I have a couple of older Dynaco PAS-3 preamps around. I used to enjoy the sound of these preamps many years ago but a few issues have kept me from using them these days. I tried to tackle the biggest problems using parts I had around.
New life for an old preamp
The biggest issue with the old PAS-3 is that the original tone control circuit depends on having exactly a 470k Ohm load on the preamp output to get flat response. A lower impedance causes rolled-off bass response, higher causes a bass boost. This makes it impossible to use this preamp with modern solid state power amplifiers that typically have from 10k to 50k input impedance.
A second issue was corrected in a later version of this preamp, the PAS-3X. When the tone controls are at half rotation on the “X” they are totally out of the circuit. In the plain PAS-3 they are not. The original circuit has 6dB less headroom at lower frequencies than high. Essentially the last stage of the preamp has a 6dB LF boost and the tone control follows this with a passive 6dB cut. The design makes it very messy to bypass the tone controls.
In the PAS-3X special controls were used; the treble control wiper was totally open at half rotation and at the same setting all 3 terminals of the bass control are shorted together.
I had changed the volume control to an Alps “Blue Velvet” model that has a beautiful feel and great tracking, far superior to the old CTS control that Dyna used. It doesn’t have a “loudness” tap, I never had much use for the loudness function anyway, and that left an unused switch on the front panel.
I used the loudness switch as a tone control bypass switch. It operates 4 “Vactrols” (LED/photocells) to accomplish essentially the same bypass technique as used in the PAS-3X. The bass control has a dual Vactrol added which shorts the 3 terminals together when it’s on and the treble control has a single Vactrol that opens the connection to the wiper when its off. The treble control has a couple of added high value resistors to discharge the tone control capacitors when the Vactrol is open so there isn’t a big “pop” when you enable the controls. The LED’s in the Vactrols are all in series and driven through a resistor from the filament supply and the former loudness switch shorts either the treble pair of Vactrols to bypass or the bass pair to enable the tone controls.
I also added a FET follower circuit to isolate the fussy tone control circuit from the load. The preamp now has a nice, low impedance output and now has no trouble driving 10k loads. A purist probably would want to use a tube but, to tell you the truth, I prefer the FET follower. It has better linearity and doesn’t need a filament supply.



http://www.normankoren.com/Audio/Spice_preamp.html
Actually there are a few more minor tweaks in my preamp beyond what I’ve outlined here so far. I’ll update when I get to it.