Friday, January 8, 2010

Revisiting the 5687 Preamp

This preamp was working well since it was built. There were a couple of mods done since then, namely, the Feedforward Shunt Regulator, and also the DACT 50K Stepped Attenuators. I was re-looking the design especially from the perspective of my entire chain, and thought I should try out a different operating point.

What I had in mind was to do a -2v bias. This meant removing one LED from each section of the 5687 and also adjusting the B+ voltage. I also wanted to try John Broskie's PS-2 solo design. I intend to reuse the 10M45S on the Feedforward Shunt Regulator and also the heatsink for this modification.

I went to Koba and got some 1% 10R and 1R resistors, Wima and Rifa capacitors, a pair of LT1085, and a pair of BI Technologies 7276 - 10 turn 50k potentiometer to be used to set the output voltage. I also threw in my Mundorf 47+47uF capacitor and bought a pair of 240R Kiwame resistors from AHFartaudio for the PS-2. All these goodies would make any power supply green with envy. ;)

The PS-2 solo worked like a charm upon power up. With a 10 turn potentiometer, it was very easy to dial in the voltage on the plate of the 5687. With this potentiometer, the working output voltage range would be from 50vdc to 250vdc. Next steps are to mount the regulator into the chassis, along with some minor wiring.

You probably want to get John's PCB for the PS-2 instead of hardwiring.

More pics here.

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