PLS help solve transformer hum

[gnd]

I have a problem with connecting 6 unbalanced synths to unbalanced mixer.  ???

Line is as follows. IIIII unbalanced synths -> (TX) -> unbalanced rack mixer -> DAW Emu balanced input

If I connect just one synth directly (no TX) into rack mixer, all is smooth, just plain flat noise on analyser.

If I connect two or more directly (no TX), there is some slight hum/buzz. Clearly some ground loop.

So I thought I'd connect them all through transformers. But buzz remains, and even increases.
I tried several transformers (Line Level Shifter, Pikatron, little RS), all increase hum. I tried it with all different cable options, no success. All TX I tried had no ground connection between primary/secondary (floating?). Probably this is as it should be, yes? TX were shielded with mu-metal (Pikatron) or in metal grounded rack (isolated from rack).

I must be doing something wrong. Why is it, that I get hum even if I connect just one synth through transformer? One synth directly is clean, no hum. But through TX, there is hum.

Pls help.

 

[ppa]

have you tried to connect the transformers' shield pins to mixer case ground too?
have you tried to connect the synths' case grounds to mixer case  ground too?
have you tried to connect the transformers' case pins to mixer case ground too?

Pier Paolo

 

[gnd]

have you tried to connect the transformers' shield pins to mixer case ground ?
have you tried to connect the synths' case grounds to mixer case  ground ?
have you tried to connect the transformers' case pins to mixer case ground ?

Pier Paolo

Pier, yes I tried different combinations.

On Pikatron I notice that it is quieter when TX can is connected at input. Pikatrons are quite fine, although they still increase hum of single synth, comparing to direct connection.

But I would preffer to use EBTech Line Level Shifter on synths, because it is 1:4 stepup, and boost level. Plus I have 4x8ch of them, so I want to use them.  : But they increase hum/buzz greatly. I will do more testing tomorrow, but, shouldn't TX be hum free? Is it possible, that TX messes grounds somehow? I mean, where could buzz come from into shielded TX?

 

[ppa]

if the shield isn't connected to input line gnd (in this case the mixer input) but not connected or connected to synth gnd then the shield induces noise in the transfomer's secondary for capacitive coupling, since the synth's ground and the mixer's ground/gnd have different voltage potential levels.

If the shield is connected to the input line gnd the shield avoid capacitive coupling with the synth gnd
However each transformer should be near to mixer , each one with the shield and the metal can connected to mixer gnd. The transformer secondary should be connected to the unbalanced input line with a (short) coax audio cable.

Pier Paolo

 

[abbey road d enfer]

I can see only two causes:
A) if you haven't floated one side of the connection, you may still have ground loops
B) the transformers may pick up hum from a mains transformer or some stray field

 

[gnd]

I can see only two causes:
A) if you haven't floated one side of the connection, you may still have ground loops
B) the transformers may pick up hum from a mains transformer or some stray field

abbey road d enfer, what does A) mean in practice?

Both sides of transformer are connected to unbalanced units. Primary and secondary of transformer are isolated.
How do I know that I have one side floated? Or how do I make it floating?

B) How do I check if hum is from magnetic field? Is it so that if I rotate and move transformers, then induced hum will change in level with rotation/moving of transformers? And if its level doesn't change while moving, then it is not induced from magnetic field, but rather ground loop?

 

[abbey road d enfer]

I can see only two causes:
A) if you haven't floated one side of the connection, you may still have ground loops
B) the transformers may pick up hum from a mains transformer or some stray field

abbey road d enfer, what does A) mean in practice?
Both sides of transformer are connected to unbalanced units. Primary and secondary of transformer are isolated.
How do I know that I have one side floated? Or how do I make it floating?

Just make sure the gnd connection to the balanced side is not connected to the ground on the unbal side.

B) How do I check if hum is from magnetic field? Is it so that if I rotate and move transformers, then induced hum will change in level with rotation/moving of transformers? And if its level doesn't change while moving, then it is not induced from magnetic field, but rather ground loop?

That's exactly it.

 

[CJ]

try a different transformer if you have one.
coil geometry can affect hum.

 

[gnd]

CJ: try a different transformer if you have one.
coil geometry can affect hum.
---------
I used Pikatrons too, and they are MUCH better, as they have metal shield. I have some Lundahls too, but didn't try them, I will use them in other places.

I have four of these 8 chanel Ebtech Line Level Shifters (LLS), and want to use them. I contacted manufacturer, Ebtech. Problem is that hum is induced from outside. Moving them arround helps a lot. Transformers in the box are not shielded. And box itself (rack unit) is not grounded. After some nice exchnange, manufacturer suggested that I scratch colour of the box and connect it to ground. I tried that, but it was a bit worse. Possibly I would have to check different ground references, and see if any improved shielding, but anyway.....

Now I managed to find position in rack, where induced hum is minimal, and I'm quite happy now. I moved units arround a bit, and moved some power adapters behind racks too, so now I get negligible hum added.

After reducing hum to minimum, these LLS's are very useful, and I hapily use two of them on synths. Another two I will use on some send/return effect units. So after getting more familiar with the units, all is good now.

Thanks all for help.

 

[CJ]

you might reduce hum by wiring the input out of phase, and then the sec out of phase, so you will be in phase, but the hot and cold leads will be swapped, possibly reducing hum even further, or make it worse.

 

[gnd]

you might reduce hum by wiring the input out of phase, and then the sec out of phase, so you will be in phase, but the hot and cold leads will be swapped, possibly reducing hum even further, or make it worse.

This is great tip. Thnx, I will try it.

This can actually reduce hum in each single transformer, due to physical properties of transformers?

Or is hum reduction coming out of turning some hum out of phase, and then when it is added on mixer, it cancels itself?

If second works, would it be good idea to wire just half of channels out of phase?