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Failed Headphone Amp


AndrewCE

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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?

Other op amp models with better specs.

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I haven't read the entire thread, but I think the failure is largely to do with this being basically a preamp kind of circuit where you need the power of a power amp circuit to drive a speaker or headphones generally. The ubiquitous LM386 meets the bill for this kind of thing with battery power as on the various "ruby" like circuits based on it...another approach is to use power transistors fed by the opamp circuit to push the power through to move speakers sufficiently.

I am not sure of this analogy, but you could look at the speaker or headphones like a big (generally 8 ohm) resistor...you may be getting the gain but not the "power" to move this obstacle at the end of the circuitry. What you are getting is a lot of voltage gain from such circuits but not current gain...so perhaps an analogy is you are getting a lot more water but not the flow rate to move the speakers when it hits that resistance...maybe an analogy would be a hose running up hill, you can have as much water at the bottom as you like, but it takes some pressure to get it to run up hill!

However, such circuits are useful if not necessary on the front end of power amps like the LM386...you need them to prevent loading down the guitars signal as generally power amps have a very low input impedance (like resistance)...such opamp or transistor circuits before the power amp stage help to condition the incoming signal from the guitar to "match" the impedance and not offer a tone robbing obstacle to the guitar itself.

Hope that hasn't confused things even further...

pete

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I don't want you to go and get a better one. I am just suggesting one with different specs might be the answer... that perhaps the op amp is "struggling" with the low-resistance load from impedence mismatch, but I doubt that is it because the op amp should not "know" what you are doing as you lower or raise a volume that is inline after the output cap. That's why I asked if you could daisy chain it with a full amp or PC speaker set, to see if the condition goes away. If it does go away, then it's a clue that it is impedence-related. If it persists, maybe you just have a defective or crappy potentiometer. The icro pots that RS sells are crap and won't last if you mess with them too much.

If it ends up being impedence or just not enough output current capability, what you do from there if you can deduce that is up to you, but high power op amps or other IC's that are designed to be audio amplifiers are out there. They are not expensive, but just a PIA to order, wait for, etc., and some of these large online distributors have a minimum order which is why I stay away.

Here's a hint as well... go to TI website, National Semi, Linear Technologies, Philips, Fairchild, etc. All these guys make these chips. They also will send you free samples of many of them to try out so long as you're not running your mail off a P.O. box.

Radio Shack doesn't have much to offer in that dept, but they do have the low-end, ever-famous (or infamous) LM386... out since the 70's I believe. It will definitely drive your headphones as it's got between a 0.5W-.7W output rating, but getting good sound quality may take a lot of tweaking as the chip is moody and requires tailoring to the rest of your circuit to avoid squealing and other annoyances.

Have you used the other side of the TL082? If not, and if the condition is impedence related, you might try a bridge-tied load configuration, which should increase output if the op amp can handle it. There's a chance the op amp may npot handle it, and instead may overheat or self-destruct.

Something else to try is changing the size of your output capacitor. that will change the character/volume of the sound. As a guideline, I've read to choose your output capacitor as 100X the value of the input capacitor, but nothing says you can't try the whole spectrum.

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I haven't read the entire thread, but I think the failure is largely to do with this being basically a preamp kind of circuit where you need the power of a power amp circuit to drive a speaker or headphones generally. The ubiquitous LM386 meets the bill for this kind of thing with battery power as on the various "ruby" like circuits based on it...another approach is to use power transistors fed by the opamp circuit to push the power through to move speakers sufficiently.

I am not sure of this analogy, but you could look at the speaker or headphones like a big (generally 8 ohm) resistor...you may be getting the gain but not the "power" to move this obstacle at the end of the circuitry. What you are getting is a lot of voltage gain from such circuits but not current gain...so perhaps an analogy is you are getting a lot more water but not the flow rate to move the speakers when it hits that resistance...maybe an analogy would be a hose running up hill, you can have as much water at the bottom as you like, but it takes some pressure to get it to run up hill!

However, such circuits are useful if not necessary on the front end of power amps like the LM386...you need them to prevent loading down the guitars signal as generally power amps have a very low input impedance (like resistance)...such opamp or transistor circuits before the power amp stage help to condition the incoming signal from the guitar to "match" the impedance and not offer a tone robbing obstacle to the guitar itself.

Hope that hasn't confused things even further...

pete

so what you're saying is i don't have enough power? All the headphone amp schems online said that low output impedance is the way to go. But now that I think about it, guitar amps and cabinets need to be matched to get the best power transfer. Do I need to match the impedance to my headphones? how do I do that? is there a way without transistors (i am a beginner and i'd rather not venture there just yet)?

and btw, how does this have anything to do with the taper of my pot? as i understand it, my headphones are altering the taper. Will increasing the amp output impedance change this effect?

