Rodeodave

According to their website, part nr. 5452258 has the following electrical parameters: Nominal current 13.5 AN Nom voltage 400 V Rated voltage (III/3) 250 V Rated voltage (III/2) 400 V Rated voltage (II/2) 630 V Rated surge voltage (III/3) 4 kV Rated surge voltage (III/2) 4 kV Rated surge voltage (II/2) 4 kV Edit: It looks like part nr. 1729131 has the same properties listed.

With differential measurement - which better corresponds to what the membrane sees if I'm not mistaken - there should be some cancellation, right. Guess I'll be breaking out the analyzer over the holidays again, currently too busy listening when time permits

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?

Input and output wiring in place: And I'm happy to report that the amp is working! No smoke, no arching, no shrapnel. After like an hour of warming up and fiddling with offset and balance, I gave it a listen with my 007mk1. Wow. I'm treating pin 1 as analogue ground btw, and circuit ground goes to the chassis earth ground point via a CL60 thermistor. Zero noise, zero hum - with my gear at least. Bias is still around 17mA for all channels, I'll get some more accurate numbers when I set up the optocoupler servo. Not sure if I'll go up to 20mA, this amp is a furnace at 17mA already.

Thanks for the encouragement, y'all. Getting closer now:

HV and LV wiring is now in place: Next up is input and output wiring.

Progress is slow during the week, but thanks to home office I can get some soldering done over the lunch break. For connecting the HV and LV PSUs and the amp boards I'm going with a star ground, held in place by wire wrap and soaked with solder: Here's how the manifold looks like in circuit: There's good clearance too: Although the currents are substantially lower than what I'm used to from solid state, I took care to have equal lengths of fat wire for each of the HV and each of the LV supplies. I hope the grounding scheme is going to be quiet, I passionately hate ground related hum.

Can you post a pic of the whole PSU so we can see cap and diode orientation and voltage references and so on?

Goop: Bolted down: Fits like a glove: Good clearance: And the full monty:

The amplifier boards are now fully populated, including the front end Jfets, for which I picked pairs of 2sk170bl from the same batch with an Idss of 7.3 and 7.4mA. Using a benchtop PSU supplying 20.00V DC I then set the constant current sources to 17.00mA: It's going to be interesting to see how much this will go up with the full B+.

Okay, moving on. Here's how I wired up the transformer: And here we see the unloaded secondary voltages: Here the PSU boards are mounted to the front plate for heatsinking. The power resistors (500R and 16k) will be used to load the PSU for testing: And finally, the moment of truth: One more view from above: So far, so good.

If I recall correctly, they are Molex Eurostyle Terminal Blocks UL certified for 300VAC, which would mean 425V DC peak. I'm not sure if there are terminal blocks rated for 600V with a 5.08mm pitch. Since I'm using the amplifier boards with a ground plane, i used a couple of layers of Kapton tape at the bottom of the terminals and used very little solder in order to avoid the arching that's been reported with these boards. Edit: I just realized that the terminal blocks aren't the Molex that I thought I had installed. I guess I should replace them, which is not going to be easy 😕 Mouser has some phoenix 5.08mm pitch terminals rated for 400V. What would be a good alternative to the screw terminals? I'm not their biggest fan either.

Here are some more build pics:

Thanks, I'll be adding 10k resistors at the inputs then. I'm making good progress with my build after it lay dormant for over a year:

Want impedance volume pot is everyone using? And those of you who aren't using a volume pot in their amp, are you terminating your inputs to ground with a resistors other than the 500k on the circuit board?