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Re: RIAA Phono Stage Project

Posted by Eli Duttman (A) on August 31, 2006 at 15:39:33

In Reply to: RIAA Phono Stage Project posted by sorenj07 on August 30, 2006 at 20:11:40:

Dude,

You show a choke I/P filter B+ PSU with a 20 H. 1st inductor. You need a 20 KOhm bleeder resistor across the 1st filter cap. to ensure you draw the critical current. Also, the standby switch to ground is unnecessary. B+ rise with a 5V4 is slow. The 'X7s will have plenty of time to get warm.

Other posters have indicated shortcomings in the topology. Many of the problems can be dealt with by buffering the 2nd gain block and using the buffer to drive both the EQ loop and the O/P. A ZVN0545A MOSFET as described in the link below will do the job for you. Given the EASIER load on the 'X7 section after buffering, you can decrease the load resistor value. Gain will still be high and the increased idle current is a good thing.

A decoupling network made from a 100 Ohm resistor, a UF4007 diode, and a 10 muF. cap. is (IMO) indicated in the B+ line between the 1st and 2nd gain blocks. Use Solens or Axons (both SCR made) for the 3rd 22 muF. filter cap. and the 10. muF. decoupling cap.

You will do better with a 12 VDC heater supply. Use the 6.3 VAC winding to drive a "full wave" voltage doubler made from 2X 50 PIV/5 A. Schottky diodes and 2X 2200 muF. 15 WVDC 'lytics. Follow the doubler with a 7812 3 terminal regulator. You spend less, get less ripple, and you acquire the option to use 7058s. NOS GE 7058s retail for about $8 each. Decent US made NOS at a price lower than current production aint something to sneeze at.

Eli D.

• MOSFET Follies   (Open in New Window)

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