[sdiy] Hum & Buzz

Michael Zacherl sdiy-mz01 at blauwurf.info
Mon Sep 18 11:18:42 CEST 2017

> On 18 Sep 2017, at 03:51, Michael E Caloroso <mec.forumreader at gmail.com> wrote:
> The pickups had changed over the production era of the Rhodes.
> On my model, any input impedance less than 1Mohm created noise.  The
> pickups are very sensitive to loading.
> You may want to use a different circuit - 20K input impedance is too
> low, and those 5532s have an input impedance of 300K.

yes, I can see that.
but in this setup it is the other way round: the mixers’s 20kΩ load is ok,
the 5532 pre-amps are very noisy.
that’s my confusion.

> On 9/17/17, Michael Zacherl <sdiy-mz01 at blauwurf.info> wrote:
>> Hi,
>> 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.
>> Nothing.
>> 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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