Op amp based preamp supply. Charge pump, single vs symmetrical & ripple - Page 1
Forum section: electronics / beginners

author: dazz
author_rank: Frequent Contributor
timestamp: November 25, 2018, 09:50:23 pm
content: I'm cloning the preamp of a Marshall Lead 12 3005 into a pedal. It's a two stage op amp job with a +16V -16V supply.The plan is to use a standard 9V power supply to power it, so I'm considering two options:1. A charge pump based Dual-Output Voltage Doubler (pic 1) based on an LT1054 to get a symmetrical supply of about +16V -16V2. A charge pump voltage multiplier with two ICL7660 or LT1054 in series for a single supply of about 24V (according to my LTSpice simulation)I'm leaning towards option 2, but I'm not sure if the reasons for that are justified. I believe that having the opamp's V- tied to ground will limit/halve the amount of ripple I'll get as opposed to having a positive and negative rails, besides, the PSRR or the V+ is much higher in the TL082 I plan to use, so that should further mitigate the effect of ripple in the power rails.I know people are using 7660S chips with the boost pin to Vcc to increase the switching frequency from 10KHz to 30KHz, outside the hearing range with good results in symmetric supply configurations, but what if I was to use the cheaper 7660 at 10KHz in single supply mode? would the fact that there's just the positive rail ripple help make up for the lower frequency?I tried crunching some numbers, but I have no idea what I'm doing. Here they go anyway: The PSRR of the TL802 in V+ is 80dB at 10KHz. The preamp's gain at 10KHz, according to LTSpice, is about 70dB. So I'm guessing the V+ ripple will appear at the preamp's output attenuated 10dB (assuming single supply charge pump with a 7660 at 10KHz). That's an attenuation of a factor of 3, give or take. Can I expect to have low enough ripple in the charge pump output that it won't whine?I've also run some simulations of the charge pump (pic 2) and with a 10mA load which I think is reasonable for a dual op amp circuit like this driving a 50K load (standard chipamp input impedance) and I get only 3-4mV ripple. That would translate to 1mV in the output. That doesn't sound like too much, does it? I mean if the voltage swing at the output is 24V, 1mV won't disturb it too much, would it?Anything I may be missing please?

author: Benta
author_rank: Super Contributor
timestamp: November 25, 2018, 10:33:44 pm
content: Quite frankly, the pain of running opamps on a single supply is not worth the effort if it can be avoided.A dual supply opamp has a wonderful signal reference: ground. In a single supply design, you need to recreate this at some DC bias point. Not easy, and usually not noise free.You have the option of running dual supplies, and I highly recommend this.Your reflections on PSRR are correct, but can be alleviated with appropriate filtering.

author: dazz
author_rank: Frequent Contributor
timestamp: November 25, 2018, 10:45:40 pm
content: Thanks Benta. Do you think the fact that symmetrical charge pumps usually have weaker V- rails (in my simulations I got something like +16V, -15V) could have unwanted effects? Wouldn't it cause the preamp to clip asymmetrically because of the imbalance of the rails, hence affecting its tone?I definitely plan on testing all these things myself once the parts arrive, should be a good learning exercise

author: Benta
author_rank: Super Contributor
timestamp: November 25, 2018, 11:11:31 pm
content: I expect your signals are at line level (~775 mV), but even if they are a bit higher or lower, supply voltage will have zero influence. Asymmetric supply voltage will also have zero impact.You're overthinking this in my opinion.

author: dazz
author_rank: Frequent Contributor
timestamp: November 25, 2018, 11:15:11 pm
content: Quote from: Benta on November 25, 2018, 11:11:31 pmI should have mentioned it's a guitar preamp/effect. So the input is much lower than line level, perhaps 100mV peak to peak. Does that change anything?And yes, I tend to overthink stuff, so it may very well be the case here, haha

author: Benta
author_rank: Super Contributor
timestamp: November 25, 2018, 11:44:44 pm
content: Your signal level changes nothing, quite the contrary. At low signal levels, a good ground reference is even more important.

author: Zero999
author_rank: Super Contributor
timestamp: November 26, 2018, 08:51:36 am
content: Quote from: Benta on November 25, 2018, 10:33:44 pmBut converting a single supply to a bipolar supply is also a pain, because it introduces more noise.Quote from: dazz on November 25, 2018, 11:15:11 pmIf it's low levels, then the larger possible voltage swing a +/-16V supply could give is not needed. A lower voltage op-amp could be used with a single supply.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 26, 2018, 09:21:02 am
content: If you must, consider heavy filtering (or investing in opamps with high PSRR at higher frequencies). The likes of ICL7660 are quite noisy indeed, and at moderate frequencies (~10kHz), which is all the more annoying. An LDO may be worthwhile to take the crud out (but again, mind the PSRR, as that's also the frequency range where most LDOs start to really suck).But there should be no need for any more than a resistor divider (or a boosted one, i.e. using a spare opamp section for a rail splitter) in an application like that.Tim

author: Zero999
author_rank: Super Contributor
timestamp: November 26, 2018, 09:31:35 am
content: Quote from: T3sl4co1l on November 26, 2018, 09:21:02 amAgreed. Let's also not forget the TLE2426.http://www.ti.com/lit/ds/symlink/tle2426.pdfIf the power supply is only powering that circuit and isn't connected to anything else, then no AC coupling capacitors are needed: the 0V from the rail splitter can be treated as the common rail for the whole circuit.

