AndrewCE

I have it hooked up literally exactly the way you do, with the exception of C6 and R6. And my R4 isn't a pot. It's a fixed 100K. And my volume pot is a 100K log pot, but it's still doing the jump-up thing. ????????

well, it's entirely clean as far as i can tell. I'm using a tech 21 blonde pedal as an amp simulator (its not great, but it works). wow, you took off work to work on an led project? sorry i just find the irony funny for some reason now, i tried to add a post-gain volume control, and keep my gain stage at a fixed gain, but here's what happens: even though i have a log pot, as i adjust it, the volume is at zero, then at some point in the sweep it jumps up to max volume. I've wired the pot up several different ways, and I can't get this problem to go away. I assume it has something to do with the opamp's low output impedance and the fact that i'm using a 100K pot. But i tried it with a 10K trim pot, and i have the same problem. any solutions?

well sorry if you misunderstood, it was only a joke. i'm actually hoping to move towards a wall wart supply, so ill keep the cap yes, i know it's "good practice", i was only saying that this specific application is an exception; i understand most other, bigger projects would be better organized if done the way you were suggesting hm, i'm not so sure about that. maybe if youve got a wall supply, where "ground" is actually connected to the ground (is it, actually, in a wall wart???), then the 0v would be more constant than the 4.5v. But with a battery, if the voltage all of a sudden jumps from 9v to 8v, i think the best way to describe what happens would be: the "9v" node jumps down by .5v, the "ground" node jumps up by .5v, and the 4.5v node stays where it is. This is because the "ground" is not actually anchored to anything big, like the actual ground. And it is best understood how the 9v battery reacts by viewing it as 2 separate 4.5v batteries, as is shown in one of my "sarcasm" drawings. This seems like more of a philosophical discussion of relativity than an application discussion, though. um, what is the "shift typing sequence"?

yeah i named em on the off chance that you would be able to see them, lol Thanks for the drawing; ive noticed two things: 1. Your r6 does not exist on my schematic. An early version of my schematic had a 10K resistor in that spot, but nobody could tell me what it did, and removing it didnt change the sound of my amp, so I omitted it. could you tell me what R6(in your schematic) does? 2. You've omitted the 220uF cap from 9v to ground that you were suggesting. I'm guessing this was just a slip-up? right, there is no voltage bias after the output cap. UNTIL you plug in a pair of headphones. Then youve got 37 ohms (DC resistance of my headphones) connecting the sleeve and tip, biasing the output side of the output cap so that the signal swings above and below whatever the sleeve is. Thus, it does not matter what the charge of the sleeve is relative to some arbitrary point such as the negative terminal of the battery. It matters what the charge of the sleeve is relative to the output tip connector. And this voltage will be 0V, regardless of where the sleeve is in relation to that arbitrary point. idk how the sleeve and tip measured 4.5v with the sleeve attatched to ground, but it did. and i repeated the experiment, and the results were the same. (as far as this little experiment, i'm puzzled) yes, and btw, i hope you dont take my stance as me trying to be difficult, or trying to disrespect you; i'm just trying to learn what i can about circuitry, and at this point i do firmly believe that the constant voltage between sleeve and the battery's negative terminal has no effect on the DC voltage b/t tip and sleeve

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ok now, i've drawn up these two new schematics. The only difference between the two is what the sleeves are connected to. Tell me which one of these 2 will work better. Anyone else care to chime in on this?

I connected the voltmeter so that 1 lead was on the sleeve of the output jack and the other lead was on the tip connector of the output jack. The way my schematic is now, with sleeve connected to "m", it read about -50mV. When the sleeve was connected to ground, it read 4.5V! Then I even connected BOTH sleeves to ground, and it still read 4.5V. If you don't believe me you can breadboard one yourself.

