Dynaco-Doctor Forum: Upgrading the Stock PAS Phono Stage by Joe Curcio

The stock PAS phono stage compromises some performance in exchange for simplicity. Plus, its RIAA compensation departs from ideal by 4db. The output impedance is quite high (30K) so low impedance loads (less than 100K) will cause LF rolloff. While it is a major undertaking to lower the output impedance, it is relatively easy to address the RIAA compensation and the LF rolloff issues.

RIAA Fix:
1. Remove the 750pf caps (both channels) from the selector switch.
2. Remove the 27K resistors (both channels) from the selector switch.
3. Replace the RIAA 68pf cap on the PC-6 pc board with a 820pf (polysty or silver mica).
4. Parallel the 100K RIAA resistor with a 2M (metal film).
5. Replace the 2750pf RIAA cap with a parallel combination of 2200pf and 470pf.
6. Replace the 4.7M RIAA resistor with a 2M (metal film).
7. FYI - DO NOT remove the 47K feedback resistors between the cathodes of each tube.

LF Rolloff:
1. Jumper the 0.1uF output cap connected from the plate of the second stage(s) with a solid wire (both chanels).
2. Change the value of the 0.1uF output coupling caps, connected between teminals 1 & 3 (left channel) and 7 & 9 (right channel) to 0.47uF / 400VDC.

After you complete these mods, you can use any of the 3 low level inputs (Phono, Tape, Special) for your turntable - all 3 will be RIAA compensated. RIAA error will have improved from +/- 2db to +/- 0.2db. Output impedance will not be changed so the original caution of low impedance loads (attached to the tape output connections) apply.