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Failed Headphone Amp


AndrewCE

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Are you getting enough overall volume? The standard opamps such as 741 have an output resistance of 75 Ohms. Headphones, such as used with mp3 players have quite low resistance, mine measure 16 Ohms, or 8 ohms if both sides are being driven. So the chip cant drive the phones to more than about 10% of the available voltage swing, which is about a 20db loss. A couple of transistors in a push/pull configuration to drive the output might help.

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Are you getting enough overall volume? The standard opamps such as 741 have an output resistance of 75 Ohms. Headphones, such as used with mp3 players have quite low resistance, mine measure 16 Ohms, or 8 ohms if both sides are being driven. So the chip cant drive the phones to more than about 10% of the available voltage swing, which is about a 20db loss. A couple of transistors in a push/pull configuration to drive the output might help.

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?

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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

OK, I just noticed a rather large design issue. I did not realize before you have a volume on the input! This is not the best way to go for signal-to-noise-ratio reasons. This is part of your noise problem, in my opinion. I would lose that altogether, so you are allowing the full guitar signal in and adjusting gain (and output volume) as necessary. Put a 100K pot in where you have the 100K resistor in the feedback loop. Where you want a volume control is on the output (after the output capacitor).

Yes, your tone will be more trebly when volume is up and it will lose highs as you go down. This will happen with any value standard pot. Not that it is a good thing, but is normal. This is just like what happens with a standard guitar volume. You can do a treble bleed mode, like show on this page... read whole page: http://www.projectguitar.com/tut/potm.htm

...or, if you want it to be less trebly, then adjust the value of the cap in parallel with the 100K resistor in the feedback loop. A larger cap should yield less treble, but make small adjustments there!

Yes, the TL082 lacks those pins because it is internally compensated and does not need offset nulling.

No, DO NOT remove the cap between "M" and virtual Gnd. It is serving the same function as the 9V bypass cap, but in a different spot. Removing it would reduce performance and increase noise. As far as you rpoposition of:

9v -> cap -> virtual ground -> cap -> ground setup. Would that do the same thing?

...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.

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OK, I just noticed a rather large design issue. I did not realize before you have a volume on the input! This is not the best way to go for signal-to-noise-ratio reasons. This is part of your noise problem, in my opinion. I would lose that altogether, so you are allowing the full guitar signal in and adjusting gain (and output volume) as necessary. Put a 100K pot in where you have the 100K resistor in the feedback loop. Where you want a volume control is on the output (after the output capacitor).

Yes, your tone will be more trebly when volume is up and it will lose highs as you go down. This will happen with any value standard pot. Not that it is a good thing, but is normal. This is just like what happens with a standard guitar volume. You can do a treble bleed mode, like show on this page... read whole page: http://www.projectguitar.com/tut/potm.htm

...or, if you want it to be less trebly, then adjust the value of the cap in parallel with the 100K resistor in the feedback loop. A larger cap should yield less treble, but make small adjustments there!

Yes, the TL082 lacks those pins because it is internally compensated and does not need offset nulling.

No, DO NOT remove the cap between "M" and virtual Gnd. It is serving the same function as the 9V bypass cap, but in a different spot. Removing it would reduce performance and increase noise. As far as you rpoposition of:

9v -> cap -> virtual ground -> cap -> ground setup. Would that do the same thing?

...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:

headphoneampschematic4.jpg

Edited by AndrewCE
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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)

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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:

headphoneampschematic4.jpg

The pot in the feedback loop is a gain control, not a volume control. If you put a pot on the output, that is a volume control. There is a difference, in that gain is a multiplier of the original signal and can never be less than one, unless the circuit is setup as an attenuation circuit, but yours is not. Volume is what allows a reduction of the total (post gain), all the way down to zero. If you can get zero, it probably is not a true zero, but if it functions to your liking, it is not a waste either.

The cap config for "M" that you mention, I have not seen that one, but do not think it makes much of a difference, other than series capacitors half the capacitance, so it doesn't sound like a good thing, since higher capacitance is better there (lower corner frequency for the low pass filter) but I could be wrong there on why they did what you saw.

The active "M" allows you to bring "M" to other parts of the circuit (like another signal entirely) without worrying about the crosstalk so much,for example if you were building a mixer out of many op amps. Right now, if you connect "M" to other signals, they may bleed into one another, but with the active "M", there's more push-to-"M" power for lack of a better description.

