Guitar preamp op amp tl072 input noise - Page 1
Forum section: electronics / beginners

author: Ale
author_rank: Contributor
timestamp: November 02, 2017, 09:06:49 am
content: Good morning,I'm trying to build a guitar preamp with a tl072. I'm learning very much trying and looking for schemas everywhere on the net, but I coulden't make any of them work properly. I really don't understand what's the problem with it, so I'm posting you the schema I made. It works, but I still have lot of noise from the input (if I ground the input the noise disappear, so I think it's not the power source). Further more, the guitar is not the problem, because it is very silent one having a faraday cage on the pickups.Can you help me to solve this problem, please?Ale

author: Andy Watson
author_rank: Super Contributor
timestamp: November 02, 2017, 10:12:43 am
content: R1 and R2 are very high values for the function they perform - I would reduce them to 100k or less and use the other op-amp as a unity gain buffer to generate GND.Pin 3, the positive input of your op-amp should have a return path to ground for the bias currents.Also, where are you getting the 12V from? Some power supplies - especially wall-warts, can couple significant mains interference into your circuit.

author: danadak
author_rank: Super Contributor
timestamp: November 02, 2017, 10:46:09 am
content: C7 will experience polarity reversal with signal, so should be non polar,or two polars back to back + - - + or - + + - connection.https://electronics.stackexchange.com/questions/161548/does-an-electrolytic-capacitor-degrade-each-time-it-receives-reverse-voltageAlso the G control circuit changes its frequency response with pot (RV1) setting.Is this what you want ?Regards, Dana.

author: sevenTech
author_rank: Contributor
timestamp: November 02, 2017, 11:44:27 am
content: Hi Ale,I don't think it is good practice to force vgnd (half of the power supply) to cable shielding...Most common is to connect input filter (resistor and capacitor) to virtual ground so that the signal will be moved ("offseted") to the working area of op-amp (around half of the power supply voltage (6V in your example)) ...Then on the output filter the signal will be moved ("offseted") back to the potential of the cable shield. As I started to draw, I've also shown unity gain buffer as Andy stated - in case if you are not familiar yet. Those R1 and R2 shall be reduced as noted before. If you will use this buffer, they can be relatively high thus reducing power consumption. Without unity gain buffer it should work too, but R1 and R2 should be waay lower values then. But often if you use dual op-amp package, you will just use one of them for such purposesBTW: Unused op amps in the package shall be properly terminated - otherwise it can add some noise.

author: Zero999
author_rank: Super Contributor
timestamp: November 02, 2017, 11:46:13 am
content: Quote from: Ale on November 02, 2017, 09:06:49 amThere's no decent path for the input bias currents. It's only working because the TL072 has tiny bias currents, so the leakage through C5 is making it work, (if you replace the TL072 with the NE5532, it won't work) but it will be very noisy. Move R6 to the other side of C5, so it connects directly to pin 3.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: November 02, 2017, 12:47:55 pm
content: R6 of 10k seems a bit on the low side, I believe most guitar amplifiers have input impedances on the order of 1 M?. Ofcourse, need to make sure you can still bias that opamp.

author: Ale
author_rank: Contributor
timestamp: November 02, 2017, 12:55:37 pm
content: Wow, thank you guys. So the schema should be something like this right? I didn't know that I should use the op amp to get a proper VGND.Anyway, the power source is a instrument power supply which should be very stable. It has many 9V, a 12V and a 18V output.Dana, I would like to change the gain through RV1, is that correct?

author: Zero999
author_rank: Super Contributor
timestamp: November 02, 2017, 01:21:51 pm
content: How did you choose the value for C6?It will have an impedance of 100k, at frequencies above 15.9Hz, which will be the upper cut-off frequency, when RV1 is set to the maximum value of 100k.

author: jm_araujo
author_rank: Regular Contributor
timestamp: November 02, 2017, 01:23:05 pm
content: It seems to me that the jack connections are reversed in the schematic. Tip is signal and sleeve is ground.

author: Ale
author_rank: Contributor
timestamp: November 02, 2017, 01:41:48 pm
content: Yes, jack connections were upside-down, I corrected them.Now the powering part is working fine, I can hear a little noise when jack input is disconnected, but when I connect it all shout down and I hear nothing.I follow the instruction of SevenTech, I used gnd of power supply for the shield, is that correct?Thanks again

author: jm_araujo
author_rank: Regular Contributor
timestamp: November 02, 2017, 02:03:29 pm
content: You should look at the cut off frequencies of the RC filtering, as Hero999 said.At the output R4/C7 have a cutoff of 159Hz which lands you right in the middle of the guitar scale. You should increase C7 at least a magnitude.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: November 02, 2017, 02:27:44 pm
content: I presume you mean hiss. Guitar inputs are prone to hiss because they are relatively sensitiveandhigh impedance. (a few mV at 47k or so) Under these conditions surprisingly it's theresistorsthat almost always generate the hiss, not the transistors. Using good quality metal film types, especially for high values in the signal or feedback path, will make a huge difference.

author: Zero999
author_rank: Super Contributor
timestamp: November 02, 2017, 03:08:11 pm
content: Quote from: jm_araujo on November 02, 2017, 02:03:29 pmYou've got the decimal place wrong. The cut-off is 15.9Hz, so it's no surprise it's not working. Reduce the capacitor to 100pF.

author: jm_araujo
author_rank: Regular Contributor
timestamp: November 02, 2017, 03:44:36 pm
content: 2nd line of my text was referring to the output high pass (DC blocking) filter, and the math was right for it (.1u C7 with 10k R4)

author: Zero999
author_rank: Super Contributor
timestamp: November 02, 2017, 04:33:41 pm
content: Quote from: jm_araujo on November 02, 2017, 03:44:36 pmOh I see, yes, no wonder hardly any signal is getting through. I'd remove R3 and make C7 100µF.

author: Ale
author_rank: Contributor
timestamp: November 02, 2017, 04:37:30 pm
content: Something like this? I will try it soon fisically to test it.

author: jmelson
author_rank: Super Contributor
timestamp: November 02, 2017, 04:54:45 pm
content: You are feeding +/- 12 V directly to the pins you mark as +6 / -6 V. Did you mean to have a resistor there? How about a Zener to drop the 12 V to 6 V?You have no DC path from the non-inverting input of the op-amp. This is likely the cause of most of your problem. You say "noise" but I'm guessing you really mean distortion, as the output may be floating at least several Volts away from ground with the floating input. Connect a relatively high value resistor to ground from pin 3.Also, the TL072 is not known for low noise. You might try my favorite, the OP275, direct drop-in pin for pin compatible.Jon

author: sevenTech
author_rank: Contributor
timestamp: November 02, 2017, 05:02:11 pm
content: Quote from: Hero999 on November 02, 2017, 04:33:41 pmR3 must be present, if just removed it will be unity gain without possibility to change gain. If shorting to virtual ground it will saturate. (Vout = (1+ (R2/R1)) * Vin)As to that output filter, I will just increase R value to 100K. C can be 0.1uF... (fc = 1/(2*pi*R*C) gives 15.9Hz). Non-polarised 100uF caps are pretty bulky and pricey.

