ground loop between two unbalanced outboard gear..

[Studiogearlover]

Hey guys..i need a second pair of eye (or more) please as I am puzzled.

I have 2 outboard gear I wish to put it into my signal chain. One EQ and one multi-comp/limiter. Both are Symetrix. Both has unbalanced in/outs... using unbalanced cables. Each of the unit works without ground loop individually.... BUT if I connect them together, voiala... massive ground loop...i am using on both units the unbalanced INs and Outs.. also, using the same PSU for both. Why I am getting the loop? Because I am sharing the same AC PSU between the two?

 

[Khron]

Any particular reason you're not using the balanced connections?

 

[Studiogearlover]

Any particular reason you're not using the balanced connections?

My e-drum module has unbalanced out... :/ I am feeding this with my vintage e-drum output...

 

[JohnRoberts]

"Ground loop" is a catch all term for signal ground contamination.

Hey guys..i need a second pair of eye (or more) please as I am puzzled.

I have 2 outboard gear I wish to put it into my signal chain. One EQ and one multi-comp/limiter. Both are Symetrix. Both has unbalanced in/outs... using unbalanced cables.

I just looked up the 201 OM and it shows both balanced and unbalanced I/O. It doesn't look like true balanced, but active differential output and differential input.

Each of the unit works without ground loop individually.... BUT if I connect them together, voiala... massive ground loop...i am using on both units the unbalanced INs and Outs.. also, using the same PSU for both. Why I am getting the loop? Because I am sharing the same AC PSU between the two?

The common power supply may be the source of your signal ground contamination. Are they quieter using separate wall warts for each SKU?

The balanced output (cough) just looks like the typical opposite polarity hot ring driven output. The balanced/unbalanced input is probably just a simple differential input, but that should work reasonably well for short signal runs.

Does the hum change with position of in/out (bypass) switch?

JR

 

[Studiogearlover]

Thanks guys for the responses... @JR this is my PSU which has these series plugs... are you suspecting this could be an issue? If I only use one unit..either the EQ or the compressor, no ground loop.

 

[Tubetec]

Make up a special patch cable to join the two devices ,
only connect the screen at the input end ,
The ground will still be made via the DC negative
your breaking the ground loop formed in the audio path though .

This was something that often happened back in the old days , it can happen if you try to connect two amps also , the dangerous way to stop the problem is to lift a ground on one of the amps , the smart way is make a dedicated patch cable for this purpose .

 

[JohnRoberts]

Thanks guys for the responses... @JR this is my PSU which has these series plugs...

Technically those plugs are wired in parallel.

Is it a factory Symmetrix PS? If yes presumably the ground current flows were sorted by the design engineers?

are you suspecting this could be an issue?

Yes

If I only use one unit..either the EQ or the compressor, no ground loop.

Again not a ground loop, but corrupted signal ground, by current flowing between the two PS ground connections.

If you had a second PS and the units didn't hum with separate PS powering them, that would confirm my suspicion. Back last century when I used truckloads of these wall wart power supplies in Peavey/AMR SKUs, I made a 6 output power supply that involved a power transformer with 6 floating 1A output windings. It was sweet but too expensive for typical Peavey wall wart SKU customers.

Just off the top of my head, perhaps try using small build out resistors in series with the separate grounds. Something like 5-10 ohms in series with each ground connection. That would allow the two PS grounds to float apart from each other and reduce inter-ground current flows. That could reduce, hopefully eliminate signal ground corruption.

I suspect Symmetrix engineers identified and solved this problem back when these SKUs were in current production.

JR

 

[Studiogearlover]

Is it a factory Symmetrix PS? If yes presumably the ground current flows were sorted by the design engineers?

No...these are aftermarket ones...16VAC 1amp

If you had a second PS and the units didn't hum with separate PS powering them, that would confirm my suspicion.

I only have one PSU...when I hook the units up separately, no hum at all.. i have a 2nd compressor (same type) and does the exact same if hook that up with the EQ... but if I power it up one by one using one PSU (the same PSU all the time) no buzz or hum or whatever it is..

Just off the top of my head, perhaps try using small build out resistors in series with the separate grounds. Something like 5-10 ohms in series with each ground connection. That would allow the two PS grounds to float apart from each other and reduce inter-ground current flows. That could reduce, hopefully eliminate signal ground corruption.

Thanks JR, I bought these units used. Would that be possible that both compressors faulty (bad caps...?) or this is simply a PSU issue i guess...?

 

[JohnRoberts]

Try adding 10 ohm resistors in series with each PS ground lead... See if that helps.

Or pick up a second PS to try. I am suspicious of common PS connection corrupting the signal ground.

JR

 

[Newmarket]

My e-drum module has unbalanced out... :/ I am feeding this with my vintage e-drum output...

You have a balanced (however implemented) input. In that case connect your unbalanced signal into that using "balanced cable" eg "mic cable'. Wire unbalanced signal to T or pin 2, Screen of unbalanced connector to R/pin 3 by the other inner core of the cable. Screen of cable also from screen of unbalanced connector to Pin1/X but try an impedance in series with this connection. eg 100R in parallel with 100nF capacitor.
To improve this add series resistor in signal to R/pin3 (prob in range 47R to1K0) and adjust for best result / CMRR. ie approximate an impedance balanced output into a balanced input.

 

[Cqwet Dbdfte]

Technically those plugs are wired in parallel.

Is it a factory Symmetrix PS? If yes presumably the ground current flows were sorted by the design engineers?

Yes

Again not a ground loop, but corrupted signal ground, by current flowing between the two PS ground connections.