author: spec
author_rank: Frequent Contributor
timestamp: November 26, 2018, 09:36:43 am
content: Crossed PostsJust so we can see whats involved here is the schematic for the Marshall Lead12-12W:https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-Lead12-12W-3005-5005-Schematic.pdfMy first thoughts are to use better RRIO opamps, so that the supply rails could be lowered and get the same or better dynamic range and open loop gain. That would possibly enable a simpler circuit with a better performance, including a lower noise, not that noise is an issue here.A 12V, 15V, 18V, or 24V wall wart should do nicely for the PSU.

author: b_force
author_rank: Super Contributor
timestamp: November 26, 2018, 10:10:47 am
content: I wouldn't go for these charge pumps anymore these days. They can be noisy, regulation is poor or limited and the output current is small.There are quite some good converters that can do split rail (TI has a few)Also you can use pretty much every buck converter to make a negative voltage rail.If efficiency is not the main priority there are also of the shelf converters available.Look for something that's at least 100kHz, but the higher the better plus some post filtering.

author: bd139
author_rank: Super Contributor
timestamp: November 26, 2018, 11:47:56 am
content: Another option - use an AC wall wart and just derive two rails from it. This works pretty well, gives you no compromise rails and keeps your circuits dead simple. Typical example of deriving three voltages cheaply (I would use regulators myself and be done with the zeners):

author: dazz
author_rank: Frequent Contributor
timestamp: November 26, 2018, 01:59:29 pm
content: Thanks everyone for your input. Lots of suggestions to explore.Quote from: Hero999 on November 26, 2018, 08:51:36 amSorry mate, I don't understand this. The input is low level, but the output at maximum gain is a square wave 30V peak to peak. I think I need to have that +/-16V supply to replicate the way the circuit distorts. Let me know if this is wrong, but the rationale is that the gain pot (attached schematic) changes the frequency response of the amp, at low gain it boosts higher frequencies more (around 10KHz) but as the gain is increased, the higher gain frequency shifts to about 1KHz. So if the original circuit with the +/-16V supply starts clipping at 10KHz with the gain knob at 50%, with a 9V supply it will start clipping very early at maybe 10% gain, and at all frequencies, making it almost unusable. I don't know if the circuit can be modified to behave the same way as the original one but with a single 9V supply. I can attenuate the gain slightly by increasing resistor R8 in between the op amps, but not enough to compensate for the lack of voltage swing.Quote from: T3sl4co1l on November 26, 2018, 09:21:02 amWell, I'm not sure I can just swap op amps like that. People will complain that all the mojo in the original LM1458 or the TL072/082 is lost. I personally have no idea if the op amp of choice plays any roll in how the thing clips and sounds. The simulations seem to suggest that the clip differently, but who knows.Quote from: T3sl4co1l on November 26, 2018, 09:21:02 amCan you please elaborate on this? What rail would the resistor divider split and what would the opamp do there please?

author: dazz
author_rank: Frequent Contributor
timestamp: November 26, 2018, 02:02:24 pm
content: Quote from: Hero999 on November 26, 2018, 09:31:35 amThanks Hero, googling that chip now. And yes, the only thing that the power supply needs to power is that dual opamp circuit.Quote from: spec on November 26, 2018, 09:36:43 amThanks for posting the schematic, should have done it myself in the 1st post.I'm still not sure how I could use lower power rails without significantly impacting the way the amp will perform. The thing has 80dB of voltage gain (with the gain pot at max) and the gain pot is part of a low pass filter so it changes the frequency response of the amp. So if the original one clips at 50% and mine clips at 10%, they won't sound the same. At least I think so.Also, a 9V supply is pretty much a requirement, since this will be a guitar pedal and most pedal power supplies are 9V

author: b_force
author_rank: Super Contributor
timestamp: November 26, 2018, 02:34:46 pm
content: I know 9V is pretty standard, but it's a little unfortunate.Otherwise it would be pretty easy to use something like a Meanwell MDD01N-15 or similar.The minimum voltage for those is either 10.8V or maximum voltage for 5.5V (for the 5V version).The second best choice would be a Meanwell DPAN02A-15 or DPAN02E-15they also switch at 100kHz, so that can easily be filtered out.How much opamps/current do you need?This is the +/- 15V version, which also gives you plenty of headroom.There are also 12V and 5V version available.

