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AndrewCE

Failed Headphone Amp

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i need to ask about why my opamp headphone amp doesn't work. and I drew up a scematic in MS Paint, but i first need to figure out how to attatch an image to this post. It only accepts URL addresses :D can someone tell me how to attatch the image?

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thanks anderekel

failedheadphoneampschem.jpg

here it is. I've tried it with and without the output resistor. all resistors are metal film. the cap is electrolytic. I'v done continuity checks. My power supply is a single 9v batter with one side connected to each opamp power supply.

My test is to hook up my strat, strum the strings, and listen through headphones to the output. I hear nothing.

I'm so upset because from what I've read this is literally the simplest project out there, and I can't get it to work.

any help would be greatly appreciated. :D

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One problem I can see is that your feedback resistor is really small, so it will have very little gain. I would try something like a 220K instead and see what that gets you.

And the other thing that you have to take into consideration is the impedance of the headphones that you plan on using.

These pages might help point you in the right direction with designing a headphone amplifier circuit:

http://www.headwize.com/projects/opamp_prj.htm

http://www.headwize.com/projects/cmoy2_prj.htm

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One problem I can see is that your feedback resistor is really small, so it will have very little gain. I would try something like a 220K instead and see what that gets you.

And the other thing that you have to take into consideration is the impedance of the headphones that you plan on using.

These pages might help point you in the right direction with designing a headphone amplifier circuit:

http://www.headwize.com/projects/opamp_prj.htm

http://www.headwize.com/projects/cmoy2_prj.htm

:D believe it or not, that 2nd link is the exact page that i was referencing when i drew up the schematic. It only uses a gain of 10, so I did too (with the 10K feedback resistor).

what changes would I have to make for my headphone impedance? (and how do i measure headphone impedance?) I thought that an opamp has theoretically an infinitely low output impedance. That should be a good thing, to drive headphones, right?

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Hmm.... OK.

Well, that circuit appears to not be designed for use with a guitar. At least not for a guitar straight into it. A passive guitar pickup needs to be amplified quite a bit first before it goes to your headphones.

This one here can be modified for use with a guitar - see asterisk at bottom of schematic in Figure I: http://www.headwize.com/projects/guitar_prj.htm

It's based around an LM386 audio amplifier IC chip, which is pretty commonly used for this sort of thing and is good for a 1/2 watt of power.

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Well, where did you get that schematic? There appears to be a major flaw in that design or a portion left out of the schematic. I ran into this exact issue when first trying to utilize op amps for sound a few months back. This is because of a few things. One, there are A LOT of bad schematics on the web. Two, even the half decent ones do not specify how to setup the power supply properly. Op amps are made easiest to use with dual supplies, and they don't tell you this in most of the op amp primers. Googling "single supply op amp circuit" will explain in further detail what I am about to summarize, but here's a links to what I consider the holy grail of single supply op amp circuits for DIY'ers...

Texas Instruments document

The circuit as is, should work with dual (series) power supplies with "GND" connected to the tap between the two. That circuit, I do not believe can work with one 9V battery as is. This is because one of the inputs needs to be referenced to the midpoint of the power rails to be able to swing above and below something in creating a AC wave. Like it is, the op amp is probably slammed to either +9V or GND all the time, hence no sound. It would as is be more of an onn/off detector, telling the op amp to send full rail in one of two states, full on or full off. More or less, you are telling the op amp to do something it can not. Once you bias to 4.5V, then the signal has a workable reference point, from which it can swing above and below nicely.

Do NOT confuse this idea with giving power at the op amps power inputs. I do realize we often leave the power inputs off the drawing as a matter of readability. However, these are 2 entirely different concepts. The power inputs just supply the op amp with power. Biasing the inverting or noninverting input to VCC/2 or virtual GND creates a "reference point".