author: sevenTech
author_rank: Contributor
timestamp: November 02, 2017, 05:15:34 pm
content: Quote from: Ale on November 02, 2017, 01:41:48 pmI think that this circuit should work...Just fix the output filter. I don't know why you hear nothing...Recheck wiring if there isn't some problem :-) And remove C6 for simplicity. You dont need that for now.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: November 02, 2017, 05:28:14 pm
content: jmelson +6V is only 12V when measured relative to the 0V and not the vitrtual ground. No issues here:)

author: jmelson
author_rank: Super Contributor
timestamp: November 02, 2017, 08:24:34 pm
content: Quote from: alexanderbrevig on November 02, 2017, 05:28:14 pmOh, I see what he's doing, using a single +12 supply to make + and - 6 V from a synthetic analog ground! Yes, now I got it.Jon

author: Audioguru
author_rank: Super Contributor
timestamp: November 03, 2017, 02:34:49 am
content: The extremely low value of 10k for the original input resistor loaded down the signal (especially high frequencies). Now that the input resistor is a normal 1M the output from the magnetic pickup will be much higher (especially high frequencies) and then the gain will be much too high. Change R3 to 10k.C6 muffles all high frequencies. Remove it.

author: Zero999
author_rank: Super Contributor
timestamp: November 03, 2017, 08:46:15 am
content: Quote from: sevenTech on November 02, 2017, 05:02:11 pmSorry I meant R4, not R3. The problem, whatever load the amplifier is connected to, goes in parallel with R4, so there's no point in having it there. The AC coupling capacitor just needs to be as big as possible. Don't use non-polarised capacitors. Use two 220µF aluminium polarised capacitors connected back-to-back.There's still the issue of C6 cutting high frequencies.

author: Ale
author_rank: Contributor
timestamp: November 03, 2017, 08:58:08 am
content: I modified the schema, is now correct?

author: danadak
author_rank: Super Contributor
timestamp: November 03, 2017, 10:26:04 am
content: RV1 at one extreme will short the output of the amp to the output of theref buffer. TL072 is protected from output shorts, but thats a bad ideato rely on that in your schematic.Also I assume RV1 is for G adjust. Therefore remove the Vgnd connectionfrom - input, connect the unconnected end RV1 to a R to ground. The R willset max gain, prevent amp from going open loop.Regards, Dana.

author: Ale
author_rank: Contributor
timestamp: November 03, 2017, 11:24:47 am
content: I made the changes you suggested and build the circuit on a breadboard. It turn out that if I connect the input shield on GND no sound are produced. If I connect the input shield on VGND I hear the sound of the guitar, but with lots of noise. Why is that?thanks alot guys

author: Zero999
author_rank: Super Contributor
timestamp: November 03, 2017, 11:28:13 am
content: Quote from: Ale on November 03, 2017, 11:24:47 amYes, that's much better. It was my fault for saying "remove R3" when I meant R4. It will only give a maximum gain of 11 though, which might not be enough. The original circuit was designed for a gain range of 1 to 100. It depends on how much gain you need.

author: jm_araujo
author_rank: Regular Contributor
timestamp: November 03, 2017, 11:58:46 am
content: It should make no big difference, the input and output capacitors take care of any DC bias. Can you post a picture of the assembled circuit?Where are you getting the 12V from and where are you connecting the circuit to?

author: Ale
author_rank: Contributor
timestamp: November 03, 2017, 04:42:24 pm
content: Ok guys, I do lots of tests and the result is the schema I attached to the post.I tried to use my smartphone with 3.5" cable and I discovered that:1-I have no noise from cables2-No noise from breadboard3-This circuit works damn good4-Using 220uF for C6 and C7 I lose too many bass freq. 470uF are better and more balanced.5-18V is better to power the tl072 (I'm using tl074, but it's the same) and get +9V/-9VThe problems are:1-with the guitar I still have lot of noise and I don't understand why. Using the same cable to my soundcard (a professional one) I have no noise at all!I also used the faraday cage around the pickups and they are very silent in every situation I played. I also tested another guitar, same problem.2-with the gain of 1 (the minumum for the non inverting amplifier if I'm not wrong) the sound is quite high, if I turn the RV1 it goes overdrive easily. I tried a resistor between C5 and pin3, but nothing change. I also tried to buffer the amp (gain 1 I think), same problem.Do you have some ideas?thanks

author: Damianos
author_rank: Frequent Contributor
timestamp: November 03, 2017, 09:22:45 pm
content: Your circuit has some drawbacks!See the attached one ... in the simulation gives good results; if I have the time, I will try it on a breadboardI will try to write some notes tomorrow...As a start, on your circuit:- remove the C6, the output is always positive against ground- remove C1 and C3, they are coupling any AC component from the power line to the input...- put the C1 in series to R3 and connect it to the GND and not to VGND, so as to not amplify the offset......BTW what you mean with "noise"? Is it thermal noise, is it hum, radio interference or something else?

author: danadak
author_rank: Super Contributor
timestamp: November 03, 2017, 11:36:41 pm
content: Tie the unconnected end of your pot to the wiper to reducepotential coupling/pickup.There should be a cap from + 6 to - 6 to shunt noise away fromopamps. A combination like C1 and C3 but strapped across 12V supply.My mistake in making non polar cap out of two polars, I kept thinkingyou were on "normal" split supply, lost focus....thanks Daminaos for catchingthat.Do you have a scope and a sine wave source so that you can feed the sineinto your layout and post a pic of the output. Or hand draw the signal outyou see.Regards, Dana.

author: Audioguru
author_rank: Super Contributor
timestamp: November 04, 2017, 12:24:32 am
content: We are still waiting for a description of the "noise". Is it hum, hiss, buzz interference from AC lighting or something else?

