The new CCS VTA ST-70/ST-120 driver board (photos)

Hmmmm....now why would he do that on April 1st?????

actually I started selling these new boards in February of 2012 . . . . Bob had leftovers of the older version until April

What is the solid state device used for the CCS? If this is proprietary I apologize for asking, thanks.

Thanks for the clarification Roy.....Of course I was being a little tongue and cheek here.
I have one of the older designed boards and since my ST70 build is still in progress - going
six years now...no kidding - when its finally complete - maybe another year or so I have to remember
the proper orientation for bias adjustment. Otherwise I'll be scratching my head wondering why
would Roy use a pot that work "backwards".

So to add a little bit of info for folks. When designing a long tail phase inverter (LTPI) there is usually a trade off between output swing and output balance. Swing refers to the maximum output amplitude before clipping and the balance is how closely do the 2 output waveforms of the phase inverter match in terms of amplitude. So the ideal balance would be identical in amplitude but exactly 180 degrees out of phase. However in real life there is no perfect balance between the outputs resulting in harmonic distortion in the power amp.

The trade off happens when you want to get more output swing and this causes a greater degree of inbalance in the outputs. This is because the general method of increase output swing is to decrease the tail resistance of the LTPI. To improve balance you can increase the tail resistance and or inbalance the LTPI plate resistors to adjust the gain of each half of the LTPI

the more superior method is to use a CCS as the tail resistance in the LTPI which allows for much better balance and more output swing using as Bob said a local feedback loop in the tail of the LTPI. Hopefully this is clear, and call me out of this is wrong.

yes, all true, and the reason I finally decided to go ahead with the CCS chip, is because it offer superior performance to just using the usual tail resistor which was previously 12K as an optimum compromise. The CCS chip give near infinite impedance, thus offering a higher voltage swing and automatically compensating for the signal and giving perfect balance (amplitude and phase) with zero adjustment needed, and lower distortion than using a simple resistor.

right on. I have done simulations of 12au7 LTPIs using just 1 BJT in a feedback loop with a tail resistor and its a significant improvement in the balance vs swing design standpoint compared to just a tail resistor. I will probably be building 1 of these kits soon, in 2013 or so, and i dig the addition of a CCS.

I can't contribute much to the technical discussion, but I have two VTA ST-70's sitting right next to each other. One has the previous driver board and the other has the new CCS board. So, I'm fortunate to have a chance to test the two driver boards with the same inputs, preamp, loudspeakers, and even moving the same power tubes back and forth. I only wish I had more 12AU7-types to test.

I always liked my first ST-70, but the new driver board is a clear improvement. The combination of lower gain and better phase splitting definitely works! The new board is quieter - at zero attenuation with no signal tube hiss and 60hz hum are absent or nearly nonexistent. I believe there is more clarity (lack of distortion?) especially on very clean vinyl and pure recordings. The new board still gives the same sense of effortless dynamics and has no problem showing off the differences between EL34's, KT66's, and KT88's without making any of them sound bad.

Sorry I can't describe the listening experience in better detail, but consider this a thumbs-up to the CCS driver board after the most controlled comparison I can perform.