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CMoy massive distortion issue

post #1 of 10
Thread Starter 

Ok so I built tangent's CMoy and 2 days ago the initial test went well. Then I added a simple bass boost circuit in series AFTER the feedback resistor (the cap was .22uf and the resistor was a 10k) and it got a slight boost and sounded ok. I then localized the bass boost circuit to the main board which caused issues at first but I believe I somewhat resolved them....it was distorted still but ok for a test setup. I then moved the components back offboard for another issue I was having and used a pot for the resistor on the filter instead and used a .1uf cap. It "kind of" worked if setup after the feedback resistor as it was yesterday. I saw somehwere that it works better if set before the resistor though. I did that today and now music is unrecognizeable no matter what value the pot is at because of MASSIVE distortion. I had to change headphones today because my original test pair went missing (I do my work at a workshop in my school and I believe someone picked them up and walked off) and the new ones are lower impedance I believe. I thought adding 47 ohms to R5 would fix this issue but it just made the distortion softer...I guess I should mention I'm using a TL082C for the opamp.....any suggestions for fixing this until my BurrBrown OPA2132PA gets here?
 

post #2 of 10

It does not matter if the bass boost circuitry is before or after the resistor it is in series with, but if it is in series with the **wrong** resistor your amp will never run right. 

 

 

This is somewhat confusing because this schematic uses different nomenclature from Tangent's schematic. 

But R5 in parallel with C2 can be swapped with R4 (in the schematic I posted)

If R5||C2 is in series with R3 your amp will never run right. 

 

 

 

 

Back to Tangent's schematic. What tangent calls R5 should ideally be a piece of wire. With certain op amps it *may be* beneficial to have single-digit-ohms of resistance here but R5 in the feedback loop is generally a bad idea. 

If you want to add output impedance (I would argue that this is a fine idea, as you can guess from the other thread) do it outside the feedback loop. 

post #3 of 10
Thread Starter 
Quote:
Originally Posted by nikongod View Post
 

 

 

This is somewhat confusing because this schematic uses different nomenclature from Tangent's schematic. 

But R5 in parallel with C2 can be swapped with R4 (in the schematic I posted)

If R5||C2 is in series with R3 your amp will never run right. 

 

 

 

 

Back to Tangent's schematic. What tangent calls R5 should ideally be a piece of wire. With certain op amps it *may be* beneficial to have single-digit-ohms of resistance here but R5 in the feedback loop is generally a bad idea. 

If you want to add output impedance (I would argue that this is a fine idea, as you can guess from the other thread) do it outside the feedback loop. 

It's built correctly but ill run through the signal path again to make sure....I may have accidentally killed the opamp the other day and that might be the real issue here but idk. Hopefully when the OPA comes in I wont have to worry about this issue any more.

post #4 of 10
Thread Starter 
Quote:
Originally Posted by nikongod View Post
 

It does not matter if the bass boost circuitry is before or after the resistor it is in series with, but if it is in series with the **wrong** resistor your amp will never run right. 

 

 

This is somewhat confusing because this schematic uses different nomenclature from Tangent's schematic. 

But R5 in parallel with C2 can be swapped with R4 (in the schematic I posted)

If R5||C2 is in series with R3 your amp will never run right. 

 

 

 

 

Back to Tangent's schematic. What tangent calls R5 should ideally be a piece of wire. With certain op amps it *may be* beneficial to have single-digit-ohms of resistance here but R5 in the feedback loop is generally a bad idea. 

If you want to add output impedance (I would argue that this is a fine idea, as you can guess from the other thread) do it outside the feedback loop. 

also in this schematic you have my R5 is a 100k pot....is that ok?

post #5 of 10
Quote:
Originally Posted by backspace119 View Post
 

also in this schematic you have my R5 is a 100k pot....is that ok?

 

Maybe. 

 

Load your values into LTspice and see what sort of range of control and frequency response you get. Are they OK? 

post #6 of 10
Thread Starter 
Quote:
Originally Posted by nikongod View Post
 

 

Maybe. 

 

Load your values into LTspice and see what sort of range of control and frequency response you get. Are they OK? 

It distorts when the pot is turned all the way up but its working now. At half value on the pot or below it has nice sound and bass even on the TL082C (although gain is turned down right now...) The new opamp will fix a lot of my issues but until then I think I'll build an output buffer (I have 2 single opamps laying around so I might as well). Also LTspice....is that this? because I'm running fedora and wine doesn't seem to want to run that program....

post #7 of 10
Quote:
Originally Posted by backspace119 View Post
 

Also LTspice....is that this? because I'm running fedora and wine doesn't seem to want to run that program....

 

Yup, that's the one. 

 

It should run in WINE. 

post #8 of 10
Thread Starter 
Quote:
Originally Posted by nikongod View Post
 

 

Yup, that's the one. 

 

It should run in WINE. 

Got it running but I'm not entirely certain how to use it...seems I'm missing how to generate an AC signal...I'll google how to use it but in the meantime I'm wanting to add an output buffer...I just so happen to have 2 single opamps from the shack (none of my mouser stuff has arrived yet). So would I just hook their noninverting inputs into the output of the amp and hook them up to V+ and V- and put a 1k resistor from their output to the inverting input? Would this work at all or am i crazy?

post #9 of 10

With a gain of ~10, bass boost, 9V supply, and CMoy virtual ground imbalance, I'm not surprised you're getting distortion at max volume.  The amp is probably clipping.

 

You should lower the amp's gain, and possibly increase the supply voltage.

 

The TL082 is also part of the problem. It's not a very "strong" chip, in terms of driving lots of current into very low impedances, like headphones. Bass boost increases this problem by increasing the voltage of the hardest-to-drive parts of the circuit.

 

Bass boost works best with buffered headphone amps.

post #10 of 10
Thread Starter 
Quote:
Originally Posted by tangent View Post

With a gain of ~10, bass boost, 9V supply, and CMoy virtual ground imbalance, I'm not surprised you're getting distortion at max volume.  The amp is probably clipping.

You should lower the amp's gain, and possibly increase the supply voltage.

The TL082 is also part of the problem. It's not a very "strong" chip, in terms of driving lots of current into very low impedances, like headphones. Bass boost increases this problem by increasing the voltage of the hardest-to-drive parts of the circuit.

Bass boost works best with buffered headphone amps.

my volume on source is down no pot control for amp volume yet. it distorts when i turn the bass boost pot up. also gain is 5.6 and supply voltage is 25v (3 9v batts (2 of which were drained a bit)) i know about the ground imbalance (read your article already wink.gif ) and will make a buffered ground after i make the buffered output as described in my previous post
Edited by backspace119 - 2/6/14 at 8:02am
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