author: Ale
author_rank: Contributor
timestamp: November 04, 2017, 08:50:59 am
content: Hello again,I implemented the new schema here, it appears a little bit better, but not perfect. Still no noise from smartphone, but lot of hiss and buzz from guitar.Anyway I noted this, when I turn the guitar tone pot down the noise redure alot, almost disappear. Of course it turn down the high frequencies too. I have the same effect adding a 0.1uF ceramic between the jack input and gnd. Turning the gain up I also increase the noise. Could it be a problem bound of hi-freq? how to rid off of that?thanks

author: danadak
author_rank: Super Contributor
timestamp: November 04, 2017, 10:00:48 am
content: Does the guitar have its own preamp ? Referring to thetone control. is that just a passive pot and cap, or anotherpreamp ?Is your TL072 grounds tied to amplifier ground ? Single pointground on your board.Try taking your design, with a set of headphones and 2 9V batteries,outside away from indoor EMI sources, like TV, Microwave, Wirelessphones, laptop, PC.... and see how that performs.I get the impression that guitar pickup is creating so little output thatyou have to run the TL072 at very high G and that amps up the noisequite a lot.One other issue, R6 creates a very hiz pickup point, you might try 10Kand see how that affects performance. 10K probably not a significantload to pickup. What is pickup, magnetic or piezo. The design of an ampquite different between the two. If its piezo you need hiz, 10K not would notbe a good idea. Piezo wants more like 10M Zload, otherwise the signal is solow level that causes much higher G to be needed which aggravates noiseperformance. In this case a typical approach is to use a JFET design phys-ically close to the pickup. Or the TL072 close to pickup.Consider locating the design right at the pickup to get rid of cabling (readantenna) coupling effects.If you can't some comments on cabling -http://www.liutaiomottola.com/PrevPubs/Piezo/CoaxTransducer.htmPost a picture of your construction technique.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: November 04, 2017, 10:45:30 am
content: One other issue, CM range. From the datasheet -QuoteSo your bias point should be 8V, not 6V, to get maximum range w/o distortion.This may or may not be an issue depending on what the pickup max output levelsare. In fact you could create a distortion control by purposely offsetting bias pointwith a pot. For another day.Regards, Dana.

author: Ale
author_rank: Contributor
timestamp: November 04, 2017, 11:32:37 am
content: Sorry I didn't mention this before, I'm using an electric guitar with magnetic passive pickups (no preamp in between). I'm testing with headphones and not with a guitar amplifier. With 10K on R6 it produce a little less noise. (Also thanks for the lutherie link, it's very interesting for me).Anyway, as you suggested I tested the circuit with 2 9v batteries in series and...no noise at all! Neither at maximum gain!And it sounds great!After that I tested 4 or 5 power supply and the best is one of an old a laptop, but still not noiseless.Is it possible to eliminate the noise from the power supply or should I use always batteries? I'm just using a powersupply with multiple isolated output I thought it was a good choise.ps, I'm posting the last schematic if anyone else from the forum needs it.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 04, 2017, 11:46:52 am
content: We sometimes use that switching node of the input connector (pin 2 of J1) to short or mute the input to avoid picking up noise when nothing is plugged in. So, instead of connecting pin 2 over to pin 1 (ignoring the switching function), we would connect pin 2 to pin 3 (ground). So that the input is shorted to ground when nothing is plugged in.

author: Zero999
author_rank: Super Contributor
timestamp: November 04, 2017, 06:48:14 pm
content: Try using the potentiometer as an attenuator before the op-amp, rather than varying the gain.Note that this circuit is no good for op-amps with a bipolar input, such as the NE5532, because the bias currents pass through the potentiometer and will generate a scratching sound, when it's adjusted, but those sorts of op-amps are noisy in high input impedance designs.

author: Damianos
author_rank: Frequent Contributor
timestamp: November 04, 2017, 07:58:51 pm
content: Quote from: Ale on November 04, 2017, 11:32:37 amSome notes:- The C3 and C6 do almost nothing for the (low impedance) ripple of the power supply; increase the impedance as I show on my schematic.- The R1 and R2 can have ten times higher values ...- The U1B, as it is used, does nothing useful! It supplies a few pA by loading the divider by ... a few pA!- The C5/R6 values do not agree with a guitar pick up characteristics and an audio amplifier...- The RV1, as it is connected, moves the operating point of the amplifier!- It is generally a bad idea to change the gain of an amplifier, it changes also other characteristics of it.- The value of C7 is very high unnecessarily.- The value of R4 is low and consumes power from the output of the amplifier.A little about the circuit I proposed:- Instead of one amplifier with "huge" gain, the amplification is performed in two stages.- The R8 and C7 constitute an additional filter for the sensitive input.- The R2 and R5 set the bias to about the middle of the operating voltage.- The C2 and C5 are blocking the DC, so as the DC voltage amplification is equal to one. They also affect the amplification at lower frequencies.- The C10 is reducing possible radio interference and has to be adjusted according to the characteristics of the guitar's pick-up.- The C8 and C9 are defining the upper frequency limit.A quick test on a breadboard:Differences from the schematic: C4=470mmF, R8=34k8, C10=0, RV1=560k fixed, R3=100k, C8=C9=22pF.Results:Amplification inside the passband: ~115Passband limits (~0.7 of the voltage on 1kHz): ~12.5Hz ... ~34kHzMaximum output voltage, before visible clipping: ~2V RMSRegards,Damianos

author: Ale
author_rank: Contributor
timestamp: November 05, 2017, 01:32:09 pm
content: Thank you Damianos, I tested your schema and it works pretty well. But the last I made worked well too.I rechecked the connections inside the guitar, cables, etc. With batteries I have a very silent preamp, but with a power supply it works really bad. Also with grounded input it produce very loud noise.The thing is... I always tried with standard ones, also my music power supply start with a switching one (I think).As I see for this kind of applications the best way is to have a toroidal power supply with multi isolated output. And I'm thinking to build one from scratch. Do you think it's a good idea? Which one to buy? I thought to build one with 4 1n4001 diode, some capacitors and lm7815 / lm7915 to regulate +15V/-15V.

author: danadak
author_rank: Super Contributor
timestamp: November 05, 2017, 03:49:23 pm
content: Of build a simple transformer based linear power supply.Regards, Dana.

author: Zero999
author_rank: Super Contributor
timestamp: November 05, 2017, 03:52:45 pm
content: Quote from: danadak on November 05, 2017, 03:49:23 pmIs a regulated power supply really necessary here? The supply rejection of the TL072 is pretty good and should be adequate, as long as sufficient capacitance is added to the rectifier.

author: danadak
author_rank: Super Contributor
timestamp: November 05, 2017, 09:41:11 pm
content: Its complicated -1) TL072 not well characterized for PSRR, no curves vs freq.2) PSRR is not symmetrical for an OpAmp, one rail to the other.3) Human ear dynamic range >> PSRR min of 072.4) No short circuit or thermal protection.But then -5) Rejection of regulator worse than TL072, but combined approaches ear response.6) Regulators cost money.7) Regulators introduce transient response issues.Regulators add noise.I give up, lets just go with the batteries that explode and burn and....Regards, Dana.