author: dazz
author_rank: Frequent Contributor
timestamp: November 26, 2018, 02:41:25 pm
content: Quote from: b_force on November 26, 2018, 02:34:46 pmIt's just a dual opamp (TL082 or LM1458). Maybe 10mA? 20mA tops I thinkOff to google those chips too, thanks for the suggestion

author: b_force
author_rank: Super Contributor
timestamp: November 26, 2018, 02:46:28 pm
content: Quote from: dazz on November 26, 2018, 02:41:25 pmIt might be tricky to get these things stable (since some converters have a minimum load needed).Obviously there are ways to build a circuit from scratch.I don't know how comfortable you are with soldering SMD stuff?

author: dazz
author_rank: Frequent Contributor
timestamp: November 26, 2018, 02:54:47 pm
content: Quote from: b_force on November 26, 2018, 02:46:28 pmThey're also pretty expensive at $10 or so. And would make the pcb a lot larger too. I'm trying to fit this thing in a 1590BB enclosureQuote from: b_force on November 26, 2018, 02:46:28 pmI've only soldered SOT23 jfets, LOL. But I don't mind trying something else if needed be.The thing is that this will be a DIY thing that I will publish, and most people prefer through hole since it's easier to assemble.But I'm very interested in how the circuit could be rebuilt to perform the same with a lower power supply. So if you could give me some pointers that would be very much appreciated

author: b_force
author_rank: Super Contributor
timestamp: November 26, 2018, 03:01:05 pm
content: Quote from: dazz on November 26, 2018, 02:54:47 pm10 bucks? Well, that's a pretty bad deal, I normally buy them for around $4-6Which isn't that much more expensive compared to developing your own stuff.Another idea i to get a basic 9-12 or 9-15 boost converter and use a flyback or 1:1 coupled inductor to form the negative voltage.

author: dazz
author_rank: Frequent Contributor
timestamp: November 26, 2018, 03:07:52 pm
content: Quote from: b_force on November 26, 2018, 03:01:05 pmYeah, I already have one of those boards I got from ebay. Maybe I could add a boost converter to my board. Something based on a LM2577-adj perhaps? Ideally something that can be got for cheap off ebay or the likes

author: b_force
author_rank: Super Contributor
timestamp: November 26, 2018, 03:29:37 pm
content: Quote from: dazz on November 26, 2018, 03:07:52 pmWell, the problem is to get a good efficiency with these low currents.Most converters are optimized for something around 100-500mA or thereabouts.The efficiency at low currents can be as bad as 50-60%. Not really ideal for a 9V battery.(I am personally getting rid of all my 9V pedals or rebuild them to accept two AA etc)What I would suggest is otherwise use something like a LT1615 (or a similar TI parts) look at page 8, or LT3460, LT1316 etc (probably better because it has a constant switch Freq)With LTSpice you can simulate this circuit and optimize it for 9V

author: Zero999
author_rank: Super Contributor
timestamp: November 26, 2018, 03:30:39 pm
content: Quote from: dazz on November 26, 2018, 01:59:29 pmThen I misunderstood. If you need 30V peak to peak, then you need a higher total supply voltage than 30V. A +/-16V supply won't do for the TL082, which needs a couple of volts of headroom on either supply rail, to avoid clipping, unless that's desired for the distortion effect.The OPA2196 will be able to give 30V peak to peak from a +/-16V supply. If does have a lower slew rate, than the TL082, but it will be the lower audio frequency components which generate the largest voltage swings, so this is a non-issue.http://www.ti.com/lit/ds/sbos869/sbos869.pdf

author: dazz
author_rank: Frequent Contributor
timestamp: November 26, 2018, 03:48:24 pm
content: Quote from: Hero999 on November 26, 2018, 03:30:39 pmYes, it's built to distort, for rock'n'roll purposes!More precisely, it's supposed to do both clean and distorted tones. That's what the gain pot is forQuote from: Hero999 on November 26, 2018, 03:30:39 pmI'm afraid it really needs to be an LM1458 as in the original circuit, or one of those TL082 with jfet inputs (which are supposed to clip more softly). Well, not really those 2, just one op amp with lots of mojo regardless of whether other types make more sense from an engeneering perspective. That's how guitar players roll

author: Zero999
author_rank: Super Contributor
timestamp: November 26, 2018, 04:20:51 pm
content: Quote from: dazz on November 26, 2018, 03:48:24 pmI admit I'm more used to avoiding distortion when building an amplifier.Do you have a specific type of distortion i.e. transfer function in mind?I think most op-amps will give fairly harsh clipping, as once the output goes beyond the limit, there will be no negative feedback. I suspect the clipping level may vary between op-amps with the same part number, because it's not a tightly controlled characteristic. By operating an op-amp at its clipping level, it's being driven past its deign specification.

