If I put my notes here, I might be able to find them again later!
Voltage Regulators for Line Level Audio. Part 11 : The Crystal M Shunt
A while back I did a series of blogs on voltage regulators. Back with a new entry today: The Crystal M, configured here for 40 V DC output and a 25 mA load.
The circuit is based on two p-channel MOSFETs, the top one is a constant current source, the bottom one a constant voltage source. As the load current changes, the voltage source adjusts its current to balance.
It's lifted directly on the Salas shunt design (as reworked by me for my own jFET phono stage), but the circuit can also be considered a distant, DC-coupled relative of the Zen amp.
I trick, I discovered, to getting it to work nicely - the attached screencap shows it well-behaved while handling a full-swing output current pulse - is the source resistor R10. This resistance dials-down the current gain of the MOSFET, damping out the overshoot.
The ripple rejection is about 70 dB over the audio bandwidth. The output impedance is about 0.05 ohms over the same frequency range.
One thing about shunt regulators: they run hottest when there is no load attached. So always calculate the power dissipation under this condition, and be careful not to leave the supply powered up but disconnected to the load: it uses just as much electricity this way as when the load is connected.
The circuit is based on two p-channel MOSFETs, the top one is a constant current source, the bottom one a constant voltage source. As the load current changes, the voltage source adjusts its current to balance.
It's lifted directly on the Salas shunt design (as reworked by me for my own jFET phono stage), but the circuit can also be considered a distant, DC-coupled relative of the Zen amp.
I trick, I discovered, to getting it to work nicely - the attached screencap shows it well-behaved while handling a full-swing output current pulse - is the source resistor R10. This resistance dials-down the current gain of the MOSFET, damping out the overshoot.
The ripple rejection is about 70 dB over the audio bandwidth. The output impedance is about 0.05 ohms over the same frequency range.
One thing about shunt regulators: they run hottest when there is no load attached. So always calculate the power dissipation under this condition, and be careful not to leave the supply powered up but disconnected to the load: it uses just as much electricity this way as when the load is connected.
Total Comments 2
Comments
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rjm:
won't q1 suffer a bit of start-up grief caused by that 22uF?
mlloyd1Posted 9th March 2016 at 04:22 AM by mlloyd1
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There is a spike for about 2-3 charging cycles where the base is brought 2-3 V above the emitter. In simulation 20 mA are drawn through the base for about 4 ms.
This is damped to insignificance when there is a capacitance on the output, which there normally is, so that the charging time of the output capacitor becomes longer than the charging time if C1.
Okay... BD135~ datasheet says abs. max base current is 500 mA, max base voltage 5 V. (I ended up using that rather than the BC546 shown, but that is anyway also 5 V.) How worried should I be?Posted 10th March 2016 at 09:38 AM by rjm
Updated 10th March 2016 at 09:44 AM by rjm
