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Simplistic Mosfet HV Shunt Regs
Simplistic Mosfet HV Shunt Regs
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Old 30th July 2009, 12:38 PM   #391
Alex M is online now Alex M  Europe
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I have only recently started reading this thread, and I fancy trying this regulator with my valve active crossover, which needs 300V at around 80mA.

I have been trying to work out how the circuit works, and there is one aspect that I don't quite understand. The voltage across the base-emitter junction of the high-voltage PNP transistor Q2 is equal to the base-emitter voltage of Q4 plus the total voltage across Q5. In other words, either Vbe of Q2 is going to be a lot higher than 0.6V, which I would expect to damage the transistor, or there is almost no voltage across Q5, which can't be the case, since it is carrying something like 4mA. What is going on?

Alex

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Old 30th July 2009, 01:08 PM   #392
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Alex M
I fancy trying this regulator with my valve active crossover, which needs 300V at around 80mA
that's a consumption of 24W.
The regulator pass and shunt may have to dissipate a similar power.
Some are saying the shunt reg sounds better if it dissipates more than the load.
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Old 30th July 2009, 01:35 PM   #393
Alex M is online now Alex M  Europe
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Quote:
Originally posted by AndrewT
that's a consumption of 24W.
The regulator pass and shunt may have to dissipate a similar power.
Some are saying the shunt reg sounds better if it dissipates more than the load.
It seems Allen Wright used to believe that too, but says he has changed his mind recently - he now recommends about 20% of load current with his SuperReg. It also, of course, depends on the stability of the AC mains voltage.

If the shunt current is 20% of the load, and the input voltage excess is 20% of the regulated output voltage, then the shunt MOSFET will dissipate 20% of the dissipation in the load, or 4.8W, which is manageable, as is the 5.8W in the CCS MOSFET.

Alex
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Old 30th July 2009, 03:51 PM   #394
AndrewT is offline AndrewT  Scotland
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I thought Allen Wright's SuperReg was a series reg not a shunt.
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Old 30th July 2009, 03:53 PM   #395
Alex M is online now Alex M  Europe
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Quote:
Originally posted by AndrewT
I thought Allen Wright's SuperReg was a series reg not a shunt.

Not so. It is a constant current circuit feeding an AD797-based MOSFET shunt. Allen is a firm believer in the superiority of shunt regulators.

Alex
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Old 30th July 2009, 04:01 PM   #396
Salas is offline Salas  Greece
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Simplistic Mosfet HV Shunt Regs
Q2 has Vout-Iq2R4 across it and Vout-Vbe on its base. You can use MPSA94 for it also, and you can keep the shunt NMosfet as hotter on top the load consumption as you feel, only allow for dynamics. The more, the better.
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Old 30th July 2009, 07:42 PM   #397
gallon is offline gallon  United States
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Default see The Tube Preamp Cookbook

Quote:
Originally posted by AndrewT
I thought Allen Wright's SuperReg was a series reg not a shunt.

Allen covers this in some depth in his 'The Tube Preamp Cookbook'
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Old 31st July 2009, 03:23 AM   #398
jameshillj is offline jameshillj  Australia
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Hi Andrew, Allex,

I wonder if a 20% of load thru the shunt would work for my F3 amp - it consumed about 1.7amps at 42 volts, so would end up with about 2amps thru the reg with a CCS of about 10W and the Shunt about 14W (42 x 20% of 1.7) for a total of 24W.
I like the idea but a shunt consumption of at least 1.7 amps@42volts is quite impractical, so put it aside.
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Old 31st July 2009, 03:33 AM   #399
Salas is offline Salas  Greece
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Simplistic Mosfet HV Shunt Regs
You must allow for peak + 20%. Is your amp using 1.7A at peak when the oscope shows the onset of flattening on a max power sinewave?

Then yes, you can use a shunt at 2A and be OK, with 0.3AX42V constant excess on the second Mosfet. Class A amps are the best customers for constant current shunts, since they have the big sinks and burn the most juice there for bias all the time. Just don't try a test without 1.7A load with the 22.6W CCS & Shunt dissipation in mind.
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Old 31st July 2009, 05:56 AM   #400
jameshillj is offline jameshillj  Australia
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Thanks Salas,
Got a "bee in the bonnet" about having more current thru the shunt than load - I remember we talked about it awhile ago.

However, I just might bang a bunch of bits together on some of that perf board and crank it up and see - I think the F3 will be transformed with it, if it works okay and the heatsinks should accept another 20+ watts (the standard 80 Watts = 22*C rise).

I have done just the one pcb and it's checked out A-okay, no problems, except still a bit too small for me (will do a much bigger design, next time) - don't know why I added all those options, as few enquiries - I had hoped someone over your side of the globe would copy it into "Eagle" and make it more user friendly. I haven't got the hang of that one at all yet.

I might just try out Iko's new V1.5 with just standard bc transistors and see what happens on the DAO headamp supplies - the current V1 with it's heavy shunt current (250mA load, 300ma shunt) is working flawlessly - no fatigue at all, yet incredible dynamics and detail plus effortless bass control with a clearly defined sense of rythmn - my sort of good sound, and not such any easy thing to manage on any equiptment.
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