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Old 20th January 2013, 12:02 AM   #21
SY is offline SY  United States
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A pointless circuit is one where additional complication is added for no particular advantage or motivation. If revintage builds this circuit, compares it to a circuit without the complication (or to just the opamp used alone), and shows a demonstrable performance advantage, that's a different story.
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Old 20th January 2013, 12:02 AM   #22
jjman is offline jjman  United States
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Would it be more sacrilegious or less sacrilegious to use a small transformer to flip the phase there?

To me the feedback is being "processed" which make it potentially less valid considering it's purpose in life.
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Old 20th January 2013, 01:38 AM   #23
Simon B is offline Simon B  United Kingdom
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As revintage has made no claim whatsoever regarding the performance of the circuit, rather he simply enquired "Has anyone tried the leftmost topology before?", it seems odd to suggest that he should provide evidence in support of it showing a "demonstrable performance advantage".

The circuit could hardly be described as complex.

"Just use the op-amp" conveniently ignores the possibility that the OP might actually want to use a valve for whatever reason, or assumes that any reason he might have is specious.

It does seem a shame that a quite reasonable open-ended enquiry should be treated so disdainfully, and certainly not conducive to sensible discussion. I had assumed that would be regarded as worthwhile, perhaps I am in error.

Surely it is possible to state that one cannot see any advantage in doing something one way, as opposed to another, without using pejorative terms like "pointless"? Or am I in error again?
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Old 20th January 2013, 07:58 PM   #24
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Quote:
Originally Posted by Simon B View Post
As revintage has made no claim whatsoever regarding the performance of the circuit, rather he simply enquired "Has anyone tried the leftmost topology before?", it seems odd to suggest that he should provide evidence in support of it showing a "demonstrable performance advantage".

The circuit could hardly be described as complex.

"Just use the op-amp" conveniently ignores the possibility that the OP might actually want to use a valve for whatever reason, or assumes that any reason he might have is specious.

It does seem a shame that a quite reasonable open-ended enquiry should be treated so disdainfully, and certainly not conducive to sensible discussion. I had assumed that would be regarded as worthwhile, perhaps I am in error.

Surely it is possible to state that one cannot see any advantage in doing something one way, as opposed to another, without using pejorative terms like "pointless"? Or am I in error again?
I whole heartily concur.
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Old 20th January 2013, 08:12 PM   #25
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SY,
Yet another pointless comment from you. If you can make a feedbacked single stage tube circuit that is not an anodefollower(low inputZ and bandwith)(to the left in the first post) I will donate 1$ to "Save the Children" in your name. You have to get around the phase reversal in some way.
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Old 20th January 2013, 10:19 PM   #26
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Quote:
Originally Posted by revintage View Post
SY,
Yet another pointless comment from you. If you can make a feedbacked single stage tube circuit that is not an anodefollower(low inputZ and bandwith)(to the left in the first post) I will donate 1$ to "Save the Children" in your name. You have to get around the phase reversal in some way.
There's a few different ways it can be done.

You have two active devices; as soon as you have the opamp in the feedback loop, it's an integral part of the network which determines the transfer function, so the tube is only reducing the performance. So you could drop the tube and configure the opamp as a 12dB gain inverter. That's probably the highest performance solution.

You could sub a tube section for the opamp to keep the same number of active devices- the topology I suggested earlier would work very well and probably outperform tube plus opamp.

You could configure the tube as a cathode follower and use a step-up transformer at the input.

You could use the tube as an anode follower with a large input resistor (100k).

You could run the tube open loop with an output transformer load.

There's probably a few dozen other ways to do the same thing with your constraint of not increasing the number of active stages.
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Old 21st January 2013, 09:41 AM   #27
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Quote:
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You could use the tube as an anode follower with a large input resistor (100k).
Terminological error on my part- what I meant was plate to grid feedback, with a 100k input resistor. It's pretty much equivalent to the circuit you drew (gain, distortion, output impedance), but without the opamp and supporting circuitry.

I appreciate the donation in my honor.
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Old 21st January 2013, 02:08 PM   #28
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Quote:
Originally Posted by revintage View Post
SY,
Yet another pointless comment from you. If you can make a feedbacked single stage tube circuit that is not an anodefollower(low inputZ and bandwith)(to the left in the first post) I will donate 1$ to "Save the Children" in your name. You have to get around the phase reversal in some way.
SY Sorry for my lack of reading comprehension in this but how is an anodefollower not an anodefollower and thus qualify for the donation ? Charles Boegli did some very good articles about the anodefollower in the 1950s . When used with a Baxsandall tone controls it work very well.
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Old 21st January 2013, 02:29 PM   #29
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As I explained, I meant a simple inverting triode stage with plate to grid feedback resistor. It's an anode follower, strictly speaking, only if it's unity gain.

100k input resistor, 400k plate to grid resistor. I'd CCS load the plate and use an LED in the cathode, but there's a lot of other options to get the open loop gain up to near the mu of the tube.

So... we have the same output impedance, same distortion, high input impedance, but no opamp and supplies.
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Old 21st January 2013, 02:53 PM   #30
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Quote:
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100k input resistor, 400k plate to grid resistor. I'd CCS load the plate and use an LED in the cathode, but there's a lot of other options to get the open loop gain up to near the mu of the tube.
SY, making anode to grid feedback, the input impedance of the stage is severely reduced because of voltage-parallel feedback type. And for make this, it is recommended to use pentodes because its higher open loop gain.

Cordially, Osvaldo.
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