Yes, jumpering the tip and the ring will give mono in both sides, but will reduce the output as well. To get around this, you can use 1/2 of the TL082 as a right and 1/2 as a left, then you won't lose volume and you'll have 2X the output you currently have.

Very importantly, in looking at your latest schematic, I do not believe you should have the sleeve of the input or output connectors going to "M". I think those should go directly to system Gnd, 0V... not the 4.5V "M". That is VERY likely to be the cause of the rest of your hiss. What is happening is you are adding a 4.5V bias to the input before you are even into your circuit, then removing it with the 0.1uF cap on the input (undoing what you've done by connecting "M" in the firstplace). Then, at the output, the 220uF cap removes DC bias (the "M"), but after that you're adding it back again. That is going to add noise and reduce the life of your speakers... they don't want a DC bias. They want to be in their natural resting position... having "M" connected to the output is liking putting an offset to the speaker cone and reduces headroom.

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The pot in the feedback loop is a gain control, not a volume control. If you put a pot on the output, that is a volume control. There is a difference, in that gain is a multiplier of the original signal and can never be less than one, unless the circuit is setup as an attenuation circuit, but yours is not. Volume is what allows a reduction of the total (post gain), all the way down to zero. If you can get zero, it probably is not a true zero, but if it functions to your liking, it is not a waste either.

The cap config for "M" that you mention, I have not seen that one, but do not think it makes much of a difference, other than series capacitors half the capacitance, so it doesn't sound like a good thing, since higher capacitance is better there (lower corner frequency for the low pass filter) but I could be wrong there on why they did what you saw.

The active "M" allows you to bring "M" to other parts of the circuit (like another signal entirely) without worrying about the crosstalk so much,for example if you were building a mixer out of many op amps. Right now, if you connect "M" to other signals, they may bleed into one another, but with the active "M", there's more push-to-"M" power for lack of a better description.

Yes, jumpering the tip and the ring will give mono in both sides, but will reduce the output as well. To get around this, you can use 1/2 of the TL082 as a right and 1/2 as a left, then you won't lose volume and you'll have 2X the output you currently have.

Very importantly, in looking at your latest schematic, I do not believe you should have the sleeve of the input or output connectors going to "M". I think those should go directly to system Gnd, 0V... not the 4.5V "M". That is VERY likely to be the cause of the rest of your hiss. What is happening is you are adding a 4.5V bias to the input before you are even into your circuit, then removing it with the 0.1uF cap on the input (undoing what you've done by connecting "M" in the firstplace). Then, at the output, the 220uF cap removes DC bias (the "M"), but after that you're adding it back again. That is going to add noise and reduce the life of your speakers... they don't want a DC bias. They want to be in their natural resting position... having "M" connected to the output is liking putting an offset to the speaker cone and reduces headroom.

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

Edited by AndrewCE
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That is what should happen with that power bypass cap when you disconnect the batttery the way you did. This is why devices incorporate power switches. The cap is discharging once you remove the battery. What it does is charge up and keeps a better DC level than the battery can, tregardless of whether it is audible. In the even of momentary spike or dip in the batery voltage (that's what you're simulating by disconnecting that battery), the cap makes up the difference by feeding what is necessary from its store. In a configuration with this cap, the power switch is usually put in place to cut either the positive or negative rail at the point it connects to the cap. That kills power and disconnects the cap at the same time, so the behavior you mentioned is not there.

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).

You are wrong about this and seem to be confused or misled on Gnds in general. I've already explained why "M" should not be connected to the sleeves, but try a different way. Even if you can get sound that you think is OK in this configuration, I don't think it is a good idea to move forward with these wrong ideas, but do what you will.

Let's start with just the guitar, no circuit. Your guitar's output connector has a tip and a sleeve. The sleeve is the Gnd at 0V. The tip can sway plus or minus about the sleeve.

The guitar's untouched signal does sway positive and negative. It is true AC. So, picture this as a sine wave centered around zero volts.

Nothing the op amp does can go negative, ever, unless 2 batteries are used, because one battery can only be 0 to 9V, all it can do is be somewhere between zero and 9V... varying degrees of positive.

