail. I was wondering if that same trick can be used in a sine generator to eliminate the ganged pot.I really like the vintage HP 207A and General Radio 1313A models that TimFox posted. I am a sucker for vintage equipment and I am trying to resist always buying vintage stuff. Now I can't get those out of my mind and I already checked eBay. But like TimFox said, I can see they are rare so they are not showing up in eBay history.I guess I'll keep breadboarding circuits that I find online. Eventually something's going to work out.Thank you all...

author: gbaddeley
author_rank: Regular Contributor
timestamp: May 14, 2020, 12:06:00 pm
content: Breadboarding until something works out? You could waste a lot of time and get nowhere. There are plenty of working wien bridge designs out there that use a bulb for amplitude stabilisation. Read the Wikipedia article on wien bridge.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 14, 2020, 12:57:51 pm
content: Quote from: gbaddeley on May 14, 2020, 12:06:00 pmIf I am interpreting your comment correctly, I think you are saying that some circuits should not be built on breadboards. Is that so?Quote from: gbaddeley on May 14, 2020, 12:06:00 pmAlong the same lines, I am interpreting your suggestion that I should just pick a circuit and build it straight on a PCB.One thing that I find difficult with building any designs in general, that I find on the internet, is the fact that I often need to order specific IC's or transistors, before I test a circuit, and then if it doesn't work out I need to order more components. To avoid paying for multiple shipping I try to compile a few candidate circuits and order all at once, then wen the time comes to build I still seem to be missing something. And often for a specific circuit that I find there's one component that none of the sellers stock. But many times I need to order some components from Mouser, some from DigiKey and there's still one or two components left that I need to look for elsewhere. There must be a better way.I've read a lot of online articles about these oscillators and I will definitely read the Wikipedia pages also.I'm still wondering if it's possible to replace the ganged pot with a single pot and OP Apm tracking circuit.

author: ArthurDent
author_rank: Super Contributor
timestamp: May 14, 2020, 02:40:55 pm
content: Quote from: TimFox on May 13, 2020, 05:40:15 pmI believe this is the most practical idea. The General Radio units are ancient but the idea is sound and has been used in many applications. This link below describes how it works. A crystal oscillator could be used for the fixed oscillator if you're interested in greater accuracy. A high stability variable oscillator could be made if it is made without switching and to cover a wide range. For instance, a 2Mhz crystal oscillator and a variable 2Mhz to 2.020000Mhz oscillator could be manually or voltage controlled to sweep any portion of the range.https://www.eeeguide.com/beat-frequency-oscillator/

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 14, 2020, 08:04:06 pm
content: I was actually working on a circuit all morning and it's a good example of what I've been struggling with.I thought I'd give it a shot at building the simple figure 4 circuit from this page...https://www.nutsvolts.com/magazine/article/op-amp-cookbook-part-3It looks simple enough and I have all the components. I tested the light bulb and it was within specs and I know the OP Amp works because I recently used it on a breadboard for another project.At first I was getting a square wave output. The output amplitude adjustment pot was working in a way that the amplitude would be increased 100x (or so) from minimum to max. and I could not adjust the 1K RV2 to get a sine wave. The ganged frequency pot worked but it was also moving too extremely.Then, after a while the square wave disappeared and I got a distorted wave on the scope. No matter how much I tried adjusting I could no longer get anything that resembled a square or sine wave.I checked and triple checked all the connections and even swapped some parts. No luck.That has been my experience trying to build a simple audio generator for the past 2 years. So frustrating I had to walk away from it all several times.Am I just having some really bad luck?

author: magic
author_rank: Super Contributor
timestamp: May 14, 2020, 08:49:19 pm
content: Use a soundcard if you really need high quality sine waves.DIY if you want to DIY for DIY's sake or if you need 1ppm linearity (you probably don't).

author: Cubdriver
author_rank: Unknown
timestamp: May 14, 2020, 08:56:36 pm
content: That output looks like a differentiated square wave, though why it's doing that I've no idea at a glance.https://learnabout-electronics.org/ac_theory/filters84.php-Pat

author: Cubdriver
author_rank: Unknown
timestamp: May 14, 2020, 09:00:10 pm
content: What happens if you DC couple the scope input? What frequency is that? (Can't see what the time base is set to.)-Pat

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 15, 2020, 12:09:02 am
content: Sorry, I already dismantled the circuit and started building figure 8 from that same page...https://www.nutsvolts.com/magazine/article/op-amp-cookbook-part-3I used the same 741 OP Amp and was able to get a sine but it's slightly distorted. Also, when I turn the output pot near either extreme the sine disappears.I'm not really interested in that circuit because it doesn't have an adjustable range.I guess what would be helpful if someone could post a link to an audio generator circuit that should not give me trouble, so that I can wrap up this project and start building amps and learning how to use the audio generator to work on amps.I did look closely at the schematic of the General Radio 1313A unithttps://www.ietlabs.com/pdf/Manuals/GR/1313%20A.pdfand I would be up for building it, but I think that it uses a FET that is no longer available. The parts list calls it a U-147 and I can't find it anywhere online. I can't even find a equivalent through thehttps://alltransistors.com/web site.If there is a way to build that circuit with some equivalent components that are still available today I'd be up for it. But I don't know how to go from here.Thanks...

author: bob91343
author_rank: Super Contributor
timestamp: May 15, 2020, 02:04:01 am
content: That FET doesn't appear to be critical. If you put in one rated for 40 V or more it probably will work. The U-147 is only rated for 20V. But try what you can find cheaply and it should be okay.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 15, 2020, 02:52:17 am
content: Quote from: bob91343 on May 15, 2020, 02:04:01 amLooking through the parts list and trying to compile an order, there are quite a few other items that would have to be substituted.Transistor 2N2188 (equivalents 2N2189, 2N2190. 2N2191) none I can find available.Mica caps 267p, 453p, 634p, 649p, 787p.... I can't find.Diode 1N4009 I also can't find and I am wondering why they used that diode and if 1N4007 might work just fine.At this time I'm also wondering why they list two types of resistors, composition and film, and how important it is to use the exact ones.Thanks...

author: ArthurDent
author_rank: Super Contributor
timestamp: May 15, 2020, 01:53:06 pm
content: Quote from: Adinol on May 15, 2020, 02:52:17 am1N4000-4007 are common rectifier diodes, 1N4009 is a fast switching diode. 1N4148/4448 might be a good choice.Quote from: Adinol on May 15, 2020, 02:52:17 amthese old composition resistors had terrible aging specs. some would jump several hundred % over time. The film ones were stable and were required in the frequency generating circuitry. The reason ancient composition resistors are mentioned is I'm guessing the book you're using might be from about 1970.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 15, 2020, 02:16:49 pm
content: Quote from: ArthurDent on May 15, 2020, 01:53:06 pmThank you for this info...Quote from: ArthurDent on May 15, 2020, 01:53:06 pmI am using the PDFhttps://www.ietlabs.com/pdf/Manuals/GR/1313%20A.pdf, so it is outdated.So, would I use any modern resistors or do I still need 2 different types?Also, what do I do about the mica caps? The values they mention sound very specific and precise.And what about that 2N2188 transistor? What would be a suitable replacement?

author: TimFox
author_rank: Super Contributor
timestamp: May 15, 2020, 03:01:07 pm
content: At the time when the 1313A was designed, silver mica was the normal type of high-precision stable capacitor for critical applications, with Q > 1000. Now, NP0/C0G ceramic (avoid X7R and Z5U for these applications) is typically used instead. For larger values, polypropylene film capacitors are good, but physically larger, with higher Q than polyester film ("Mylar"). Also, molded carbon composition resistors were the common resistor for non-critical applications, but they have the problems discussed above. You can certainly use metal-film to replace carbon resistors in audio-frequency applications, being careful about power rating. In my own audio work, I use metal film for lower-power applications, and wirewound for higher powers. The worry about self-inductance of wirewounds in audio applications is often overstated.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 15, 2020, 04:40:58 pm
content: Wide range oscillators often are made as voltage-controlled oscillators (VCO). Look at that way.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 16, 2020, 02:43:20 am
content: Quote from: TimFox on May 15, 2020, 03:01:07 pmI am still having trouble finding capacitors of any kind in values 267p, 453p, 634p, 649p, 787p. Since these are not rounded off number I assume that it's important to use a cap of 267p instead of 265p, and so on. Or do these numbers represent old standard values and it's OK to use rounded off values? I know that if I have to recap an old radio it doesn't make much of a difference if I use a bigger cap. But I never touched mica or ceramic caps in old radios, so I don't know if those have to mach the factory values closely.I would hate to spend a couple of hundred dollars on part and have my circuit behave oddly. For some reason I'm not having much luck building oscillator circuits.

author: TimFox
author_rank: Super Contributor
timestamp: May 16, 2020, 01:45:22 pm
content: General Radio could order mica capacitors with close tolerances made to their specific values.If you want to proceed along these lines, you need to obtain a good LCR meter (such as the DE5000, much discussed on this forum). Purchase standard-value C0G/NP0 capacitors lower than the required value and measure them. Also purchase lower-value units and carefully measure them. To make 267 pF, for example, start with 220 or 240 pF, and add maybe 47 or 27 pF, depending on what you measure, in parallel. For many such circuits, the match between two capacitors is more important than the value itself (e.g., shoot for 0.2% match between 1% values). The cost of the extra capacitors will be much less than the DE5000, which is roughly $100. To be authentic, you could stick with silver mica, but they are now expensive and rare compared with the C0G ceramics which are cheap and plentiful.Old corollary to Murphy's Law: self-starting oscillators won't.

author: MarkF
author_rank: Super Contributor
timestamp: May 16, 2020, 04:48:58 pm
content: Quote from: Adinol on May 14, 2020, 02:18:36 amWhen you get to building a Function Generator with a DDS chip, I can offer the circuit I did with a AD9834 DDS and PIC18F2550. It has Sine, Triangle, Square, Sine Sweep Up & Down, and PWM. Frequency range is 1Hz to 15MHz.I overlaid a picture of the OLED display.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 17, 2020, 02:26:12 am
content: Thank you again for the info.Let's say that caps can be resolved in that way. That still leaves the JFET that 2N2188 transistor.I can't find any 40V JFETs but I did find a 30V SMD JFET on DIgiKey, lsited as PMBFJ177,215. Mouser's filters don't filter by voltage, so I don't know how to search there. But I guess the PMBFJ177,215 should be good, right?However, I still don't know what replacement to use for that 2N2188 transistor.Quote from: MarkF on May 16, 2020, 04:48:58 pmThat function generator looks good. Is this a kit that you sell?

author: TimFox
author_rank: Super Contributor
timestamp: May 17, 2020, 02:46:52 am
content: The 2N2188 was a Ge PNP. Back in the day, PNPs tended to be Ge, while NPNs tended to be Si. Probably, any general purpose Si PNP would work, like a 2N2907. Look at the circuit conditions to verify ratings.Similarly, what is the actual voltage across the FET? There is only +32V applied to that part of the circuit, and the quiescent voltage across the FET is about 14V. The parameter to compare for a substitute is Idss, the zero-bias drain current. It must be higher than the actual drain current, so that the FET operates with reverse gate bias. Note that the original device is a P-channel JFET. This is left as an exercise for the reader.

author: MarkF
author_rank: Super Contributor
timestamp: May 17, 2020, 04:27:51 am
content: Quote from: Adinol on May 17, 2020, 02:26:12 amNo. It's a one-of. You're welcome to build it.You might want to make at least one change. (i.e. A SMD oscillator instead of the through-hole)Edit:. I will provide the Gerber's and Hex file.I don't have a BOM, but could wack one together if you really need it.

author: gbaddeley
author_rank: Regular Contributor
timestamp: May 17, 2020, 05:55:59 am
content: You’ve hit a couple of common snags. Old designs using 741 (went out of common use 20+ years ago) and old test equipment (uses unobtainable custom values and parts) and wien bridges that really only work with specific bulbs. Try finding more recent designs.

author: gbaddeley
author_rank: Regular Contributor
timestamp: May 17, 2020, 06:18:21 am
content: sound-au.com, I built project 86. There are a couple of other oscillators under Test Equipment.

author: TimFox
author_rank: Super Contributor
timestamp: May 17, 2020, 01:19:07 pm
content: Back to the G-R 1313A. If you really want to clone it, you would probably do better using a good operational amplifier, running on +30V or +/-15V power, instead of the discrete circuit used in the original. With luck and some creativity about the thermal stabilization, you could make it work.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 17, 2020, 02:10:08 pm
content: Quote from: gbaddeley on May 17, 2020, 05:55:59 amTo be honest, I actually am very attracted to old stuff, perhaps because I'm also (getting) old, LOL. At home I have tube radios, reel to reel players, and even three Tefifon players (forgotten technology).The 741 designs were just what I was finding on the internet and even if they are old (which I know now) I'm guessing they still should work.Regarding the bulbs, I have a few different ones and some circuits specified some parameters and I did find a match. Some circuits just said to use any bulb within whatever voltage range (or I forgot whatnot). I would still like to put together a bulb oscillator, at least for fun and learning.So, basically, even if I'm testing circuits with a 741 or with a bulb I'd still like to put together circuits that work out, even if they never end up in an enclosure. I guess at the end of the day, to have a reliable piece of test equipment, I might have to look into more upgraded stuff.Quote from: TimFox on May 17, 2020, 01:19:07 pmI'd like to clone that circuit and last night I already started compiling a cart on DigiKey. For now I put all the semiconductors in that cart and I was going to compile the caps today.I would also love to put together an upgraded clone that uses and OP Amp, but my knowledge of electronics is so limited that I would need a lot of guidance on the way. In fact, I wouldn't even know where to start. But let's say the goal would be to build a circuit that produces a natural sine wave (not a converted square wave) and to have the control pot be able to dial in the entire spectrum, without having a separate decade switch. And if it's an upgrade I might want to have a multi turn pot do that job.I'm thinking, I should probably start a separate thread just on building the General Radio 1313A.Quote from: TimFox on May 16, 2020, 01:45:22 pmI'm not finding anything listed as COG or NPO on DigiKey. I'm sure they are stocking them, but they list their caps as follows:Aluminum - Polymer Capacitors (10941 items)Aluminum Electrolytic Capacitors (126208 items)Capacitor Networks, Arrays (3022 items)Ceramic Capacitors (741074 items)Electric Double Layer Capacitors (EDLC), Supercapacitors (1733 items)Film Capacitors (69732 items)Mica and PTFE Capacitors (9102 items)Niobium Oxide Capacitors (476 items)Silicon Capacitors (267 items)Tantalum - Polymer Capacitors (13141 items)Tantalum Capacitors (104071 items)Thin Film Capacitors (4197 items)Trimmers, Variable Capacitors (2659 items)Thanks...

author: MarkF
author_rank: Super Contributor
timestamp: May 17, 2020, 02:41:47 pm
content: Look atDigikey Cermanic Capacitorsfor example.C0G, NP0 is the capacitor dielectric or temperature coefficient.You will see most common C0G, N0G, X7R, Y5V, ZSU for example.See Alan's video on capacitors (at time 7:45 for ceramic caps)

author: TimFox
author_rank: Super Contributor
timestamp: May 17, 2020, 05:07:09 pm
content: The capacitors are C-Zero-G or N-P-Zero, not Oh. The "0" refers to nominal 0 ppm/K temperature co-efficient. NP0 is the old commercial term, C0G is the more modern term. These will be found under capacitors-ceramic in most catalogs. C0G is available in leaded or surface-mount, as "MLCC" monolithic construction. Small values of NP0 can be found as disc ceramic leaded construction.

author: ArthurDent
author_rank: Super Contributor
timestamp: May 17, 2020, 05:26:04 pm
content: One note about trying to clone the GR-1313 oscillator is that it uses a dual section variable air capacitor that may be hard to find unless you disassemble an old AM radio from 40 years ago. I don't see where the capacitor range is specified in the manual and the shape of the blade may have a very unusual shape to give a more linear frequency change per degree rotation. See photo below.https://api.utmel.com/Upload/Images/Article/b0cadc01-6e26-4734-8c09-ddfd0da89b86.jpg

author: TimFox
author_rank: Super Contributor
timestamp: May 17, 2020, 05:47:14 pm
content: The dual-capacitor plates in the 1313 are, in fact, an interesting shape since the scale is logarithmic (decades per degree of rotation) to cover the full audio range in 180 deg of rotation.I own one of these and had some trouble getting it to work, since I could only find a manual for the "normal" Wien-bridge (multi-range) unit at the time. It now works, but it takes an inordinate amount of time to achieve a stable oscillation, after which it seems to work well. I was intrigued by the circuit used to get the full range without resistor switching.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 18, 2020, 02:19:35 am
content: Quote from: ArthurDent on May 17, 2020, 05:26:04 pmWell, that's probably the most challenging part to obtain. I can see that the schematic says C401A and C401B, which means it's a 2 gang variable air cap. And as you said, the range is not specified anywhere. I guess one way to find out what the range might be is to build the entire circuit, minus the C401 cap, then swap some caps in and out of circuit to see what range is needed to get 10Hz to 50KHz out of the circuit. But then what would be the next step? If those fins are really odd shape then the only way to obtain one might be to build one from scratch. I would actually be able to do that as I have a mini machine shop, but I wouldn't know what the physical specifications of the needed cap would be.

author: TimFox
author_rank: Super Contributor
timestamp: May 18, 2020, 02:56:34 am
content: If you have the tooling to build such a capacitor, and you know the capacitance for a discrete set of frequencies that covers your range, you can fit a curve and do the geometry for the required plate overlap area as a function of rotation angle. It shouldn’t be more difficult than Kepler deriving his laws of planetary motion...

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 21, 2020, 11:58:07 pm
content: I think I made some progress on the GR-1313 clone project.A friend of mine just let em have a tube radio chassis that has a 3 gang tuning capacitor with asymmetrical fins. I didn't remove the capacitor from the chassis yet, but here are some pictures.Would it be safe to say that even if it's not the proper value capacitor, I should still be able to make that circuit work and get some decent frequency range out of that capacitor? Then if the circuit works out with this cap I can look into figuring out the best value for the tuning cap and work on obtaining (or making) one?Also, I am thinking of building this circuit, as well.http://www.seekic.com/circuit_diagram/Signal_Processing/Oscillator_Circuit/20_Hz_TO_200_kHz.htmlBut I do not have any 2N4340 JFETS. I do have some other JFETS in my bin MPF102, J201 and J112.What would be the process of figuring out if any of those FETS would be suitable replacements for that circuit? And of so, which one would be the best choice?Thanks...

author: bob91343
author_rank: Super Contributor
timestamp: May 22, 2020, 12:24:29 am
content: A DIY variable capacitor project has some pitfalls. The first to come to mind is the means of connecting the rotor (usually to ground) with low impedance and minimal noise. In commercial units there is a clip that does that job. Look closely at any unit and find that clip, usually at the end opposite the ball bearing. High performance units for high grade lab gear use several clips per section. Sometimes they are made of beryllium copper.End play can create unstable capacitance. The material of the plates and frame is important, as its temperature coefficient of size affects the oscillator. The nonconducting supports also affect capacitance and losses. One of the rotor plates is usually slotted to allow for fine adjustment of tracking.The balls in the bearing need to be free of lumps, which could cause bumpy tuning.The shape of the plates will affect the dial calibration of course. So that needs to be planned carefully to obtain a linear dial. This will be compromised by trimmer capacitors but generally not too seriously.High grade units have an adjustable rear bearing to obtain smoothest operation.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 22, 2020, 12:37:57 am
content: Quote from: bob91343 on May 22, 2020, 12:24:29 amAnd I thought this would be a relatively simple project for a machinist. Of course, a lot of work, but still doable.I'm surprised to hear about the beryllium. That's the same stuff that in some magnetrons in microwave ovens. It's important to make sure not to chip the beryllium parts as fractured parts can release beryllium particles into the air and it's nasty stuff if you inhale it. They really used some dangerous stuff in old equipment, even in children's toys.I think it's common knowledge that in the era of tube radios those guys were real experts. Well, this is not to say that people aren't experts now (or during other decades) but it's interesting to see that in the early days of electronics those people designed really good circuits and they were in fact real experts. That's one reason why I like old equipment.Thanks for alerting me to that possibility of beryllium being present in those capacitors. I really had no idea.Thanks...

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 22, 2020, 08:02:46 pm
content: So, I removed the tuning capacitor from that radio chassis and measured a range of about 130p to 560p on each of the gangs. I built that circuit that I mentioned above and it is a complete failure. I don't get an oscillation whatsoever. Just a flat line on the scope.It seems that no matter what oscillator circuit I try to build I just don't have any luck. Although, this time I did substitute the JFET's because I do not have any 2N4340 in stock. So I tried with MPF102, and J112 JFETs to see what I get. But all I get is a completely flat line.Is there something wrong with the schematic?Now that I went through the trouble of obtaining a tuning capacitor I really want to build a circuit that makes use of it. If the circuit I was using is no good, could anyone suggest another simple circuit that I could build using components that I am likely to have. I basically have what I believe should be the "usual suspects" in terms of transistors and OP Amps, and the MPF102, J201, J112 JFETs.Thanks...

author: TimFox
author_rank: Super Contributor
timestamp: May 22, 2020, 10:12:40 pm
content: Two comments about the variable capacitor:1. These tuning capacitors for the MW broadcast band were usually nominal 365 pF. Your range of 130 to 560 pF is suspicious, since the minimum value is far too large. The mica compression trimmers typically found on the side of each section should not be so large as 100 pF (typically 30 pF). As a typical example, the single-gang unit in this linkhttps://www.tubesandmore.com/products/capacitor-365pf-variable-single-sectiondoes not have built-in trimmers, and has a range of 14.9 to 384.2 pF, for a delta of 369.3 pF.2. How did you insulate the case of the three-gang capacitor from ground? The case is common to the two gangs, and will connect to the gate of Q1 in your circuit.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 23, 2020, 01:12:39 am
content: Quote from: TimFox on May 22, 2020, 10:12:40 pmI don't see any damage on the capacitor and I also carefully used compressed air to clean it. Perhaps the measurements are not correct due to the method I used to measure. I recently watched a YouTube video that explained that you will red different values, depending on what frequency is used by the measuring device. Is is possible that my BK Precision DMM is simply using the wrong frequency for measuring this type of capacitor?However, the measurements were quite consistent between the gangs, which leads me to believe that the unit is OK. I think if there was damage to the unit, each gang would read something completely different.Quote from: TimFox on May 22, 2020, 10:12:40 pmYou bring up a good point. In fact, I was a bit unclear about how this should be connected, so I actually tried various combinations. Perhaps that's the reason why I get no waveform.What I did was, connect the lead wire from the rotor to the gate of Q1. Then I connected one of the gangs to ground and the other gang to what is marked as point A of the selector switch.Did I do that correctly?Thanks...

author: gbaddeley
author_rank: Regular Contributor
timestamp: May 23, 2020, 06:54:22 am
content: The frame of the big ganged variable cap is a fairly high impedance point, connected to the gate of a fet? Good luck with that! A Small plastic ganged cap from a transistor radio might work better.

author: TimFox
author_rank: Super Contributor
timestamp: May 23, 2020, 01:10:00 pm
content: The frame of the capacitor is, in fact, a problem for this circuit, since it goes to a high-impedance node. It must be carefully insulated, including the shaft through an insulated coupling to the knob, to avoid difficulties.I would suspect the DMM method of measuring small capacitances. Does your unit have the mica compression trimmers mounted? Note that in the photograph I showed, there is provision on the side, but they were not mounted. If the screws on the trimmers are tightened all the way, they may be damaged or at an excessive capacitance. For this application, I would remove them entirely.Did you check the DC voltages inside the circuit? The voltage from the negative rail (ground) to each FET drain should be roughly between 1/3 and 2/3 of the positive supply voltage. If the Idss of your FETs is totally different from the original ones, the quiescent point may be way off.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 24, 2020, 12:20:05 am
content: I read some more on the internet about building these kinds of audio oscillators and came across an old article that said this was difficult to do and the many people get discouraged. Clearly I am not making great progress so I guess that's what they meant.I measured the DC voltages between ground and drains of the FETs. When the circuit is powered up by 18V I get 2.47V at Q1 and 2.72V at Q2. So, it is not between 1/3 and 2/3 of supply voltage. That's when I use MPF102 JFETs.When I switch to J112 JFETs I get 2.16V at Q1 and 2.58V at Q2.So, perhaps one thing that is definitely wrong is the fact that I am not using the correct 2N4340 JFETs.I am having trouble understanding what the issue is with the frame of the capacitor. I searched the internet to make sense of it and I looked at many images but I can't find any similar capacitors that do not have a metal frame. Is this circuit perhaps designed for that other kind of variable cap that comes in a plastic frame?My capacitor does have trim pots mounted on top. One of the gangs does not and the other two gangs do. I connected the gang without the trim pot to the B side, as seen on the schematic, and one of the gangs that has the trim pot to the A side.But I really wish I could make something work, now that I went through the trouble of removing the capacitor from that radio chassis. I can put it back into the chassis, but I would much rather use it to build an audio oscillator that uses this cap.I am attaching an image that shows the capacitor and how I connected it to the breadboard.There must be some other relatively simple circuit that can use this capacitor.Thanks...

author: Cubdriver
author_rank: Unknown
timestamp: May 24, 2020, 01:26:24 am
content: I can't quite tell from the image - are you connecting to the frame? Do a resistance check between your connections and the cap plates, and make sure that one side connects to the stator plates, and the other to the rotor (obviously, do this carefully so as not to bend or otherwise damage the plates).-Pat

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 24, 2020, 02:12:53 am
content: Quote from: Cubdriver on May 24, 2020, 01:26:24 amI did a continuity check when I was cleaning the capacitor and trying to measure the capacitance (as well as possible on my DMM). The rotor plates are connected to the rotor shaft, which is also electrically connected to the frame. There are three leaf springs, mounted on the frame, which provide secure electrical connections of the shaft. Each leaf spring extends to the bottom and ends with a soldering tab. There is a black lead wire soldered to all three leaf spring soldering tabs. Of course, these leaf springs are also touching the metal frame, but so is the shaft itself. I connected that black lead wire to the gate of Q1, as per the schematic.Each stator is isolated (I checked continuity and capacitance by turning the shaft) and each stator has one white lead wire. Only two of the stators have trim caps. I connected the lead wire of the stator without the trim cap to ground and the lead wire of one of the other stators to point A on the schematic.Regarding the trim cap. Those have a solder tab. So, I tried connecting that solder tab to point A and I also tried to test the circuit without that connection(as I don't know if the trimmer is already internally connected).I believe I did all those connections correctly. Here is the schematic again (for convenience).The plates are not bent nor damaged. Although I do know that in some old radios the outer rotor plates are slotted and in fact bent at the factory for accurate tracking. I did not attempt to straighten these outer rotor plates. Only the outer (slotted) rotor plate of the gang without the trim cap is a bit bent. (I hope I'm explaining clearly).I still don't understand what is the technical issue with the metal frame as I am still under the impression that all of those types of capacitors, from that period, are built in the same way. At least that how it looks visually, but I never did any continuity tests on any of them.Thanks...

author: TimFox
author_rank: Super Contributor
timestamp: May 24, 2020, 02:46:20 am
content: Virtually all multi-gang capacitors of that type have all rotors connected to a common frame that would be grounded in the original application but must be insulated for the Wien bridge circuit.The drain voltages you measure indicate that the FETs are fully ON and incapable of amplification. You can verify that by measuring both drain and source voltages. To measure all three voltages on Q1, select the 20k resistors. If the FETs can’t amplify, then oscillation is impossible.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 24, 2020, 05:33:18 am
content: Quote from: TimFox on May 24, 2020, 02:46:20 amAlright, I completely dismantled the capacitor. There's some light surface rust on the shell, so I'll have to clean it off with hydrochloric acid, tomorrow.The shaft can easily be isolated from the frame, by replacing the steel balls with non conductive ones. I happen to have acrylic balls of the exact same diameter, which was 1/8 inch. There are 9 balls on the face side and a single ball at the rear. That one will eventually have to be a glass ball because there is a lot of pressure on it.I also removed all the leaf springs.Tomorrow I'll finish the work and post some sequential pictures.If this circuit doesn't work I at least hope to be able to use this modified capacitor for another circuit, perhaps even when I get to making that GR-1313 clone. But I really want a simpler circuit to work out first, because I want to make sure that I'm capable of putting together an oscillator of this type.Quote from: TimFox on May 24, 2020, 02:46:20 amOnce I reassemble the capacitor I'll be able to measure all the voltages.I guess I still need to resolve the FETs. Mouser has 2N4340 FETS in stock for $12.16. Is there any chance that MPF102 or J112 FETs can work by changing some of the resistors in the circuit?Thanks...

author: AVGresponding
author_rank: Super Contributor
timestamp: May 24, 2020, 08:25:16 am
content: Quote from: Adinol on May 22, 2020, 12:37:57 amBeryllium is alloyed to softer metals like copper and gold in order to stiffen them without work hardening them.In this case that would be to make the copper clips retain their springiness over time.I should think the chance of releasing beryllium particles to be vanishingly small, unless you decide to use an angle grinder on them or something.

author: TimFox
author_rank: Super Contributor
timestamp: May 24, 2020, 01:45:13 pm
content: Insulating the shaft from the frame with non-conducting balls is probably not a good idea. It's much easier to insulate the frame from ground.You can measure the voltages on the circuit FETs without the capacitor connected: there is no point in trying to make it oscillate before the amplifier is working.Are you sure you have connected the FET leads correctly? While the D and S can usually be interchanged, exchanging the G with one will result in a useless transistor.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 24, 2020, 03:07:22 pm
content: Quote from: TimFox on May 24, 2020, 01:45:13 pmNow the capacitor is dismantled, so I might as well do some cleaning.I do not understand how else to insulate the frame from ground. The shaft is electrically connected to the frame with physical connections. One of those connections are the metal balls.I never connected the frame or rotor shaft the the ground on the breadbaord. The only thing I connected to ground was the stator of one of the gangs.I am clearly not understanding how I have to insulate the frame from ground.Quote from: TimFox on May 24, 2020, 01:45:13 pmI triple checked the pinouts, on the datasheets as well as on the transistor tester.Perhaps I should just flip the drain and source and see what happens.Quote from: AVGresponding on May 24, 2020, 08:25:16 amI guess the beryllium copper clips are what I called leaf springs. I am glad I know about the beryllium. Those solder joints were not easy to heat up and it is not out of the realm of possibilities for someone to reach for a mini grinding wheel on a Dremel tool to cut through the solder joints.

author: TimFox
author_rank: Super Contributor
timestamp: May 24, 2020, 03:50:50 pm
content: Before proceeding further, you need to find out what the DC voltages on the two-FET amplifier are. The drain voltages you measured indicate total failure of the amplifier circuit. It wouldn't hurt to verify that the frame is insulated from circuit common with a simple ohmmeter check.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 24, 2020, 06:30:20 pm
content: Quote from: TimFox on May 24, 2020, 03:50:50 pmIf I read the datasheet correctly, I believe the MPF102 FETs are rated for 25V.https://www.onsemi.com/pub/Collateral/MPF102-D.PDFI already cleaned up the capacitor and I am attaching some images. I used hydroclhloric acid and when I was done I used a baking soda solution to neutralize the acid. After I rinsed with water and used an air gun to dry it off. I managed to remove a lot of the surface rust. I am attaching some images.I am not sure if the mica trim caps survived.I reassembled and tested that there are no shorts. I did end up using the acrylic balls, since the steel balls are in fact rusty. As a result there is no more continuity between the rotor and the frame. To connect the capacitor to the breadboard I can just use a jumper straight from the rotor. Later I will reattach the spring clips.Quote from: TimFox on May 24, 2020, 03:50:50 pmFirst I tested the circuit by swapping the drain and source. Results were the same. Then I completely dismantled the circuit and reassembled using different parts of the breadboard. Again, results were the same.I took some voltage measurements.With 18V applied to the circuit I get the following.Q1Drain - 2.28VSource - 2.09VQ2Drain - 2.77VSource - 2.44VNothing changes if I turn any of the three pots.

author: TimFox
author_rank: Super Contributor
timestamp: May 24, 2020, 07:46:34 pm
content: With only a few tenths of a volt between your measurments of "Drain" and "Source" voltages, I still suspect that you have mis-wired the device. Note that the Gate is not the center connection, but one end. In that circuit, the gate must be negative with respect to both Source and Drain, to reverse-bias the gate-channel diode. You did not state the actual voltages at the two gates: in this circuit working properly, the gates are connected to ground through relatively large resistors. With a miswired gate, the nodes might be at substantial positive voltage, which might explain the bad voltages on your D & S measurements.By the way, if you insulate the shaft from the rotor with plastic balls, won't the leaf springs make the connection again?

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 24, 2020, 08:59:16 pm
content: Quote from: TimFox on May 24, 2020, 07:46:34 pmIt is a real mystery because I did in fact check multiple times that this is wired properly. The gate is in fact pin 3 and I did wire the gate of Q1 to the rotor of the big cap and I wired the gate of Q2 to the wiper of the R12 pot. I also checked (several times) on the transistor tester that the FET pinouts are correct and that they match the datasheet. I feel like I am loosing my mind.Perhaps one of my breadboarding jumpers is bad. I am going to rewire everything.Quote from: TimFox on May 24, 2020, 07:46:34 pmThe voltages from the gate to ground read 0V on both transistors.Quote from: TimFox on May 24, 2020, 07:46:34 pmThe Gate of Q2 is on a wiper, so wouldn't that mean that the gate could potentially be shorted to ground if the wiper of R12 is turned all the way towards ground?Quote from: TimFox on May 24, 2020, 07:46:34 pmWhen I initially dismantled the cap I was planning to reinstall the spring clips on some kind of non conductive material. At this time I did not do that. I simply used a jumper from the rotor to the point on the breadbard.I am still confused about how the case of the capacitor needs to be separated from ground. I never wired it to ground. I I only wired the stator of gang B to ground, as per the schematic.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 24, 2020, 09:04:19 pm
content: OK, I built another circuit on another breadboard, using all new components and all new jumpers. Tested each component before using it.This time I get different voltage readings but still a flat line on the scope.With 18V supply:Q1:S - 1.69VD - 5.17VG - 0VQ2:S - 0.72VD - 13.94VG - 0VI don't believe I wired up anything differently, but as you can see I am getting different voltage readings.Does this make any sense?Thanks...

author: TimFox
author_rank: Super Contributor
timestamp: May 24, 2020, 10:04:47 pm
content: Those voltages look reasonable. If you disconnect the feedback network (tuning capacitor still removed, only R5 to R8 switched from the gate of Q1 to ground) and feed a small audio signal (say, 100 mV rms at 1 kHz) to the gate resistor, what do you see at the output? Sometimes you need to check subcircuits before connecting them all together.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 25, 2020, 02:45:19 am
content: Quote from: TimFox on May 24, 2020, 10:04:47 pmSince I currently don't have an audio generator I had to rig something up. I used a sine wave generator board that I made a while ago and used a 5k pot to adjust the output to 100mV RMS. The frequency happens to be 1.165kHz.What I see on the output is an inverted, slightly amplified, signal. The image shows what I see when the output control pot R14 is dialed to maximum output and the waveform adjust pot R12 is dialed up, so that the signal from drain of Q1 goes straight into gate of Q2. When I dial down R12, so that gate of Q2 gets shorted to ground the output is a flat line, as expected.When I measure the RMS voltage at the output it reads 145mV.I'm not sure if that's what's expected.Thanks...