If you had a second PS and the units didn't hum with separate PS powering them, that would confirm my suspicion. Back last century when I used truckloads of these wall wart power supplies in Peavey/AMR SKUs, I made a 6 output power supply that involved a power transformer with 6 floating 1A output windings. It was sweet but too expensive for typical Peavey wall wart SKU customers.

Just off the top of my head, perhaps try using small build out resistors in series with the separate grounds. Something like 5-10 ohms in series with each ground connection. That would allow the two PS grounds to float apart from each other and reduce inter-ground current flows. That could reduce, hopefully eliminate signal ground corruption.

I suspect Symmetrix engineers identified and solved this problem back when these SKUs were in current production.

JR

Add to this some wall warts have poor isolation. You can easily measure leakage current with a DMM on AC current range between wall warts (+) output and AC mains ground.
Wall warts and power supplies do not get much respect, expect to find a lot of junk.
Using a non contact voltage indicator on output DC cords can yield interesting results .

 

[JohnRoberts]

Add to this some wall warts have poor isolation. You can easily measure leakage current with a DMM on AC current range between wall warts (+) output and AC mains ground.
Wall warts and power supplies do not get much respect, expect to find a lot of junk.
Using a non contact voltage indicator on output DC cords can yield interesting results .

I remain suspicious of the common power supply ground corrupting the signal ground.

In theory the differential input should ignore chassis ground contamination but I suspect these SKUs predate the broad industry awareness of "pin 1 problem" . The TRS i/o don't even have a pin 1.

I repeat try adding some impedance in series with power grounds. After that maybe experiment with a separate ground wire bond between the two chassis but presumably the audio wires will provide that.

It might be easier to source a second wall wart. At least try borrowing one to confirm the problem source.

JR

 

[Ike Zimbel]

Hey guys..i need a second pair of eye (or more) please as I am puzzled.

I have 2 outboard gear I wish to put it into my signal chain. One EQ and one multi-comp/limiter. Both are Symetrix. Both has unbalanced in/outs... using unbalanced cables. Each of the unit works without ground loop individually.... BUT if I connect them together, voiala... massive ground loop...i am using on both units the unbalanced INs and Outs.. also, using the same PSU for both. Why I am getting the loop? Because I am sharing the same AC PSU between the two?

1) What frequency is the hum, 60Hz, or 120Hz? 60Hz supports the ground loop theory, but 120Hz may indicate that the two units are drawing more current than the PS is capable of supplying, and going out of regulation. If that's the case you need a second supply for the 2nd unit.
2) If you are "daisy-chaining" the two units, try using a TRS-TRS balanced patch cord between the two and see if that helps. A shorter cable should be better than a long one in that position.

 

[[email protected]]

The Symetrix 2XX units derive balanced power supply rails from the 16VAC power input in a clever way. the 16VAC input must float relative to power suppy/audio common. The draw on the rails may be slightly different due to LED indicators etc. in the two different units so the offset of 16VAC input lines to power supply/audio common may be different with unpredictable results.
This power supply is also quite demanding of the filter caps, make sure they're good.

Bottom line, you must have separate power supplies for each unit.
-Mac

 

[JohnRoberts]

Bottom line, you must have separate power supplies for each unit.
-Mac

I have only made that suggestion a couple times so far.

A differential input should still be capable of ignoring chassis ground differences but the common PS connection is somehow coupling into signal grounds, perhaps internally after that input stage.

Back last century I wasted some brain cells thinking about a multiple output electronic power supply design with active floating outputs but that required much higher output voltage and higher current/power dissipation capability, again not price competitive for customers of value products.

The individual wall wart power supplies were pretty cheap when purchased by the truckload.

JR

 

[Voyager10]

Make up a special patch cable to join the two devices ,
only connect the screen at the input end ,
The ground will still be made via the DC negative
your breaking the ground loop formed in the audio path though .

To expand on this: Given that the I/O is "balanced", use a twisted pair + screen cable (like a mic cable) and use TRS ("stereo") jack plugs.

Wire 'tip' to 'tip', 'ring' to 'ring', and connect the cable screen to 'sleeve' at one end only.

This gives worse protection from RF noise than a continuous screen, but you might get away with it. Separate power supplies is the 'correct' answer IMO.

 

[Studiogearlover]

Thank you so much for your help and suggestions,


I will get a better PSU and separate PSU's for each unit. Just for a "why not" I did check this crappy PSU plug and my DMM is giving me continuity reading between the -ve and +ve at the plug end. That can not be right i guess...

 

[Newmarket]

Thank you so much for your help and suggestions,


I will get a better PSU and separate PSU's for each unit. Just for a "why not" I did check this crappy PSU plug and my DMM is giving me continuity reading between the -ve and +ve at the plug end. That can not be right i guess...

Don't read anything into that continuity indication. You can get "dc continuity" at the output of a smpsu when it's not powered.
Similarly with dc continuity indicated across a transformer winding.

 

[Tubetec]

Well the post did specify unbalanced connections ,
what I meant was a short TS patch cord with the screen only connected on one end .

It was also mentioned that both devices work fine independantly , its only once the signal ground is made between the two devices the problem appears ,

 

[ruffrecords]

Well the post did specify unbalanced connections ,
what I meant was a short TS patch cord with the screen only connected on one end .

It was also mentioned that both devices work fine independantly , its only once the signal ground is made between the two devices the problem appears ,

The interesting thing is both are powered by an single AC transformer. This means each must have its own rectifier and PSU internally. I am not convinced that you can successfully power two independent circuits from a single secondary and expect them to behave when you connect their analog 0V together. I strongly suspect each unit is supposed to be powered by its own transformer.

Cheers

Ian