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Low impedence is the way to go. What you don't want is a low impedence input pulling from a high impedence output. The reverse is fine. There is a difference between the LM741 and the TL082 in this regard. The TL082 is a better choice, having very high impedence JFET inputs. Someone before mentioned the LM741 has an output resistance of 75. I could not verify this on the datasheet, but perhaps am missing something.

The pot taper gives the characteristics of how the volume goes up/down when you adjust it. Variations in the manufacturing process could give you the behavior you describe.

I built the TL082 headphone amp tonight (1 side only). PSW is right that it is a preamp circuit. Really, the only difference is the power output, general purpose op amps being low output current, so the chip is better suited to where it doesn't really have to drive anything, but I do believe the TL082 is sufficient for your application (unless maybe you're trying to drive biga$$ over-ear headphones) after trying some things tonight. It worked OK with my strat, but was weak IMO. I experienced no pot taper issues, but I used different value caps and resistors than you as i did not have the same cap values on hand. I think your pot is suspect.

I drastically improved the performance and noise of the unit I built by using dual 9V supplies. This also allowed a drastic reduction in component count (and therefore necessary space), down to the 2 batteries, 1 chip, two resistors, one pot and zero capacitors. Design is much simplified when using true dual supplies. The circuit actually sounded pretty kicka$$ through my Bose headphones, which are pretty high output headphones.

Incidentally, the dual supplies worked for my LED project as well, so my mood is improved as well. :D

I've always been intimidated by dual supplies, but now that I've seen how much easier it is to use op amps with dual supplies, I'm not going back, at least for audio applications. The space savings alone with regard to the PCB is enough. Without the need to create a VCC/2 and all the extra capacitors, the tradeoff is well worth it.

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I think your pot is suspect.

i measured resistance of my pot from i pin to wiper at degrees of rotation in 60 degree increments (it's actually a bit less than 60. maybe 57). Here is the chart:

deg. Kohms

0 1.5

60 1.5

120 4.45

180 10.5

240 24.9

300 84

360 95.5

looks pretty standard logarithmic to me, considering it is a rough test. (except that 240-300 stepup?)

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I think your pot is suspect.

i measured resistance of my pot from i pin to wiper at degrees of rotation in 60 degree increments (it's actually a bit less than 60. maybe 57). Here is the chart:

deg. Kohms

0 1.5

60 1.5

120 4.45

180 10.5

240 24.9

300 84

360 95.5

looks pretty standard logarithmic to me, considering it is a rough test. (except that 240-300 stepup?)

It looks like 240-300 (55.1 jump) and 300-360 (11.5 jump) are defective and should be reversed and even then are not so smoothly logarithmic, so that might exhibit a jump in perceived volume, not to mention that you should have 0 at 0, but that alone would not give you the jump behavior. Do you have a linear pot or another log pot you can test perceived volume with?

Would you say you are getting some volume the moment you turn a small amount from 0 degrees? Please estimate the degree amount where you first notice sound and the degree amount that the jump occurs.

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It looks like 240-300 (55.1 jump) and 300-360 (11.5 jump) are defective and should be reversed and even then are not so smoothly logarithmic, so that might exhibit a jump in perceived volume, not to mention that you should have 0 at 0, but that alone would not give you the jump behavior. Do you have a linear pot or another log pot you can test perceived volume with?

Would you say you are getting some volume the moment you turn a small amount from 0 degrees? Please estimate the degree amount where you first notice sound and the degree amount that the jump occurs.

ok i went buy another radio shack pot, 100k audio, and i get the same exact problem. I've tried wiring it forwards, backwards, etc. no luck. I'm using the wiper terminal to connect to the headphone jack tip.

I think the problem is really that the impedance is too small on the headphones. they are altering my pot taper. how can this be fixed?

the jump occurs about where the chart indicates: 240-300 deg.

i'll try a smaller value pot; 10k.

also note that the chart i gave is very very vauge. i did not have a protractor on hand.

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my 10k pot is linear, so forget that.

what i did is put a 47k resistor between the wiper of the 100k pot and the headphone jack tip. This made my pot a bit smoother, although not completely smooth, however there is volume loss. I dont think it's a good idea to put a resitor there,but what I found out is this: headphone impedance WAS the issue. putting the resistor was increasing the impedance, therefore having less effect on pot taper.

i have not been able to translate this into a solution, though. i don't wanna have to use special ultra-high impedance phones for my amp. i wanna be able to use any phones.

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I've always been intimidated by dual supplies, but now that I've seen how much easier it is to use op amps with dual supplies, I'm not going back, at least for audio applications. The space savings alone with regard to the PCB is enough. Without the need to create a VCC/2 and all the extra capacitors, the tradeoff is well worth it.

well, i would like to be able to power this with a 9v wall adapter. How would this work with dual supplies? Like this?:

headphoneampschematic5dualsupplyand.jpg

but what if it was daisy chained with other single supply units? (i have a power distributer) Wouldnt that screw it up?

