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Old 17th October 2013, 08:11 PM   #1
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Default Opamp Line Attenuator and Buffer Question

If I were to build a simple 100k audio taper dual pot in front of a stereo opamp inverting buffer (av = 1), would the .03% distortion of the opamps I have on hand, TL082, add to the .003% of my equalizer or multiply it ?

Does distortion get noticeable to keep adding opamps in the signal path ?
I already bought TL082's but I do know there are lower distortion amps out there.

Now also, is there an easy way to add a balance pot and not exaggerate the audio taper of the pot I already bought ? I do not plan any input resistor to ground, but the balance pot may affect the taper of the volume pot if both are at the input.

It looks like I would need to put the balance pot at the input, go through a buffer, feed the volume pot then through another buffer to isolate any interaction of the pots. Then comes my reason for the distortion question. .03 + .03 + .003% = .063% ?
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Old 18th October 2013, 10:03 AM   #2
DF96 is offline DF96  England
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Combining distortions in different stages does not have a simple answer, as they can interact, and of course they are signal-level dependent. However, for low amounts of distortion you can assume that the worst case is that they simply add up.

How best to combine a volume pot and a balance pot depends on how aggressive a balance control you want. If +-3dB is sufficient balance variation then a linear balance pot of the same value as the log volume pot can be wired as a variable resistance in series with the top of the volume pot. At mid position the balance pot will be 50k (in your example) so you lose about 3dB of signal at max volume.

PS this ought to be in the Line Stage forum

Last edited by DF96; 18th October 2013 at 10:03 AM. Reason: add PS
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Old 18th October 2013, 10:09 AM   #3
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by DF96 View Post
PS this ought to be in the Line Stage forum
It is now
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Old 18th October 2013, 11:29 AM   #4
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Can't put a schematic up for now, but here is my hope :

Balance
One 50k linear pot, 1 end to the L channel from CD, opposite end to the R channel from CD, and the wiper to ground.

Volume, one end of 100k log pot to L channel from CD (same point as above) opposite end to gnd. Wiper to inverting input of L channel buffer (AV = 1).

Same for R channel.

I have never found things that simple, so I figure on the balance pot at the input, then from the output of the first balance buffer, feed the top of the 100k volume pot, bottom to gnd, and wiper to a second inverting buffer.

I'll just have to try a single stage. So what, it inverts, the Australians live upside down and they don't notice.
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Old 18th October 2013, 12:09 PM   #5
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Never ever use TL082 in a device that must sound good. There are so many better choices. OPA2604, LM4562 (bipolar) etc.
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Old 18th October 2013, 01:21 PM   #6
DF96 is offline DF96  England
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Quote:
Originally Posted by Wavewhipper
Balance
One 50k linear pot, 1 end to the L channel from CD, opposite end to the R channel from CD, and the wiper to ground.
That is not a balance control, but a variable load. You will see almost no effect until the wiper reaches an end-stop; at that point one channel will almost disappear.

Read my post 2 very carefully. Write out a circuit diagram of what you think I described. Think about how it works.
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Old 18th October 2013, 02:07 PM   #7
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I'm a novice myself, but personally i would use a buffer stage between the volume and balance pots.

There may be a way to skip an intermediate buffer, but as i said, I'm a novice and I don't know of a way.

With devices like LM4562 and LME49710/20/40 i doubt an extra buffer is much of a concern as far as total THD is concerned.
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Old 18th October 2013, 02:49 PM   #8
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Ok, I can do it like in post 2, I think that would take 3 pots, 1 ganged volume and two single pots for balance, one each channel. That would be the easy way out of two amps/channel. And it looks like I'll put the TL082's in a socket and find a better direct upgrade on my next order.
I am trying to use what I have for now, packing to move in a few weeks and want to get it started before my stuff gets lost in the boxes.
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Old 18th October 2013, 03:10 PM   #9
DF96 is offline DF96  England
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No, one ganged pot for volume, one ganged pot for balance. You wire the volume pot in the same way for each channel, and the balance pot 'backwards' for one channel. If you get it the right way round then turning the balance pot to the right means you boost the right channel etc.
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Old 18th October 2013, 05:19 PM   #10
Mooly is offline Mooly  United Kingdom
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I was going to say look at sound.westhost.com (Rod Elliots site) and search for potentiometers but the site looks down just at this moment.

That shows how to wire the balance control as DF96 suggests (the dual gang pot for balance is the best option, single pots can allow crosstalk due to the small but finite and variable wiper contact resistance)
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