[sdiy] Hum & Buzz

[Michael Zacherl]

> On 18 Sep 2017, at 18:58, Ingo Debus <igg.debus at gmail.com> wrote:
> 
>> Am 17.09.2017 um 21:18 schrieb Michael Zacherl <sdiy-mz01 at blauwurf.info>:
>> 
>> 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.
> 
> Are you sure it’s only hum? Have you checked that the preamp doesn’t oscillate?

to answer the question: no I’m not sure.
OTOH I had a very close look at the signals with the oscilloscope and the oscillation would have to  be in the mV range to get cover in noise (hiss).
To understand correctly: your question is implying the oscillation only would occur with a signal present at the input, i.e. the ins not tied to ground?

The audible hum is a very prominent 50Hz signal.
On a side note, I noticed a 60Vpp hum signal when just touching the tip of the oscilloscope’s probe (w/ floating gnd) which is way above of what I’ve seen elsewhere.
Thanks for replying, Michael.