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Old 11th October 2012, 10:14 AM   #1
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Default chipamp - Volume control and input impedance

Dear all, I have several questions concerning the potentiometer/attenuator used as volume control.

1. The attached diagram is ok or do you suggest better topology?

2. If R1=1k, R2=20k, R3=20k (=R2 for minimal DC offset), R4=1k, what are recommended values for potentiometer P and why?

3. What’s the best way to have constant input impedance if P is in parallel with R3 ?
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Old 11th October 2012, 10:31 AM   #2
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Why not use the R3 as the potentiometer (20k)?
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Old 11th October 2012, 11:12 AM   #3
AndrewT is offline AndrewT  Scotland
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if Pot is parallel with R3 then a ratio of >=10:1 gives a substantially constant input impedance.

eg. your Rin = 20k.
1/10 of that is 2k.
If the output impedance of the Source is <=2k then you are in the ballpark
A 8k pot has a maximum output impedance of ~2k.
The closest but lower value pot to use is 5k.

5k can be driven by any headphone output from any source.
Some line level outputs from rubbish equipment may struggle to drive 5k and the cable joining them.

If you decide to use a 10k pot then I'd suggest you use a Rin >= 25k and preferably 47k or 51k.

Post2's suggestion of a 20k pot for a Rin=20k is asking for very non constant input impedance.
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Old 11th October 2012, 11:49 AM   #4
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Quote:
Originally Posted by AndrewT View Post
if Pot is parallel with R3 then a ratio of >=10:1 gives a substantially constant input impedance.

eg. your Rin = 20k.
1/10 of that is 2k.
If the output impedance of the Source is <=2k then you are in the ballpark
A 8k pot has a maximum output impedance of ~2k.
The closest but lower value pot to use is 5k.

5k can be driven by any headphone output from any source.
Some line level outputs from rubbish equipment may struggle to drive 5k and the cable joining them.

If you decide to use a 10k pot then I'd suggest you use a Rin >= 25k and preferably 47k or 51k.
Thank you, I was thinking more what if P is towards 50k...100k. What would be acceptable value?
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Old 11th October 2012, 11:50 AM   #5
AndrewT is offline AndrewT  Scotland
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That would be the complete opposite to what I led you through as an example.
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Old 11th October 2012, 11:52 AM   #6
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Ah... Sorry...
I was simply trying to suggest what can be seen for example in the LM3886 data sheet...
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Old 11th October 2012, 11:55 AM   #7
AndrewT is offline AndrewT  Scotland
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work it out.
At full volume the Source sees POT//Rin
At minimum volume the Source sees POT.
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Old 11th October 2012, 04:21 PM   #8
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Quote:
Originally Posted by AndrewT View Post
work it out.
At full volume the Source sees POT//Rin
At minimum volume the Source sees POT.
If the logarithmic potentiometer P=50k or P=100k is in parallel with R3=20k, will be the resulting curve still logarithmic?
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Old 11th October 2012, 04:24 PM   #9
AndrewT is offline AndrewT  Scotland
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sort of.

But that was not what you asked.
You asked about near constant input impedance and I gave you a rule of thumb that gives near constant input impedance.

If you do have varying input impedance then any preceding filters will have varying filter turn-over frequency.
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Old 12th October 2012, 03:37 AM   #10
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Quote:
Originally Posted by AndrewT View Post
sort of.

But that was not what you asked.
You asked about near constant input impedance and I gave you a rule of thumb that gives near constant input impedance.

If you do have varying input impedance then any preceding filters will have varying filter turn-over frequency.
Correct, I pressed the "quote" button by mistake; this is entirely new question:

If in the same setup as in first diagram the logarithmic potentiometer P=50k or P=100k is in parallel with R3=20k, will be the resulting curve still logarithmic?
If not, what would be the solution still using a passive potentiometer or attenuator?
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