Extremely tight packing, it was a challenge! The new / additional PSUs are at bottom left, DAC step up transformers centre top and tube buffers / constant current sources at top right. The DAC chip is a BB DSD1791 which has differential outputs at 1VRMS. I took the outputs from the DAC directly to Lundahl LL1674 amorphous core step-up transformers, configured as 1:2+2 (visible in the picture just above the CD drive). To avoid the use of output capacitors, I decided to use an XLR output configuration (balanced). The voltage gain is due to the transformers; the output from each transformer is buffered using a pair of cathode followers. The negative end of each pair of cathode resistors is fed by a (depletion mode) mosfet cascode constant current sink (CCS) akin to a differential amplifier. The mosfet cascode CCS has an extremely good DC stability and high impedance that is very flat across and beyond the audio passband. (Check out the test results I obtained when developing a CCS for the 6AS7G PP amp.) (Akin to a differential amplifier, the CCS forces signal current balance in each phase. However, unlike a differential amplifier where the output signal voltage balance is controlled by the matching of the load resistors; in this case the cathode coupling is via the cathode resistors (64.9k each) and so the stage must be fed with balanced signals.) The effect of the extremely high impedance of the CCS cathode feed is that no signal current flows in the B+ supply loop with dramatic consequences for the sound which is incredibly articulate and free from blurring. I allowed the plate supply to float and referenced the cathodes to ground using matched resistors. All four cathodes are within a few mV of ground. I installed a linear DC regulated 12V supply for the heaters (bottom left in picture). This includes 15V regulator which feeds the existing 12V regulator in the switched PSU board, the switcher being disconnected. This (now linear) supply feeds the DACs and existing output op-amps. A separate plate supply for each channel is sourced from twin secondaries on a back-to-front transformer connected to the secondary of the heater transformer (visible just to the right of the heater supply at bottom left in the picture.). The tube sockets are mounted over a blank section of board, using spacers from the XLR socket mounting screws. A bit fiddly but it all fits together very nicely. The sound of the unit completely belies the starting price of $150!
To the left are the (linear) replacement PSUs for the DAC and tube heaters - to the right is the dual B+ supply for the tube transformer output buffers.
Lundahl LL1674 signal transformers (from K&K Audio) with the tube output buffers mounted on dual XLR sockets.
Top right, DN2540 constant current sources (from K&K audio).
Ventilation holes over the linear PSUs and tubes. The unit runs very cool. I finished the cover using self adhesive vinyl tile to damp the cover.
Self adhesive vinyl tile used to reduce the 'bonginess' of the chassis. The whole assembly is now quite dead (mechanically that is...).
Blue Tack (audiophile grade blue snot) used to damp the disc clamp/bearing. Blue tack came onto the market in Britain years ago for sticking posters on the wall. It was very inexpensive. When audiophiles started using it, the price mysteriously rose....
Click to see the schematic: