Balanced volume control

[Cqwet Dbdfte]

Rather than using two vanilla OP amps and a dual pot, (tracking?) could you use an instrumentation OP (diff in, diff out) with a single resistor control?

 

[ccaudle]

could you use an instrumentation OP (diff in, diff out)

The traditional instrumentation amp configuration is diff in, SE out. Fully differential amps do not have the high CM impedance that you can get with a instrumentation amp configuration, so there are some concerns with CMMR. In this particular use case that may not matter, the input cables are probably going to be pretty short and the output cables long, but just using unity buffers is easy and cheap, and is the optimal topology, so doesn't seem to have any down sides.

with a single resistor control?

In the traditional instrumentation amp topology when the resistor is open you get unity gain, and adding a resistor increases the gain.
Since in this case the OP wants to run a high signal level that is optimal for his signal processor, then attenuate on the output to control monitor level, the suggested instrumentation amp design would be the opposite of what is desired.

If your concern is with the dual pots, the topology I drew has no connection to the circuit common, it is a true differential attenuator, so the worst that will happen if the pots don't track very closely is the hot and cold signal balance shifts slightly. That has zero influence on the CMMR or noise rejection of the downstream diff input since it does not affect the output impedance balance of the buffers, so isn't a concern .

 

[ccaudle]

what do I need to filter ins and outs please ?

Low value capacitors (50pF to 100pF) attached directly at the connector pins 2 and 3 to chassis connection, ferrite bead connection with high saturation current rating to connect pins 2 and 3 to your circuit. Pin 1 directly to chassis with as low an inductance connection as possible.

 

[RoadrunnerOZ]

I know that the auxiliary outputs of my mixer are balanced, I have already said that I only use balanced cable, Jack TRS to XLR that I made myself so I know perfectly well that they are balanced. the routing of my aux is in pre-fader and there is actually no problem on this side and even no problem at all, I think I have poorly explained my request. I am only trying to manage the volume of my stage monitors from the DBX outputs and not with the volumes of the mixer's aux outputs because that would influence the DBX input level and therefore their effectiveness.

I have no problem as such, neither level nor cabling, I just need volume controls at the output of my DBX.

In your posts you made no mention of the cabling from desk to DBX only balanced out from the DBX but also said you had to have your Aux levels right up to get enough level to drive your DBX - without knowing the setup or the mixer it made it difficult to determine why you get such low level and as such the losses from passive attenuation for balanced line control of volume from the output of the DBX would seem to make that not an option if you’re not getting enough level to your monitors.
But if there’s plenty of level to drive your monitors then a passive line attenuator for each Aux’s DBX output could be the simplest way to go.
There are plenty of ready made balanced passive controllers out there and lots of simple schematics for these - I don’t see the concern about impedance to the monitors being a problem - passive monitor controllers have been used in studios around the world for years - no artifacts or coloration of the sound.

 

[ccaudle]

why you get such low level

In post #5 already the OP explained that the signal processing worked optimally with higher input level, that there was no problem getting level to the signal processor, but that those levels were higher than desired for the monitor level.

I don’t see the concern about impedance to the monitors being a problem

That depends on the impedance of the passive level controller, and the distance between the console and the monitor amplifiers. Studios would generally have pretty short runs to the amplifiers, live sound not necessarily.

 

[RoadrunnerOZ]

In post #5 already the OP explained that the signal processing worked optimally with higher input level, that there was no problem getting level to the signal processor, but that those levels were higher than desired for the monitor level.



That depends on the impedance of the passive level controller, and the distance between the console and the monitor amplifiers. Studios would generally have pretty short runs to the amplifiers, live sound not necessarily.

Ok. Understood. With a well matched balanced controller with accurate tracking on both sides of a dual pot it should be ok for long runs shouldn’t it? There is the other method of having a single gang log pot bridging the hot and cold after a pair of series resistors, one on each hot and cold, that are matched 1% or better, of a value around 4K7 that I’ve seen - would that work on a long run? Some of the controllers I’ve seen in studios have had 30 - 40 metre runs - no hum or interference.