Using a dual10K pot on ballanced line level.

[Coldsnow]

Hi,
Would it work to use a simple duel 10K pot in between a ballanced out to a ballanced in as an attenuator without messing up the phase, sound etc.?

 

[McCroskey42]

In a word, no.

A balanced attenuator is only as good as the matching between the two resistors, and a simple dual pot won't match closely enough. Many manufacturers will quote you 20% tolerance...you need it to be a whole lot closer. Doing it with a dual pot would attenuate the signal, but you'd shoot your CMRR to hell. This is what keeps the nice folks over at Goldpoint in business.

Do a search here for balanced attenuators...plenty of folks around here have built balanced attenuators like the Goldpoint, albeit with somewhat cheaper switches like Lorlin or Alpha. For example, the famous (and sorely missed) New York Dave's version

EDIT: Read on page 2 of that link for PRR's comment about a single pot attenuator, and NYDave's response. Probably closer to what you're looking for.

 

[Natural]

you could probally use a LM13700 VCA, not?

http://atosynth.blogspot.com/2015/01/the-xonik-vca-revisited.html

https://sound-au.com/articles/vca-techniques.html

just connect R13 and R14 to Q1

 

[ruffrecords]

A single pot will do fine. We used them at Neve on balanced floating monitor outputs back in the 70s.

Cheers

Ian

 

[JohnRoberts]

you could probally use a LM13700 VCA, not?

http://atosynth.blogspot.com/2015/01/the-xonik-vca-revisited.html

https://sound-au.com/articles/vca-techniques.html

just connect R13 and R14 to Q1

A 13700 is an OTA (operational transconductance amplifier) not a VCA.

Not very quiet or low distortion (trade off distortion for s/n when setting operating levels).

JR

 

[Natural]

well, the LM13700 is easier to find.

alternately you have the ...

SSM2164
https://www.analog.com/en/products/ssm2164.html

or the newer
cool audio V2164D ... quad VCA

https://www.coolaudio.com/features-page.php?product=V2164D
a Behringer subsidiary

 

[RuudNL]

Nice, but to me it looks like killing a mosquito with a machine gun...

 

[JohnRoberts]

Hi,
Would it work to use a simple duel 10K pot in between a ballanced out to a ballanced in as an attenuator without messing up the phase, sound etc.?

The resistance matching and tracking is not good enough to use in series but in a shunt leg, or legs of an H pad, the tracking will not imbalance the interface.

JR

 

[RuudNL]

At the radio station I used to work for, we simply did it like this.
And yes, the inpedances are not constant, but as long as the load impedance is high compared to the source impedance, it is no problem.

 

[Natural]

Nice, but to me it looks like killing a mosquito with a machine gun...

well, it could be solution.
using a single gang pot and controlling both channels.

https://www.analog.com/media/en/technical-documentation/data-sheets/SSM2164.pdf
as figured on page 8.

 

[Bo Deadly]

Using a VCA as a balanced attenuator is exactly what I would do if I were designing the rube goldberg machine of balanced attenuators.

As JR eluded to, you could just use two precision resistors and then a one section pot to make a balanced attenuator that also has good CMRR. Although the output impedance will be lame at settings with small amounts of attenuation. If you are doing at least 10dB it could work. Something like two 1K with an 1K pot as the shunt element maybe. Not sure about the taper. A switch or 3 pos on-on-on toggle could make for a decent solution.

 

[Natural]

Using a VCA as a balanced attenuator is exactly what I would do if I were designing the rube goldberg machine of balanced attenuators.

As JR eluded to, you could just use two precision resistors and then a one section pot to make a balanced attenuator that also has good CMRR. Although the output impedance will be lame at settings with small amounts of attenuation. If you are doing at least 10dB it could work. Something like two 1K with an 1K pot as the shunt element maybe. Not sure about the taper. A switch or 3 pos on-on-on toggle could make for a decent solution.

https://www.thonk.co.uk/shop/coolaudio-v2164-all/
cost £2.40 (Excl. VAT

a 0.1% resistor will cost me 1.10€ + vat ... if precision is required.

 

[scott2000]

exactly what I would do if I were designing the rube goldberg machine of ...

.... Had to look that one up.... pretty funny

 

[Natural]

.... Had to look that one up.... pretty funny

yeah, i heard that expression also for the first time this weekend on discovery on the show: junkyard empire

 

[abbey road d enfer]

A balanced attenuator is only as good as the matching between the two resistors,

Good in what respect?

Doing it with a dual pot would attenuate the signal, but you'd shoot your CMRR to hell.

CMRR is very overrated this year. 
Seriously, very often it just doesn't matter. Using a single pot between two balanced connections works fine in many cases. Indeed, it won't for long distance or EMI/RFI noisy environment. Since the OP didn't describe the validity domain, we can't give definite answers.
My "in a word" answer would have been yes.

 

[mjrippe]

There are inexpensive dual 10k stepped attenuators from Hong Kong that fit the form factor of  a traditional pot.  My buddy just installed two of them yesterday in a piece of gear and I'm waiting to hear how they work.  They use surface mount resistors and gold plated contacts.  $12USD each.

https://ebay.us/PavPlz

 

[Bo Deadly]

Mmm, those parts look interesting. Make-before-break. 21 steps. I could see that design being used for a lot of things and not just attenuators. If you can reverse the shaft and get a reverse log, that could be very useful for a parametric EQ.

 

[mjrippe]

They have other values/tapers/etc available.  I sent their info to a manufacturer I was doing a repair for and they are going to try them for their products.  Could be a great resource!