 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
68.202.191.115
In Reply to: SCA-35 - slow moving train posted by kurt@bauer.org on April 1, 2007 at 17:20:49:
In the phono preamp section, you show a 510K (!) gate resistor in the FET source follower. I'd lose that huge resistor because it will create a pole (roll-off) with the FET's Crss that is low enough to create voltage-variable phase shift within the audio band. If someone is trying to use this pole to balance out the ultrasonic RIAA response, that's simply not a good design choice. I'd rather see a wimpy 12AX7 CF in the output than this FET as wired. Consider a 12AT7 CF with fairly low current to keep the PS from being overloaded. So, either lose the 510K (replacing it with a 1K to 10K gate stopper) or lose the FET altogether and stay pure tube. Just MO.
_____________
Brian
Follow Ups:
Brian,510 K is a TYPO. 510 R is what the schematic should show.
Eli D.
OK, big difference! Maybe I should have guessed this was a typo. Me? I'd still use a triode CF. At first this MOSFET seems appealing because it promises 100mS of transconductance at 100mA, but here it must be used at a mere trickle of 1.8mA of drain current to avoid overloading the stock transformer and supply. At this low drain current gm will be in range of 2mS or less, hardly better than most tubes at the same current. Even a 12AX7 will come close! The low transconductance in either case may not be a problem here if the phono stage only has to drive the power amp in the same chassis. The one advantage of the MOSFET is that it doesn’t need a heater supply. But a single heater supply could be elevated at a compromise voltage between the grounded cathode 12AX7s and the elevated 12AX7 CF. For me if would be worth the trouble to stay 100% tube, but YMMV…
_____________
Brian
Brian,You are absolutely correct about mediocre transconductance at the low drain current. The BIG edge for the FET in this particular situation is its small physical size (TO92 case). PCB space is at a PREMIUM in the SCA35 "box".
The WIMPY 'X7 can drive the SMALL capacitances of the ZVN0545A with ease. If I was building a stand alone phono preamp, I'd buffer the little FET with a big FET, whose operating transconductance (gfs) would be > = 10 mA./V. LOTS of drive there.
Eli D.
Eli,Yes, space is a problem in the SCA-35 (I've got one too). I'm not even sure why we need a CF in the phono stage so long as we're driving only the power amp section, which should have a very high input Z. Sometimes a CF is useful in a phono stage to drive the feedback RIAA filter, but we don't have that here. It just comes down to one's own tastes and listening experiences. I happen to think that FETs are generally audible. If you're going to build a tube circuit, then I say use tubes (except for housekeeping functions like power supplies). But each to his own...
_____________
Brian
Brian,The rebuild in question is going to use the ST35 driver circuitry and eliminate the tone controls, etc. The basic RCA circuit forces the use of 250 KOhm volume controls. 250 KOhms at the grid of a common cathode 'X7 section is a prescription for HF rolloff, due to interaction with the LARGE Miller capacitance. The 50 K controls shown are a reasonable compromise between I/P impedance and low Miller interaction. In order to use 50 K controls, the RCA phono circuit needs to be buffered. The little FET is the best practical choice in this situation.
Many FETs are unfavorably audible and should not be employed. OTOH, FETs with small and reasonably stable reverse transfer capacitances make very good voltage followers that are CONVIENENT.
Eli D.
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]
