I have updated my Mac 1900 receiver output section with no input cap and resistor, higher bias of 200ma, lower feedback, reduce or eliminated the freq compensating caps at the trasistors, and paralleling a few Green Sanyo electrolytic cap for the feedback loop cut off cap. The sound is really very satisfying; compare to my Classe 700, 6V6 SE, and a dozen other amps. It already sounds decent before the mod, with the bias at ZERO ma. The topology is a simple pnp diff input, follows by npn which collector drives npn/pnp emitter follower, which in turn drives np pair. The only special feature is that the npn driver has 110V B+ with 6.8k as the load, and the diff pair has the emiiter connected to a 30k and 110v B+. My experiences with MC2600, Mac1700 all proved that pure class B can sound good.
I have also built a small JLH amp with 2N3054 type NPN outputs at 14V 1A power supply. While this amp has beautiful sound, it could not sound good until fully bias, ie, around 1 A.
So I just wonder, what make the Mac sound so good at low bias?
I am not really a believer in differential input, after a long unsatisfying experience with many Mark Levinson, Jeff Rowland amp, but I must say the Mac amp is really good. I hope to try bypassing the Q403, the negative side of the diff input transistor, by adding a cap between the B and the E, to hear the different. Wonder any one tried it?
Attached diagram is for Mac 6100 which is the same as 1900
Best rgds
I have also built a small JLH amp with 2N3054 type NPN outputs at 14V 1A power supply. While this amp has beautiful sound, it could not sound good until fully bias, ie, around 1 A.
So I just wonder, what make the Mac sound so good at low bias?
I am not really a believer in differential input, after a long unsatisfying experience with many Mark Levinson, Jeff Rowland amp, but I must say the Mac amp is really good. I hope to try bypassing the Q403, the negative side of the diff input transistor, by adding a cap between the B and the E, to hear the different. Wonder any one tried it?
Attached diagram is for Mac 6100 which is the same as 1900
Best rgds