[sdiy] Hum & Buzz

Michael Zacherl sdiy-mz01 at blauwurf.info
Sun Sep 17 21:18:55 CEST 2017

I’m currently helping a friend experimenting with a couple of tines and pickups of a pretty much destroyed Fender Rhodes he acquired a good while ago.
For this I built an amplifier stage (gain 2-5x) and and a mixing/output stage based on some 5532 circuits from Doug Self’s book. Nothing fancy actually.
It’s built partly on perfboard and partly on small PCBs I did to speed up the process.
The PCBs are mounted on the perfboard as would be the parts themselves if I'd have had them handwired.

Also it’s “on site” i.e. at my friend’s artist workshop, which turns out to be electrically very noisy.
And that’s where trouble starts.
In his project the pickups are individually wired to each pre-amp via a longer un-balanced shielded cable.

ATM I’m trying to focus to understand a particular situation in this fragile setup:

Currently only two inputs and one output are wired in.
After experiencing lots of hum and buzz, as one would have with a ground loop, I disconnected the pickups and wired the two inputs to ground to see how the bare circuit would perform.
To my surprise the hum and all other noise was almost gone.

So, suspecting the pickups and their long connections I hooked up a cheap-ish Klarkteknik [1] mixer, connected the pickups via the wired 1/4” TS-plugs to the mixers line ins, the mixer’s main outputs to the power amp and expected hum.
Cranking up the amp, as I did in the previous 0-input test, just exposed lots of hiss of the mixer itself. No hum at all.

There is a difference in input impedance: the mixer’s one is typically 20kΩ which is 15 times lower than the 5532 can offer in the circuit.
So I’m not sure if the circuit I’m using is “too good” for this application allowing all the dirt becoming audible.

The pickups deliver a signal in the 350-500mVpp range, which is good, IMHO.

Would could be my next step to track down the trouble?

Thanks a lot, Michael.

[1] https://media.music-group.com/media/PLM/data/docs/P0ALP/QX1202USB_QX1002USB_M_EN.pdf

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