A bit of further progress today as managed to build two Salas Shunt HV regulators (SSSHV2) with the slight tweaks I tried recently. Both regulators will provide a very stable voltage reference (+280V) for stacked supplies. Now time for completing the wiring of the 46 drivers and do some further testing…
Thought it was going to be an easy task as I’ve done it before many times and building a Shunt regulator seems to be not the challenging part of this amplifier build. We all know that life brings surprises and specially when we are not expecting them. My 4-65a SE amplifier requires a very stable DC as part of the DC-coupling design. The Salas Shunt Regulator version 2 (a.k.a. SSHV2) is a good choice for this task.
After building it very quickly I struggled to get it to work. To cut a long story short which involved some IRF840, PNP and JFET replacements, I discovered that the stabilising RC wasn’t connected as the 330nF MKP capacitor was not properly soldered to the right holes. The PCB has multiple holes to accomodate capacitor sizes, however only the top two correspond to one capacitor pin and the remaining bottom ones are for the other. My logic of placing the capacitor in the centre clearly didn’t work and the capacitor was disconnected in the end. Finally, when hooking the regulator to the raw supply and switching it on, the whole thing produced the unwanted smoke particular of sand devices getting blasted. What happened? The maximum input voltage to the regulator evidently exceeded the CCS voltage and the top FET (M1) blowed away and therefore the regulator cascode CCS (J1) and the pass FET (M3) as well. My PCB was already suffering from multiple solder work and was reaching to its usable life. I looked at using HV parts as hand to increase the robustness of the regulator. The pass-FET was replaced by a 1kV part (STE5NK100Z) and the Mosfet CCS DN2540 pair for an IXTP01N100D which is also 1kV part:
All worked well until I realised that the differences between DN2540 and 01N100D’s VGS(th) and gm made the CCS maximum to be limited to about 40mA given the test point resistor value. As M2 can be a simple DN2540, I replaced it back and all worked well to get 60mA and deliver about 280V @ 40mA rock-solid!
It took me probably half an hour to build this brilliant shunt regulator. I waited for a long time until picked up this board and stuffed all components. I will use it in my next generation of DHT preamps.
Output ripple is below 5mV, can’t even measure it given the noise in my workbench. The test gig included my variable power supply (600V) feeding the shunt regulator which was set for 40mA. output load is three 3K3 power resistor clads (50W each).
I adjusted the regulator to provide about 160V, so current is about 16mA. Interesting to seer the harmonic profile to have a higher peak at 200Hz compared to the 100Hz harmonic….