Volume control pots

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Signaling

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Joined 2014
Hi, just put together a little 2w stereo amp based on the TDA2822m and its working fine however I've noticed that the right channel is always louder.

Did some trouble shooting and found out that the 10k dual gang volume control pot from Maplin is poorly matched with the louder side always measuring about 500 ohms lower than the other.

Now being a basic voltage divider this normally wouldn't be a problem but I've got 4.7k resistors further up at the input forming part of the RC low pass filter so that probably throws the whole voltage dividing part out of wack.

tl;dr Can anyone recommend some good, well matched dual gang volume control pots? I'm looking for values in the region of 10-33k and preferably available in the UK.

Thanks.
 
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Enzo

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Joined 2004
Probably? Do the math. 4.7k/10k divider versus 4.7k/9.5k. I get for the 10k side the divide point is 0.68 of the input, while the 9.5k side is 0.67 of the input. That doesn't seem like a large difference. And a dual pot matched within 5% sounds pretty good to me. How large a difference is there?

Apply a steady signal to both inputs. Is there a difference in signal level at the hot leg of each half of the control? Is the signal the same entering those 4.7k resistors? We can't just assume the resistance difference is the problem, we need to measure signal and find out.
 
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Signaling

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Joined 2014
Enzo said:
Probably? Do the math. 4.7k/10k divider versus 4.7k/9.5k. I get for the 10k side the divide point is 0.68 of the input, while the 9.5k side is 0.67 of the input. That doesn't seem like a large difference. And a dual pot matched within 5% sounds pretty good to me. How large a difference is there?

Apply a steady signal to both inputs. Is there a difference in signal level at the hot leg of each half of the control? Is the signal the same entering those 4.7k resistors? We can't just assume the resistance difference is the problem, we need to measure signal and find out.
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You're onto something here Enzo, scoped the input pins of the TDA2822m and both channels are a nice clean matching signal within a few mV RMS of each other.

But when I scoped the output the right channel is fine, but on the left I get a much lower amplitude and there is a phase shift of nearly 90 degrees! It is definitely lagging the input signal.

Just my luck, on the one occasion I decide to not use an IC socket everything goes belly up :sad:

Any ideas what could cause such a large input-output phase shift?
 
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Signaling

Member
Joined 2014
Update:

Good news is I fixed it, bad new is I don't know how! I took all components related to the one channel out and tested them, but they tested good so I just replaced them anyway and spaced them slightly further apart from each other this time.

Tested it all again and now both channels are identical in amplitude and phase.

I can see how a bad solder joint would cause a lower amplitude but what on earth could have caused that phase shift?
 
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Signaling

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Joined 2014
kamis said:
Bourns 91A potentiometers are better sounding than Alps Blue Velvet . Alps are soft, less open and focused than conductive plastic Bourns.All better professional eq. uses conductive plastic pots., very often Bourns.
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Thanks, I'll check them out. Even though the original issue is technically fixed I still don't really like these Maplin pots too much as they have a habit of going scratchy even after a squirt of contact cleaner.
 
S

Signaling

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Joined 2014
kamis said:
Bourns 91A are long life , sealed, without need to clean .
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Eldam said:
what about cermet with a Sfernice P11 ?
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scottjoplin said:
omeg eco pots from cpc farnell in the UK are very good omeg eco pots - Search Results | CPC
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Thanks! That's certainly plenty of options I can choose from

AndrewT said:
Scratchy.
Have you fitted a DC blocking capacitor to keep DC current off the wiper contact?
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Yes, 3.5mm input jack>rc low pass filter>dc blocking capacitor>top leg of pot>wiper>ferrite bead>TDA2822m input pin.

Does this sound right? Could a small resistor between wiper and input pin help reduce it?
 
S

Signaling

Member
Joined 2014
Update: Looks like it's crap pots after all, turns out the metal shaft doesn't always make electrical contact with the metal body so when you touch it and rotate its like a capacitor being continuously connected and disconnected. The capacitor being the part of the shaft that rotates and the human body capacitance, I guess the lubricant used to get a smooth rotation feel could be to blame for this.

Connecting an alligator lead to the shaft and metal pot body as a test stops the crackling all together and I've tested another identical pot and it crackled just like this one until I connected the jumper wire.

Here is the pot in question so you can avoid similar problems 10k Ω Miniature Log Dual Gang Potentiometer | Maplin

An externally hosted image should be here but it was not working when we last tested it.


Or maybe they are ok and its just maplin selling the production rejects to save a few pennys.

Also I managed to make the amp pic up a local radio station with all the messing with jumper wires + scope ground around the input pin. :wiz:
 
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Signaling

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Joined 2014
I managed to solve the issue by putting an old spring over the shaft so it always makes electrical contact with the rest of the body, the result is no more crackling noise when the volume is adjusted.

Pic related:
 

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S

Signaling

Member
Joined 2014
AndrewT said:
Yes or a hair-spring fixed to the chassis and leaning on the side of the metal shaft.
But this would be outside the panel securing the pot. Not convenient unless you have a big knob, or a second panel to hide it.
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I bent the ends of the spring so they force inwards slightly and press into the metal, the enclosure is going to be plastic to save cost but I think the spring is good enough.

Should I solder a wire to the pot casing and connect it to ground or is it better to leave them floating? I wonder how they do it in commercial speakers with plastic cases :confused: even with a plastic knob there can still be capacitive coupling between the finger and shaft.

I won't be buying this model of maplin pot again that is for sure.
 
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