I have now pulled some numbers from the circuit. And I took more pretty pics, which I‘ll start off with:
Now for some numbers:
PSU voltages:
V_HV_pos=402V, V_HV_neg=-403V
V_LV_pos=14.88V, V_LV_neg=14.88V
Current sources:
Right channel voltage drop over the RN60C 49R9:
V_ccs_pos=857mV, I_ccs_pos=17.17mA
V_ccs_neg=856mV, I_ccs_pos=17.15mA
Left channel voltage drop over the RN60C 49R9:
V_ccs_pos=862mV, I_ccs_pos=17.28mA (adjusted to 857mV/17.17mA)
V_ccs_neg=856mV, I_ccs_pos=17.15mA
On the left side the initial B+=20V current source adjustment was the following:
V_ccs_pos=845mV, I_ccs_pos=16.93mA
V_ccs_neg=847mV, I_ccs_pos=16.97mA
So going from a B+ of 20V to full a 402V led to an increase in current of a meager 1%.
Next up is gain, THD and clipping. Input is a single ended 0.1V at 1kHz from the Boonton 1121. The – input of the carbon is grounded through a 220R resistor (equivalent the output impedance of my preamp). The output is measured unloaded.
L+: V_out_rms=42.5V, gain=52.56dB, THD=0.3%
L-: V_out_rms=46.0V, gain=53.26dB, THD=0.095%
R+: V_out_rms=41.9V, gain=52.54dB, THD=0.38%
R-: V_out_rms=46.6V, gain=53.37dB, THD=0.09%
The THD is higher than I would have expected (the Boonton‘s baseline is at 0.00085% btw). It‘s mostly 2nd harmonic though.
Here’s how I took the gain and THD readings:
And here’s what the THD spectrum looks like (notched out fundamental at 1kHz, peaks at 2kHz and 3kHz, and maybe even 4kHz):
Here we see the output just before clipping:
CH2 in purple is the input signal to the amp, 1kHz at like 1.5Vpp (518mVrms):
CH1 in yellow is the output signal. 764Vpp. The probe is rated for 600Vpp at its 10x setting, so that was stretching it a bit…
And here we see clipping with an input signal of 1.9Vpp (668mVrms):
I also took temperature readings after the amp has been running for 2hrs or so. Ambient is at 24°C.
Right behind where the SiC Fets are mounted, the heatsink temperature is 45°C (113°F). The corner the farthest away from where the bracket connects to the heatsink is as 41°C, so the heatsink gets utilized reasonably well. Inside the case it read 43°C.
I realize that the brackets are not positioned ideally on the heatsink (too far up), which is why I have used chunky brackets, thinking that the thick material would conduct heat nicely. Seems to work well enough.
Seeing the 21°C rise in temp makes me think I could try upping the current. Calling it a furnace was overstating it a bit perhaps, but the whole enclosure gives off an impressive amount of heat already.
I also employed the opto servo btw, going up to 17.5V positive offset before putting in the jumper. The amp now comes on with like 1V offset, which goes down to a few mV after a couple of minutes. I don’t think I could hear any sonic difference between going raw vs. opto servo.
So, two questions remain:
Why is THD so high? I would have expected a lot less. Is it the frontend perhaps?
And what's up with the gain imbalance? It’s consistent between polarities (single ended input, - input grounded). Could that be the cause?