When you add "M" to the raw guitar signal,you're biasing it to 4.5V, so instead of having true AC centered around zero, you have 4.5VDC that has an AC riding on it. The AC is shifted up by 4.5V. There are times when you do want this and times when you don't. You don't want this on the sleeves ever.

Let's pretend though that you did want it on the input sleeve. So now instead of a pure AC, our signal at the tip is now centered around 4.5V. Once you pass the 0.1uF capacitor on the input, you remove the 4.5V. Remember, the cap blocks it, so then we're back to a true AC! Let me ask you then, what was the point of adding "M" to begin with? There isn't one. Not only is it useless on the ring, but adding an imperfect variable in and then using an imperfect method to remove it has to add noise.

"M" is added after the input capacitor. Doing it there does not allow it to bleed backward into the guitar signal because of the input cap. This is where we DO want "M", to move that signal from being centered around zero to centered around 4.5V. This accomodates the 9V battery and this is the only reason to do this.

On the output side, before the signal passes the 220uF cap, it has the DC offset of 4.5V on it. The purpose of the output capacitor is to remove that 4.5V and bring the signal back down to a true AC, centered around 0V. So like I asked before about the input side... on the output side, why would we remove the DC with a capacitor only to add it again at the ring? Removing it with the output cap is correct. Adding it back in at the sleeve is not.

Incidentally, both the input and output sleeves should be connected to the systerm Gnd.

Again, here's how it should be:

Unless your guitar is all opened up, the closes thing you have to Gnd is the guitar's output connector sleeve. The op amp negative power input should be connected to the output connector's sleeve should connect there too, as well as the end of the line in your feedback loop. "M" should not go to any of those.

As a last piece of evidence for this, study the MXR schematic again. Find their "M" made by 1M resistors and a 1uF cap. How many places do they add that signal to? Only one, on the signal AFTER the input cap. All other places are system Gnd at a theoretical 0V.

What I think you've done is make one mistake, then make a 2nd, so you have a case of 2 wrongs that actually do make a right.

Yes, you're wrong about the treble bleed mod. It is not something to apply to the gain pot. It would be something to apply to a volume on the circuit's output, after the circuit's ouput capacitor. The treble bleed mod allows a certain amount of highs to forgo being sent to ground as you roll off the volume, preserving a lot of what gets lost in turning down a traditional volume knob.

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On the output side, before the signal passes the 220uF cap, it has the DC offset of 4.5V on it. The purpose of the output capacitor is to remove that 4.5V and bring the signal back down to a true AC, centered around 0V.

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.

Edited by AndrewCE
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On the output side, before the signal passes the 220uF cap, it has the DC offset of 4.5V on it. The purpose of the output capacitor is to remove that 4.5V and bring the signal back down to a true AC, centered around 0V.

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.

As close to 0V as theoretically possible.

You are making my point by making the assumption that "M" is the ground you should be using on the ouput side. You took out the DC with the output cap and are then putting it back in. That is bad for reasons I've explained 3X now. You should be using system ground everywhere except 1 place and the only reason to use it in that place is input biasing. Everything else should be a 0V Gnd.

If you don't believe me, get a volt meter. You will see 4.5 V on your output. This is not good. I am only trying to help, but unfortunately,don't know how else to convince you. Good luck.

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If you don't believe me, get a volt meter. You will see 4.5 V on your output. This is not good. I am only trying to help, but unfortunately,don't know how else to convince you. Good luck.

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.

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If you don't believe me, get a volt meter. You will see 4.5 V on your output. This is not good. I am only trying to help, but unfortunately,don't know how else to convince you. Good luck.

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.

Nice filenames on those drawings. :D

Neither of those drawings is correct. This one is. I should have just drawn it that 1st day to avoid the decline in mood.

Notice all negatives and Gnds are common, with VCC/2 (your "M") connected to only 1 place... not 2, 3, or 4, just the 1.

Headphone_Amp.jpg

4.5VDC can not make it past the output cap. That defies the nature of the capacitor.

It is not theoretically possible for you to have moved both sleeves to Gnd and measure 4.5V from signal to Gnd after the output cap. Before the output cap, yes, after it, no... unless, you have VCC/2 applied to Gnd somewhere else (which you do, the end of the feedback loop is another error) or your output cap is faulty.

