Proprius Amp tweaking......

goldiver wrote: I'm still wondering if there's a technical reason for the difference in sound. Is it simply that employing the Proprius pot meters, as opposed to leaving them on full, adds an extra circuit (resistors) to the mix?  Or is the 'current' to the speakers otherwise affected?

Yes, there is a technical reason, but just leave it at the position it's now set at for four or five days and let me know how it sounds then.

Forgive me for not "techsplaining" too much (although I will) but such subtleties are more involved than you'd think.

There is no "extra circuit" just the pot, and although I go to every length I can to make the sound the same at every tiny increment, the "mathematics" change when a variable is introduced -- that variable being the use of a potentiometer, period.

For example, during development of the Majestic the sound changed as the volume control pot was put into different positions. It was nothing to do with the pot, or the following stage, but because of the output stage of the AD712 op-amps that had been selected at the time. Although being technically brilliant, the emitter-follower output stage, like all EF's isn't perfectly linear, and the increasing load as the volume was turned up, obviously had some effect. It was replaced by an AD823 op-amp, which has a complementary feedback pair output stage (CFP), which is more linear versus output level, and no difference in the sound was noticed.

On the Proprius, the voltage amplifier section uses a traditional feedback pair, and its input transistor "sees" any input impedance as being part of its base resistance. Due to its hFE (its current gain) that base resistance appears as part of its emitter resistance, and that subtly changes the gain of the amplifier.

So, by using the pot to change the gain setting to reduce it, it has a very subtle effect on the "fixed" gain of the amplifier as well.

Any change to the fixed gain alters the bandwidth very slightly, and in your case, has led to a noticeable difference using your speakers. All of this being perfectly normal and it hasn't changed the laws of physics (cap'n).

So, what you do, is set the variables (the pots on either item) to a position where it's just right for you.

(to understand more about amplifiers, buy my book, that I'm still writing...) 

Has anyone measured the the Peak or Transient output of the Proprius? I read Bruce's (BAK) comments that into 8ohm load this could be approx. 50 to 75 W resp. It just seems that the Proprius design with it's 25 (Yorkshire ) W has plenty of capacity to handle the most onerous speaker loads and music demands, so I'd be interested to understand more.

It's an academic  question for my listening, where in a small room with nearfield positioning I can't be using 1W output for my little Harbeths to be singing.