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Old 15th October 2011, 02:19 PM   #1
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Default Q: Converting 5k pot to 20k pot

I have a very nice quality 5k stereo audio potentiometer. However the input impedance of my amp is 20k ohm. So I need a potentiometer of 20k.

With my limited knowledge of electronics I came up with the following as a solution: Simply add a 15k (metal film 1% 1/4 watt) resistor to the input of the variable voltage divider--and that's it.

Is my thinking correct? Would really appreciate some feedback thanks!
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Old 15th October 2011, 02:50 PM   #2
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It's not necessary to match the input impedance of your amp. It's only important the the device feeding the pot is okay working into a 5k load.
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Old 15th October 2011, 07:02 PM   #3
DF96 is offline DF96  England
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Very rough rule of thumb is that the pot should not be greater in resistance than the amp input. Also significantly greater than the output impedance of whatever comes before. Note I really mean 'output impedance' and not 'recommended load impedance' - people writing blurb often get these two confused.

Many devices have an output impedance in the range 100ohms to 1k, so the pot would typically be 10-50k.
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Old 15th October 2011, 07:24 PM   #4
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Quote:
Originally Posted by DF96 View Post
Very rough rule of thumb is that the pot should not be greater in resistance than the amp input. Also significantly greater than the output impedance of whatever comes before. Note I really mean 'output impedance' and not 'recommended load impedance' - people writing blurb often get these two confused.

Many devices have an output impedance in the range 100ohms to 1k, so the pot would typically be 10-50k.
I agree with DF96. If your amplifier has a 20k input resistance, I would not use a pot with a higher resistance than, perhaps, 10k. Depending on the output impedance of your source, you may want to use a higher resistance pot. My experience has been that the pot should be an absolute minimum of 10X the output impedance of the source.
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Old 15th October 2011, 08:19 PM   #5
forr is offline forr  France
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audiogeekess
I have a very nice quality 5k stereo audio potentiometer. However the input impedance of my amp is 20k ohm. So I need a potentiometer of 20k.

Not at all. Keep your high quality 5 kOhm pot and use it normally. Feeded with a low impedance source, the output at the cursor will never be no more than 1.25 kOhm (for the -6 db position) and this will drive a power amp ok. The less the impedance seen by the amp input, the better.
By the way, what's your pot ?
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Old 15th October 2011, 08:37 PM   #6
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^ What they said.

Keep it, use it. Check your source impedance, if it's >500R see if i can be modified; if not, then it might be worth building a buffer.
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Old 15th October 2011, 09:14 PM   #7
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Thanks for the help

Turns out my pot measures 20.2k -- don't know why I thought it was 5k (that was when I measured it while it was still connected in another circuit I knew nothing about--ripped it out).

So the pot is 20.2k and the input impedance of amp is 22k. The amp used to have two cheap 20k pots in it -- one for each channel. I took them out and made a fixed voltage divider of (0k, 20.0k) -- i.e. max gain in the amp. I used a metal film resister 1/4 watt, 1% tolerance which -- both measure exactly 20.0k ohm each.

So would it sound better if I go down to 10k stereo pot or just use the one I have?
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Old 15th October 2011, 09:18 PM   #8
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If the amplifier originally had 20k pots ... use the new ones. No need to change the resistance of the input pots.
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Old 15th October 2011, 10:21 PM   #9
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I have the voltage dividers fixed (0,20k) in the amp for max gain, and i need to attenuate what i feed into the back of the amp via the rca's. so use this 20.2k pot for that?

I have the fiio d3 as the source and here are the specs on that:

Specification

Power supplyDC 5V
Output amplitude1.6V0dB Fs
Frequency Response20Hz~20KHz
Signal to Noise Ratio: 90dB
Crosstalk: 75dB
Total harmonic distortion: <0.01%(10mW)
Size: 61.6mm49.1mm21mm
Weight:50g

It doesn't say anything about output impedance.. should I be able to calculate that by the output amplitude?

Last edited by audiogeekess; 15th October 2011 at 10:28 PM.
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