Edited by AndrewCE
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I've always been intimidated by dual supplies, but now that I've seen how much easier it is to use op amps with dual supplies, I'm not going back, at least for audio applications. The space savings alone with regard to the PCB is enough. Without the need to create a VCC/2 and all the extra capacitors, the tradeoff is well worth it.

well, i would like to be able to power this with a 9v wall adapter. How would this work with dual supplies? Like this?:

headphoneampschematic5dualsupplyand.jpg

but what if it was daisy chained with other single supply units? (i have a power distributer) Wouldnt that screw it up?

If you had the ability to set it up as 2 4.5V cells, then yes,but one 9V, no. There is a difference, but I am no good at explaining how/why.

Is your 100K log in that drawing what we've been discussing as your "volume"?

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Is your 100K log in that drawing what we've been discussing as your "volume"?

oh no, currently i've got a 100K fixed resistor there. My volume is a standard passive volume after the capacitor. By the way, when I had my variable resistor in the gain stage, the taper was perfect.

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Is your 100K log in that drawing what we've been discussing as your "volume"?

oh no, currently i've got a 100K fixed resistor there. My volume is a standard passive volume after the capacitor. By the way, when I had my variable resistor in the gain stage, the taper was perfect.

I'm at a loss, unfortunately. If no one else can help you, you may as well go back to just a variable gain if that worked perfectly.

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Is your 100K log in that drawing what we've been discussing as your "volume"?

oh no, currently i've got a 100K fixed resistor there. My volume is a standard passive volume after the capacitor. By the way, when I had my variable resistor in the gain stage, the taper was perfect.

I'm at a loss, unfortunately. If no one else can help you, you may as well go back to just a variable gain if that worked perfectly.

well the only problem is, idk how to put a treble bleed mod on the gain control.

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oh and btw i get some really nasty clipping when the gain is up. turning down my guitar volume helps this a bit, but i'd rather not have to.

Make the 1K resistor bigger. Max gain is limited by the ratio of the 100K to the 1K (por whatever is in that spot) + 1. As is in the last drawing, max gain is a theoretical 101X. The more you limit the ratio to being smaller at max gain, the more you reduce likelihood of clipping, which will occur when ever you approcah the limits of the op amp swing, the power supply swing, or both. You can figure the theoretical max gainif you know what the op amp's true swing is and you know what your max input swing is. This is where"rail to rail" op amps and those designed for single supply use and with audio application in mind accel over general purpose ones like the LM741 or TL082.

Remember, you can get free samples of just about anything from most IC manufacturers. I just got 5 IC's for my LED project today and will not be paying for anything unless they work for my application and I decide to buy more.

Regarding your previous post, I don't think you can do a treble bleed on your circuit in the gain section, at least not the way that is familiarly done on a volume pot. What you can do is tweak frequency response through choice of capacitors. Or, broaden your circuit design to add bands of boost, but this would add a lot of components, which I'm sure you don't want to do.

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well idk how to really measure my guitar's max voltage swing, i think i'm just gonna have to get by with the variable gain config.

my setup now (with the 100k gain rheostat) is really really dark on the low gain settings, then theres that fast jump up to LOUD, and then it starts to clip. I think i'm gonna have to set this thing right on that edge, and find the sweet spot, and if it's clipping just back off on my guitar's volume. It's too much work, but oh well

what opamps are better than the TL082 i'm using? i was told that was an "audio" opamp. which are the better ones?

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well idk how to really measure my guitar's max voltage swing, i think i'm just gonna have to get by with the variable gain config.

You'd need a multimter with a sample and hold peak detector. Pickup manufacturer's sometimes post this information, but it's a big window at best.

my setup now (with the 100k gain rheostat) is really really dark on the low gain settings, then theres that fast jump up to LOUD, and then it starts to clip. I think i'm gonna have to set this thing right on that edge, and find the sweet spot, and if it's clipping just back off on my guitar's volume. It's too much work, but oh well

That's unfortunate. I'd really hoped to see this worked out. Have you tried playing with capacitor values?

what opamps are better than the TL082 i'm using? i was told that was an "audio" opamp. which are the better ones?

As we said in the beginning, the TL082 is better than an LM741. The TL072 is a lower noise version of the TL082. Still, all of these are general purpose "penny" chips because they are outdated by newer technology and you can buy them on the penny.

Here's an interesting page, though it's 5 years old. Even then, the TL082 was outdated. http://tangentsoft.net/audio/opamps.html

Take a look at this and order yourself some free samples... http://www.linear.com/pc/productDetail.jsp...009,C1026,P1293

This one looks just right for you IMO... it should be more output, less noise, and, you can get free samples... http://focus.ti.com/docs/prod/folders/print/opa627.html

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well idk how to really measure my guitar's max voltage swing, i think i'm just gonna have to get by with the variable gain config.

