aleph 30 project

[dslsa666]

Zen Mod, Bill, thanks for taking time to help with this!

Cheers,

Boris

[fulvio]

Hi, I’ve finished to build an Aleph 30 some days ago; the main differences respect the original Pass schematic are the power devices I used (IRFP350 instead of IRFP240) and the source resistance of the power devices (0.6 ohm instead 0.47).
The power supply is 24.0 volts, and the biasing is 1.5 A (0.5 per mosfet); AC current gain is near 50%.
I experienced the same behavior of dslsa666 Aleph, that is a clean clipping at about 16 Vrms at 1 KHz, but a bad waveform distortion at higher frequency, beginning over 13.5 Vrms at 10 KHz.
The distorted waveform is exactly the same as the ones in the dslsa666 post (a notch just before the top of the positive peak), so I don’t attach any picture.
I used a 8 ohm load but the I noticed that the distortion remains the same also without load (output open circuit), so I don’t believe it is a current problem; in any case I tried to increase the bias and modify the AC current gain, but without a real improving in the waveform.
The only change that seems “positive” is to increase the capacitor across the feedback resistor (C104 in the 1994 Nelson Pass drawing of Aleph 30, initial value 10 pF) but this produces a roll-off in the frequency response; more over this roll-off can reduce CMRR when the amp is used with balanced input.
Maybe the notch in the waveform is the beginning of a high frequency parassitic oscillation?
With a 43 pF (10 + 33) capacitance value, the loss at 20 KHz is about 1 dB; I measured the following THD values:

C104= 10 pF

1 KHz 15Vrms THD = -48 dB
14Vrms THD = -54 dB
13Vrms THD = -56 dB

10 KHz 15Vrms THD = too high
14Vrms THD = -36 dB
13Vrms THD = - 44 dB

C104= 43 pF

1 KHz 15Vrms THD = - 47 dB
14Vrms THD = - 52 dB
13Vrms THD = - 55 dB

10 KHz 15Vrms THD = too high
14Vrms THD = - 41 dB
13Vrms THD = - 47 dB

Please note that THD is measured in the band 0 – 600 KHZ so it could be higher than THD in audible range 20Hz - 20 KHz, especially for 10 KHz fundamental.
In the next days I’ll do more measurements, picking up the single harmonic level.
A funny thing: making distortion measurement, I noticed that THD value sometimes showed fluctuations of 3 or more dB; at the end I realized that it was a consequence of the mains voltage fluctuations that, in turn, caused fluctuations of the DC power supply voltage: when you work near clipping, also little variations of supply voltage are significant.