How to do this:

Create a virtual GND or a VCC/2. This is usually a simple voltage divider, utilizing 2, series 100K resistors that are connected between +9V and GND. The midpoint of the resistors is 4.5V. That midpoint is your virtual GND or VCC/2. To cut noise, a cap in parallel with this midpoint and going to GND shunts unwated AC to GND by creating a low pass filter. Incidentally, you should do this low pass thing any place you add + voltage to a chip as well, meaning the op amps positive supply pin should have a low pass filter on it.

VCC_2.jpg

Bias one of the inputs to virtual GND or VCC/2 (4.5 volts).

In this picture, from the Texas Instruments document I linked earlier, the virtual GND gets attached to the line marked (VCC/2).

SingleGain.jpg

In your drawing, I'd try adding the VCC/2 point right at the noniverting "+" input. This should get you SOME sound, whether or not your gain feedback loop is properly set up... that's another hurdle to figure out after you try what I've outlined above, but I suspect the design may also need other work-overs in that dept.

Lastly, your circuit is going to need an output capacitor as well to ditch the DC bias coming out of the op amp. What the output will have as AC is actually 4.5V, plus or minus some swing. The swing is your AC wave. An output cap removes the DC bias, leaving only the AC portion. To do this, add a 1uF cap onto the output after your output resistor, and probably ditch the output resistor entirely. Not doing this will likely put your headphones out of ideal operating range, if not causing them to fail entirely.

Edit: I just looked at the 2 links in th eprevious posts and confirmed that yes, those schematic assume you are eiyhrt using dual supplies (2 batteries with a center tap) or creating a virtual GND, using a VCC/2, so none of those circuits should work without doing this. Funny how they expect us to just know this!

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Edit: I just looked at the 2 links in th eprevious posts and confirmed that yes, those schematic assume you are eiyhrt using dual supplies (2 batteries with a center tap) or creating a virtual GND, using a VCC/2, so none of those circuits should work without doing this. Funny how they expect us to just know this!

Yeah, that's a good point you make there. I don't know that much about opamp design, but I do see a lot of stompbox schematics that don't use a virtual ground. Many times they are simply non-inverting opamps with input biasing.

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ok, i just added the power supply donovan drew up in the gray box (except with 15K resistors and a 2.2uF cap), and now all the things in the original circuit that WERE connected to ground are now connected to "m" (my name for the vcc/2). I even connected the "ground" terminals of the input and output jacks to "m". Was i supposed to do this?

p.s. I also am using a 200K feedback resistor (gain of 200) and a 2.2uF output capacitor instead of the output resistor.

and it still gives me no sound :D any thoughts?

btw, great resource, donovan! thanks. i read almost the whole thing!

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ok, i just added the power supply donovan drew up in the gray box (except with 15K resistors and a 2.2uF cap), and now all the things in the original circuit that WERE connected to ground are now connected to "m" (my name for the vcc/2). I even connected the "ground" terminals of the input and output jacks to "m". Was i supposed to do this?

p.s. I also am using a 200K feedback resistor (gain of 200) and a 2.2uF output capacitor instead of the output resistor.

and it still gives me no sound :D any thoughts?

btw, great resource, donovan! thanks. i read almost the whole thing!

15K resistors will eat battery quicker,but should work.

Everything that was connected to GND should still be connected to ground, with the exception that MAYBE the 1K resistor could go to "M", but I don't think so. Try it going to GND first. Make sure the op amp negative power input goes to GND, not to "M".

The 10K and 100K resistors... not sure about the 10K, but the 100K and the 1uF cap make a high-pass filter. The 10K may be noise-minimizing or impedence-related. Both of those resistors should be able to be removed without killing function.

Lastly, I THINK that another thing they are not telling you is that for your gain to work,you'll need another cap. In your drawing, insert a cap directly to the right of the 1K resistor. I believe this is th eonly way to get gain working in the config you have.

Do a search for "MXR Distortion Plus Schematic" and check out how the opamp is set up. It is as bit more complex, but shows how a good gain is set up.

What op amp are you using?

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15K resistors will eat battery quicker,but should work.

Everything that was connected to GND should still be connected to ground, with the exception that MAYBE the 1K resistor could go to "M", but I don't think so. Try it going to GND first. Make sure the op amp negative power input goes to GND, not to "M".

