Here's a little teaser pic of what we have been working on for the upcoming Moreplay upgrade. With the current BeePre2 and Moreplay, there's much to be gained by going to a constant current source load on the primary voltage gain triode, as the loading is quite low to begin with and the plate load and volume pot are in parallel with each other and both acting as loads simultaneously. By swapping out the stock plate load resistors with a C4S, one of those loads almost entirely goes away, and the 6V6 operates much more happily. In looking at before and after shots of the distortion spectrum, the second harmonic pretty much stays as-is, but the higher order nasties disappear.
The C4S loads get fed by a solid state shunt regulator that's somewhat similar to what's used in our Integration upgrade for the Seduction. This helps to isolate the power supply from carrying signal current and keeps the power supply noise out of the signal path.
The back half of this large upgrade board houses a low voltage DC supply that feeds two individual 1085 regulators to feed super quiet DC to each 6V6, which reduces the 60Hz noise that was somewhat present in the stock circuit.
Up front I have designed a PC board that holds the standard Alps balance and Alps volume pot. The commonly available parts have slightly different shaft lengths, but by mounting them on a sub plate below the chassis, we are able to negate that issue up top and the knobs fit back on evenly. The Alps balance pot has zero insertion loss compared to the stock balance pot setup, and this further assists the signal to noise ratio of the preamp. There is also a center detent, so there's no need to fiddle with adjusting the knob up top to get it pointed just right for even channel balance. The Alps volume pot is very well regarded and offers a nice improvement in channel balance and clarity.
Ultimately when collecting data and running the numbers on how to make our typical coarse/fine attenuator work with this specific circuit, the Alps pot ended up making a whole lot more sense compared to the tradeoffs we were looking at otherwise.
