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Revised shunt power supply for SRX Plus

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Sorry to start a new topic on this, but I thought if I posted it on a previous thread it would get lost. The issue with the original power supply was that it was noisy at times. 

I had originally measured < 2 mV power supply noise, and 3 - 5 mV noise at the amp outputs. But on re-measuring, the power supply noise was > 5 mV with the amp output noise measuring around 50 mV. In fact, the noise measured before and after the regulator was similar. Thinking the regulator was noisy, it was rebuilt with new devices, without improvement. 


Since the current source (CCS) in the positive supply line should block noise from the raw positive supply, the likely remaining point of entry was the negative line.  Adding a CCS there did the trick - noise dropped ten-fold. However, with a CCS in both lines, the output resistors were unable to keep the B+ and B- voltages balanced – the voltages shifted towards one polarity, until the voltage across its CCS dropped to less than 5 volts, compromising its isolation and resulting in rising noise. So, the single shunt directly connecting B+ and B- had to be discarded in favor of separate regulators for each polarity, to control B+ and B- and maintain isolation.


Each regulator is similar to the original circuit, using an SPX431A (the A suffix has a tighter 0.5% tolerance) IC regulator to set the DC voltage, with associated capacitors to stabilize its internal IC, reduce its output noise, and restrict its function to DC control. The shunt MOSFETs counteract AC variations and noise by compare B+ and ground (positive regulator), or ground and B- (negative regulator) via capacitors connected to gate and source. I used 23N80s, but 19NM50s should also work. Positive and negative CCS should be adjusted to pass the same current. The schematic is posted below. It is still simple enough to be built point-to-point.


With the revised supply schematic, voltage initially rose above the final value, then settled to steady state in 5-10 seconds, demonstrating the slow response of the 431 to AC perturbations, and was stable to within 0.2 volts (0.03%) thereafter, with essentially no warm-up drift. With a Fluke 189 meter, unweighted noise measured between B+ and B- was generally < 0.5 mV rms, with occasional spikes up to 1 mV rms. Unweighted amplifer output noise was <3 - 5mV rms. It is stable and relatively low noise, but not particularly low impedance. However, due to the constant load of the amplifier, with its differential design and multiple current sources, the latter is less important.



1)            The 10 ohm test point resistors for both CCS should be 0.1% tolerance to allow the current sources to be set as closely as possible to the same current.  When set to 0.43 volts (43 mA) across the test resistors, the shunt current will be around 3 mA.


2)            The shunt supply will burn about 5-10 watts while the amp is running.  However, with solid state rectifiers, the shunt supply will burn 35-40 watts at turn on for about 10-20 seconds, until the tubed amp circuit starts drawing power. It is ESSENTIAL that the shunt MOSFETs have sufficient heatsinking to absorb 35-40 watts at least for a short time, otherwise sooner or later they WILL BLOW UP. 


3)            The 100k output resistors and 200 ohm shunt resistors at least 1 watt rating, gate resistors ¼ watt, all other resistors ½ watt.  I used 0.22 μf/400V Cornell Dubilier polypropylene caps and 22 μf /630V Solen polypropylene caps for my build. 


4)            The resistor chain values are shown for +/-350 volts nominal. To change to other voltages, you can alter the value of the 1.8 kilohm resistors connected between the SPX431 reference and anode terminals. To calculate the resistor R for voltage V, use: 

V = 2.5*(248k + R)/R, 

or R = 248k/[(V/2.5) – 1]




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Great to hear you did not give up on the shunt PSU!!  I have a SRX build that I want to use this PSU for, and I hope to build it point to point in the coming few months.  Thanks, Jim!!  And still loving your SRX Plus design using the current BHUltraMini PSU.  



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