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Old 31st October 2007, 01:58 PM   #11
Alex M is offline Alex M  United Kingdom
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How about Allen Wright's Super Linear Cathode Follower?

FVP5 circuit

If you look at the output stage (the right-hand three valves) the middle one is a cathode follower, the lower one is a current sink, and the top one bootstraps the output to the cathode of the CF so that it has a constant voltage between cathode and anode.

A CF is substantially improved by the current sink on the cathode, as it makes it a more of a bidirectional device, and the bootstrapping improves the linearity.

I built the differential RTP3, and it sounds great to my ears.

Alex
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Old 31st October 2007, 02:07 PM   #12
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by philpoole
I was probably going to use a complimentary feedback pair acting as an emitter follower, with a two transistor current source instead of a load resistor. I've used a similar circuit in a headphone amp and it seemed to work well. Are there any obvious ways to improve it?
Sounds fine. Except you mean "complementary", not "complimentary". Unless, of course, you mean you were given the PNP/NPN transistors for free...
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Old 31st October 2007, 02:08 PM   #13
MRupp is offline MRupp  Germany
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Quote:
... However, I'm more comfortable below, say, +-30V.
If you want tubes ECC86 / 6GM8 is about perfect for that voltage range. Maximum anode voltage is 30 volts and accepts much lower voltages, e.g. 12 Volts. If you build an Aikido or a SLCF stage with bipolar supplies you may not need an outpout cap eiter. All necessary information can be found at the Tubecad pages.
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Old 31st October 2007, 02:12 PM   #14
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Fair point. I think the NPNs were from Farnell and the PNPs off of Ebay - neither were free of charge

To be honest, I was really pleased with the circuit first time round and will probably just use it again. It might be foolish, but I'd rather have each circuit perform a single task, so the IV stage should do just that, and an additional follower circuit should be used to drive the interconnect IMO.
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Old 31st October 2007, 03:51 PM   #15
Jeb-D. is offline Jeb-D.  United States
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Quote:
If you build an Aikido or a SLCF stage with bipolar supplies you may not need an outpout cap eiter
I was going to suggest the possibility of direct coupled as well. But , that could cause a problem if the tube fails or is mis-matched.
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Old 31st October 2007, 04:01 PM   #16
MRupp is offline MRupp  Germany
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Fair point and I did not think if it. So either the next stage can survive an offset voltage or even rail voltage during a tube failure or a cap may be necessary. But maybe there are other means of protecting against this ?
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Old 2nd November 2007, 10:41 AM   #17
Gordy is offline Gordy  United Kingdom
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Quote:
Originally posted by EC8010


Feedback reduces distortion. An emitter/source/cathode follower operates with 100% negative feedback.
It was absolutely obvious from borges initial post that the desire was for no global feedback.

EC8010 if you are going to wade in with the "operates with 100% negative feedback" type of statement relating to local feedback then you could have at least had the common courtesy to explain the difference. (So go on then...)

(Perhaps people might question why you just waded in to demonstrate your 'superior knowledge' without any attempt to actually help at that point).

Borges, philpoole, etc., the emitter resistor in an emitter-follower acts in effect as local feedback. It's not a problem and is just part of normal circuit design.

(There are several threads on this forum where people get into childish pi55ing contests about what is and what is not feedback, and silly arguments based on semantic miss-interpretation, all of which does no more than confuse those who simply wish to learn).
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Old 2nd November 2007, 11:10 AM   #18
borges is offline borges  Norway
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Quote:
Originally posted by Alex M
If you look at the output stage (the right-hand three valves) the middle one is a cathode follower, the lower one is a current sink, and the top one bootstraps the output to the cathode of the CF so that it has a constant voltage between cathode and anode.
Thanks Alex,

I must ge getting a bit rusty because I had forgotten about the bootstrap.

So I guess what I'll end up doing is use a cathode follower, feed it a constant current and give it a nice bootstrap to keep it happy. That should be as linear as anything that can be made with as few components.

The output buffer is for a differential signal that comes from a passive IVC. The topology allows me to bias the grid with a DC voltage through a resistor in order to keep the output DC at 0V. I'm actually contemplating to feed the output DC back through an integrator to have the whole differential output DC coupled. I have too little experience with output caps and transformers. I guess this can be done if the active devices are well matched.

Borge
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Old 2nd November 2007, 11:19 AM   #19
Alex M is offline Alex M  United Kingdom
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For your interest, there is a solid-state version of the SLCF here . This uses JFETs for the follower and the current sink, and a MOSFET for the bootstrap.

I plan to use this circuit in the low-pass leg of my active crossover.

Alex
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Old 2nd November 2007, 11:34 AM   #20
borges is offline borges  Norway
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That looks like a nice place to start! However, I would probably drop the passive filter and opamp after the follower. Do you know if the follower itself has the ability to drive a line output?

Borge
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