Centered around 0v? are you sure about this? When I passed the signal through the input coupling cap, all the DC was removed. There was only AC. And I was required to put the 100K resistor to "m" to tell the signal what constant voltage to swing above and below. It doesn't just know what DC voltage to use. So why would the signal after the output coupling cap "know" what DC voltage to use? It doesnt. You assume that the AC signal would choose ground by default, but it doesnt. It'll use whatever you give it. Here I gave it "m" via the sleeve, so it swings above and below "m", which is perfect, because the sleeve is at voltage "m". It works exactly the same as if i connected the sleeve to ground. If you don't believe me, look at that underlined sentence and try substituting "ground" everywhere you see "m". It holds exactly the same truth value! remember, voltages are relative. They are not values of charges but instead they are differences between charges. If an AC signal is riding on 4.5v, the 4.5vDC is irrelevant as long as the place to which the AC is flowing is also at 4.5v. Note that the entire post-output-cap circuitry is not even connected to ground(except by the 100K power supply resistors which can be ignored). So the current isn't flowing from 4.5v to 0v, it is in fact flowing from 4.5v to 4.5 v. So there is 0 DC current.

i'm ok with just having a gain control; I dont need zero output, and if I do, I'll just unplug the unit. as far as the capacitors, i've put a 220uF across the 9v potential, and i didnt hear the noise change at all. but when I unplugged the battery, the cap took a second to decharge, and the output made a quick flanging-like sound. i dont know if the slowly changing supply voltage is HURTing the circuit or not, but I'd rather not chance it if i dont have to. especially since the cap doesnt make any audible difference. i dont think i'll need to use an active "m" nor will i have to use the other half of the chip; my output is good, i'm only dealing with one signal, as far as where the sleeves are connected, i'll play around with it but i really think it should be "m". In an actual ground circuit, you wouldnt connect it to the -9v, you would connect it to the spot halfway b/t the 2 supply rails. [Actually, i dont think it should make any difference.] as the input sways between +-3mV (or however big the guitar signal is) that is basically telling the circuit to go that far above and below "m". the 2 caps isolate DC from the circuit, and the headphone recieve the difference between output and "m". Since the input swings above and below "m", so will the output, and so the voltage across the headphones will always be +-10mV (or whatever the amplified guitar signal is). oh and one last thing, that treble bleed mod. it seems like the 220uF feedback cap is already acting as a treble bleed mod for the 100K pot. i'm sure the value is off, but what i'm saying is, to add a treble bleed mod would be pointless, because it would effectively be the same as increasing the value of the feedback cap. wouldnt it? correct me if i'm wrong... i would like to have a more even eq across all volumes if possible

man, i thought this thing sounded decent before, but WOWWWWW! I just replaced the 741 with a TL082. godddangggg it sounds so crisp. there is still hissing there, but it is reduced. i think all i need to do is that treble bleed mod, then solder up (unless you have another suggestion based on my last post)

...can you draw that? I'm not quite following you there. However, since the TL082 is a dual op amp, what some designs have is one half of the TL082 doing a different version of an active "M" that can serve as a "virtual Gnd" for many other op amps, whereas your passive "M" can not do so without affecting the rest of the circuit. Then, they use the other half for the gain section. yes, someone had already notified me of the pot issue. I put a 100K log pot in the feedback loop. are you saying i should put a second volume control after the output cap??? you know the 2 100K resistors between 9v and ground (creating the virtual ground)? well one of those resistors already has a cap in parallel with it. I have seen some power supplies where both resistors have a cap in parallel with them. So there is one cap b/t 9v and m, and another cap b/t m and ground. would this do the same thing as adding the cap from 9v to ground? I looked up what you mean by a "version of an active m". What advantage would that give me over a passive "m"? question: i have a 1/8" stereo out jack. if i just solder a jumper connecting the left and right channels, will that make it mono (and give me sound in both ears)? edit: this is what it currently looks like:

well i'd really like to do this with no more active elements than a single op amp. especially since this is my first self-design project. Also, i was led to believe that opamps had really low output impedances. i guess 75 ohms is pretty low, are you saying it's not low enough? how else can i fix that problem without transistors?

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

Thanks guys, here's the current schem: 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.

One quick question: i got some assorted orange ceramics, and they only have numbers on them. What is the implied unit?

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]

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. ???

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?

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!!!)

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?

and btw, this is prob a dumb question, but what is the purpose of the 10K and 100K resistors at the input part of the circuit?

well, i do know that if I, or another person, is standing next to the guitar or holding it, and the person is not grounded via the strings, then that person is acting as a satellite dish for hum. the hum bounces off of him and gets all up in the pickups. a truss rod could possibly do that. but i'm not sure to what extent...

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

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?

thanks anderekel 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.