The 10K and 100K resistors... not sure about the 10K, but the 100K and the 1uF cap make a high-pass filter. The 10K may be noise-minimizing or impedence-related. Both of those resistors should be able to be removed without killing function.

Lastly, I THINK that another thing they are not telling you is that for your gain to work,you'll need another cap. In your drawing, insert a cap directly to the right of the 1K resistor. I believe this is th eonly way to get gain working in the config you have.

Do a search for "MXR Distortion Plus Schematic" and check out how the opamp is set up. It is as bit more complex, but shows how a good gain is set up.

What op amp are you using?

update: i actually got it to "barely" work now without any changes. The key was to get a loop pedal to feed the circuit, instead of trying to strum it while probing (duhhh). Also, it's nice to now have working 9v and output jacks for a solid connection.

THERE IS a problem, though. The sound is very buzzy. It almost sounds like a square wave. I was under the impression that this amp would be super clean; almost disgustingly clean. Any ideas on how to clean it up? I think I'm gonna try that feedback capacitor thing donovan was talking about.

a single 741 opamp

p.s. Do all these caps need to be electrolytic? What is the best type (metal film, carbon, electrolytic, nonpolarized electrolytic)? And is there a "best type" of resistor?

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ok, i added a 1uF cap in parallel with my 200K feedback resistor, and another 1uF cap from inverting to ground. This gives me a fairly clean sound. for some reason i removed the resistor from inverting to ground(theoretically that should take away my gain of 200, but after doing it i am still able to hear the sound *???*), and that magically got rid of the buzziness.

My only problem is that I don't have enough volume. I can hear the sound, but I want it to be louder.

I'll draw up a schematic to clarify my problem. (But i'm getting excited, it's almost up and running!!!)

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headphoneampschematic2.jpg

all polarized caps are electrolytic. all resistors are metal film. all nonpolarized caps are metalized film. "m" is the virtual ground. My headphones measure 37 ohms DC.

the signal is clear, but not loud enough. i've tried short circuiting the inverting input to "m", and that forces it into clipping (square wave buzziness). I've tried short circuiting the inverting input to ground, and I get no audible signal. I've tried putting a 1K resistor from inverting to ground and I get no audible signal. I've tried putting the 1K resistor from inverting to "m" and the signal is audible, but just as weak as it is when there's no 1K resistor there(how the circuit is in the schematic).

any help on how to up the gain cleanly?

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headphoneampschematic2.jpg

all polarized caps are electrolytic. all resistors are metal film. all nonpolarized caps are metalized film. "m" is the virtual ground. My headphones measure 37 ohms DC.

the signal is clear, but not loud enough. i've tried short circuiting the inverting input to "m", and that forces it into clipping (square wave buzziness). I've tried short circuiting the inverting input to ground, and I get no audible signal. I've tried putting a 1K resistor from inverting to ground and I get no audible signal. I've tried putting the 1K resistor from inverting to "m" and the signal is audible, but just as weak as it is when there's no 1K resistor there(how the circuit is in the schematic).

any help on how to up the gain cleanly?

I think you need to do a few things.

1) Where you have your feedback loop connecting to "M", I believe that needs to go to ground, meaning the same place the battery negative goes. Also, there really needs to be a resistor where you removed that one. That, along with your 200K resistor are what sets the gain. In your current configuration, there is no gain loop, at least in the conventional sense. I think you are getting "open loop" gain, or something close to it, which is the op amp going wide open. What most setups have instead of an actual resistor (where you removed yours) is a potentiometer. Gain in your configuration is G = R1/R2 + 1, where R1 is your 200K and R2 is the one you removed. A 200K pot in the spot you removed that resistor from yesterday would let you vary gain from 2X to whatever the open loop gain of the op amp data sheet says it is, within the bounds of the power supply.