author: Zero999
author_rank: Super Contributor
timestamp: November 05, 2017, 10:44:15 pm
content: Quote from: danadak on November 05, 2017, 09:41:11 pmYou raise some interesting points. Another alternative to regulator(s) is an emitter follower with a RC circuit. It drops some of the supply voltage but can work quite well. I've done it before for the TDA2003, which has a very poor supply rejection, of just 36dB and was a poor choice for a mains powered amplifier. It should be more stable and better at rejecting high frequencies, than a linear regulator.

author: Audioguru
author_rank: Super Contributor
timestamp: November 06, 2017, 02:39:47 am
content: An inductive magnetic guitar pickup resonates with the capacitance of the shielded cable that connects it to the amplifier. The amplifier input impedance has always been about 1M ohms in vacuum tube days so that the pickup resonance flattens the frequency response, or a higher amplifier input impedance is used to give some high frequency boost. But in this thread the load for the pickup was as low as 10k or a 100k volume control that muffles high frequencies and reduces the output level.

author: Ale
author_rank: Contributor
timestamp: November 06, 2017, 09:52:36 am
content: Yes Dana, it was exactly what I was thinking about. Anyway, this evening I will try to build this and test with and without regulations, evenually also the emitter follower. Hero999, the schema you posted is for reduucing ripple, right? is it possible to use on a transformer which gives +15V/-15V or should I use only the positive and then the voltage divider?thanks again

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2017, 10:00:11 am
content: Quote from: Ale on November 06, 2017, 09:52:36 amIf will work fine for a single supply, as is. If you want to use it for a dual supply (+/-15V) then a use a PNP transistor with the capacitor's polarity reversed, for the negative supply.

author: Damianos
author_rank: Frequent Contributor
timestamp: November 06, 2017, 03:18:06 pm
content: Quote from: Ale on November 05, 2017, 01:32:09 pmFor this circuit, it is not needed a regulated power supply, anything between 12 V and 30 V will do the job. In addition, it is not necessary a bipolar power supply, as we have already seen.The problems are filtering and shielding.Put the circuit inside a metallic box and connect all the common/ground referenced parts to one point.In the past, when I played with sensitive audio circuits, like this one, I found a simple solution (I am a "fanatic" of simplicity):- Separate power supply for the sensitive circuit.- An external power supply has serious advantages, for safety and interference.- Shielding of the amplifier. A small tin box of candies will make a big difference.- Additional filtering of both lines of the power supply. Due to the low current consumption (less than 10 mA), this can be performed by using RC circuits.In the example that shown in the picture, increasing the values of resistors and C1, filtering and isolation of interference is improved.

author: Damianos
author_rank: Frequent Contributor
timestamp: November 06, 2017, 03:29:24 pm
content: Quote from: Hero999 on November 05, 2017, 10:44:15 pmIn this circuit it is needed a resistor from the base of the transistor to ground, because if the capacitor is charged to the peak voltage then it can not do something and the ripple will pass from the collector to emitter ...

author: Damianos
author_rank: Frequent Contributor
timestamp: November 06, 2017, 03:36:23 pm
content: Quote from: Audioguru on November 06, 2017, 02:39:47 amAdditionally to the loading resistance, I think that an adjustment of the capacitance is also needed, for the correct frequency response. I am not an expert on that but I have already mentioned it!

author: macboy
author_rank: Super Contributor
timestamp: November 06, 2017, 06:06:31 pm
content: If you want to try a lower noise, high quality FET input dual audio op-amp, look at TI OPA2134, OPA2132, OPA2604. These are Burr-Brown designs (TI acquired them).

author: Ale
author_rank: Contributor
timestamp: November 06, 2017, 09:10:00 pm
content: Well, tonight I hadn't enough time to try all. But I build a simple power supply with an EI transformer I have here, without regulators, all over a breadboard, without crossed cable or shielding and... it works pretty well, there is some noise, but really low and acceptable. Now I'm eager to try with a toroidal one, with shielded cable, etc... but I have to buy onemacboy, the OPA you post seems to be very very good, but they are really expensive. I will probably try one in the futureMeanwhile, thank you all, I learnt alot here and you are all very kind. Now I have to order some stuff, eatching the board and so on, it will take me daysI will post the finished project as soon as possible.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: November 06, 2017, 10:02:58 pm
content: Quote from: Ale on November 06, 2017, 09:10:00 pmPlease post the final schematics when it's ready. I would like to see how it works in spice. Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2017, 09:18:49 pm
content: Quote from: Damianos on November 06, 2017, 03:29:24 pmAnother resistor may help, but it's not a necessity. There will be some voltage drop across R1, due to the base current.There's a compromise between voltage loss and ripple rejection. The main reason why I added it to my circuit is because the ripple was noticeable on the output of the amplifier, especially at the higher harmonics. I didn't measure the ripple on the supply but estimate it was on the order of 100mV.Quote from: Ale on November 06, 2017, 09:10:00 pmThere are better op-amps around but I think the TL072 is good enough for this. It just needs to be configured correctly.Here's another circuit. This one uses an op-amp as a buffer and a 1M biasing resistor, before the potentiometer.EDIT:DO NOT BUILD THE ATTACHED THIS CIRCUIT. IT WON'T WORK!

author: Damianos
author_rank: Frequent Contributor
timestamp: November 08, 2017, 10:31:45 pm
content: What about the operating voltage range of the second amplifier?

author: Audioguru
author_rank: Super Contributor
timestamp: November 09, 2017, 12:54:59 am
content: The second opamp is biased WRONG at GND instead of being correctly biased at half the supply voltage like the first opamp.HINT: add 1 coupling capacitor and one resistor to the half supply voltage divider.

author: Zero999
author_rank: Super Contributor
timestamp: November 09, 2017, 09:17:08 am
content: Yes, the common mode range of the second op-amp is exceeded. The lower side of RV1 should have been connected to half the supply voltage. Note that wouldn't be a good idea with a bipolar op-amp because the high bias currents would result in noise when the pot is adjusted, but it's a non-issue for a J-FET op-amp.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: November 09, 2017, 11:11:26 am
content: Perhaps someone may find this interesting.

author: Zero999
author_rank: Super Contributor
timestamp: November 09, 2017, 01:49:46 pm
content: Quote from: HoracioDos on November 09, 2017, 11:11:26 amThe amplifier is overdriven so it's clipping. It may not behave exactly the same, in real life, if you conducted the same test with a signal generator, because this kind of behaviour might not be modelled accurately. Reduce either the input signal or the gain of the amplifier.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: November 09, 2017, 01:52:41 pm
content: Quote from: Hero999 on November 09, 2017, 01:49:46 pmAdvice taken!!

author: Ale
author_rank: Contributor
timestamp: November 11, 2017, 09:23:04 am
content: Sorry Hero999, I don't understand why to use the first amp as a buffer. Does it add more stability to sound? reduse noise? as far as I know it should change the impedance from high to low. Is it an important step to do befor amplify the signal?thanks

author: Damianos
author_rank: Frequent Contributor
timestamp: November 11, 2017, 06:10:08 pm
content: Quote from: Ale on November 11, 2017, 09:23:04 amIn my opinion, the input buffer is a wrong choice. It degrades the signal to noise ratio, because it adds its own noise without amplifying the signal. Additionally, it drives a load that is of the same order of magnitude as is its input. So, there seems to be no reason to use it.Also, high amplification, in one stage only, decreases the available bandwidth of the amplifier...As there are two amplifiers, it is better to share the amplification work between them, as I have already shown.By the way, what are the specifications of the pickup? Is there a datasheet? My memory does not help me to remember many things, from a project years ago!Also what is expected from this amplifier?

author: Zero999
author_rank: Super Contributor
timestamp: November 11, 2017, 07:26:25 pm
content: Quote from: Damianos on November 11, 2017, 06:10:08 pmThe reason for the buffer was to increase the input impedance from 100k to 1M. It will make a negligible contribution to the noise of the amplifier, because it's configured for unity gain. Splitting the gain between two amplifiers, will not make any difference to the noise. For low noise, the amplifiers should be connected in parallel but I doubt noise is much of a concern. The thermal voltage noise, in a pick up with a DC resistance of 15k, at room temperature will be 15.6nV/Hz0.5, which will be added to some thermal acoustic noise, in the air and strings, as all matter above zero temperature vibrates randomly. There will be more noise in the volume control (maximum when set to 50%) which could reduced in value, if it's a concern.I agree about spreading the gain across both amplifiers, to increase the bandwidth, although this would require more components. I don't know what the highest frequency of interest is for a guitar amplifier. I don't think it's 20kHz. The TL072 has a gain of around 300 at 10kHz, so a gain of 100 is doable, although more negative feedback is good, so reducing the gain and splitting it may help.

author: Ale
author_rank: Contributor
timestamp: November 12, 2017, 07:39:52 am
content: The guitar frequency range is about from 80hz to 1200hzAnyway it seems to have also some harmonics aboveWhat I had in my mind is to build a first pedalboard stage with one input for the guitar and one for the voice (with differential op-amp configuration to drive signal from balanced to unbalanced). Both the signal, guitar and voice should be slighly amplified and should have overdrive and fuzz (I will add that with a couple of fast switching diode). After this stage the unbalanced signals will pass through the other pedals, modulation, delay, then one output to the guitar amplifier and the voice to a DI Box (to rebalance the signal) and to the mixer. I don't exclude to add some tone control or a 3 band equalizer in the middle.

author: Zero999
author_rank: Super Contributor
timestamp: November 12, 2017, 05:35:07 pm
content: Going by those graphs, most of the energy is below 6KHz, so a bandwidth of 10kHz is more than sufficient.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: November 12, 2017, 08:48:41 pm
content: I don't know about those values..?Here's a random pinch harmonic from a DI recording of my guitar. I play a C5 (523 odd hz) with a pinch harmonic on G5 (783 odd).You can see that there's a lot of content above the theoretical sine max.There's pinch harmonics, natural harmonics and other fun abuse. It's a fun challenge, next time I'm recording I'll check what my max freq isBand-pass just added for reference.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 12, 2017, 09:54:28 pm
content: Quote from: Hero999 on November 12, 2017, 05:35:07 pmAccording to the pitch chart, a guitar highest fundamental is only ~1.5KHz and virtually no instruments reach ~5K fundamental.Note, however, that it is the HARMONICS ("overtones" in musicspeak) that distinguish one instrument playing A 440Hz from another playing the same pitch. Else everything would sound like a boring sine-wave from an oscillator. The standard for "conventional" audio bandwidth is the 10 octaves from 20Hz to 20KHz. However there are fewer and fewer listeners who can hear over 10KHz, and MPEG compression does no great benefit to very high frequency response.https://en.wikipedia.org/wiki/Template:Vocal_and_instrumental_pitch_ranges

author: Audioguru
author_rank: Super Contributor
timestamp: November 13, 2017, 02:32:36 am
content: Most musical instruments have harmonics that reach at least 15kHz. But an electric guitar has a magnetic pickup that resonates with the cable capacitance at 5kHz and makes a pretty high peak in the response that the cheap speaker reduces in level to be fairly flat to about 6kHz.The magnetic pickup produces a level so high that this low gain preamp is used:

author: HoracioDos
author_rank: Frequent Contributor
timestamp: November 13, 2017, 04:04:53 am
content: What about this?

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2017, 09:03:13 am
content: Quote from: HoracioDos on November 13, 2017, 04:04:53 amMuch better.Quote from: Audioguru on November 13, 2017, 02:32:36 amSo you think the gain is too high?Wouldn't it be better to put a pre-amplifier inside the guitar, right next to the pick-up? That would avoid any resonance or noise pick-up from the cable.

author: Ale
author_rank: Contributor
timestamp: November 13, 2017, 09:14:53 am
content: @HoracioDos I don't understand the node RV1 and RV2, one is to control the volume which comes from the first amp to the input of the second one, but the other?

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2017, 09:52:06 am
content: Quote from: Ale on November 13, 2017, 09:14:53 amIt represents the potentiometer, which is split in to two resistors because LTSpice doesn't have a potentiometer model.

author: Audioguru
author_rank: Super Contributor
timestamp: November 13, 2017, 01:47:24 pm
content: Why are the values for C6 and C7 so high? They will cause gain at earthquake rumbling frequencies at 0.8Hz and lower. Use 220nF film capacitors for each cutoff at 36Hz then both create a cutoff at 72Hz.EDIT: Why is the output capacitor value so high that it will take weeks to charge? Calculate a suitable value for the load resistance it will have (100k? Then the capacitor will be 100nF).The gain required by the opamps depends on the sensitivity of the pickup and how hard you strum the strings.High frequency resonance is used and is needed because guitar speakers have poor high frequency response like an old telephone or AM radio.

