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Old 6th June 2017, 01:18 AM   #1
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Default Volume control pots

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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Old 6th June 2017, 02:31 AM   #2
Enzo is offline Enzo  United States
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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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Old 6th June 2017, 03:20 AM   #3
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Get an eBay DACT, cant be beat under $100, maybe much higher.
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Old 6th June 2017, 03:27 AM   #4
rayma is online now rayma  United States
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Quote:
Originally Posted by Signaling View Post
Can anyone recommend some good, well matched dual gang volume control pots? 10-33k
These work quite well, and track perfectly.
DACT Type 21 Stepped Attenuator / Volume Control (Hi Fi Grade) * D shape Shaft * | eBay
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Old 6th June 2017, 03:30 AM   #5
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Quote:
Originally Posted by Enzo View Post
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.
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

Any ideas what could cause such a large input-output phase shift?
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Old 6th June 2017, 04:08 AM   #6
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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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Old 6th June 2017, 04:09 AM   #7
kamis is offline kamis  Serbia
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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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Old 6th June 2017, 04:27 AM   #8
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Originally Posted by kamis View Post
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.
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.
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Old 6th June 2017, 05:17 AM   #9
kamis is offline kamis  Serbia
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Bourns 91A are long life , sealed, without need to clean .
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Old 7th June 2017, 06:19 PM   #10
Eldam is offline Eldam  France
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what about cermet with a Sfernice P11 ?
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