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 26, 2018, 04:31:57 pm
content: Another thing with using charge pumps, is that your negative output won't be completely symmetrical.As the application circuit shows, there will be the diode's forward losses, plus any losses in the converter itself.And indeed if not filtered correctly, the plain 7660 will generate an audible whine when used with sensitive audio circuits.

author: b_force
author_rank: Super Contributor
timestamp: November 26, 2018, 04:33:04 pm
content: Quote from: dazz on November 26, 2018, 03:48:24 pmYou don't want any distortion/clipping from your opamps. Also really don;t see where "sound descriptions" come from.I have building these circuits for years.Only thing you need to take care of is be able to drive enough capacitive load if it's a pedal.

author: dazz
author_rank: Frequent Contributor
timestamp: November 26, 2018, 04:46:07 pm
content: Quote from: Hero999 on November 26, 2018, 04:20:51 pmWell, it should be as close to the original as possible. Would posting LTSpice screenies help? Or perhaps I should post the .asc? I'm using a couple of "custom" models but I can zip themQuote from: Hero999 on November 26, 2018, 04:20:51 pmWell, there are many guitar effects and preamps that use opamps for clipping, I also heard that they clip harshly but many Marshall amps use them. The Tube screamer too, although I think the clipping diodes do most of the distortion there. I dunno, the Lead 12 works like that, so who am I to question Jim MarshallQuote from: schmitt trigger on November 26, 2018, 04:31:57 pmYeah, the asymmetry of the charge pump is what made me lean towards using it as a voltage multiplier to get from 9V to as close to 30V as possible, then use a heavily filtered virtual ground to ensure symmetry. I believe the load from the op amps will remain fairly stable so I don't expect the voltage supply to sag very much

author: BravoV
author_rank: Super Contributor
timestamp: November 26, 2018, 04:57:23 pm
content: Alternative consideration ...https://www.analog.com/en/products/ltc3265.html#product-overview

author: Audioguru
author_rank: Super Contributor
timestamp: November 26, 2018, 04:58:37 pm
content: Yes, for the 50 years old extremely distorted sounds, you use 50 years old 1458 opamps (dual 741 old opamps). A fairly old but much newer TL082 or the lower noise TL072 are NEVER used for severe clipping because they produce the problem called Opamp Phase Inversion where the output suddenly goes positive when the input voltage goes to within a few volts from the negative supply. Its distortion is completely different to having ordinary clipping.I see the circuit as an ordinary 2-opamps preamp with a passive tone controls circuit at its output. The 2 opamps can easily use a single polarity supply and the tone controls circuit can have an added input coupling capacitor.

author: b_force
author_rank: Super Contributor
timestamp: November 26, 2018, 05:00:36 pm
content: Yes, you do proper clipping with diodes, not relying on how the opamp clips.That is asking for trouble.

author: dazz
author_rank: Frequent Contributor
timestamp: November 26, 2018, 05:10:15 pm
content: Quote from: BravoV on November 26, 2018, 04:57:23 pmThanks! I'll look into itQuote from: Audioguru on November 26, 2018, 04:58:37 pmOh, thanks for the info. Maybe Tl072 were being used in other amps, not this Lead 12. I'll socket them to try them both

author: dazz
author_rank: Frequent Contributor
timestamp: November 27, 2018, 01:05:05 am
content: Damn, I believe I made an incredibly stupid mistake. When I argued for the single supply charge pump instead of the symmetrical one because I would avoid ripple in the V- supply pin of the opamp, I completely overlooked the fact that the ripple from the positive rail would go straight to my virtual ground and to the opamps inputs. So actually the charge pump ripple will get massively amplified if I'm not mistaken. FML, am I dumb or what

author: spec
author_rank: Frequent Contributor
timestamp: November 27, 2018, 02:50:52 am
content: Quote from: dazz on November 26, 2018, 02:02:24 pmNo probs posting schematicIf you have a RRIO opamp, the output will swing to the positive and negative supply rails. With non RRIO opamps, like in the Marshall amp, the output only swings to within approx 2.5V of the supply rails. There are other aspects too, but not necessary to go into that. Just to give and example, using +-15V rails a TIL072 will produce a maximum output of 25V peak to peak, whereas an OPA191, for example, will produce 30V peak to peak. So for the same dynamic range as the TIL072 you could run the OPA191at +- 12.5V rails.About the circuit sound, I get your point 100%. I was forgetting that this is a guitar pedal where the sound of the circuit is most important. The TIL072 is sweet sounding opamp, even in clipping whereas the OPA191 would give less distortion, but it would be much harsher in clipping.I find all the above posts very interesting and, after all the discussions, can I summarize that the specification is to clone the Marshal circuit exactly, to get the same sound. This includes +-15V rails. The power input to the circuit must be 9V DC to be comparable with standard pedals. I think you actually said most of this in your original post.As well as the opamps, other factors affect the sound. These include: type of capacitor (especially), type of resistor, and the way that the supply rails perform dynamically.One suggestion is to use turn-pin sockets to mount the opamps. That way you can do opamp rolling if you ever want to. To use surface mount opamps, just mount them on DIL headers that plug into the socket, like standard DIL opamps.To summarize then, all you need from us is an approach to derive +-15 rails from a 9V input.I would strongly recommend the use of symmetrical split supplies, for the reasons so well put by Benta in reply #1One question, and apologies if this has already stated, but will you be running this pedal from a 9V PP3 battery?

