John Curl's Blowtorch preamplifier part II

[kamis]

Originally posted by janneman


Well, I would never use an open loop shunt (I'm not sure I know what that is even, is that just a fixed DC current source?). But a shunt can be very usefull if placed near the load because it allows you to keep all ground currents local.

Even with a perfect series regulator, the ground current varies with the signal and has to go back to the supply caps. If more than one stage shares ground wires that can increase noise and distortion.

The ground current with a shunt back to the supply cap is constant (DC) because the sum of load current and shunt current is constant (= the CCS current). So mutual influence between stages is much less. But it anyway depends a lot on the implementation, you really have to know what you are doing of course.

Jan Didden

Open-loop regulators, series or shunt, is here accepted name for no-feedback regulators. Mr. Perez regulators, showed here, are open-loop types. They consist of zener voltage source and shunt transistor, without any error amplifier. Constant current source or resistor are used with both feedback and no feedback shunt regulators. No feedback regulators have very high output impendance . Jung super regulators current/sense connecting method was developed to eliminate mutial influence between stages you have mentioned. Blowtorch preamp uses LM317/337 series IC regulators and then feedback-free shunts to improve HF regulation, and even cap. multiplier near the audio circuits.
But Jung/Didden regulators are much better, especially in current/sense connecting mode.

[KSTR]

Bear,
Those (the bjt's) are self-biased constant current sources biased by other self-biased current sources (the FET's) so to speak, and therefore you need a bit of voltage drop (a few Volts or so) on the pass bjts to get the FET ccs's to operate properly, additional to the needed drop along the emitter-R's + Vce's abve saturation.

- Klaus

[bear]

Jan said: "The ground current with a shunt back to the supply cap is constant (DC) because the sum of load current and shunt current is constant (= the CCS current). So mutual influence between stages is much less. But it anyway depends a lot on the implementation, you really have to know what you are doing of course."

Bear wonders out loud: "well sure, on paper... but what if the tracking between the two is less than ideal in the real world? ...what would the 'difference' waveform look like, would it be nasty higher order stuff? And, of what magnitude? If the series regulator has more sinusoidal ground currents, might that end up sounding better? So, in practice how good is the tracking of the shunt requlator, and what sort of HF response and bandwidth does it need to have?..."

_-_-bear


PS. Jan I think your diagrams alone are not accurate, they really need to reflect the constant DC claim for the shunt going to the main return, and the AC varying current with the series regulator??

[syn08]

Originally posted by bear
Perhaps someone could explain the function of the the series pass transistor and fet combination that seems to sit on the rails in post http://www.diyaudio.com/forums/showt...57#post1870157 ??

I suspect that many readers are unsure how this bit functions without an external reference, or what it is intended to do...

_-_-bear

That would be a very simple current source. The JFET is injecting Idss in the diodes. One Vbe drop divided by the emitter resistor defines the current throungh the series bipolar transistor.

A relatively poor configuration, due to the dependency on temperature, pretty low output impedance and mediocre frequency response. Replacing one diode with a 1.2V bandgap reference will do some good.

[bear]

Originally posted by KSTR
Bear,
Those (the bjt's) are self-biased constant current sources biased by other self-biased current sources (the FET's) so to speak, and therefore you need a bit of voltage drop (a few Volts or so) on the pass bjts to get the FET ccs's to operate properly, additional to the needed drop along the emitter-R's + Vce's abve saturation.

- Klaus

Oh, it is so confusing!!

So why does one want a "current source" in series with my load? I thought I want a low Z source, not a highish Z source??

What is the idea/purpose behind this scheme?

[KSTR]

Jan's diagrams are perfect, except for misuse of a few current source symbols for voltage sources

[syn08]

Originally posted by kamis

Open-loop regulators, series or shunt, is here accepted name for no-feedback regulators. Mr. Perez regulators, showed here, are open-loop types. They consist of zener voltage source and shunt transistor, without any error amplifier. Constant current source or resistor are used with both feedback and no feedback shunt regulators. No feedback regulators have very high output impendance . Jung super regulators current/sense connecting method was developed to eliminate mutial influence between stages you have mentioned. Blowtorch preamp uses LM317/337 series IC regulators and then feedback-free shunts to improve HF regulation, and even cap. multiplier near the audio circuits.
But Jung/Didden regulators are much better, especially in current/sense connecting mode.

You might be dissapointed, but all those regulators have negative feedback. They are not open loop, although the loop gain is not very large. The parallel regulator (MOSFET) has a significan current gain and acts both as error amplifier and regulator.

EDIT: the small signal output impedance at low frequencies is 1/Gm where Gm is the MOSFET transconductance. That's a hell of a poor regulation, at least for the current levels involved in a preamp (and the required MOSFET ratings thereof).

[bear]

Well, I seem to think I disagree! Once you know in advance what Jan is aiming at, the diagram makes sense, but there is current flowing on the shunt regulator diagram back to the main PS, none shown by arrows, yes?? The issue is nominal DC (constant) vs AC (varying), yes?


Without the text, it is unclear, was to me. Maybe I am unfamilliar enough with common engineering style diagrams to miss the idea with just the diagram...?

_-_-bear

[KSTR]

Originally posted by bear
What is the idea/purpose behind this scheme?

The shunt regs provide the low Z and the stable output voltage, the ccs in front of them try to isolate AC components to the amount they act as ccs (vs. freq), and the series regs bootstrap the ccs to help them isolate even more.

Basically a shunt reg also works with a simple series resistor, fed from noise supply... but not that good.

BTW I don't quite get is why people don't use like circuits instead of complementary ones, when using two-pole designs.

And as for the Perez schemo, don't chop the mains xformer capacitance with individual bridges, even with common-mode chokes. Workaround: 4 secondaries or capacitive bypasses at the right place.

- Klaus (off to lunch now)

[bear]

Originally posted by KSTR
The shunt regs provide the low Z and the stable output voltage, the ccs in front of them try to isolate AC components to the amount they act as ccs (vs. freq), and the series regs bootstrap the ccs to help them isolate even more.

- Klaus (off to lunch now)

Mmmm... of course in modern solid state consumer work, small size is of paramount importance, but for those in high-end not.

So I am thinking why not a choke or a pi filter in the same spot?

Why not a second high performance series reg in the same spot?

Inquiring minds want to know!!

_-_-bear