Any measure of 4.5VDC should be confined to within the op amp circuit (the input/output caps) with both sleeves connected to system Gnd.

Instead of me building it your way, I suggest you build it my way. I've got over a dozen of these types of circuits built and need not prove the theory to myself.

Please don't take any of this as badgering. I am only trying to help and would not want to feel like I am letting you go with misinformation, so if I can steer you right, I am going to, as painful as it may get for either of us.

If this last part is correct the way I describe, I bet all of your detectable noise will be gone and wouldn't that be excellent? If not, and I'm wrong, well then at least you have something that works in a way you are satisfied, right?

Edited by Donovan
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:D 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?

4.5VDC can not make it past the output cap. That defies the nature of the capacitor.

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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:D 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?

4.5VDC can not make it past the output cap. That defies the nature of the capacitor.

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

I admit I felt some negativity in that last post when i saw the filenames, but I am to blame as well as my own frustration of trying to explain things which are approaching the limit of my technical jargon, but OK, today is a new day and now we're on a better track.

1) No, I can not tell you with surety what R6 does, but it appears to be a standard thing to do frommany resources. I have copied it (in your case I scaled it with the rest of your values). It does appear in the MXR Distortion + that I provided previously, as well as the DOD 250, and the active mixer schematics on this page: http://www.harpamps.com/schematics/. In the case of mixers, I found an explanation saying "The resistors further the isolation between inputs, i.e. serve to limit the back-feeding of input signals."

2) I omitted the cap from the 9V to Gnd as you said you already tried it and it had no effect. I do put that on all my circuits. I test all my 9V circuits with a 9V AC adapter. Those supplies are much noisier than a battery and I find I can discern a huge difference with that cap in that mode, which BTW they call a "supply bypass" cap, so I've included it in everything since as a default. I should also mention there are 3 varieties of this. The first one is, for me, usually a 100uF and goes directly across the battery + and -. The 2nd is smaller, usually a 1uF, directly across the + and - power inputs on any op amp or other IC in the circuit. The 3rd is the one going from "M" to ground. They all do the same thing. The one on the battery is the main one. The op amp power one and the "M" one both serve to cut any noise that was picked up in the line between the battery and the location of interest because these lines can pick up noise in even those short distances. Even though you could not hear a difference when you tried them, if you try it with an AC adapter, you may be pleasantly surprised at the difference and decide to go the mojo rooute as I have.

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.

If this is what you meant 2 posts ago, OK, that is what I would expect... a 4.5VDC swing with ouput connected and with an input signal running through it. With it just powered on and quiescent there should be 0VDC, only the residual AC quiescent noise. With it connected and running the 4.5VDC swing should only be with it cranking some music.

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.

It does matter. It is just good general practice to use one Gnd wherever possible. If you can use one massive Gnd, it will be a more effective Gnd. Everything else will be in reference to it and when there is an errort-type spikes/dips due to imperfections in the supply or components heating up, etc., they will be more efficiently sunked, for lack of a better term, and there will be less of a divergence or more of a convergence. Using "M" as a Gnd in those extra places, though you may not hear it, is less than ideal because "M" is not really static as a 0V Gnd is. "M" is an ever-moving (however slight) product of the current formed within a voltage divider and has its own noise associated with the fact that there are wires, resistors, and a current. Does that make sense? If you do insist on using "M", it would be a step up to use a zener diode regulator or a voltage regulator to form the midpoint as they have their own error-correction by nature. Also, there is a tradeoff relationship in the resistors you choose to form the "M". Lower value resistors have as a potive that they create less noise than higher value resistors. They have as a negative that they will consume your batery faster. The converse is true for higher value resistors used there. The same scenario goes for within your feedback loop.

I am limited in being able to explain any of that better as I don't have any formal education on design. What I have is limited to what I learn from the resident nerds at my workplace, what I research online, and what I tinker with. So, I can only try to explain these things in the contexts they were explaonied to me or how I preceived them. The more I try to explain this Gnd stuff, the more dangerous it feels, so I'll stop here, but will be on standby if there's anything more I can help you with.