author: dazz
author_rank: Frequent Contributor
timestamp: November 27, 2018, 03:13:14 am
content: Quote from: spec on November 27, 2018, 02:50:52 amThanks mate. Yes, that's exactly the goal, to replicate the original preamp as faithfully as possible.I think you're right, Benta hit the nail in the head and somehow I managed to miss the point badly. I'm exploring my options for a dual rail solution now. Maybe a mc34063 if I can get away with +9V/-9V by increasing the value of R8 to attenuate the signal since all of the clipping is done in the second opamp. Problem is that would put the signal in between the opamps well bellow 0dB and I fear that might make it prone to picking up noise, not sure.

author: spec
author_rank: Frequent Contributor
timestamp: November 27, 2018, 03:44:41 am
content: Thanks for reply.I think you quoted from my post while I was still editing it and the bit about the use of a battery is missing.Could you please confirm that you will be using a 9V PP3 battery to power this pedal?In any case, the approach that seems to be the most economical and lowest risk is to use a 9V to +-15V power supply module.I havn't done a good search but something like this is what I have in mind:https://www.aliexpress.com/item/8W-2-5-15V-to-15V-Dual-Output-power-supply-DC-DC-Step-up-Converter-module/32789671683.htmljust out of interest, what guitars do you have?PS: if you run the opamps from +-9V, as you suggested, the sound would change, not due to headroom or noise considerations but because the opamp itself is operating from lower rails. This effect is not so marked in newer opamps, but more so with the older opamps.

author: dazz
author_rank: Frequent Contributor
timestamp: November 27, 2018, 04:10:49 am
content: Quote from: spec on November 27, 2018, 03:44:41 amThat's up to the user/builder of the pedal. I personally never use batteries, I have a cheap 9V PS that I slapped a linear regulator in front. I don't require much current at all for the few pedals I use.Quote from: spec on November 27, 2018, 03:44:41 amI ordered a couple of those just two days ago, hehe. In principle for testing purposes, I'd prefer to integrate the converter in the board if I can find a reasonable solution, but using one of those is certainly a possibility.Quote from: spec on November 27, 2018, 03:44:41 amJust a 90's Fender Mustang MIJ and a cheap chinese strat copy. I'm not a great guitar player at all and I only play at home these days, don't really need more than that.Quote from: spec on November 27, 2018, 03:44:41 amDamn, I see. Thanks for letting me know, that seals the deal for anything but a full +/-15V solution then

author: spec
author_rank: Frequent Contributor
timestamp: November 27, 2018, 04:14:11 am
content: I mentioned about capacitors affecting the sound. Just in case, the CDE 715 and 716p orange drop polypropylene capacitors are very much favored in the guitar amp world (without going crazy on price). Data sheet attached.At the other end of the spectrum ceramic capacitors give a gritty sound which some players like. y5U dialectic in small leaded or surface mount versions have the greatest effect.

author: dazz
author_rank: Frequent Contributor
timestamp: November 27, 2018, 04:23:38 am
content: Damn it, look at this in the product description of those boards in aliexpress:1 Due to the diode partial pressure, the voltage of -Vo is lower than +Vo2 In order to obtain a more stable voltage, it is recommended that the output is greater than 15MAI guess I could add a diode in series with +V to make it more symmetric, and a resistors in parallel with the preamp to increase load and go above 15mA?Quote from: spec on November 27, 2018, 04:14:11 amThanks, I will include that info in the build instructions

author: spec
author_rank: Frequent Contributor
timestamp: November 27, 2018, 04:40:08 am
content: Quote from: dazz on November 27, 2018, 04:10:49 amYou can mount the PSU modules on the board, just like a component, or am I missing something?Quote from: dazz on November 27, 2018, 04:10:49 amYamaha FG350W (1981), Washburn WLD20S, and Crafter ST120 strat copy- can't play any of emJust sniped a Fender Deluxe 112+ combo for a song on ebay. The voice coil connecting wire has pulled of the cone and snapped under the dust cover- must get around to fixing it.

author: spec
author_rank: Frequent Contributor
timestamp: November 27, 2018, 04:51:08 am
content: Quote from: dazz on November 27, 2018, 04:23:38 amThat module is just the first one in Google. It is just an example. But all the same, the minimum current draw is no big deal and is a requirement for many PSUs. (even the LM317, LM337 three-terminal regulators).Minor differences in the PSU rail voltage should make little difference- you will see more variation in sound between different samples of the opamps, especially different ages and suppliers.

