balanced preamp - Single attenuator?

Hi

Im playing around with some OPamps trying to put together a "nice" and simple (good or better) fully balanced preamp.

( and please save the sarcasm about Opamps :))

I would like some help/suggestions/bright ideas about how to implement a good volume control in the project WITHOUT using two double ELMA switches or similar. I want just one knob for controlling the volume.

I guess one way is to use ELMA switch and relays (expensive, clumsy etc etc), or making a "digital" switch or using some voltage-controlled IC. But I want something good. Is there anyone out there with ideas?

/Stefan
 
Singe attenuator for each balanced channel

What you could do is get yourself 2 high quality precision resistors (which will be used all the time). Place one per line followed by the attenuator shorting between them. The type of resistor used in shorting is not nearly as important as the series resistors, and so you might want to use a regular pot to try it with.

What you could also do if you wanted to was to create 2-4 ranges, where you would switch in the required pass element (high quality resistors).

This method removes the need to work against ground. Try it, you might like it -- and it does seem to solve your problem.

Petter
 
I think this is the way to do it:
 

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Hmm I think I got it now... THANKS

If I did understand it its working because the two signals is 180 degrees phase-shifted, which means that when the shunting resistor/pot is zero the signals neutralise each other?

I just happen to have a holdplated ELMA switch of audioquality in my component box from my last preamp project. Already equipped with 1% metallic resistors. Any thoughts is 3k maximum might be too small?

/Stefan
 

arnach

Member
2001-04-07 10:03 pm
USA
Dact

DACT makes some rather large attenuators for balanced operation. Be prepared to pay, however,

Hoffemeyer's alternative will sound quite good, for alot less money. Use caddock or vishay resistors. Holco is no longer making non-magnetic resistors (cry.)

-- Aaron
 
Hi

Hoffmeyer has a simple and elegant solution for a balanced volumecontrol, as long as it is a true balanced signal.
But I do not agree with Petters statement that the quality of shunt resistors is less important than the quality of serie resistors, both resistors is a part of the same signal path and Kirchhoff's law makes no exeptions. "The sum of the currents entering any junction must equal the sum of the currents leaving that junction". So the shunt resistor can make just as much harm as the serie resistor. You have to use high quality resitors all the way.

Claus
 

arnach

Member
2001-04-07 10:03 pm
USA
hmm..

Interesting, I also agree that all resistors must be of the high quality.

However, Balanced Audio Technology's very costly preamplifier uses a high quality resistor and then uses a CMOS IC or switch, according to a Stereophile review, and we all know CMOS ICs are not high audio quality.

If anyone wants the exact text I'll go look for it.

-- Aaron
 

Brett

Member
2002-01-07 6:02 pm
Hi Aaron,

I'd be curious to read it. I can't find any BAT reviews in the $phile archives, and the ones at BAT's website are partials.

I am building a modified Allen Wright pre, and have been looking into this idea. I built a shunt att years ago using relays in a pre, and experimented with SS switches in a broadcast device I developed, but I never really listened to it that closely. Eventually used a VGA.

BAT had a full page ad in $phile a while back showing the inside of the pre, and it looked to me that there were a lot of ss switches in there.

Cheers
Brett