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2017, 02:17:14 pm
content: Quote from: Audioguru on November 13, 2017, 01:47:24 pmThe high value capacitors are presumably relics from my design, which had much lower value gain setting resistors.The value of the output capacitor is unknown because no one knows what load it is going to be driving and it's better it being way too big, than too small.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: November 13, 2017, 02:49:54 pm
content: Few more changes.

author: Audioguru
author_rank: Super Contributor
timestamp: November 13, 2017, 02:57:31 pm
content: On your new schematic, why do you have R3 that throws away half your signal level?

author: HoracioDos
author_rank: Frequent Contributor
timestamp: November 13, 2017, 03:11:05 pm
content: Quote from: Audioguru on November 13, 2017, 02:57:31 pmYes, I was trying to flatten frequency response towards 100Hz. Does it make sense?

author: HoracioDos
author_rank: Frequent Contributor
timestamp: November 13, 2017, 03:39:21 pm
content: I'm sorry for hijacking the thread. Rollback to V2 with proposed capacitors values.

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2017, 03:50:06 pm
content: Lots of those AC coupling capacitors are unnecessary.

author: Audioguru
author_rank: Super Contributor
timestamp: November 13, 2017, 07:28:09 pm
content: Hero: "Lots of those AC coupling capacitors are unnecessary."Without those capacitors then the two opamps are a low frequency oscillator with positive feedback from the output to the input.HoracioDos: "Yes, I was trying to flatten frequency response towards 100Hz. Does it make sense?"You reduced the input resistance of the preamp to 1.5M and kept the same 1nF coupling capacitor but you attenuated the signal with a voltage divider. The 5kHz peak is probably needed when the input resistance of the preamp is 3M and the value of the coupling capacitor can be changed to produce any cutoff frequency you want. It was crazy to use a voltage divider to reduce the signal from the pickup.Now I am guessing because I cannot make quotes on this website and I cannot see the thumbnails while I am replying.

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2017, 07:46:54 pm
content: Quote from: Audioguru on November 13, 2017, 07:28:09 pmPositive feedback from where? C3 and C4 firmly bypass R3, R4 and R6 to 0V, attenuating any feedback. The Bode plot shows no signs of anything which would cause oscillation, between 10mHz and 10MHz.By the way, I've had no problems with making quotes on this site.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 13, 2017, 09:36:23 pm
content: Quote from: Audioguru on November 13, 2017, 01:47:24 pmIt is trationally pretty large becausewe don't knowwhat is the load impedance. It could be as low as 100-300 ohms, but more typically 600-1000 ohms, or even 10K. If we set the lowest expected load impedance at 300 ohms and the lowest frequency of interest at 70 Hz, we would need a 35uF capacitor to maintain a 3dB rolloff at 35Hz.QuoteAnd that is why a "drain resistor" of perhaps 1~10K from the output side of the capacitor to ground is commonly seen. And/or using the switched node of the output connector to short the output capacitor to ground when there is no load applied.QuoteAnd it would appear that even modern guitar speakers continue to have terrible frequency response (by modern hi-fi standards).But the lousy frequency response and peaky frequency response is all part of the sound of the instrument.So "improvement" it to modern standards is apparently not very desirable in this context.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 13, 2017, 09:43:16 pm
content: Quote from: Audioguru on November 13, 2017, 07:28:09 pmYes, that is one of the unfortunate things about this forum.Dunno whether is is a fundamental design of SMF, or just how this forum is configured?In either case, pretty annoying. I have to open a second copy of the thread so that I can see the graphics in the previous posts.

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2017, 09:49:43 pm
content: Oh, I see the issue with seeing schematics whilst replying. That's one of the reasons why I usually make my attachments inline, using  tags.Quote from: Audioguru on November 13, 2017, 07:28:09 pmYes, C5 and C8 are too small. I can't believe I missed that, especially with the Bode plot staring me in the face.

author: Audioguru
author_rank: Super Contributor
timestamp: November 13, 2017, 11:49:12 pm
content: Quote from: Hero999 on November 13, 2017, 07:46:54 pmWithout seeing the schematic during my reply I forgot that the gain of this preamp is only 7 times and much less at low frequencies so of course the filter of the virtual ground prevents positive feedback.QuoteThis quote looks different than before. I think the quote I am making in this reply will work.

author: Zero999
author_rank: Super Contributor
timestamp: November 14, 2017, 12:00:53 am
content: Now the gain is much lower, there's no point in having the gain split across two amplifiers, as the bandwidth of one, is more than enough. The circuit can be simplified greatly.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: November 14, 2017, 12:47:15 am
content: Amazing shape!

author: Ale
author_rank: Contributor
timestamp: December 04, 2017, 06:17:45 pm
content: Hello everyone! I've been a little busy lately, but finally I made out. This is the final circuit I build, on the breadboard it works great, with very very (very) little noise if I use an external power supply (EI, I'm still waiting for the toroidal and the switch psu is still noisy), almost nothing with batteries. I'd like to thank you another time, I really learn alot and now I could experiment easilly.Ah, I was forgetting to say... I put the 24V because I made one psu (unregulated) like that, I still don't hear any difference with or without R4 and neither with C3. I added some diodes to prevent the circuit to explode with reverse voltage. R1 and C2 are used to decoupling and lower the DC noise (I found many videos on youtube with this schematic).

author: Audioguru
author_rank: Super Contributor
timestamp: December 05, 2017, 01:35:23 am
content: The 470uF output capacitor value is way too high and will take 23.5 seconds to almost fully charge by R4. Calculate a value suitable for 50Hz and the load impedance of minimum 2k ohms parallel with R4 (1.9uF). A 2.2uF capacitor will pass 50Hz pretty well and will fully charge in 0.022 seconds.R4 charges the output capacitor when power is applied to the circuit. Then there is no loud POP when a load is plugged into the output jack.You show the value for C3 as .22nF which is impossible. If it is 22nF then it cuts frequencies above 35.2kHz. It will barely reduce 7kHz.

author: Ale
author_rank: Contributor
timestamp: December 05, 2017, 09:22:17 am
content: Yes, it is 22nF not .22, my badI will reduce the C7 value and make some tests.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: December 06, 2017, 06:26:44 pm
content: Please post final schematic when it's ready. I'm very interested in what you are doing but sadly I don't enough time right now to build it.Thanks in advance.

author: grants225
author_rank: Newbie
timestamp: December 07, 2017, 01:49:53 pm
content: Guys, what about changing RV1 of 100k to 1M fixed resistor, volume/gain controll moving after opamp (or if we still have possibility to use second part of TL072, put potentiometer in beatween of two cascades) and drop R5 to 10k-20k rate?Current gain of x10 is too much as some guitars may output up to 1Vrms. To avoid overload in second amp stage, RV1 should be turned down and what happens that poor weak input signal first should be divided by <~10 in order to be later on amplified by x10. Higher rate of set opamp gain adds distortion and additionally RV1 on input can became possible noice pickup source due to hard to implement proper grouding and shielding (again, picked noise goes straight into x10 amplification)

author: Ale
author_rank: Contributor
timestamp: December 07, 2017, 02:50:10 pm
content: What do you think about this?Grant225, I place a buffer before the real amplification stage now the RV is between them, were you think somethink like that?I also added a low pass filter and high pass filter in the first stage, is that a bad idea? are they correct like this?