author: dazz
author_rank: Frequent Contributor
timestamp: November 27, 2018, 04:56:08 am
content: Quote from: spec on November 27, 2018, 04:40:08 amYeah, sure. In fact I have an extra pad in the pcb design to connect the boosted voltage from an external converter to give that option to the user in case s/he doesn't want to use the charge pump. Yes, I'm so stupid that I couldn't wait to make the pcb until I had a closed design. Now I'll need to modify it to use dual rails and whatever I put there to implement themQuote from: spec on November 27, 2018, 04:40:08 amOuch! I wouldn't know how to fix that. Congrats on the bargain!

author: dazz
author_rank: Frequent Contributor
timestamp: November 27, 2018, 05:07:32 am
content: Quote from: spec on November 27, 2018, 04:51:08 amSo if my preamp draws say 5mA, I need to add a dummy load to increase the current, right?Quote from: spec on November 27, 2018, 04:51:08 amOk, I was concerned that an imbalance in the rails might induce asymmetrical clipping and change the tone.Apparently people are usingLT1054 based charge pumps for the dual railswith good resultsI might just put that in my pcb and then the V+/V- pads to feed +15/-15V from an external board

author: spec
author_rank: Frequent Contributor
timestamp: November 27, 2018, 05:16:44 am
content: Quote from: dazz on November 27, 2018, 04:56:08 amThanks- it was a bargain. Apart from some work by a gorilla inside, the amp is in good condition- I was expecting some beat-up wreck.Fixing the speaker is fairly simple:Heat the dust cover to melt the glue and pull it off.Remove broken tail wire.Solder new tail wire to protruding voice coil wire.Araldite voice coil wire, solder joint, and tail wire to coneRe-glue dust cover.Of course, that is in theory- I changed the car battery yesterday- took four hours.But I have done steps 1, and 2 already, mainly to establish the fault.

author: spec
author_rank: Frequent Contributor
timestamp: November 27, 2018, 05:24:24 am
content: Quote from: dazz on November 27, 2018, 05:07:32 amYes, spot on. But if you were using batteries it would be benificial to get a PSU that had no minimum drain requirement.By the way, having a constant resistive load is good design practice- it helps all PSUs.Quote from: dazz on November 27, 2018, 05:07:32 amWith non RRO opamps, like the TL072, you get non-symetrical clipping even if the supply rails are exactly equal in magnitude.Quote from: dazz on November 27, 2018, 05:07:32 amSounds good to me.

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 27, 2018, 02:49:56 pm
content: Quote from: dazz on November 27, 2018, 04:23:38 amChinglish translations at its best.

author: dazz
author_rank: Frequent Contributor
timestamp: November 27, 2018, 05:52:05 pm
content: OK, how about this for a dual rail supply? It's based on two MC34063. The first one boosts the voltage to 18V, the second inverts it to -18V, then I can regulate them with some 7815 + 7915.Or maybe I can do what b_force suggested here and get rid of the inverting 34063?Quote from: b_force on November 26, 2018, 03:01:05 pmI have no idea what a flyback or 1:1 coupled inductors are, but that's what google is for, isn't it?Pros and cons of this last solution seem to be:-PROS-1.MC34063&220uH inductorsand of course the obiquitous 7815 & 7915 are pretty cheap and readily available2. Noise free regulated rails-CONS-1. Component count increases. I still have two DIP8 IC's, and i don't need the voltage divides and the coupling caps for the opamp bias, but the power supply section gets significantly more complex with 7 resistors, 10 caps (including the ones in the regulators) 2 diodes, 2 inductors and two linear regulators. If I could do the flyback thing this may not be an issue2. What the hell are those 0.22R & 0.24R resistors all about? Apparently they need to be that precise at those ridiculously low values or else the converter doesn't work.ETA: Of course I wasn't the first one to come up with this idea:https://www.diyaudio.com/forums/chip-amps/206788-using-2-mc34063-cmoy-virtual-ground.htmlDave even has a video on it apparently:

author: dazz
author_rank: Frequent Contributor
timestamp: November 27, 2018, 07:14:20 pm
content: Quote from: dazz on November 27, 2018, 05:52:05 pmWait a minute, there's feedback in those MC34063, so they're regulated, and if I calculate my values to have a high enough ripple frequency that it's not audible it won't matter for my application and I won't need those linear regulators, is that right? Because that would significantly simplify my circuit

