Balanced volume control

[PN0707]

Hello everyone, with a view to modifying a DBX AFS224 feedback suppressor, I would like to have your opinion on the simplest and most effective way to add a volume potentiometer to the output of the device. It would therefore be necessary to have a volume control for each balanced output.

Thank you all for all.

 

[ccaudle]

Do you have the schematics? Likely the device is unbalanced internally with an output buffer, so the easiest way would be to add the pot at the input of the output buffer circuit. That avoids the need for maintaining balance in the output control, and also maintains a low output impedance no matter what level selection is used.

 

[PN0707]

Unfortunately I can't find any schematic for this unit.
I think the best way is to add a balanced line balanced buffer with volume control (line preamp with unity gain at max volume ?) after the output (even in a separate rack enclosure it's not a problem) could someone have schematic for that may be ?

 

[ccaudle]

What is the actual problem you are trying to solve? You might just be able to use a balanced passive attenuator at the input of the next device. The problem with passive attenuators is that they increase the output impedance, and make it variable with setting. That is a problem if you are driving a long cable, not so much if you are just driving a few inches/cm of cable. So if you really need or want a passive attenuator, put it as close to the input as possible for best results.

 

[PN0707]

The problem I want to solve is the following: the DBX AFS224 needs a fairly high input signal to function optimally and given that it is connected to the mixer output (auxiliary outputs for stage monitors) I have to considerably increase the levels of the auxiliary outputs. So I lose the possibility of adjusting the volume of my stage monitors from my mixing desk. I cannot use a passive attenuator because I will indeed induce impedance problems between my attenuator (which will have to be in control near me) and the stage monitors. The DBX AFS224 is just perfect and magical for feedback suppression but this small defect (missing two volume potentiometers in the design) makes it very difficult to use... This is why I think I need a balanced buffer at line level with adjustable volume.

 

[RoadrunnerOZ]

What is the actual problem you are trying to solve? You might just be able to use a balanced passive attenuator at the input of the next device. The problem with passive attenuators is that they increase the output impedance, and make it variable with setting. That is a problem if you are driving a long cable, not so much if you are just driving a few inches/cm of cable. So if you really need or want a passive attenuator, put it as close to the input as possible for best results.

Putting a balanced attenuator at the output would probably require the signal to come into the wiper of the pot to maintain constant output impedance, but then you have the problem of the line driver prior being able to handle a short circuit at zero volume so you also would need to have a feed resistor to each wiper. Whichever way you do it going to the wiper or top of the pot you lose level going passive or have varied output impedance. Best off putting the pot prior to the output driver internally or getting balanced line driver boards to come after the pot in a remote box or rack case.

 

[ruffrecords]

According to the dbx manual the nominal input level is 0dBu with peaks up to +10dBu. That is not a high level. Are you sure this is not a problem with your mixer? Which mixer are you using?

Cheers

ian

 

[RoadrunnerOZ]

There are also these, they have level control - it’s likely your Aux sends will be unbalanced from the desk so you come into the ART unbalanced and go out balanced with volume control and feed that to your DBX
https://artproaudio.com/product/cleanbox-pro-dual-channel-level-converter/If you need more gain you could use one of these - balanced or unbalanced in same for out
https://www.artproaudio.com/product/scc/

 

[RoadrunnerOZ]

You’re not using a balanced jack to whatever you’re using into the DBX lead for an unbalanced output from the desk are you? - that would leave the ring floating and very low signal at the other end whether TRS or XLR if going into a balanced input on the DBX - this will happen if your Aux outs are unbalanced Tip and Sleeve.

 

[PN0707]

The Art CleanBox Pro seems like an excellent initiative, but does anyone have its schematic or a DIY equivalent? Because ultimately if I want to use the 6 aux outputs of my desk for monitors and therefore switch to 3 DBXs I would need 6 volume adjustment channels and I think that it would be simpler to make that (plus the ergonomics of the ART CleanBox Pro don't seem terrible to me, which is normal given the size of the device).

 

[ccaudle]

You’re not using a balanced jack to whatever you’re using into the DBX lead for an unbalanced output from the desk are you?

Maybe OP could specify the model of mixer used and it would be easy enough to verify the specs of the aux outputs.

 

[ccaudle]

If you have a bipolar power supply available something like this would work. This is really basic idea, a real circuit would need power supply bypass caps, preferably some RFI filtering on in/out connectors, etc.

 

[RoadrunnerOZ]

The Art CleanBox Pro seems like an excellent initiative, but does anyone have its schematic or a DIY equivalent? Because ultimately if I want to use the 6 aux outputs of my desk for monitors and therefore switch to 3 DBXs I would need 6 volume adjustment channels and I think that it would be simpler to make that (plus the ergonomics of the ART CleanBox Pro don't seem terrible to me, which is normal given the size of the device).

 

[PN0707]

If you have a bipolar power supply available something like this would work. This is really basic idea, a real circuit would need power supply bypass caps, preferably some RFI filtering on in/out connectors, etc.

View attachment 117802

Thank you so much ! This is what I needed I think ! I´ll make 6 circuits like this in a separate rack enclosure to plug my 6 DBX outputs in to go my 6 stage monitors. Thank you !! I will use regulated and filtered bipolar 15v psu, what do I need to filter ins and outs please ?

 

[RoadrunnerOZ]

What desk are you using?

 

[PN0707]

What desk are you using?

Presonus studiolive 16.4.2ai

 

[RoadrunnerOZ]

Looks like your Aux sends are balanced - what cabling are you using to get into the DBX?

 

[RoadrunnerOZ]

Also there must be Aux masters for each Aux number - are those and the individual sends all up full? Are they running from pre-fader not post? It specs +18 for those sends.

 

[PN0707]

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.

 

[thor.zmt]

Hi,

Hello everyone, with a view to modifying a DBX AFS224 feedback suppressor, I would like to have your opinion on the simplest and most effective way to add a volume potentiometer to the output of the device. It would therefore be necessary to have a volume control for each balanced output.

Internally I'm pretty sure the unit uses a SE signal path up to the final driver stage, the easiest way would be to trace the schematic from the XLR Out backwards and find the best place to add a volume control.

An alternative would be to add a separate stand alone level control. Something often called a (passive) monitor controller in it's own box.

Monitor Controllers

Thor