author: Ale
author_rank: Contributor
timestamp: December 07, 2017, 03:00:04 pm
content: Perhaps it's better like this, with the hi pass before and low pass after the buffer.

author: Audioguru
author_rank: Super Contributor
timestamp: December 07, 2017, 05:22:56 pm
content: If you modify the awful buffer then might as well do it right:

author: Ale
author_rank: Contributor
timestamp: December 07, 2017, 06:09:24 pm
content: Yes of course, but you are using the inverting configuration, I was using the non-inverting. Is it not possible to do the same with the non-inverting? I read that the non-inverting is the less noisy configuration, that's why I was using it. Is it better to use the inverting when apply filter because the bias on the non-inverting input is isolated right?Another question... the 470nF and RV1 are forming an high pass filter right? but the cutoff frequency is 1hz at 100k and 11hz at 30k (but it also dimish the volume) but it's not important because it is too low hz value, am I wrong?thank you, I'm ready to modify the schema with your suggestion.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 07, 2017, 06:19:16 pm
content: Why would non-inverting be less noisy? I never heard of that before.

author: Audioguru
author_rank: Super Contributor
timestamp: December 07, 2017, 06:34:28 pm
content: I used an inverting opamp so that the lowpass filter is a simple integrator. I doubt that anybody cares about its 180 degrees of phase shift.I was going to use a 100nF capacitor to feed the 100k volume control but it adds another -3dB at 16Hz and affects up to about 80Hz.

author: Ale
author_rank: Contributor
timestamp: December 07, 2017, 06:38:10 pm
content: Don't remember where I read it, sorry... This is the new schema of the buffer.I have a question: if I end the first stage with the 470nF cap, can I remove the .1uF cap on the second stage?thanks Audioguru, I'm eager to try it out

author: Audioguru
author_rank: Super Contributor
timestamp: December 07, 2017, 07:19:39 pm
content: Quote from: Ale on December 07, 2017, 06:38:10 pmThe 470nF capacitor can be 100nF for a cutoff frequency of 16Hz.Your new schematic does not have a second stage so I am guessing that its .1uF capacitor is its input coupling capacitor.If the volume control that is connected to 0V connects directly to the input of the second opamp then the input of the second opamp will be at 0V and this opamp will rectify the audio causing severe distortion. Actually if the DC input of a TL071 opamp is within about 3V from the negative supply (which is ground in this circuit) then the output goes as high as it can and produces no sounds but maybe popping sounds on the loudest sounds that exceed 3V. The input of the second opamp must be biased at half the supply voltage so that it can swing up and down with the signal. Then the signal from the volume control (which has 0VDC) can feed it with a coupling capacitor.

author: Zero999
author_rank: Super Contributor
timestamp: December 07, 2017, 08:22:18 pm
content: Quote from: Richard Crowley on December 07, 2017, 06:19:16 pmThe only reason I can think of is it's possible to design a non-inverting amplifier, with a high input impedance, yet still have low value resistors, giving lower noise, that what you'd get with an inverting amplifier, with the same gain and input impedance.

author: Ale
author_rank: Contributor
timestamp: December 09, 2017, 07:10:31 am
content: I made some changes, is it possible to remove C5, R6 and R2?

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 09, 2017, 08:10:36 am
content: As drawn in the latest diagram, C5 will effectively short the audio signal to ground. It seems unlikely that is the desired effect. Perhaps you meant C5 to be in series with the audio path instead of in parallel? As a DC-blocking, AC-coupling capacitor.R2/R6 form a voltage divider to set the operating point of U1D at VCC/2 (i.e. +12V)Circuit design by committee.

author: Ale
author_rank: Contributor
timestamp: December 09, 2017, 01:11:21 pm
content: For semplicity I inverted the second stage. Now it should be ok. What do you think?thanks

author: Audioguru
author_rank: Super Contributor
timestamp: December 09, 2017, 03:30:54 pm
content: It looks like you do not know the simple formula for calculating the value of a capacitor in your inverting opamp circuit: 1 divided by (2 x pi x R x C).1) C1 is so small that it cuts all frequencies below 73kHz. Therefore you will have no audio, just ultrasonics from bats.2) C3 is so large that it cuts all high frequencies above 73Hz. Therefore if C1 would pass it you will have only deep bass and earthquake sounds.3) R3 is so small that it shorts the volume control.C15 can be 100nF or 220nF.

author: Ale
author_rank: Contributor
timestamp: December 09, 2017, 04:02:08 pm
content: eheheeh you are right, I'm learning and I missed that formulaAnyway, I made some maths to cut under 80hz and over 6khz. I hope it is correct... I have doubts on C3.We are almost there I think

author: Audioguru
author_rank: Super Contributor
timestamp: December 09, 2017, 04:35:11 pm
content: Your preamp is still cutting low and high frequencies to sound like an old AM radio. Each RC causes its calculated frequency to be reduced -3dB which is half the power.For the high frequencies you have two RC cuts which total -6dB which is 1/4th the power at 7.3kHz. The cuts are gradual so even 2.5kHz will be noticed to be cut a little. Most of us can notice a cut of -2dB at 20kHz. Why are you cutting the high frequencies?For the low frequencies you have three RC cuts at 80Hz which will be at -9dB which is 1/8th the power. 230Hz will be noticed to be cut a little.The value of R3 is still so small that it is still shorting the volume control. If you reduce the resistance of the volume control to 5k then the C15 capacitor feeding it must be huge to pass low frequencies.

author: Ale
author_rank: Contributor
timestamp: December 09, 2017, 06:12:57 pm
content: Ok, so... since I just have on the first stage the hi-pass (70Hz) and low-pass (6kHz) I think there's no need to have another on the second stage, right? I removed the low-pass on the second stage.If I understood well RV1 can cause problems when it goes down to gnd, that's why C1, but also need R3 to avoid shorting, right?Is there a simple solution to this?Thanks for your patience.

author: Audioguru
author_rank: Super Contributor
timestamp: December 09, 2017, 10:13:27 pm
content: The voltage gain of the second opamp is R5/R3 which is 100k/10k= 10 times. The 100k volume control is shorted by the low value for R3 so make R3 100k and make R5 1M. Then the cutoff frequency of C1 and the new 100k for R3 is too low at 8Hz so make C1 100nF then the cutoff frequency is 16Hz and will not affect the 73Hz cutoff frequency of C11 and R16.Why are you cutting frequencies above 6kHz? We can hear harmonics as high as 20kHz. Without hearing the harmonics then the sound is muffled like an old AM radio. But a guitar speaker probably does not produce frequencies above 6kHz anyway.

