Tuesday, November 18, 2008

Upgrading a Dynaco PAS tube preamplifier

I was searching for a cheap preamplifier to use with my recently bought McIntosh 250 amplifier when I stumbled upon a nice Dynaco PAS-3X on Ebay. I've owned a few of these back in the day and even though they aren't the ultimate in definition, they certainly are pleasant to listen to. With Tube rectification, simple circuitry and decent reliability the PAS has been used throughout countless systems. In fact the very first tube preamp I've ever owned was a PAS. Since this is for a second system that is used by the rest of my family, I decided to go for it based on the ease of use and AC switching for the McIntosh 250. The upgrade I did was simple and I did not go the route of the complete rebuild (ala Curcio).

First thing to do is to replace the filament diodes and power supply electrolytics. Triode Electronics carries an SDS board that does both.
SDS Board

I then bypassed the tone controls using instructions on the Curcio website
Tone Control Bypass

While I was in there I also removed the loudness switch. Basically removing the connection from the tap on the volume control to the loudness switch. The volume control seemed to track well between channels so I did not replace it (yet).

After that it was time for a capacitor upgrade. In the line section I replace the .02uF caps with some .022uF Russian K40s I had on hand. Just about everything else left in the signal path I replaced with Obbligatos from DIY Hifi Supply. I've found the Obbligato to be the best 'bang-for-the-buck' budget capacitor out there. Not the best of breed, but its sins are few compared to most metalized capacitors.

I plugged everything in and everything worked without any smoke or sparks. Always a good sign. This particular unit came stock with three Telefunken and one GE 12AX7s. Since I mostly listen to CD, I opted to keep the Telefunkens in the linestage while using a pair of Shuguang 12AX7C9s for the phono. Sound in my second system is not ultra-fi by any stretch of the imagination, but the Dynaco PAS has proven to be a good combination with the McIntosh 250 amplifier.

The modifications proved to be a success - especially the tone control bypass and capacitor upgrade. The unit now sounds cleaner, less grainey and with a subjective wider frequency response. The 'magic' of the tubes is still there providing a natural midrange and vocals.