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.

 

 

Larger schematic – opens in a new tabIf you’re more ambitious and would like to really do things the right way, check out my friend Norman Koren’s detailed analysis and redesign of this preamp. He made a bunch of real improvements to the original circuit design including his “purist” version which completely removes the tone controls. Sometimes tone controls are nice but they can also get in the way of transparency. This is especially true of the old Dynaco design.

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.