2) If you can, try changing the cap values. They play a really big role in what gets amplified and what gets attenuated. I think your values are too large, with the exception of your output cap. Since the cutoff frequency is F = 1/(2*pi*R*C), you can see that F becomes smaller as R or C becomes larger, so what this means is that for a low pass filter (formed in the feedback loop), the bigger the cap, the lower the cutoff frequency, and the more that highs will suffer.

a) Try a 0.1 uF in place of your 1 uF input cap.

:D Try a 0.01 uF in place of the cap that's in parallel with your 200K resistor.

c) Try a 0.047 uF in place of your cap that goes directly to "M", but again, that should not go to "M", but rather to ground, ground being same as batery negative.

Lastly, in answer to your question, none of these caps ned to be electrolytics. In fact, you will have a hard time finding electrolytics in the values you most need. You need smaller values, probably a good idea to get a cheapo $5 kit from Radio Shack . You need values from .001 uF up to 1uF to experiment.

Look at this circuit, I believe I got this from DIY stompboxes.com. It has the same general setup that you need to accomplish with regard to ground, "M", and the gain feedback loop. I built this and it works kicka$$

mxr_distortion_plus.jpg

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I think you need to do a few things.

1) Where you have your feedback loop connecting to "M", I believe that needs to go to ground, meaning the same place the battery negative goes. Also, there really needs to be a resistor where you removed that one. That, along with your 200K resistor are what sets the gain. In your current configuration, there is no gain loop, at least in the conventional sense. I think you are getting "open loop" gain, or something close to it, which is the op amp going wide open. What most setups have instead of an actual resistor (where you removed yours) is a potentiometer. Gain in your configuration is G = R1/R2 + 1, where R1 is your 200K and R2 is the one you removed. A 200K pot in the spot you removed that resistor from yesterday would let you vary gain from 2X to whatever the open loop gain of the op amp data sheet says it is, within the bounds of the power supply.

2) If you can, try changing the cap values. They play a really big role in what gets amplified and what gets attenuated. I think your values are too large, with the exception of your output cap. Since the cutoff frequency is F = 1/(2*pi*R*C), you can see that F becomes smaller as R or C becomes larger, so what this means is that for a low pass filter (formed in the feedback loop), the bigger the cap, the lower the cutoff frequency, and the more that highs will suffer.

a) Try a 0.1 uF in place of your 1 uF input cap.

:D Try a 0.01 uF in place of the cap that's in parallel with your 200K resistor.

c) Try a 0.047 uF in place of your cap that goes directly to "M", but again, that should not go to "M", but rather to ground, ground being same as batery negative.

Lastly, in answer to your question, none of these caps ned to be electrolytics. In fact, you will have a hard time finding electrolytics in the values you most need. You need smaller values, probably a good idea to get a cheapo $5 kit from Radio Shack . You need values from .001 uF up to 1uF to experiment.

Look at this circuit, I believe I got this from DIY stompboxes.com. It has the same general setup that you need to accomplish with regard to ground, "M", and the gain feedback loop. I built this and it works kicka$$

mxr_distortion_plus.jpg

yeah i actually found that exact same schematic, but I didnt have the right value caps on hand. I'm hittin up radio shack tomorrow (but damn, radio shack is expensive).

you know, i still don't get what the cap in parallel with the 200K DOES. it seems to decrease the gain as frequency increases. why is that important?

and i connected that other cap to ground instead of "m". it sounds exactly the same. I think it's actually supposed to go to m, because, if you think about it, connecting it to ground would be the same as: if there were no "virtual ground" and the cap was connected to -15v (and that wouldnt make much sense).

i've done all of your modifications except for 2B, which i'll do as soon as i get the right cap. and it still sounds exactly the same. I wonder if i'll need more than 200K ohms.

p.s. what do i need to know about impedance matching? some sources say the impedance needs to match the headphones, and some say you want the lowest possible output impedance. ???

