stereo sidechain link grounding noise problem

[beatnik]

i am finishing a pair of nice compressors based on the urei 175b design. the compressors sound very nice but there is a noise problem when in stereo mode

a jack connector has been added on the back panel that goes to the sidechain outputs (C5 on the schematic) with an on/off switch to connect the sidechain signals together and select between two compression modes - stereo or dual mono

i believe the noise is a result of some mistake in grounding the two units together. i have done many tests but i cannot understand what would be the right way to eliminate this noise
if i momentarily connect the two units enclosures together with a piece of wire, the noise drops considerably, but if it's still higher compared to when the sidechain link is out.


right now i have the two units grounded this way:

- pin 1 of input and ouput xlr tied to ground tab on the respective connector
- signal grounds are tied to chassis at one point only
- mains earth is connected to chassis

i have tried both insulated and non insulated jacks for the link connection, but that didn't make a difference

i have done many experiments but can't find the right solution, would be great to hear any suggestions

 

[JohnRoberts]

Too much homework... I found a schematic and it took me a few minutes more to find C5.

It appears that is transformer input and output so audio should be well isolated from ground issues.

The side chain and C5 is all unbalanced so could be corrupted by ground currents.

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Just a WAG (wild a__ guess) How about connecting the signal ground (I hate calling it that) to chassis and PS ground through a modest value resistor 50-100 ohm in each unit.  Then hard wire connect the signal ground between the two units to each other. Side chain jacks should be insulated from chassis. Perhaps add small resistances to those jack grounds to prevent ground contamination from other gear connected to the side chain.

This is just a guess, I have no idea if it will work. I AM NOT A TUBE GUY.

JR 

 

[beatnik]

the side chain jacks are insulated from the chassis. just the tip is making a connection between the two side chain signals, at the moment the shield is not connected to ground and temporary connecting it to chassis doesn't make a difference

if the two units sit on top of each other the noise drops in level, because the two chassis are making a connection, but even in this case there is a little more noise in stereo link than in dual mono, which is dead quiet

 

[beatnik]

Just a WAG (wild a__ guess) How about connecting the signal ground (I hate calling it that) to chassis and PS ground through a modest value resistor 50-100 ohm in each unit.  Then hard wire connect the signal ground between the two units to each other. Side chain jacks should be insulated from chassis. Perhaps add small resistances to those jack grounds to prevent ground contamination from other gear connected to the side chain.

This is just a guess, I have no idea if it will work. I AM NOT A TUBE GUY.

JR

What you mean by hard wiring the two signal ground together ? Since the circuits are in two separate chassis how can i achieve that ?

 

[beatnik]

I have made a simple drawing to better explain how i wired up these compressors, maybe you can spot something wrong

 

[beatnik]

and here is a gut shot

 

[radardoug]

This all revolves around where you have applied the main ground to the electronics. If you have grounded at the power supply, you may have rectifier peak currents in your grounding. You would be better to ground at a common point in the amplifier proper.
Or you could try partly isolating the electronics ground in at least one chassis. Use a low value resistor in series with the ground.
Because the unit is transformer isolated in and out, the internal ground can be at any potential.

 

[JohnRoberts]

Just a WAG (wild a__ guess) How about connecting the signal ground (I hate calling it that) to chassis and PS ground through a modest value resistor 50-100 ohm in each unit.  Then hard wire connect the signal ground between the two units to each other. Side chain jacks should be insulated from chassis. Perhaps add small resistances to those jack grounds to prevent ground contamination from other gear connected to the side chain.

This is just a guess, I have no idea if it will work. I AM NOT A TUBE GUY.

JR

What you mean by hard wiring the two signal ground together ? Since the circuits are in two separate chassis how can i achieve that ?

Your stereo link switch can carry a "signal" ground and a side chain signal.

If you introduce a compliance (modest value resistance) between each units signal ground and PS/chassis ground, that will not interfere with each units operation (you have transformer inputs and outputs).

When you link them in stereo mode you can link both the side chain voltage and the two signal grounds together. Since the hard wire connection is lower impedance than the resistors added between each signal ground and power supply/chassis grounds the two signal grounds will be forced very close to same voltage. 

I would use insulated jacks for the side chain so you don't corrupt this floating signal ground with currents from chassis ground.

or not....

JR   

 

[beatnik]

Your stereo link switch can carry a "signal" ground and a side chain signal.

do you mean i should use a double pole switch, one side for the side chain signal , one side for the 0V signal ? right now there is a spst that only switches the side chain signal

i will try experimenting tomorrow by adding resistors like you suggested and report back

thanks for helping out

 

[JohnRoberts]

If my idea works, you can leave the signal grounds connected, but if you have a 2 pole switch you can break both conductors.

JR

 

[beatnik]

the 100ohm resistor seems to be doing the trick very well, the ground noise is absolutely gone in any of the compression settings

thanks a lot for your suggestion it definitely got the issue sorted out. thumbs up

i have been doing a lot more reading on the subject, grounding is definitely a critical aspect.

i never had this problem before, just connecting the signal 0V to chassis ground on one single point. but this time there were two units connected together so i guess a new set of problem came in

do you think it's always better in any case to separate the signal 0V from chassis ground with a 100ohm resistor ?

I also read about adding a 100nF capacitor in parallel to the resistor

 

[beatnik]

by the way, i have used a 1/2W resistor, is that safe enough or i need a bigger wattage ?

 

[JohnRoberts]

the 100ohm resistor seems to be doing the trick very well, the ground noise is absolutely gone in any of the compression settings

thanks a lot for your suggestion it definitely got the issue sorted out. thumbs up

i have been doing a lot more reading on the subject, grounding is definitely a critical aspect.

i never had this problem before, just connecting the signal 0V to chassis ground on one single point. but this time there were two units connected together so i guess a new set of problem came in

do you think it's always better in any case to separate the signal 0V from chassis ground with a 100ohm resistor ?

I also read about adding a 100nF capacitor in parallel to the resistor

I am reluctant to give cook book advice regarding grounds, as different situations require different strategies. 

There should not be much current in that resistor so 1/2 W is probably more than needed.

Glad this worked, I am not a tube guy but transformer isolated inputs and outputs made this easier.

JR