Strange preamp output noise behaviour

[grid_stopper]

I have a friend with a pair of yamaha pm1000 channel strips he had racked up. the output section is the standard feed taken from c37 and connected to a jensen je-123 output transformer, configured as 1:1. They are in the same enclosure, and when the strips are used one channel at a time, their is no noise, however when used as a pair one channel develops a nasty buzz, this buzz does not change with a cable change/swap, it stays on the same channel. flipping the phase at the channel strips input does nothing. to fix it, however flipping the phase down stream of his preamp, say on his interface does remove the noise. to me, this clearly indicates that the issue is somewhere in the output, but im wondering how this combination of things is possible.... would anyone care to speculate as to what might be going on there?

 

[Voyager10]

You may need to describe what "used as a pair" means, possibly with a diagram/picture showing exactly how things are connected when the buzz occurs.

For instance, you might have some grounding problem where connecting two inputs or outputs to the same piece of equipment creates a ground loop (i.e. reveals a "pin 1 problem").

 

[grid_stopper]

You may need to describe what "used as a pair" means, possibly with a diagram/picture showing exactly how things are connected when the buzz occurs.

For instance, you might have some grounding problem where connecting two inputs or outputs to the same piece of equipment creates a ground loop (i.e. reveals a "pin 1 problem").

sorry for the lack of details! let me clarify;
when i say pair, i just mean that two channels have been racked in the same enclosure. however when only one out of the two strips is used- there is no noise, regardless of wether its channel one or two. however, when both 1 and 2 are used simultaneously there is a loud buzz(waiting on a measurement from the owner to see wether its mains noise or something more broadband).

I put together a quick diagram showing how the output is wired. Shield is connected only at pin 1 of the output on each channel, and pin 1 has continuity with the steel chasis on both outputs. From what I can understand, the input is connected in a similar manner. pin 1 is connected to the cables shield at that end only, and hot and cold connect to the primary with no shield connection

 

[Evertide]

When used as a pair , meaning powered on or connected ?

 

[grid_stopper]

When used as a pair , meaning powered on or connected ?

both channels live in the same enclosure, so if the unit is powered on, they are always recieving power. as far as i can tell, the problem only occurs when the inputs/outputs have a connection/are terminated with a load

 

[Evertide]

So it can be a power problem , to small , not able to power two channels.

"pin 1 problem"

 

[grid_stopper]

So it can be a power problem , to small , not able to power two channels.

"pin 1 problem"

im fairly certain its not a power supply issue, transformer is more than adequate for the load of both channels. and i am aware of the pin 1 problem conceptually- what i dont understand is how flipping the polarity on the interface this preamp is connected to fixes that problem, if that is indeed whats happening

 

[Khron]

Magnetic coupling between transformers?

 

[abbey road d enfer]

What happens if you disconnect pin 1 of one of the cables?

 

[grid_stopper]

The owner doesnt feel comfortable opening the unit up on his own, so its getting dropped off in the next day or two. with that in mind

Magnetic coupling between transformers?

How would I test for this, and would magnetic coupling manifest as a buzz/hum on one channel but not the other?

What happens if you disconnect pin 1 of one of the cables?

I will give this a try, thank you. Am I correct in am assuming you mean pin 1 of the one of the outputs in this instance?

 

[abbey road d enfer]

I will give this a try, thank you. Am I correct in am assuming you mean pin 1 of the one of the outputs in this instance?

Correct.

 

[Evertide]

How would I test for this, and would magnetic coupling manifest as a buzz/hum on one channel but not the other?

If the build of the channels is not the same , one can interfere with the other.

It is a miracle also to us , sometimes wrong wiring 1, 2, 3 mixed up...

 

[grid_stopper]

If the build of the channels is not the same , one can interfere with the other.

It is a miracle also to us , sometimes wrong wiring 1, 2, 3 mixed up...

from the pictures provided to me the wiring to and from looks to be identical, on 2 channel strips that were taken from the same console. the XLR outputs are wired identically, however from the pictures i cant ascertain whether the other end of that connection is potentially flipped at the transformer secondary side, should be finding out soon however. But even then, what im struggling to wrap my mind around here is if say, the second output had pins 2+3 swapped from the first output, would that not just mean the two channels would be 180º out of phase with one another assuming an identical input? im really having trouble understanding what principle is at play that allows a simple polarity inversion to correct the noise. I understand an answer isnt likely forthcoming until i can get my hands on the unit and take a look, but this is really messing with my conception of how things should work!

 

[radardoug]

If two channels are mixed together and one is phase reversed, and both channels have buzz, the buzz will cancel. Could be that. But at the moment it is all complete speculation based on the hearsay evidence from your client. It might have been better to wait till you got them on the bench, and then give us your findings. Clients notoriously get it wrong.

 

[Khron]

from the pictures provided to me

... And which are seemingly kept secret..?

That being said, no guarantees thw gremlins are not further upstream in the circuit / wiring... But indeed, disconnecting pin 1 from either (or rather both) inputs would be a first, and probably simplest, troubleshooting step.

 

[Voyager10]

When used as a pair , meaning powered on or connected ?

This is still the million dollar question.

It would help greatly to know, step by step, how we get from 'one channel working' to 'two channels with a buzz'. What is the actual step (plugging something in?) which makes the buzz first appear?

My best guess so far is that the power supply arrangements for the two channels are slightly wrong, and that connecting two grounds together, somewhere, is causing a noise current to get in the signal path.

 

[grid_stopper]

ok, so i have the unit in front of me and can dig in a little bit.

something ive found is that the noise is being injected after the gain control, but before the eq. seems to be centred around the 100ish hz area, and boosting the the low band increases the noise on channel 1, however channel 2 is dead silent at any boost setting. rotating the toroidal power transformer reduces but does not remove the noise.

the noise only appears with two mics connected. the two strips each have a wire that connects to the designated ground point on the channel strip, each of these wires connect to a ground point on the psu, whcich then connects to the chassis.

still have some poking around to do, but these are my findings so far.

 

[StarTrucker]

the noise only appears with two mics connected. the two strips each have a wire that connects to the designated ground point on the channel strip, each of these wires connect to a ground point on the psu, whcich then connects to the chassis.

Bad ground scheme between the channels? Shield noise injected into the circuit somewhere after your gain control? Send pics, are they two separate PCBs?

 

[Voyager10]

the noise only appears with two mics connected.

So plugging in a mic into a second channel makes the noise appear on both channels? What sort of mic? What about just an XLR lead, nothing connected to the other end?

 

[grid_stopper]

ok, i figured this out. it was a combination of things- a funny ground connection in the PSU, and a cap that went microphonic on one of the channels. i corrected both of these, and everything is working as intended!