The circuit of both boards is basically the same with a few minor differences ...
1. The resistor set is different and the board is now set up for the lower gain 12AU7 or 12BH7 driver tubes instead of 12AT7 medium gain driver tubes. The newer board has somewhat lower gain than the older board .. This is a good thing especially if you have very efficient (93 dB or higher) speakers.
2. The phase splitter now uses "CCS" a Constant Current Source in the tail of the phase splitter. In the older board there was an AC balance control for each channel which the user would set in the center of its rotation. Set this way and using a matched pair of output tubes, the AC balance was near optimum. The newer board now uses a small integrated circuit which provides a local feedback loop that feeds an error correction signal to the tail of the phase splitter. The signal fed is inverted so any deviation is cancelled. This type of phase splitter is more accurate in maintaining AC balance on the phase splitter section of the driver board. This CCS phase splitter was created by (the late) Fred Nachbaur in 2002. A more technical explanation by Fred is here > CCS phase splitter - Fred Nachbaur
3. In the older board the four bias pots worked "counterintuitive". Turning the pots to the right or clockwise REDUCED the bias voltage and turning the bias pots to the left or counterclockwise INCREASED the bias voltage. The newer board has the the bias pots set up so that clockwise rotation increases bias voltage and counterclockwise rotation decreases bias voltage.
4. On the new board two filament jumpers are now used to send the 6.3 volt AC filament voltage from V3 to V2 to provide filament voltage for the center voltage amplifier tube. Also, the driver tubes are now designated V1, V2 and V3 from left to right which is slightly different from the older board.
As a side note I have found this board to be very flexible in regards to what LOWER GAIN driver tubes may be used. I have used 12AU7, 12BH7, 5963, 5814 and 6189 driver tubes and found that they ALL work well in this newest iteration of the VTA driver board.
Older VTA driver board
Newer VTA driver board
Last edited by Bob Latino on Thu Oct 25, 2012 6:03 pm; edited 1 time in total