author: Zero999
author_rank: Super Contributor
timestamp: November 27, 2018, 10:08:53 pm
content: You could build your own power supply but it's a pain. Just use a DC:DC converter module. Some examples are linked below, which have a regulated, isolated output. Additional filtering will probably be necessary to prevent audible noise, due to rectification of the RF in the op-amps. No doubt there are cheaper modules available off ebay, Amazon etc.https://uk.rs-online.com/web/p/isolated-dc-dc-converters/1714747/https://docs-emea.rs-online.com/webdocs/168f/0900766b8168f657.pdfhttps://uk.farnell.com/xp-power/itw0515s/dc-dc-converter-1w-15v-0-033a/dp/2422847http://www.farnell.com/datasheets/1847138.pdf?_ga=2.154705917.435636270.1543350785-1037805362.1481052103

author: John B
author_rank: Frequent Contributor
timestamp: November 27, 2018, 10:22:43 pm
content: Don't use the MC34063, far too much quiescent power wasted.I'm in the middle of some similar experiments with the 7660s, to see if I can reduce the switching noise. Probably a mix of both input and output filtering.It's going to be far more complicated than anything implemented in a commercial musical product, like pedals or guitar preamps, but for a DIY application, parts count isn't the problem, its noise and efficiency.You're going to have to filter the MC34063 anyways.I'm a little confused with your input power requirements. So you're not going to run it off batteries ever? Always an adapter? If that is the case, simply supply your pedal with a wide enough +/- range in the first place. Either with 2 floating DC adapters or an AC transformer supply. The input can be filtered and linear regulated, both on the + and - rails.Your dual power supply idea seems like the way to go at first, but you end up spending far too much time, effort and space trying to make it work, not to mention probably about 30% efficient

author: dazz
author_rank: Frequent Contributor
timestamp: November 28, 2018, 12:34:03 am
content: Quote from: Hero999 on November 27, 2018, 10:08:53 pmThose are just great simple, compact solutions, than you. If I can find those for cheap it's a done dealQuote from: John B on November 27, 2018, 10:22:43 pmAccording to the datasheet the quiescent current is 3mA. 6mA if I was to use two chips... just as much as the opamps themselves, so yeah, not ideal at all I guess.Quote from: John B on November 27, 2018, 10:22:43 pmWell, I have an adapter cable to plug a 9v battery or two Li-Ion cells to the 3.5mm socket in case I need it but what I was referring to is that I personally hardly ever use anything but a wall wart 9V supply. There's no requirement in the design for that though, If someone else wants to build the pedal and power it with batteries, that should be an option. I'll just provide a 9V/DC input in my board, at least that's the initial plan.Are you having trouble with noise with those 7660S's? Even using the boost to 30KHz pin? Bummer. I was hoping I wouldn't need to filter my supply if I could get the ripple frequency above 30KHz or soBTW, I said before that for my application (dual opamp) I would have an almost constant current draw from the supply, which I now think is BS. Wouldn't that be a problem with something like a 7660S since it would sag pretty bad (a couple of volts) if you get a 5mA spike in your current draw?

author: John B
author_rank: Frequent Contributor
timestamp: November 28, 2018, 01:14:25 am
content: For your application I'd probably skip the 7660 since it's max input voltage is around 12V depending on the model. Good for a 9V battery powered device though. Yeah, you can do voltage doubling like it says in the datasheet, but it's more hassle and space taken up. Obviously the manufacturer is happy for you to buy 4 chips vs 1.The boost pin brings the switching frequency to around 60kHz from memory, but its a wideband burst of noise that propagates to the output of the op amps. I have used them some simple designs (a splitter and mixer for smartphone earbuds/mics) where audio quality was not a concern, but haven't put them in studio quality gear yet.What I was getting at is that because you do not have to adhere to any standard for power, ie 9V DC input or batteries, you can cut a lot of hassle by simply providing enough voltage in the first place, and spend your effort on the signal path instead.

author: dazz
author_rank: Frequent Contributor
timestamp: November 28, 2018, 01:36:19 am
content: Quote from: John B on November 28, 2018, 01:14:25 amI just thought that by not adhering to an almost de facto standard like the 9V supply for guitar pedals I would be limiting the user's options. I was going to put there a 9V input in case you want to implement the power supply in the pcb, or two +16V, -16V for those who prefer to use an external converter, or a dual power supply of their own in which case they don't need to populate the power supply section in my pcb.Maybe I didn't make myself clear enough, I'm just designing the PCB, not the whole pedal. Well, it's designed to fit in a 1590BB enclosure but it could also go into a larger case with a complete power supply and power amp to make an amp instead of a pedal. Hope that clears that up

author: Zero999
author_rank: Super Contributor
timestamp: November 28, 2018, 09:52:51 am
content: Quote from: dazz on November 28, 2018, 12:34:03 amWhat are the budget constraints?Is just just a one off hobby project? Will it be made and sold commercially in small quantities i.e. 5 or so? Or are you going to make 1000s? It's often cheaper to use a module, once the cost of all of the parts, assembly and board space is taken into account.QuoteHow about using two 9V batteries or a 12VAC mains adaptor?A voltage doubler circuit with the 78L08 and 79L08 could provide a +/-8V supply when running off the mains and batteries can give +/-9V directly. It's possible to get sockets with a built-in switch to disconnect the batteries, when running off mains power.

