|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
84.241.211.28
In Reply to: RE: Any reason I have not seen a fixed bias phono stage??? posted by TomWh on December 09, 2014 at 08:03:04
Depending on the design, fixed bias on the first stage could create problems when it results in bias voltage applied to the coil of the cartridge, or the secondary of any step-up.Other then that, I'm always in favour of fixed bias. Indeed eliminating at least a resistor. Bypass caps I don't ever use. Fixed bias also has the benefit of not raising Zout of the stage, which could be beneficial when using LCR's.
I moved from battery bias to filament bias in my phono stage. Yes, it introduces noise, but results are fabulous. I might try using Schottky diodes in the cathode once. If this results in the same sonic results while loosing the noise, then that would be great.
NC
Edits: 12/09/14 12/09/14Follow Ups:
So I should be able to fix bias the second and third tube because we do not have to worry about the cart or sut?
Thanks Tom
As long as you avoid bias on the cartridge it should be fine. Bias on the SUT might be acceptable.
The bias supply needs to be VERY quiet though.
NC
A variation on the battery bias theme might work well. Take the 6SC7, with its common cathode connection. Place a small NiMH cell or 2 in the line between the cathodes and ground. Use 1 triode of the twin in each channel. Without a cap., there is no DC offset in the grid circuit to create problems with a cart. or SUT.
Charge the NIMH part(s) before use. The 2 mA. or so trickle of regular use will keep things in order.
BTW, I like grid leak biased 2nd gain blocks. As pointed out by Thorsten Loesch, the huge (20 Mohm) grid to ground resistor VERY lightly loads the EQ network.
Eli D.
Zeners can be pretty noisy. Try a LM4040 instead. Or, even better, use a LM4040 to bias a PNP or P-FET as the shunting element (like here although we used an LED).
A schottkey diode should work great.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: