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Balanced Volume Control Using Stereo POT - impedance calcs

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Re: Balanced Volume Control Using Stereo POT - impedance calcs

Postby Martin Walker » Sat Jan 13, 2018 10:51 pm

ef37a wrote:Personally I would buy the best 2 gang log pot I could find and then use high grade op amps, LM4562 say around it or some of the audiophile amps Hugh mentioned.

Dave.

I'd be inclined to do the same, as in my experience a reasonable quality dual-gang log pot is fairly well matched from 10% to 100% of its travel - it's only when you find yourself in the first few degrees of rotation that they tend to go out of whack between L and R :headbang:


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Re: Balanced Volume Control Using Stereo POT - impedance calcs

Postby Folderol » Sat Jan 13, 2018 11:12 pm

Blue velvet audio taper pots are the mutt's nuts, and not particularly expensive.
They have a very nice feel too - hmmm, not sure I should have said that :lol:

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They used to be stocked by RS, but not any more :(
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Re: Balanced Volume Control Using Stereo POT - impedance calcs

Postby Martin Walker » Sat Jan 13, 2018 11:29 pm

Folderol wrote:Blue velvet audio taper pots are the mutt's nuts, and not particularly expensive.
They have a very nice feel too - hmmm, not sure I should have said that :lol:

Just tracked down this supplier in Hong Kong who sells dual-gang 10k log version at £6.99

Image

https://www.ebay.co.uk/itm/1pc-Japan-Al ... XQMmJRWVzb

Must be worth a punt at that price!


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Re: Balanced Volume Control Using Stereo POT - impedance calcs

Postby ef37a » Sun Jan 14, 2018 8:28 am

Martin Walker wrote:
ef37a wrote:Personally I would buy the best 2 gang log pot I could find and then use high grade op amps, LM4562 say around it or some of the audiophile amps Hugh mentioned.

Dave.

I'd be inclined to do the same, as in my experience a reasonable quality dual-gang log pot is fairly well matched from 10% to 100% of its travel - it's only when you find yourself in the first few degrees of rotation that they tend to go out of whack between L and R :headbang:


Martin
And there's a fix for that last point Martin. Stick in a 20dB (say) fixed attenuator then you not only have a 'dim' function but that puts the pot back into its well matched range if you want very quiet listening.

Dave.
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Re: Balanced Volume Control Using Stereo POT - impedance calcs

Postby Hugh Robjohns » Sun Jan 14, 2018 12:11 pm

Yep. Standard feature of all decent monitor controllers.

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Re: Balanced Volume Control Using Stereo POT - impedance calcs

Postby Martin Walker » Sun Jan 14, 2018 2:45 pm

ef37a wrote:And there's a fix for that last point Martin. Stick in a 20dB (say) fixed attenuator then you not only have a 'dim' function but that puts the pot back into its well matched range if you want very quiet listening.

Bravo Dave - of course that's a perfect fix in the cunning guise of a useful extra feature :clap:

For the very reasonable cost of two resistors and a DPDT switch 8-)


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Re: Balanced Volume Control Using Stereo POT - impedance calcs

Postby ef37a » Sun Jan 14, 2018 4:17 pm

Martin Walker wrote:
ef37a wrote:And there's a fix for that last point Martin. Stick in a 20dB (say) fixed attenuator then you not only have a 'dim' function but that puts the pot back into its well matched range if you want very quiet listening.

Bravo Dave - of course that's a perfect fix in the cunning guise of a useful extra feature :clap:

For the very reasonable cost of two resistors and a DPDT switch 8-)

Four resistor Mart. Two per.
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Re: Balanced Volume Control Using Stereo POT - impedance calcs

Postby Martin Walker » Wed Jan 17, 2018 12:55 am

ef37a wrote:Four resistor Mart. Two per.

I was thinking of a switchable 91K resistor in series with the existing 10K pot per channel, so only 10% of the signal appears across the pot - have I missed something? ;)


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Re: Balanced Volume Control Using Stereo POT - impedance calcs

Postby ef37a » Wed Jan 17, 2018 5:58 am

Martin Walker wrote:
ef37a wrote:Four resistor Mart. Two per.

I was thinking of a switchable 91K resistor in series with the existing 10K pot per channel, so only 10% of the signal appears across the pot - have I missed something? ;)


Martin

Your maths is spot on Martin! Just that 91k will produce 103dBu of noise which is worse than many modern AIs whereas a 2 resistor attenuator of about 2k2 (which an LM4562 can easily drive) whacks the noise down to 120dBu which is about the limit for even the most esoteric devices?
I admit I am being picky! The 91k noise will be attenuated anyway.

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Re: Balanced Volume Control Using Stereo POT - impedance calcs

Postby Folderol » Wed Jan 17, 2018 9:02 am

Personally, I'd go for 1.1k shunting the pot, with 9.1k in series.
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Re: Balanced Volume Control Using Stereo POT - impedance calcs

Postby ef37a » Wed Jan 17, 2018 9:32 am

Folderol wrote:Personally, I'd go for 1.1k shunting the pot, with 9.1k in series.

Or, whatever happens to be in the tin Will!

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Re: Balanced Volume Control Using Stereo POT - impedance calcs

Postby Folderol » Wed Jan 17, 2018 2:10 pm

ef37a wrote:
Folderol wrote:Personally, I'd go for 1.1k shunting the pot, with 9.1k in series.

Or, whatever happens to be in the tin Will!

Dave.
Surely not! You mean you don't have the full range :o
Besides, I've never forgotten a simple little rule an old guy taught me about 20dB step attenuators to maintain constant loading. Divide by 9 shunt, times 9 series.
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Re: Balanced Volume Control Using Stereo POT - impedance calcs

Postby blinddrew » Wed Jan 17, 2018 2:39 pm

Folderol wrote:Divide by 9 shunt, times 9 series.
Mornington Crescent!


Sorry, wrong thread.
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Re: Balanced Volume Control Using Stereo POT - impedance calcs

Postby Sam Spoons » Wed Jan 17, 2018 3:18 pm

:clap: :clap: :clap:
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