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John Curl's Blowtorch preamplifier part II
John Curl's Blowtorch preamplifier part II
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Old 3rd February 2012, 02:10 PM   #20261
SY is offline SY  United States
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John Curl's Blowtorch preamplifier part II
Perhaps your "students" might want to have more than two possibilities with a leading premise.
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Old 3rd February 2012, 02:30 PM   #20262
zinsula is offline zinsula  Switzerland
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For the CCS part, readers should know these articles.

It is obvious, that at least up to 200 kHz, a competently designed CCS will guarantee a constant current flow in the entire loop. The degrading CCS performance higher up can be compensated by an ....

The only varying currents are in the shunt element and in the load itself. It's obvious that those should be close to each other.

I think this is a massive difference to a series regulator supplying a current modulating load, with the modulated current flowing through the whole loop, rectifiers, transformer secondaries and (induced) primaries.

Of course, you could make your active circuit a more constant current like load.....go balanced....and/or folded cascodes.... but that's another (Blowtorch-) story.
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Old 3rd February 2012, 02:32 PM   #20263
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by ThorstenL View Post
Hi,
I didn't. Didn't have to.

I have been using shunt regulators since the last millenium for precisely these reasons...

But it seems some others would be well advised to familiarise themselves with the content of said column...

Ciao T
Indeed. That is why I was surprised that JC said 'Amazing!'. I though he would be well aware of it too....

jan
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Old 3rd February 2012, 02:34 PM   #20264
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by zinsula View Post
[snip]Of course, you could make your active circuit a more constant current like load.
Yes, and if you place a shunt reg near the load, or at least consider them as a single 'load', that's exactly what a shunt does!

jan
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Old 3rd February 2012, 02:49 PM   #20265
jan.didden is offline jan.didden  Europe
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Actually, that begs the question: if the purpose of a shunt is to keep the total current constant, why do we design them as a voltage regulator?
If, otoh, we would design them as a current regulator, we would also negate the effects of any wiring between the shunt and the load!
No?

jan
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Old 3rd February 2012, 02:51 PM   #20266
Bonsai is offline Bonsai  Europe
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Quote:
Originally Posted by zinsula View Post
For the CCS part, readers should know these articles.

It is obvious, that at least up to 200 kHz, a competently designed CCS will guarantee a constant current flow in the entire loop. The degrading CCS performance higher up can be compensated by an ....
.
.
.

Of course, you could make your active circuit a more constant current like load.....go balanced....and/or folded cascodes.... but that's another (Blowtorch-) story.
A heavy localized supply line filter gives similar results: reduction in supply line frequency harmonics before the filter (because load current changes above the filter cut off frequency are localized around the load and active stage). Running active stages in class A also helps. The final point to remember is that signal load return currents should be terminated back at the driving source.
 
Old 3rd February 2012, 03:04 PM   #20267
Bonsai is offline Bonsai  Europe
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Quote:
Originally Posted by janneman View Post
Actually, that begs the question: if the purpose of a shunt is to keep the total current constant, why do we design them as a voltage regulator?
If, otoh, we would design them as a current regulator, we would also negate the effects of any wiring between the shunt and the load!
No?

jan
Jan, I think if you are using an active local shunt, you will get better theoretical results if you feed it from a CCS. I think the main benefits in this set up is almost complete removal of AC on the supply line, so no radiation, so no noise coupling etc. For opamp based amplifiers, if you feed them from a CCS, for the local shunt you can use a zener or TL431 with a simple RC filter straight afterwards (to kill the zener or 431 noise at HF) to create a 'zero AC' supply feed. The high supply rejection of the opamp at low frequencies will take care of LF below the cut off frequency of the filter, while above the cut off, the opamp PSSR is aided by the filter. Expect over 110dB from a decent modern opamp at LF. Discrete based designs will usually not be as good as this, so more care with the shunt arrangement is needed.
 
Old 3rd February 2012, 03:12 PM   #20268
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Bonsai View Post
Jan, I think if you are using an active local shunt, you will get better theoretical results if you feed it from a CCS. I think the main benefits in this set up is almost complete removal of AC on the supply line, so no radiation, so no noise coupling etc. For opamp based amplifiers, if you feed them from a CCS, for the local shunt you can use a zener or TL431 with a simple RC filter straight afterwards (to kill the zener or 431 noise at HF) to create a 'zero AC' supply feed. The high supply rejection of the opamp at low frequencies will take care of LF below the cut off frequency of the filter, while above the cut off, the opamp PSSR is aided by the filter. Expect over 110dB from a decent modern opamp at LF. Discrete based designs will usually not be as good as this, so more care with the shunt arrangement is needed.
Sure, you can use all kinds of refinements. I was asking the question if designing a shunt as a current regulator would not give better results than designing it as a voltage reg. For one thing, at the face of it, the current regulating shunt will also 'make invisible' the wiring between shunt and load which a voltage regulating shunt can't.
Maybe should sim both cases with similar parts, to see if there is a difference.

BTW You cannot 'kill' noise or 'take care of LF'. It is always a matter of more or less noise, or more or less LF. For a given set of parts or parts layout, some topologies may be better than others.

jan
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Old 3rd February 2012, 03:15 PM   #20269
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Bonsai View Post
Jan, I think if you are using an active local shunt, you will get better theoretical results if you feed it from a CCS.
Maybe, maybe not. The concept circuit I scrabbled above does that, of course. It turns the combo shunt+load into a CCS

jan
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Old 3rd February 2012, 03:30 PM   #20270
ThorstenL is offline ThorstenL  Germany
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Hi,

Quote:
Originally Posted by janneman View Post
Actually, that begs the question: if the purpose of a shunt is to keep the total current constant, why do we design them as a voltage regulator?
If, otoh, we would design them as a current regulator, we would also negate the effects of any wiring between the shunt and the load!
No?
The shunt has the job to keep the voltage constant. If it is set as current reg it does not do that.

It is the job of the CCS to keep the current constant.

Incidentally, as no-one wanted bonus credits.

An optimally set up LM317 as series regulator offers around 80dB ripple rejection.

If the 317 is used as CCS and with a TL431 shunt the ripple rejection is around 126dB.

Ciao T
 

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