author: b_force
author_rank: Super Contributor
timestamp: November 28, 2018, 11:03:56 am
content: What happened to the LT1615 , LT3460, LT1316 idea?

author: Zero999
author_rank: Super Contributor
timestamp: November 28, 2018, 11:24:34 am
content: Quote from: b_force on November 28, 2018, 11:03:56 amGoing by the prices of those ICs in Mouser, it was probably deemed too expensive.I don't see the point in doing hobby electronics on a shoe string budget. If you really want to save money, there's plenty of cheap junk available.

author: dazz
author_rank: Frequent Contributor
timestamp: November 28, 2018, 02:01:02 pm
content: Quote from: dazz on November 28, 2018, 12:34:03 amI meant to write "thank you" there, sorry about that.Quote from: Hero999 on November 28, 2018, 09:52:51 amIt's not a commercial product, I'm simply designing the PCB and publishing the gerber files for DIYers, so one-offs typically. Re: budget constraints, well, I don't have a figure in mind, just thought it made sense to use cheap components that are readily available regardless of where you live. The idea of building this thing was posted in a spanish forum by someone else, I believe using components that can only be purchased at mouser or digikey, with shipping costs and all, doesn't make too much sense. There's already a "cheap" solution available here:http://tagboardeffects.blogspot.com/2014/01/bipolar-voltage-converter.htmlhttps://tagboardeffects.blogspot.com/search?q=3005Quote from: Hero999 on November 28, 2018, 09:52:51 amI'm going to put a 9V input for those who want to use the pcb's supply and also V+ and V- inputs for those who prefer to use an external supply like those you suggest, or one of those cheap ebay +15V/-15V converter boards. The pcb is going to be 80x40 mm anyway because it's meant to be fitted in a 1590BB enclosure with pcb mounted potslike theseSince I need those pcb dimensions to get proper spacing between pots (see pic 1 for initial pcb design) I have plenty room to include the optional power supply.I think I might be able to make it work with +9/-9 but spec said that it wouldn't sound the same as running it at +15/-15:Quote from: spec on November 27, 2018, 03:44:41 amQuote from: b_force on November 28, 2018, 11:03:56 amQuote from: Hero999 on November 28, 2018, 11:24:34 amPoint taken. But it's not just about the money, smd components complicate things for DIYers.

author: Zero999
author_rank: Super Contributor
timestamp: November 30, 2018, 08:48:39 am
content: Since you want a symmetrical power supply, then how about using a boost converter to get 24V to 30V, then a rail splitter, such as the TLE2426 to get the 0V point?http://www.ti.com/lit/ds/symlink/tle2426.pdf

author: dazz
author_rank: Frequent Contributor
timestamp: November 30, 2018, 12:29:43 pm
content: Quote from: Hero999 on November 30, 2018, 08:48:39 amAccording thothisI can do pretty much the same thing with an opamp as a voltage follower, is that right? I'll try both anyway. I've ordered 7660s, 2577's, and MC34063's to try for the boost converters/inverters in different configurations. Will also try 7815s, 7915s, 7824s to regulate and filter the rails when needed, pi filters, and I'll get a few flyback transformers to try this:https://www.hobby-hour.com/electronics/flyback-regulator-lm2577.php.But yeah, I think the simplest approach is boost converter + filter + rail splitter. I already have a preamp based on TL082's with a virtual ground, not even a rail splitter, just a resistor divider, and it works great as long as the supply rail is properly filtered (using a 7809 for that one)At the end of the day, as always, the ultimate objective is to learn and I have plenty things to try already.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 30, 2018, 04:48:21 pm
content: If you're going to the trouble of a boost converter, you might as well do a buck-boost or SEPIC or the like, with a coupled inductor, so you get everything in one stage without any fumbling with rails.Tim

author: dazz
author_rank: Frequent Contributor
timestamp: November 30, 2018, 11:12:52 pm
content: Quote from: T3sl4co1l on November 30, 2018, 04:48:21 pmI had no idea that such a thing existed, thank you.

author: dazz
author_rank: Frequent Contributor
timestamp: December 22, 2018, 07:56:45 pm
content: I've been trying the MC34063 today to build a negative rail. It works great... except for the current draw. I know you guys said it's an old chip and very inefficient, but come on... the thing is drawing almost 300mA to convert 9V to -9V! I must be missing something

author: dazz
author_rank: Frequent Contributor
timestamp: March 10, 2019, 08:29:14 pm
content: Success! I eventually managed to pull this off with an MC34063 based boost converter and some LC filters. Works a treat!Attached pics of the schematic and the pcb. Thanks everyone for your help