Peterson Suitcase hum, solution with sound isolator?

[valentenet]

Dear all, how are you?

I know this topic has already been explored a lot. I read all the discussions as well as videos and other forums, but I still come across the problem of electrical network noise or perhaps a ground loop and I would like to exhaust the attempts and discussions about this problem.

I'm working on a Rhodes Suitcase 80w - Peterson design (4 pins). I re-capped it in addition to other components except the power transistors, which are very good (apparently) and the 0.01uF capacitors next to the power transistors.
I redid all the wiring with good new shielded cables in addition to the connectors, I carefully recovered the printed circuit boards, tracks and soldering points.
Result, the piano is incredible and so is the amplifier. In isolation, the piano works without noise when connected to outputs other than the suitcase amp. The problem occurs when I connect the two together then the hum appears.
To know:

1. The piano has the stage conversion kit (VINTAGE VIBE
Fender Rhodes 4 Pin Cheek Block Power Supply).

2. When using the piano in stage mode (without the suitcase amplifier) connected to other amplifiers (e.g. Fender Rumble) or on the soundboard with a D.I. the sound output is perfect, leaking a minimum line hum that is totally tolerable and equalizable.

3. When turning on the amplifier without the piano connected to it, there is absolutely no noise, hum or hiss, completely clean.

4. When connecting the piano to the amplifier, the mains hum (60hz) is immediately audible, regardless of the piano's volume. The hum does not change with the piano volume, it has a fixed volume. Of course, if you increase the volume, the hum is disguised. But if you connect the amp outputs to a soundboard (concurrent with the amplifier), even with a DI the hum comes through very strong.

5. Some ground loop tests (unsuccessful) were performed such as:
5.1 Isolate (cut) the ground wire of the 4-pin cable between the piano and the suitcase ampli.
5.2 Isolate (cut) the power supply wire (25v) of the 4-pin cable between the piano and the suitcase ampli and use the external VV source.
5.3 Connect the piano in stage mode using P10 cables connected between the piano and the suitcase ampli (without the 4-pin cable) and use the external VV power supply.
5.4 Isolate the fixation of the p10 jacks from the ampli suitcase housing body.
5.5 Add more filtering capacitors to the source input (4x 3300uF).
5.6 Add capacitors to the 25v output to reduce possible ripples if the voltage drops.
5.7 Test another (electronic) keyboard connected directly to the ampli suitcase.
5.8 Use power sockets with a grounded line filter (from the studio), invert the poles and test by eliminating the ground as well.
5.9 Turn off all other equipment in the surrounding area, lights, move the piano, also testing other amps to make sure there was no grounding fault in the space.
5.10 I reanalyzed all the wiring, disconnecting the internal circuit from the housing body (both the pre and power amp).
5.11 Temporarily removed the VV stage conversion kit from the instrument.
5.12 I checked all the harp connections, possible differences in grounding, poor contact, oxidation and damper grounding.

In conclusion, I did not obtain any change in the result. It's a mystery!

My next guess is to try to make a modification to the preamp output by installing a sound isolator module (with transformer or octocoupler), or perhaps changing the entire powersuply with a new, more modern circuit.

Has anyone ever experienced this? Has anyone tested a sound isolator between the pre and power amplifier? Or am I the one who is too obsessed with this hum and in this case it is inherent to the instrument?

Thanks

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[Jenzz]

Hi :-)

What about the cable you're unsing from the cabinet to the piano? Is it 4 wires + shield? Be aware that the shield should only be attached to the connector housing of the cabinet side, piano side should be open. Otherwise you get a ground loop.

Inside the cabinet, tie the chinch cable together and keep them as far away from the transformer as possible.


Jenzz

[valentenet]

Hi Jenzz

Was exactly what I did before.
I'm going to move the transformer far away from the cabinet to check if it is causing interference. I'll also try another power supply.


Thanks.

[drpepper]

I would be tempted to rig up some batteries to power the preamp. That would remove any ground loops.

Good luck, Dean