Dual Pot Source for Passive Volume/Monitor Controller

[Matt C]

The schematic (idea taken from Ian Bell's post in another thread) definitely works as drawn if the destination is balanced.  But if I just leave the neg phase disconnected when sending to an unbalanced output, I get signal but the volume control does not work properly. 

I have a copy of the Morgan Jones book, I'll take a look to see if there's anything applicable in there.

I think the easiest solution is just to keep the neg phase disconnected and go unbalanced ,not ideal ,but simple at least

 

[Gene Pink]

There seems to be a disconnect in this thread, I have not seen what type of "balanced" the source is, unless I missed it. Floating or fixed?

Floating, is a transformer output with no reference to ground, or a cross-coupled feedback pair of op amps with almost no reference to ground.

Fixed, would be a transformer with a center tap connected to ground, or a pair of op amps of opposite polarity, each referenced to ground.

Each would be handled differently for this.

Gene

 

[Matt C]

There seems to be a disconnect in this thread, I have not seen what type of "balanced" the source is, unless I missed it. Floating or fixed?

I did mention it, but the answer is both - one source is floating transformer balanced, the other is active balanced (two op amps)

 

[Gold]

I use a passive monitor control setup. It's a good choice for a "closed system" where you have all known sources and you can modify any sources that don't work with your scheme. Unless you have this I think a passive solution is not a good choice because you won't know whether the monitor is functioning properly with an unknown source. The pot could be loading the source or not suitable for the device in question, like Gene Pink pointed out.

 

[Matt C]

My system is closed enough that a passive controller is working pretty well for me (the sources don't change)

The only sticking point I've run into is finding a way to send two different types of balanced sources to an unbalanced receiver. 

It's not necessarily a deal breaker since this unbalanced amp is just feeding a tiny cube speaker I rarely use anyway, but it would be nice to find a way to make it work.

 

[Gold]

but it would be nice to find a way to make it work.

Convert the whole system to unbalanced. I imagine this isn't wanted or practical. I'd buffer it with a line receiver IC.

 

[abbey road d enfer]

I dont think that schematic is going to work ,at the full volume position your going to get phase positive max volume, and in the middle it will cancel out completely

What?

Your phase negative output is connected directly to ground in the case of unbalanced connection.

Yes, and depending on the output circuit's architecture, that may be a big problem , or not. Most recent equipment is equipped with floating balanced drivers that can drive unbalanced lines as well as balanced.

Usually when you use a pot on a balanced output you have padding resistors ,and the pot is wired across the phases like a rheostat, 

This arrangement has the advantage of retaining balance when balanced outs and ins are used; some balanced inputs have such poor CMRR that a standard potentiometer connection would not provide the necessary attenuation.

this only allows a certain gain range depending on the magnitude of the padding resistor in relation to the pot value . 

The range is theoretically the same, since it goes from minus infinity to something. What changes is the level diagram. At the max position, there is necessarily some attenuation, which may or may not be an issue. There's always some sort of compromise in passive "controllers"; the choice must be based on the analysis of the operating conditions. There is not one single "best" answer.

 

[Matt C]

Thanks for the input everyone, but I think I've solved the "problem".  Or actually realized I was inventing a problem where there wasn't one to begin with. 

The source that I thought was active ("fixed") balanced is, in fact, not.  It's "psuedo-balanced", with the cold side  connected to ground through a small resistor.  So grounding it will not cause an issue. 

 

[supersonic]

I bought a passive monitor controller once, about 15 years ago. Inside was a 2.5k 4-ganged pot IIRC. Most DAW interfaces have a low output impedance, and the monitors had a 47k input impedance, so no problems there either. 2.2k-4.7k for the pot is OK I think, if you go that route.

 

[Matt C]

I bought a passive monitor controller once, about 15 years ago. Inside was a 2.5k 4-ganged pot IIRC. Most DAW interfaces have a low output impedance, and the monitors had a 47k input impedance, so no problems there either. 2.2k-4.7k for the pot is OK I think, if you go that route.

I ended up using a 10k dual-gang linear pot (Alps RK27) slugged with 5.1k resistors to approximate a log response.  FWIW I've got the thing built and it's working great, although Paul's point about it being best used within a closed system is good advice.  I'm sure in many ways an active monitor controller can give better performance but I am a sucker for simple solutions.