You'd need a multimter with a sample and hold peak detector. Pickup manufacturer's sometimes post this information, but it's a big window at best.

my setup now (with the 100k gain rheostat) is really really dark on the low gain settings, then theres that fast jump up to LOUD, and then it starts to clip. I think i'm gonna have to set this thing right on that edge, and find the sweet spot, and if it's clipping just back off on my guitar's volume. It's too much work, but oh well

That's unfortunate. I'd really hoped to see this worked out. Have you tried playing with capacitor values?

what opamps are better than the TL082 i'm using? i was told that was an "audio" opamp. which are the better ones?

As we said in the beginning, the TL082 is better than an LM741. The TL072 is a lower noise version of the TL082. Still, all of these are general purpose "penny" chips because they are outdated by newer technology and you can buy them on the penny.

Here's an interesting page, though it's 5 years old. Even then, the TL082 was outdated. http://tangentsoft.net/audio/opamps.html

Take a look at this and order yourself some free samples... http://www.linear.com/pc/productDetail.jsp...009,C1026,P1293

This one looks just right for you IMO... it should be more output, less noise, and, you can get free samples... http://focus.ti.com/docs/prod/folders/print/opa627.html

yes, i've tried playing with capacitor values. i either get much noise or a dark sound. i opted for the small value cap ("much noise") and i can keep tabs on the noise by rolling down the gain knob.

thanks for all those resources. i would get one of those better chips, if i hadn't just finished soldering my PCB and prepping my enclosure. this thing should be finished by this time tomorrow (hope it works!)

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thanks for all those resources. i would get one of those better chips, if i hadn't just finished soldering my PCB and prepping my enclosure. this thing should be finished by this time tomorrow (hope it works!)

No problem. If you do any more of these things, get a socket so you can change the chips out to try different ones. They usually follow a standard pinout so they are "plug and play" in a sense.

What are you using as an enclosure?

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headphoneampschematic6final.jpg

grrrrr

i got it set up, but i get no signal through the headphones. just pure hum. i'm gonna check all my connections, but here's my schematic as of now. does anybody see where i went wrong? I added a DC jack for a 9v input. there are 2 connectors that are "normally closed", and when the DC plug is in, one of them is the shell and the other is not connected. the node labeled "dc jack center pin" is not involved in the switching.

the reason i added this is so that when my 9v adapter is plugged into it, the adapter powers it. then when it is removed, it goes to battery power. right now i get no signal on either power source. any help? ive done plenty of continuity checks

i wonder, was i supposed to use a heatsink when soldering my chip? cause i didnt. could i have burned out the chip? and how do i check for that?

ps. when i touch the pot shaft (or even put my finger near it) the hum gets louder. im using a plastic enclosure(all radio shack has, and my altoids tin won't fit a 1/4 in jack).

EDIT: ok, i didnt change anything, and now with the battery supply it sounds like an extremely low-fi fuzz box. and the pot taper has problems, which, btw, it has never done while in the gain-control configuration. there is no sound in the power supply operation (but i'm in the process of correcting that) can anyone spot the reason for my fuzz ?

Edited by AndrewCE
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That's your latest configuration?

I thought you said before that your gain pot was where the 1K resistor is and that what you're showing as a 100K log is actually a 100k resistor. I think that is where it should be, not where it is shown in that drawing. The way it is now, it will try to go to gain <1, which will make it unstable. I also think it should be a linear pot when you're working gain. Making that a linear pot and moving it will help with the jumpiness.

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That's your latest configuration?

I thought you said before that your gain pot was where the 1K resistor is and that what you're showing as a 100K log is actually a 100k resistor. I think that is where it should be, not where it is shown in that drawing. The way it is now, it will try to go to gain <1, which will make it unstable. I also think it should be a linear pot when you're working gain. Making that a linear pot and moving it will help with the jumpiness.

no the gain is "r1/r2 + 1" so the gain is never less than 1.

well before i worry about the pot taper i have to figure out why it sounds like a lo-fi fuzz box.

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no the gain is "r1/r2 + 1" so the gain is never less than 1.

well before i worry about the pot taper i have to figure out why it sounds like a lo-fi fuzz box.

Yep, you're right... sorry, my bad. Gain is as you said in your "noninverting" configuration. I had it confused with the inverting gain circuit, which gain can be unstable in that configuration.

Back to your problem...

If you've not changed anything from your test setup and your test setup was working fine, then I can think of only two things as possibilities. You've either made a wrong connection or have a damaged/defective component. Use your meter to trace through the circuit. Any of your components could have failed or be on the verge of complete failure, without visual evidence.

Edited by Donovan
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