author: TimFox
author_rank: Super Contributor
timestamp: May 25, 2020, 01:16:52 pm
content: Looks reasonable. For the next diagnostic, replace the variable capacitor with two equal fixed capacitors and see if you can make it oscillate.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 25, 2020, 04:25:55 pm
content: Quote from: TimFox on May 25, 2020, 01:16:52 pmThe smallest value fixed value caps I have on hand are 1nF. I tried with those, and some higher value ones, and I do not get oscillation.However, I did at least figure out what one of the problems was.For the second circuit that I built I used MPF102 FETs from a different batch. When I plug those into the first breadboard I do get that circuit to work the same way as the second breadboard. And when I plug the FETs from the first breadboard into the second breadboard, then the second breadboard no longer works.I have a few of each type of MPF102 in stock and none from the first batch work but every one from the second batch that I tried works.Below is a photo of the two, side by side.The one on the left is from the second batch (the one that works) and the one on the right is from the first batch.If I recall, I bought both batches from Jameco, but at two different times.

author: TimFox
author_rank: Super Contributor
timestamp: May 25, 2020, 05:22:45 pm
content: 1 nF = 1000 pF is a bit high, but I would think it would work. If you can find some 100 to 220 pF caps, they would be worth trying.When the "oscillator" flat-lines, are the DC voltages the same as when the feedback is connected?It's conceivable that the two batches have different pinouts, but that would be strange. Probably just defective parts.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 25, 2020, 09:27:50 pm
content: Quote from: TimFox on May 25, 2020, 05:22:45 pmI managed to find some 220p caps I didn't know I had. When I connect them I still do not get oscillation.I did the DC measurement tests.Test 1. When the 100mV signal is connected to the gate of Q1 and there are no 220p caps in circuit.Q1:D - 5.14VS - 1.69VG - 0VQ2:D - 13.92VS - 0.73VG - 0VTest 1. When I connect the 220p caps.Q1:D - 5.125VS - 1.69VG - 0VQ2:D - 14.02VS - 0.72VG - 0VHope these measurements make sense.Thanks...

author: TimFox
author_rank: Super Contributor
timestamp: May 25, 2020, 10:22:08 pm
content: OK--it looks like connecting the feedback has no effect whatsoever--I would carefully re-check the connections around the feedback network to the gate of Q1--something is missing.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 26, 2020, 01:23:57 am
content: Quote from: TimFox on May 25, 2020, 10:22:08 pmIs it possible that the schematic is wrong? I know 100% that my circuit is built exactly as the schematic shows.However, I question that resistor R10. There is no standard 4K value is there? I put a 3K9 resistor there. Perhaps that should actually be a light bulb and somehow that detail got lost in translation when someone copied the schematic.Could that be the problem?Thanks...

author: TimFox
author_rank: Super Contributor
timestamp: May 26, 2020, 02:19:57 am
content: The standard Wien bridge has a reactive leg (R + C) feeding (R || C) for positive feedback and a resistive leg (2R feeding R) for negative feedback. The 4k R10 is the "R" in the resistive leg, and should be half of R11 = 2R to get an open-loop gain of +3, while the reactive leg has a gain of +1/3. Therefore, the variable resistor R11 should be set to 8k to balance the bridge. Usually, R10 would be a temperature-sensitive thermistor. However, with a 3.9k fixed resistor, changing R11 should achieve oscillation of some kind, either increasing to a distorted waveform or decreasing to flat line. If R11 is too small (due to a bad part or connection), then the oscillator will not start.Circuit signal tracing: with the capacitor network disconnected from the gate but connected to C4, do your gain test again from the gate of Q1 to C4, then to the junction of the two "C2s". You should get a gain close to unity from the gate through C4, through the reactive leg, to where the gate was disconnected. The magnitude matters: oscillation requires an exact loop gain of unity at zero phase. The feedback through the reactive leg should be +1/3 at the frequency determined by the R and C: f = 1/(2piRC).To simplify the explanation of the reactive leg: at very low frequencies, the upper series capacitor does not pass AC; at very high frequencies, the lower shunt capacitor shorts out the AC. In between, the feedback signal reaches a maximum, and the circuit will oscillate of the resistive leg and amplifier have sufficient gain.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 28, 2020, 06:26:24 pm
content: I tried to do a lot by myself, without asking for help every step of the way. I am trying to be respectful of everyone's time and I don't want to come across as expecting people to troubleshoot every single step of the way.So, I did get the circuit to oscillate. But through a bit of an unexpected process.I basically accidentally knocked off the 10K feedback adjust pot and I saw a very distorted waveform on the scope. This led me to believe that I had to increase resistance beyond the 10K, so I used a 100k pot to figure out the approximate resistance value. It turns out that I need to add a 15k to 20k resistor in series with the 10k pot, to keep a good range on that pot. That's the only way I get oscillation.But there were other issues.The variable capacitor must be acting like some kind of antenna because as soon as I approach that capacitor with my hands the oscillation fades away, the closer my hands are to the capacitor.Also, when I lower the capacitance of the variable cap I lose oscillation and I have to keep readjusting the 10k feedback pot. But when I go back to increase the capacitance of the variable cap I get distortion.I tried some other very similar circuits using OP Amps and although I do get oscillation out of some circuits, every circuit I breadboarded is unreliable.I think the fact that I am building these on solderless breadboards has a lot to do with all of this.I am still tempted to just go ahead an build a clone of the GR-1313 by simply copying the layout of the PCB (with minor mods) and seeing what happens. But that is all very experimental in nature and my intention is to actually end up with a reliable piece of equipment, so that I can start building guitar amps.For that reason I decided to split my projects. I just bought a function generator on eBay, so that I have a reliable piece of equipment, to start building amps.However, I still want to build my own audio signal generator. But that's just going to be a pet project that I'll have to do between other projects.There is a lot to learn from this and I already learned a lot.With that said I wish to thank everyone, especially TimFox, who have contributed to this thread and tried to help me. I learned a lot from all of this and no doubt I will be asking for more help on this project, as time goes by.I much appreciate all the help.Thank you...

