Last fall I decided to try some improvements to the SCHA I was building. My phones are highly modified AKG K340, dynamic driver paired w electrostatic element for the tweeter. These phones have a reputation of being hard to drive; hence the mods.
1. Use the LME 49860NA opamp from Texas Instruments; (This replaces the metal can version & requires no heat sinking).
2. Use a power transformer that is rated at least 20 - 20 vac @ 400ma.
3. Set the regulators for 20vdc + & -.
4. Change the 4.7 ohm 1/2 watt emitter resistors to 3.9 ohms 1/2 watts
5. Aside from the above mods, build the board using Sandy K's modifications.
P.S. For R.G. members that want to do another build or a first build of the SCHA, I can supply PCB's foe both the amplifier & the combined regulated supply/ripple eater circuits.
NOTE: I do not have a price on the boards as of this date but will post it here in the next week.
I have been running the SCHA wth 3.9 ohm 1/2 watt emitter resistors in place of the 4.7 ohm units originally specified on the output transistors with no problems, for the last 3 months. This seems to make the output slightly more robust.
Being such an easy mod to do I'm interested in what you think the improvement is here.