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In the Forum: Analog Playback
In the Thread: The last phonocorrector: “End of Life" Phonostage
Post Subject: Air caps?Buffer?Posted by N-set on: 11/29/2011
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 Romy the Cat wrote:

Did you try just for test to increase the caps value that short the plate to ground? Will it amount and the character of the noise change?


Just like one would expect: 0.1uF shorting cap gives approx 4x more ham than a 0.47u one.
The character is the same--quite a nice sinus (shorting cleans some HF obviously)

 Romy the Cat wrote:

If I were you then I would use elimination techniques. Get rid of the output buffer and see if the noise still there. Replace the air caps in feedback with mike or alike and see if noise is there. The air caps are in a way antenna and they suck anything from air.


Ok, this gave some info:
0) disconnecting the air caps completely reduces 50Hz by some 10dB, but the resulting noise is very ugly, irregular, high peak-to-peak (a bit higher than with the loop closed), the spectrum suggest a white noise; the RIAA eq is out so all the dirt is present in the full glory; where the fuck does it come from? a cheap chinese 12AX7 noise?

1) shorting the buffer (either the plate or the cathode to gnd) or the output changes absolutely nothing
2) but taking the buffer out completely and taking the signal from the second stage's plate via 0.47uF reduces the ham by some 20dB;
the noise is quite "nice", the noise floor drops by some 15dB or so
3) simultaneously disconnecting the air caps from V2's grid reduces ham by additional 3-4dB

Moreover, if the scope is at the output, the phono switched off, touching the 330pF gives a super strong ham
(touching the 150pF has a very-very mild effect in turn). Caps as antena ok, but why the buffer?
I keep scratching my head...

Cheers,
N-set


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