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New Dual pot Chanel mismatch?

Oneoff

Oneoff

Active Member
I’m trying to figure out if it’s only in my head and what is actually the problem.

So, I had to replace an old scratchy dual motorized Alps pot in my musical fidelity A5
Integrated. I was 100% happy with the sound of that amp before, but I didn’t have any choice because the old pot couldn’t be cleaned, and I had some loud pops In my speakers.

I ordered the new RK27 50kA dual pot from eBay in Germany, the seller looks legit, the unit arrived and it looked like a very nice quality, even better then the old one installed. I desoldered the old and installed the new.

First impression- the motor is smooth, a little more mechanical resistance, feels ok
But... something is slightly different with the sound I can’t say exactly what,
The soundstage changed, stereo image, the sound is maybe brighter now?
Hard to tell without a proper A-B comparison with the old one...

Measuring the pots outer solder lugs shows:
On the old pot the left channel total value is around 700 Ohm less then the right.
On the new pot the left channel value is almost 800 Ohm more then the right.

Some thoughts and questions:
-Could this mismatch be the source of my problem with the sound?

-How this difference translates to the output? Is 800 ohm can turn into an audible difference?

-Is this difference in the pre amp stage usually get compensated by the manufacturer somewhere in the circuit?

-Would it be a good idea to solder a resistor across the right channel outer lugs in order To match it with the value on the older pot? Will this change the logarithmic nature of the pot?

-Could some of it be related to the need of some ‘burn in’ time of the new pot?

I’m planning to order some dummy load resistors and measure the outputs on the amp.

I’d love to hear your thoughts and ideas,
Many thanks! And sorry for a longish post...
 
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I would measure the output of the amp.

The Alps RK27 has a specified mismatch as high as 3dB across most of the usable range. I've never seen one higher than about 0.5dB though. You can easily determine how much imbalance exists between the channels by measuring the Volts out. If your DMM is not a true RMS unit just keep the input frequency low like 60 - 100Hz. If you do not have a dedicated sine generator you can use a laptop/phone and any one of a multitude of freeware programs to generate a frequency.

You may also want to measure the output of whatever you are using to generate a signal to make sure it doesn't have an imbalance as well.
 
Thanks!
That would be a first good thing to check.
I’ve already tried to connect one channel
With a sine wave from a phone app.
But at 1K Herz at a moderate volume, it was
A kind of annoyance to move on with the speakers hooked in (I don’t have 4 ohms dummy resistors yet).
Good idea to check out the source amplitude.

Not sure if my bench fluke 8840A is a true RMS. I’ll try again with 60hZ sine.
 
So I finally measured each Channel at 3 different (increasing) levels with a sine wave from my phone.
The source was measured before with a scope For the right symmetry.
The highest difference measured and converted (If I understand right the V to DB conversion here:
http://www.sengpielaudio.com/calculator-volt.htm )
Was around -0.11DB which I guess is not as bad as I thought it’ll be... and is maybe normal?

This calculated difference in DB, is consistent in all different levels that were measured for the channels.

It’s worth mentioning that I mistakenly measured the sine wave at 160 hz
My meter is not true RMS so I hope it’s
Still a valid measurement...


So I guess the change in the sound After replacing the pot is probably from something else.
Thanks for the advice!
 
Last edited:
Oneoff said:
Was around -0.11DB which I guess is not as bad as I thought it’ll be... and is maybe normal?

This calculated difference in DB, is consistent in all different levels that were measured for the channels.

It’s worth mentioning that I mistakenly measured the sine wave at 160 hz
My meter is not true RMS so I hope it’s
Still a valid measurement...
Click to expand...

Yeah, 0.11dB is really good and this is after the gain stages too, so nothing to sneeze at for sure.

About your meter,
As long as both measurements were made with the same meter you're good. Even if there was a measurement error in the meter it would've been consistent between channels and for measuring the channel mismatch this would be fine. I was primarily worried you might try to measure say a 20kHz signal with a low end meter and when that measurement was extremely low you might think there was something wrong.
 
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