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In the Forum: Analog Playback
In the Thread: The last phonocorrector: “End of Life" Phonostage
Post Subject: Ignorance is bliss...Posted by N-set on: 11/26/2011
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I've spent two more nights examining and fighting my ham (I'm vegetarian and I want my phono to be so as well).
1) I did Romy's excercise with shorting the stages via a 0.47u cap to GND. Shorting the plate of V2/grid of V3 resulted
in the biggest reduction of ham (around 8-10x); shorting V1 plate or V2 grid produced moderate reduction of 3x-4x;
shorting V3 (plate to cathode) and the output produced no result. So the largest pick up seems to be at the second stage.
Small changes of wiring around there gave no result.
2) The ugliest discovery and most probably the source of ham: if ignorance is a bliss, don't look into your power supply operation!
Below is an example how ugly my operation is:
This is the current in the secondary of the HT transformer (red trace; taken at the resitor in series with the input choke)
and a signal from a 100mH RF
search coil placed close to the transformer (blue trace). A very very ugly current waveform, with large oscillation, caused
by the big 20H input choke. I'll probably try snubbers a la Morgan Jones to tame this oscillation.
The blue spikes (produced by the inductance of the transformer at the zero crossing of the voltage, when the rectifiers switch)
is with high probability what couples to my interference-sensitive signal layout (it's fucked up, sorry to admitt that,
I wanted to avoid steel bolts and being unable to solder to 1.5mm Cu plate directly I had to rise the components
too much off the ground plane, creating areas which catch whatever is in the air; it's not compact enough).
As I understand it I can't really do anything to kill the spikes, only shield them and try to kill the oscillation.
My only way is to shield heavily the signal box and place the PS box 10km away.
Cheers,
Shit-set
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