Finally finished the gain-reduction and hi-pass-filtering work, which took lots longer than I initially imagined.
Replaced the gain-reduction pots in the LF amps with single resistors in series, which reduced their Voltage gain by 19dB (to c. 11dB), and then selected a resistor to feed the UF amps' pots to match that Voltage gain. Now the pots work beautifully, allowing me to reduce the upper-frequency energy by a dB or 2 quickly and repeatedly. The hi-pass-filter-cap value eluded me until I determined that the value of the series resistor
preceding the cap affected the filter point just as the following load resistor did. A pair of old 4700pF Siemens StyroFlexes created a filterpoint right where I wanted it...just over 60Hz.
The pic shows the StyroFlex in and the pot no longer in the LF amp, and in the HF amp, the new BlackGate decoupler on the front of the board and the tops of the final SoniCap Platinum output-stage-powersupply bypasses, 3 Teflon-film caps totaling 1.503uF in each pole.
I'm thrilled to be able to say that my system has NEVER sounded better--more transparent AND smooth*. An SACD I love, of string suites played by the Rotterdam Chamber O--TELARC, # SACD-60623--contains the Elgar Serenade for Strings. The first time I heard this disc, with the improved Monarchy SE-160s, I thought it sounded great. The first time I heard it with the Atma-Sphere M-60s driving the 5As, my jaw dropped open at how smooth AND transparent this recorded music sounded, but only in several short sections at low levels. Since then, the system with these MA-24s has sounded at least that good. This week, the
majority of the 10-minute work sounded that jaw-droppingly transparent and smooth. I think I'm on to something great.
Finally I can see me actually completing the improvements to these amps.
Have ordered new, all-Neotec-UPOCC-silver-conductor speakercable for the upper frequencies.
* That is, NOT edgy or grainy.