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yeah i actually found that exact same schematic, but I didnt have the right value caps on hand. I'm hittin up radio shack tomorrow (but damn, radio shack is expensive).

you know, i still don't get what the cap in parallel with the 200K DOES. it seems to decrease the gain as frequency increases. why is that important?

and i connected that other cap to ground instead of "m". it sounds exactly the same. I think it's actually supposed to go to m, because, if you think about it, connecting it to ground would be the same as: if there were no "virtual ground" and the cap was connected to -15v (and that wouldnt make much sense).

i've done all of your modifications except for 2B, which i'll do as soon as i get the right cap. and it still sounds exactly the same. I wonder if i'll need more than 200K ohms.

p.s. what do i need to know about impedance matching? some sources say the impedance needs to match the headphones, and some say you want the lowest possible output impedance. ???

The cap in parallel with 200K forms a low pass filter, rolling off the highs... the right value attenuates frequencies above the audible range (20kHz).

I am holding that that other cap does need to be connected to ground. Study the MXR schematic a bit more. Notice after that cap (a .047uF), they have some more resistance (a 4.7k followed by a 1M potentiometer). That is KEY! ...not the exact values, but the premise. You're blocking DC going to ground, and allowing AC to go to ground. This is what tells the op amp that it needs to boost AC and not boost DC. Op amps amplify difference between the inputs.

Also notice that in the MXR schematic, their "M", formed in the top half of the drawing, uses 1M resistors to form the 4.5V, then yet another 1M resistor in seried before connecting it to the + op amp input. You might try that. In any case, using higher values there than your 15K's will save battery life AND reduce noise because higher values will allow less curent through and bring the low pass filter's F to a lower number, providing a cleaner "M" reference voltage.

At this point, I am convinced your cap values are your biggest problem.

I do not think you need to worry about impedance matching. Op amp inputs are high impedance, so they won't mess with your guitar in a backward manner. The outputs are low impedance, so they should be able to match high and low impedance on the next stage without a problem.

What might be an issue is that the op amp's output current is not all that high, so it might have trouble driving headphones, I'm not sure as I've not tried it. I used the LM741 to drive as a preamp, not an actual amp, and it works excellent. You might want to try hooking up the output to alligator clips and clip them onto another guitar cable going into your normal amp in order to test easier, at least for sound quality and to see if you are getting any working gain.

Get some potentiometers when you go to RS. Yes, their prices are about 100X higher than buying online, but nothing beats them for the sheer convenience of it... mine is a 20 minute drive, which equals no wait for delivery and no minimum orders like you may find online.

Anyway, I think you're on the right track. This can be a long and frustrating road your first time, but once it clicks, imagination is the only limit to the kinds of projects you can embark on.

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The cap in parallel with 200K forms a low pass filter, rolling off the highs... the right value attenuates frequencies above the audible range (20kHz).

I am holding that that other cap does need to be connected to ground. Study the MXR schematic a bit more. Notice after that cap (a .047uF), they have some more resistance (a 4.7k followed by a 1M potentiometer). That is KEY! ...not the exact values, but the premise. You're blocking DC going to ground, and allowing AC to go to ground. This is what tells the op amp that it needs to boost AC and not boost DC. Op amps amplify difference between the inputs.

Also notice that in the MXR schematic, their "M", formed in the top half of the drawing, uses 1M resistors to form the 4.5V, then yet another 1M resistor in seried before connecting it to the + op amp input. You might try that. In any case, using higher values there than your 15K's will save battery life AND reduce noise because higher values will allow less curent through and bring the low pass filter's F to a lower number, providing a cleaner "M" reference voltage.

At this point, I am convinced your cap values are your biggest problem.

I do not think you need to worry about impedance matching. Op amp inputs are high impedance, so they won't mess with your guitar in a backward manner. The outputs are low impedance, so they should be able to match high and low impedance on the next stage without a problem.

What might be an issue is that the op amp's output current is not all that high, so it might have trouble driving headphones, I'm not sure as I've not tried it. I used the LM741 to drive as a preamp, not an actual amp, and it works excellent. You might want to try hooking up the output to alligator clips and clip them onto another guitar cable going into your normal amp in order to test easier, at least for sound quality and to see if you are getting any working gain.

