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Old 5th November 2006, 10:08 PM   #11
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Shoog's Avatar
Join Date: Aug 2002
Location: Eire
I bit the bullet and decided to rebuild in a new case.
I had a suitable case so it didn't take long. There is a great deal more distance between the power transformer and the main circuit, there are also two grounded screens between as well. I introduced a high value inductor into the power supply ( I tried a smaller inductor but it introduced a resonance at about 50hz). I also placed another 65K uf of smoothing on the heater supply. I finally replaced the ECC83's base with a screened can base.

Its lashed together until I can get some decent phono plugs.
Still, the hum is all but gone at any sensible listening level. Its been well worth the frustration of debugging as I now feel confident I will have a phono preamp that I can happily live with for years. Without the residual hum it sounds very clean and forward.

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Old 6th November 2006, 02:24 AM   #12
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Kofi Annan's Avatar
Join Date: Jan 2004
Location: US
Comin' in late on this one, but you might want to try Thorsten's dirty diodes trick. Note that the grounding scheme for his phono pre design that I posted has the bridge rectifier 90 degrees INCORRECT, but if you shift it as he mentions, you'll be right on.

I've screwed up almost everything in my life, just ask my dad, but I got this thing dead quiet.

If you try this, let us know how it worked out.

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Old 6th November 2006, 10:54 AM   #13
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Join Date: Aug 2002
Location: Eire
Thanks for that Mr Annan (what plans for your retirement ?).
I tried that this morning. Took away almost all of the remaining hum. Would you believe that I had tried that before but had got it into my head that it wouldn't work and had shorted it out. So I already had the little board to put in. I use a slightly different implementation. I have back to back diodes bypassed by a 0.68uf cap and a 100R resistor. The cap allows a perminent path to earth for radio frequencies, and the 100R is insurance should the diodes die.
I have the same arrangement in my main preamp. Works a treat there to.


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