author: Ale
author_rank: Contributor
timestamp: December 10, 2017, 06:21:39 am
content: Well, the maximum guitar frequencies are from 80 to 1200 Hz more or less, but as you say there are still lots of harmonics. Anyway over the 6kHz they are very low in volume and very poor, since I'm building this for a live situation and not for studio recording I think is acceptable to cut freqs to minimize the noise. Further more, I'm usually play rock music with distortions and sometimes chorus, so I will receive additional harmonics from them.Of course I should make some field test, in the worst case I will get rid of the filter in the 2.0 version

author: Zero999
author_rank: Super Contributor
timestamp: December 10, 2017, 10:39:28 pm
content: Quote from: Ale on December 10, 2017, 06:21:39 amThat looks much better, than the other designs. I'd say go with it.

author: Audioguru
author_rank: Super Contributor
timestamp: December 10, 2017, 11:15:09 pm
content: Most amplifiers for electric guitars made by Marshall, Fender, Gibson and etc. had high input impedance of at least 1M ohms so that the low impedance electric pickup could resonate and produce a fairly high level peak at about 5kHz. They did not use an amplifier with a low input impedance that would produce a flat frequency response and they did not cut the highs. They liked the peak that helped the poor high frequency response of guitar speakers.

author: Zero999
author_rank: Super Contributor
timestamp: December 10, 2017, 11:21:00 pm
content: Quote from: Audioguru on December 10, 2017, 11:15:09 pmWell that's what he's got there, in the latest schematic: an input impedance of 1M. To minimise noise, it has an upper roll-off of 7.2kHz which is easy to change, if it's an issue.

author: derationalize
author_rank: Newbie
timestamp: December 12, 2017, 08:00:07 am
content: I have been breadboarding something similar so I have been following this thread. I have a few TL072s and wanted to make an internal preamp inside the guitar to boost the signal level. Having the opamp run in inverted mode screws up the phase relationship. This is important because I'm planning on using one channel of the TL072 to amplify the neck pickup and the other to amplify the bridge pickup separately to be able to balance the volume/gain. Since this is running off of 9v batteries I would think limiting it to a single TL072 would keep power consumption low, though there is enough space available for 2 9v batteries. If I'm using 2 batteries though, wouldn't it be smarter to just wire them up for 18v for the increased headroom? Sorry for hijacking the thread.

author: Ale
author_rank: Contributor
timestamp: December 12, 2017, 09:06:41 am
content: Wouldn't be easier to use 2 simple 2n3904, also for the space problem?

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2017, 09:27:54 am
content: Quote from: derationalize on December 12, 2017, 08:00:07 amHow does an inverting op-amp screw the phase relationship? As long both channels are inverted, it won't matter.The guitar will sound the same through an inverting amplifier, as it will through a non-inverting amplifier.Quote from: Ale on December 12, 2017, 09:06:41 amI agree, a discrete solution will do. I'd probably suggest J-FETs though because the input impedance can be higher, than BJTs.

author: Ale
author_rank: Contributor
timestamp: December 12, 2017, 09:49:03 am
content: Maybe a 2n3819 with a schema like this?

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2017, 10:55:49 am
content: Yes, you could even have the source resistor and AC coupling capacitor, at the other end of the cable, so a two wire connection can be used.

author: Ale
author_rank: Contributor
timestamp: December 12, 2017, 11:00:24 am
content: You mean like this? This is basicaly a DI Box, I'm working on this lately...I also build a simple soundcard oscilloscope sunday and tested out this schematic because I also need a DI. I still have question on that, but maybe is better to open another thread, right?

author: HoracioDos
author_rank: Frequent Contributor
timestamp: December 12, 2017, 11:21:45 am
content: JFET PreAmp examples with calcs.http://www.rason.org/Projects/jfetamp/jfetamp.htmPD: I was going to move this post tohttps://www.eevblog.com/forum/beginners/single-mosfet-power-amplifier/but I realized it was about mosfet not jfet.

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2017, 11:59:53 am
content: Quote from: Ale on December 12, 2017, 11:00:24 amI meant using only two wires: one for 0V and the other for both power and signal.

author: Ale
author_rank: Contributor
timestamp: December 12, 2017, 01:06:49 pm
content: Like in the phantom power?

author: Audioguru
author_rank: Super Contributor
timestamp: December 12, 2017, 05:13:15 pm
content: The neck and bridge of a guitar are some wavelengths of sounds apart so their phases are very different at different frequencies. I doubt you can hear if one signal has its phase reversed and if both signals have their phases reversed then they will have the same phase.Instead of amplifying each pickup separately, why not use a single amplifier with a simple balance control.A magnetic guitar pickup produces a very high output level. Then an opamp or two Jfets are not needed. The preamp input impedance must be high to allow the pickup to resonate and produce a level boost at about 5kHz so ordinary low input impedance transistors cannot be used. Use a single Jfet like this attachment:

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2017, 09:18:42 pm
content: Quote from: Ale on December 12, 2017, 01:06:49 pmI meant like audioguru's circuit, but with the JFET, R1 and R2 separated from the rest of the circuit with a cable, consisting of two cores.

author: Audioguru
author_rank: Super Contributor
timestamp: December 12, 2017, 09:31:09 pm
content: You can separate the Jfet from the parts on the right side if they are connected together with shielded (coaxial) audio cable.The value of the 6.8k resistor depends on the spec's of the Jfet.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 12, 2017, 10:54:14 pm
content: Quote from: Hero999 on December 12, 2017, 09:18:42 pmNote you can do that with 1-core shielded cable. In fact that is how virtually all electret condenser microphone circuits are designed. Your common, 82-cent jelly-bean electret condenser mic capsule is just a FET that has its gate connected to the electret mic element. The power required by the FET is fed through the same conductor as the signal comes back through. And that powering convention is called "plug-in power" by most consumer gadgets that have external mic connectors. Phones, recorders, camcorders, etc. etc.

author: Zero999
author_rank: Super Contributor
timestamp: December 14, 2017, 11:41:25 am
content: Quote from: Richard Crowley on December 12, 2017, 10:54:14 pmYes, that's true. With that sort of signal level, it's possible you might be able to get away without screened cable. I've recently used a common emitter amplifier to boost the signal level of a dynamic mic and it worked fine with an unscreened cable, even in a fairly high noise environment: the cable was over 10m long and ran past several switched mode power supplies. The impedance of the amplifier was lower at a 510R, rather than 6k8, though the signal level was slightly lower too.