Splitter Hum Problem

[lars-musik]

Hi Folks,

I cooked up a mélange of two different circuits – the ROG Splitter-Blender with PRRs improvements and RGs "Transformer Coupled Splitter" in order to operate two guitar amps through the two outputs and testing different circuits (EQs/Booster) in the two loops (red/green).
Now I got it all put together and I have a nasty ground hum, no matter if the transformer are in or out of the circuit.
Could you tell me if I made a crude mistake somewhere?

Thanks



https://postimg.cc/bsvQ7cCq

[iainpunk]

have you checked for ground loops?
if you have both send and returns internally ground connected, and a few patchcables and pedals complete a parallel ground path, you are creating a ground loop.
in ab loops/mixers with send and returns, don't ground the return jack, just leave it.

cheers, Iain

[antonis]

Have you check it with phase reversal switch both ON and OFF..??

[lars-musik]

have you checked for ground loops?
if you have both send and returns internally ground connected, and a few patchcables and pedals complete a parallel ground path, you are creating a ground loop.
in ab loops/mixers with send and returns, don't ground the return jack, just leave it.

cheers, Iain

Hey Ian,

it is all in one enclosure, no other pedals/cables involved. Not very pretty, but the wires are not excessively long.

[lars-musik]

Have you check it with phase reversal switch both ON and OFF..??

Yes, I did.

[lars-musik]

I think I narrowed the problem down to that part of the schematic. If I take the transformer (1:1, TY-141P) out of the circuit by grounding the sleeve of the output jack and connecting the tip directly to the 10uF cap everything works fine.

I admit I have not a good conception of transformer-driving circuits - do you spot potential humming issues here?

[iainpunk]

what was your testing set up?
did you connect the send and returns with patch cables for testing? still a ground loop, despite the short cable length.
are the jacks 'normalized' or grounding? (that they pass signal between them if no cables are inserted or is the return jack switched to ground so ensure silence if no cables are inserted)

what are the DC voltages on your opamps?
have you decoupled the opamps? (using polyester or mylar caps of about 100nF placed as close as possible to power pins of the opamp)
it could be that the transformers themselves pick up the hum. are you near high power loads, like fluorescent tubes or washing machines? what is the power source you use? wall warts could be noisy/hummy

cheers

edit: are you actually using the send and return capabilities of the circuit? if not, you have atleast one redundant opamp and with a bit of wiggling, you could eliminate another one if you want.

[PRR]

What are those MUTE switches doing? They short the opamp, the destination gets full line crap.

[lars-musik]

What are those MUTE switches doing? They short the opamp, the destination gets full line crap.

The two outputs are meant to go to two guitar amps. If one should get no signal the output gets send to ground. How does the opamp gets shorted if it is isolated by the transformer?

[antonis]

How does the opamp gets shorted if it is isolated by the transformer?

It isn't isolated 'cause PRI_1 & SEC_3 are shorted via BRIDGE switch 4 - 6 lugs..

[PRR]

If you are not using an output, leave it open. (The transformer does not protect against shorts; it floats the ground.)

If you are not using an input, short it to ground.

[lars-musik]

If you are not using an output, leave it open. (The transformer does not protect against shorts; it floats the ground.)

If you are not using an input, short it to ground.

Ha! I didn't think of this. As always, you thought the crucial step further. Thanks!

However, the idea is not to use the "shortcut" via the bridge anyway and route the signal through the isolating transformer (the jumper to cut the transformer out was meant for testing purposes because I wanted to test how the frequency range is cut by the transfromer),

[lars-musik]

Embarrassingly enough I found a threat with the same problem. Looks like I will try RG's solution of "grounding" the output sleeve via a 100R to 1K resistor to the chassis.

After the read I am still not quite sure if and how much that would impede the isolation.
https://www.diystompboxes.com/smfforum/index.php?topic=109086.0