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Old 30th December 2006, 09:59 PM   #1
lndm is offline lndm  Australia
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Default Calculating shunt reg source impedance

Is it correct to put rail voltage delta over current delta?

I have filtered the input so the supply rail is steady (420V) with the amp idle. Whilst running max clean output (20Hz sine) through the amp the rail fluctuates 5mVp-p. I have lifted ground with a 1 ohm resistor for sensing current and the amp draws (30mA) with fluctuations of 200uAp-p.

This equates to 25 ohms, or at 20Hz, a capacitor of 300uF

The circuit I am simulating is a cascode and a pass device (using 3x 6SN7 for the moment), similar to the T-Rex/Broskie power supply but with a series pass resistor. The cascode is passing 8mA and the pass device, 3mA.

I want to add a stage of error gain. Where should I put it and what type of stage, particularly in order not to negate the benefit of the cascode.


Edit: due to an oversight, I have changed the figures.
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Old 31st December 2006, 09:06 AM   #2
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Default Re: Calculating shunt reg source impedance

Quote:
Originally posted by lndm
Is it correct to put rail voltage delta over current delta?
Yes.

PSU circuit?
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Old 3rd January 2007, 12:59 AM   #3
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Here's what I'm working with which leads on from a recent thread. Never mind the PS before the 5k pass resistor, I'd like to use a FW vacuum diode & CLC or similar. The rest of the design is still rough.

I don't know whether there would be further improvements to be had WRT hum and distortion coming from load regulation. The cascode is not optimal, so there's that, and if there's no need to go further then I'd be keen to learn that.
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Old 3rd January 2007, 09:23 AM   #4
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On a practical note, do you have enough HT for those 6SN7 to operate correctly? I'm uneasy about all that AC coupling and wondering what will happen when there's a step in the mains.
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Old 3rd January 2007, 10:21 AM   #5
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I am feeling that I should aim the HT toward 500V. This way (with a little more work), each of the 4 6SN7s in the amp section would have 150-200V on them and be passing around 8mA.

On extending the time I can see what appears to be motorboating out of the second stage. Is that what you're getting at with the AC coupling? I could regulate the first two stages and power the last from a separate non-regulated leg, I don't feel the mu-stage would suffer much. (edit: that helps the motorboating)

If I were conventionally filtering, I would use an independent leg per stage. Ideally I would like to regulate the first two stages using separate regulators. Does this seem like overkill, like can I get 90% of the benefit by either regulating the first two stages together, or regulating the first and filtering the second separately do you think?
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Old 3rd January 2007, 10:34 AM   #6
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500V should allow the 6SN7 to work comfortably. The AC coupling in the audio department is almost inevitable, no, it was the AC coupling in the power supply that worried me, although motorboating will always be a danger when two audio stages share a supply. In your situation I might regulate to the second stage and RC filter to the first. You have rather large coupling capacitors, leading to large time constants. Simply reducing these to the more conventional value of 100nF and 1M might cure your problem.
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Old 3rd January 2007, 11:33 AM   #7
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WRT the AC coupling I could easily use bypassed zeners to set the 'screen' supply
and the shunt device bias (or LEDs depending on which shunt device I decide on). The other two caps seem as tricky to get rid of as in the amp itself.

Point taken about the coupling caps (I usually sim with ideal voltage nodes), I'll make them as small as I reasonably can.

Is it safer to set the regulator's rolloff lower or higher than the amp?

Quote:
I might regulate to the second stage and RC filter to the first.
By this do you mean tap the RC from the regulator? and what lead you to this way of thinking?

If 500V/15mA is 33k and 220nF is 33k@20Hz, I figure I could use a 2u2F reservoir. If I feed from a 1k resistor (wary of lost voltage), 1k & 2u2F filters at 70Hz but the regulator is helping with the hum. The reservoir cap and the stage have a time constant of just over a cycle at 20Hz but the cap and the series resistance have a time constant 33 times shorter WRT the reservoir remaining full. Is this a logical argument for the 2u2F/1k combo?
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Old 3rd January 2007, 01:42 PM   #8
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You want the ratio between the audio time constants and the power supply time constants to be as large as possible. It's usual to make the power supply time constant the larger one. Having said that, I've often found that regulators ought to have their speed-up capacitor time constant set to 100Hz or more.

The power supply time constant is the capacitance multiplied by the resistance seen by that capacitance, so if you feed the capacitor from a 1k resistor connected to AC ground (regulator), that's the resistance it sees.
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Old 3rd January 2007, 10:23 PM   #9
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Hmmm...OK then if I know what I want for the first stage and its HT, then the audio TC is set. Making the first stage PS (RC) TC larger can involve increasing the 1k series resistor (this seems counter-intuitive ), or increasing the capacitance which the more I avoid, the higher quality cap I can use.

Do you think I should reduce the 100uF error sampling capacitor to 5uF (I thought a speed-up capacitor was a BJT Cbc thing )
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Old 4th January 2007, 09:06 AM   #10
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Yes, you might want to juggle the value of your error sampling capacitor. I have to say I'm uneasy about that entire ripply reduction circuit, but then, if we all did it the same, where would the interest lie?
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