Get some potentiometers when you go to RS. Yes, their prices are about 100X higher than buying online, but nothing beats them for the sheer convenience of it... mine is a 20 minute drive, which equals no wait for delivery and no minimum orders like you may find online.

Anyway, I think you're on the right track. This can be a long and frustrating road your first time, but once it clicks, imagination is the only limit to the kinds of projects you can embark on.

so, let me get this straight... the feedback capacitor is a lowpass filter, and the cap to ground is a highpass filter? I think i'm finally getting that. Is that correct?

[me thinking out loud] if it is, then the whole thing is a bandpass filter, and decreasing the size of each capacitor shifts the edge of the "band" up in frequency... increasing the cap size shifts it down... right?

i already changed over to 100K resistors for the virtual ground. I was hoping on the off chance that my lack in gain was due to that network draining my battery. No luck. and in retrospect, it sounds pretty stupid. I'll keep them there anyways, they didnt change the sound.

yes a pack of assorted low value caps are first on my list for tomorrow.

I did see that extra 1M and i was wondering about it. I'll try it tomorrow, but until then, um, could you explain what it does?

btw, thanks for all this help youre giving me. and sorry if i ask too many questions [my mind is a sponge] :D

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Thanks guys, here's the current schem:

headphoneampschematic3final.jpg

I'm about ready to call this thing "final". The gain is good, the only problem is, it's noisy. There's a constant hissing. The only way I've figured to make it go away is to increase the value of the feedback cap, but then the tone gets all dull. Unless someone has a few quick tips on how to reduce noise, I'll move this thing from breadboard to PCB.

btw, I believe the MXR Dist+ equivalent of my 100K to "m" is MXR's extra 1M that goes into the network of 1M resistors. This answers both the question of where to connect the other end of the 100K and why my design doesnt include that extra resistor (it does!).

and i'm assuming that a ceramic cap reading "10" means 10pF. correct me if i'm wrong.

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the only problem is, it's noisy. There's a constant hissing. The only way I've figured to make it go away is to increase the value of the feedback cap, but then the tone gets all dull.

What opamp are you using? With headphone amps, you need to use an opamp designed specifically for audio use (or for lowest possible noise) because it will have the lowest noise floor that you can get. Something like an OPA134, OP275, NE5534, etc. A 741 or something like that will be very noisy in comparison.

and i'm assuming that a ceramic cap reading "10" means 10pF. correct me if i'm wrong

Yes, that should be a 10pF cap. But that's a very small capacitor to use in a feedback loop. What does a 100pF do for you? That should lower the noise a bit more but not kill the high end much.

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First off, congratulations on getting where you are. You should be proud. This stuff frustrated me for a LONG time.

Paul is correct. The LM741 is a "general purpose" op amp, not specifically intended for audio. You are probably always going to have some noise unless you modify your choice of chip. However, I do think we can do 2 more things to drastically reduce the noise you have now. I have not mentioned them previously because we were already throwing a ton of info at you and may have been overwhelming and I considered it much more important that you actually get sound and gain first.

The best news of all here is that if the following 2 things don't do the trick, then most of the other 8-pin DIP package op amps share the same pin configuration, sort of a standard, so switching from the LM741 is a simple pop-out-old, pop-in-new.

1.Condition the battery power going into pin 7 of the LM741. To do this, add a 100uF (or larger) cap from pin 7 (V+) to Gnd. This forms yet another low pass filter, sending a great deal of the AC spikes/dips from the battery to Gnd and allowing just the bulk of DC to get to the op amp's power input. Not doing this makes a pretty big difference in my experience. The connection to pin 7 should be as close to the body of the component as possible, NOT at the end of a long lead, to avoid any other induced currents long the way to the pin.