author: TimFox
author_rank: Super Contributor
timestamp: May 28, 2020, 07:45:43 pm
content: As you suspected earlier, usually a resistor (likely the 4K in your diagram) should be a thermistor (positive tempco) to stabilize the oscillation amplitude. Was there anything in your original source about that? The classic -hp- generators had a light bulb in the cathode of the first tube for that purpose. IIRC, it was a 3W 120V bulb, which would be 4800 ohms at full brightness, but operated far below that temperature.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 28, 2020, 10:24:00 pm
content: The original source where this schematic was published did not mention the light bulb in place of that 4k resistor. I came to that conclusion my self when comparing various circuits.As I mentioned, I experimented with various circuits, in the past few days, and I actually had the best results with the following one.However, I changed values and components. I used a light bulb in place of the R4. The bulb that worked out was one that measured 68 Ohms. Then I uses a 1K pot in place of R3.Also, I changed values of R1 and R2 and I used my variable cap in place of C1 and C2. However, when I dialed down my tuning capacitor I lost the oscillation, so I kept 220p caps in parallel and I was able to get a decent tuning range.At some frequencies the amplitude was not steady, but as you explain, this is because I don't have the right value bulb.What is interesting is that I had to change the OP Amp to make it work. I used a 741 and the first one I used was not working although I know that OP Amp is fine because I used it in other circuits I was breadboarading. On a hunch, I switched the OP Amp for another 741 and that made it work.There were many failed experiments in the past few days and I don't remember every details. But I find this fascinating and I will definitely eventually build my own audio oscillators. I want to learn to build various ones, some with the tuning capacitor and some with a pot instead. Some with OP Amps and some with FETs or BJTs.Like I said, when I have time it will be a pet project of mine. For now I had to make the practical decision and I bought a function generator. It's a IEC F54A.I hope it works, and if not I guess I'll have to fix it.Thanks for everything... seriously... I learned a lot here.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 28, 2020, 11:47:39 pm
content: Wine bridge oscillators are very sensitive to many things and can be unstable.If somebody wants an easy and stable sine wave oscillator then here it is, in the attachment.It is sold on eBay too.

author: Frankentronics
author_rank: Regular Contributor
timestamp: June 05, 2020, 10:16:35 am
content: The function generator I bought on eBay does not work.However, I did make some progress doing more tweaking of the circuit I last mentioned......but it would need more tweaking to make it work reliably over the entire range. At higher frequencies I get distortion that make the sine wave more of a triangular wave.I did find another circuit...http://www.valvewizard.co.uk/siggen.htmlThis circuit uses an LDR that is in close proximity to an LED - essentially a DIY opto coupler. Since I do not have any LDRs on hand I was wondering what kind of modification to the circuit would be required so that I can use a regular opto coupler instead. The opto couplers I have on hand have a built in photo transistor.Thanks....

author: TimFox
author_rank: Super Contributor
timestamp: June 05, 2020, 01:23:52 pm
content: The LDR circuit you show really requires a photoresistor, for two reasons:1. The photoresistor (also a thermistor or light bulb) is a two-terminal symmetric device where I(-V) = -I(+V), not necessarily a linear function.2. Photoresistors have a slow response to the light compared with phototransistors. This can be exploited as part of the control design.

author: Frankentronics
author_rank: Regular Contributor
timestamp: June 05, 2020, 07:47:08 pm
content: Quote from: TimFox on June 05, 2020, 01:23:52 pmThank you for this clarification.OK, I breadboarded this last circuit and it works really well. This is in fact my most successful attempt at building a sine generator, so far. I am going to call this good and build a PCB for it.This design even has one of my initial requirements, which is the use of a single gang pot.There is, however, one detail that the designer describes, that I am not 100% clear on.QuoteWouldn't a better solution be to just buy a reverse log pot?Thanks...

author: TimFox
author_rank: Super Contributor
timestamp: June 05, 2020, 08:36:41 pm
content: Log-taper pots are far easier to find, since they are the normal audio volume control. Turning ccw to increase frequency is a reasonable method. Cheap log-taper pots are not very accurate, so you need to carefully calibrate the dial. Accurate volume controls are usually stepped attenuators (totally unsuitable for your application) or stepped voltage dividers (which will work as a stepped resistor), which you may find awkward ( typically 24 steps).

author: Frankentronics
author_rank: Regular Contributor
timestamp: June 05, 2020, 09:13:11 pm
content: Quote from: TimFox on June 05, 2020, 08:36:41 pmI think so too, because one could think of it as decreasing the wavelength. Although the dial would not actually be marked that way.Quote from: TimFox on June 05, 2020, 08:36:41 pmI can already see how this would be tricky to do so that it's accurate for every decade.Perhaps it's more practical to just use a frequency counter that displays the exact frequency. Would that work?Quote from: TimFox on June 05, 2020, 08:36:41 pmMy hope is to have a signal generator to test audio amps. What peek to peek voltage range should I have for that application?Also, I see on Mouser that they have pots categorized as reverse audio and also as inverse log. Isn't that the same thing?Thanks...

author: Frankentronics
author_rank: Regular Contributor
timestamp: June 21, 2020, 04:02:40 am
content: After days of trying, and dozens of failed attempts, at making a PCB using a toner transfer method, I finally did manage to make a half decent board and migrated my components from the breadboard onto the PCB.I kept revising the PCB layout so many times that I ended up missing one trace, so the unit did not initially work. But I found the mistake and soldered a jumper.I ended up expanding the range and the rotary switch allows me to select 5 decades. Each decade overlaps with the previous one.At this time the signal generator is being powered from my bench power supply and I quickly put together a temporary chassis.One idea that I came up with is the use of gears (salvaged from a printer) to reverse the direction of the frequency tuning pot. I actually ended up selecting a ratio for these gears so that I end up with a multi turn knob. I know it's not the same as a multi turn pot, but I'm not sure that it's even possible to find a multi turn reverse log pot. So, this multi turn knob does actually give me a bit more control over fine tuning to a desired frequency.I can't use a regular scale, but I guess I could end up building a mechanical scale like on an old radio. I'd have to make a flywheel and it would be a fun project. Perhaps one day I'll even tackle it. But for now I'm thinking of just installing a cheap frequency counter off of eBay. Is there any reason why that shouldn't work.Any thoughts?Thanks...