One other thing Paul M mentioned a page or two ago... the TL072 is basically the same as the TL082 but lower noise. You can't get it at your local RS, but it, as well as many newer op amps designed specifically for audio, may serve you well. The LM741 and TL082 are general purpose op amps with specs that aren't so hot. You may see a large difference in perceived noise just by upgrading the op amp, though if you do go that route, no point in stopping at the 072, might as well talk to a specialist at Philips, Fairchild, or LT and get something really new and pretty.

I feel the need to mention how much I am beginning to hate the SHIFT " typing sequence. :D

Edited by Donovan
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I admit I felt some negativity in that last post when i saw the filenames,

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

less than ideal because "M" is not really static as a 0V Gnd is. "M" is an ever-moving

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"?

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Well, what's the performance like now anyway?

I'm assuming the sound you got is entirely clean or does it have some dirt to it as well?

um, what is the "shift typing sequence"?

The quote character...ugh. I must have typed it 2000 timed in the past 3 pages. :D

I admit I felt some negativity in that last post when i saw the filenames,

well sorry if you misunderstood, it was only a joke.

Thanks, but not necessary... I think I came off as uber-persistent.

less than ideal because "M" is not really static as a 0V Gnd is. "M" is an ever-moving

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.

Some, but not all adapters. Obviously, none of the 2-prong deals are. I believe some of the 3-prong deals have the Gnd tapping the transformer.

We can agree to disagree. I am going to back off on this Gnd discussion as my knowledge level is at its bounds and if you're happy with what you have, then I feel satisfied.

If you're going to use an AC adapter, maybe look into rechargeables and a charging circuit, then you can be mobile when it's inconvenient to plug in.

I'm off to enter my own frustration with an LED project I'm working on. I've been trying to do something similar to your project, but trying to get enough gain to rectify it and re-amplify the DC enough to drive LED's brightness. I took the day of from work and have been at this for about 8 hours already, unsuccessful and very annoyed... time to unplug. My 3 year old is playing my strat that is sitting on the living floor with about 7,000 resistors all over the place. Time to intervene. :D

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Well, what's the performance like now anyway?

I'm assuming the sound you got is entirely clean or does it have some dirt to it as well?

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? :D 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?

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Well, what's the performance like now anyway?

I'm assuming the sound you got is entirely clean or does it have some dirt to it as well?

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? :D 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?

That does sound kind of bad, doesn't it. Well, I originally took off because I was dead tired. My wife works 12 hour shifts on Sundays and Mondays, leaving me as a babysitter as well as 2-way transportation... doesn't make for a good Monday, especially when you add a doctor's appt to that. The LED project is just something I brought myself to do during the early morning hours as I've been excited about it and working on the concept and drawing for about a week, but today ended horribly after a crapload of trials, no sucess because the op amps can not give me enough gain to drive more than one LED. I am going to have to add a crapload more components. If I do get it going, it's going to be an RGB spectrum analyzer with color shifting to pitch, mounted in a strat pickguard, between the pickups.

I use 100K pots as well. If the problem was the pot value, I think going higher would be better than lower. I haven't run into your exact problem, but I have run into something similar very similar where the last bit on the high end seems to jump up a bit suddenly,but I always though it was because I was not using log pots. Something else to try... maybe a longshot, but move your gain pot to outside the loop, where the resistor to ground goes and vary it there, then use a 100K fixed resistor inside the loop instead. That's how I've seen it most, though I did not think the way you have it should cause a problem. Can you draw how/where you've got it hooked up?

I connect the volumes like this. This drawing also has the noise-reducing caps we talked about.

Headphone_Amp_2.jpg

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I connect the volumes like this. This drawing also has the noise-reducing caps we talked about.

Headphone_Amp_2.jpg

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.

????????

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OK, for some reason I thought you had your gain setup the other way around.

Maybe like you said and someone mentioned back on page 1'ish, it is the addition of the headphones that is doing it. Can you try hooking the output up to a different system, like a guitar amp or a set of PC speakers to see if the result is the same?

Edited by Donovan
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OK, for some reason I thought you had your gain setup the other way around.

Maybe like you said and someone mentioned back on page 1'ish, it is the addition of the headphones that is doing it. Can you try hooking the output up to a different system, like a guitar amp or a set of PC speakers to see if the result is the same?

i'll try it, but, well even if that is the problem, it doesnt really seem fixable. i mean, how would i be able to have a headphone amp... without using headphones?

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