2. Balance the inputs. The LM741 (and all differential op amps) are subject to minute variations in the manufacturing process that makes them "less then idea". This means they have some error in the inputs, referred to as input offset current. When you apply gain to a signal, the error is amplified along with the signal. To get around this, the manufacturer provides pins for measuring and "nulling" the offset, sort of like a calibration procedure which restores the op amp to a more perfect balance, thus reducing any imbalance, reducing error, and reducing noise on circuits with gain. Look at this page and go about 3/4 down, check out Fig. 11 and the procedure there. Unfortunately, you need a multimeter to do this.

http://www.uoguelph.ca/~antoon/gadgets/741/741.html

I think you are right about the one other resistor. I could not find any documentation to support this, buyt I seem to remember that there should be a resistor somewhere in the circuit that is the parallel sum of your R1/R2 gain resistors. I have never bothered with these as I never had a static gain setting, I always use a potentiometer for variable gain, so that resistor value would need to change every time I moved the pot... not practical.

If you've been going to Radio Shack for your components... try the TL082 op amp. It is better quality and provides 2 op amps in one 8-pin chip, so you could do stereo and get the benefits of pre-balanced inputs and JFET inputs, which are better than the bipolar transistors the LM741 utilizes.

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First off, congratulations on getting where you are. You should be proud. This stuff frustrated me for a LONG time.

Paul is correct. The LM741 is a "general purpose" op amp, not specifically intended for audio. You are probably always going to have some noise unless you modify your choice of chip. However, I do think we can do 2 more things to drastically reduce the noise you have now. I have not mentioned them previously because we were already throwing a ton of info at you and may have been overwhelming and I considered it much more important that you actually get sound and gain first.

The best news of all here is that if the following 2 things don't do the trick, then most of the other 8-pin DIP package op amps share the same pin configuration, sort of a standard, so switching from the LM741 is a simple pop-out-old, pop-in-new.

1.Condition the battery power going into pin 7 of the LM741. To do this, add a 100uF (or larger) cap from pin 7 (V+) to Gnd. This forms yet another low pass filter, sending a great deal of the AC spikes/dips from the battery to Gnd and allowing just the bulk of DC to get to the op amp's power input. Not doing this makes a pretty big difference in my experience. The connection to pin 7 should be as close to the body of the component as possible, NOT at the end of a long lead, to avoid any other induced currents long the way to the pin.

2. Balance the inputs. The LM741 (and all differential op amps) are subject to minute variations in the manufacturing process that makes them "less then idea". This means they have some error in the inputs, referred to as input offset current. When you apply gain to a signal, the error is amplified along with the signal. To get around this, the manufacturer provides pins for measuring and "nulling" the offset, sort of like a calibration procedure which restores the op amp to a more perfect balance, thus reducing any imbalance, reducing error, and reducing noise on circuits with gain. Look at this page and go about 3/4 down, check out Fig. 11 and the procedure there. Unfortunately, you need a multimeter to do this.

http://www.uoguelph.ca/~antoon/gadgets/741/741.html

I think you are right about the one other resistor. I could not find any documentation to support this, buyt I seem to remember that there should be a resistor somewhere in the circuit that is the parallel sum of your R1/R2 gain resistors. I have never bothered with these as I never had a static gain setting, I always use a potentiometer for variable gain, so that resistor value would need to change every time I moved the pot... not practical.

If you've been going to Radio Shack for your components... try the TL082 op amp. It is better quality and provides 2 op amps in one 8-pin chip, so you could do stereo and get the benefits of pre-balanced inputs and JFET inputs, which are better than the bipolar transistors the LM741 utilizes.

thanks, i'll do those mods. i'm also gonna remove the 10k pot, and to adjust volume i'll get a 100k audio pot for the feedback. at this time my tone gets more trebly as i turn up to volume; i dont think it should with the new gain-changing setup. (or should it?) Also, since i'm changing my gain, will i need to change some other resistor? (you mentioned something about the r1/r2 parallel value?)

i got a TL082. one question: it doesnt have any offset null pins; do i need to adjust the offset null? the package say "internally compensated". does this mean that i dont need to?

also, if i add the cap between 9v and ground, can i remove the cap b/t "m" and virtual ground? And I've seen schematics that just have a 9v -> cap -> virtual ground -> cap -> ground setup. Would that do the same thing?

p.s. i dont think i'll be doing the stereo thing; i really only have one input signal to work with: my guitar signal

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