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Old 9th January 2008, 05:28 AM   #1
CLS is offline CLS  Taiwan
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Default Power Transconductance Amp with Tubes, How?

A little background...

Supposedly most of you have read this:
http://www.passdiy.com/pdf/cs-amps-speakers.pdf

It's fascinating to drive those low Q driver with "additional power" to the region of bass resonance peak. Doing it elegantly with high output impedance. While "normal" amp can only provide a fraction of rated power on these low frequency high impedance peaks with their voltage output...

Why do I need this? I'm going to build an OB bass module, I guess this might be a good alternative to the "regular amp +massive EQ" combination.

So I'm thinking how to build this "power transconductance amp" with tubes.

Or, simply doing it by modifying an existed amp?

By reading the text book (or the likes), a transconductance amp can be formed by "current series" feedback. Simply put, taking the output current to feed the input.

So, here it is.

This is a simplified circuit of Altec 1568A:
Click the image to open in full size.
You can see the NFB is formed by a separate winding on the OPT, and feed into the cathode circuit of the 1st stage by the 47R/1k5 divider.

Except for the independent feedback winding, it is a typical voltage feedback type, thus a "voltage amplifier".


Now I'm trying to modify it into a "transconductance amp" by:
Click the image to open in full size.

Taking the output current to feed input, simple enough?

But how can I figure out the "??" in the cathode circuit?

Will this work?
Am I too naive on this?
Any foreseeable disaster?

Any comments are welcome.
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Old 9th January 2008, 06:18 AM   #2
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My understanding is that a no feedback pentode is a natural transconductance amp.

dave
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Old 9th January 2008, 06:23 AM   #3
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www.tubecad.com has an article about it, as I recalled.
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Old 9th January 2008, 06:27 AM   #4
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Thanks.

But pentodes have too much intrinsic distortions, wouldn't they?
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Old 9th January 2008, 06:48 AM   #5
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I would look at the voltage you are feeding back in the voltage feedback case. Suppose that for a 1V output on the speaker you feed back 0.1V (I'm just making the numbers up). Then, in the current feedback case, with the same 1V output, I want the same feedback level. So, with the 1V into the nominal 8 ohms, I have an output current of 125mA. This 125mA through the ?? should give that 0.1V so ?? = 0.8 ohms. Sanity check: we are looking for 0.1 times the drive as feedback. With a load on nominal 8 ohms, we came up with a sense resistor of 0.8 ohms so this looks allright.

Then, at resonance, the nominal speaker load increases above 8 ohms -> the current through ?? decreases -> less feedback -> more output level -> that's what you wanted.

Jan Didden
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Old 9th January 2008, 07:06 AM   #6
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Quote:
But pentodes have too much intrinsic distortions, wouldn't they?
I believe that might only apply when using them in a constant voltage amp.

dave
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Old 9th January 2008, 07:14 AM   #7
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Quote:
Originally posted by planet10
I believe that might only apply when using them in a constant voltage amp.

dave

Dave, I don't understand your comment?
The way I explained it, would lead to two amps, one voltage source, the other current source. Both would play equally loud at mid frequencies where the speaker impedance approaches a nominal value. Then, at resonance, where the speaker imp would rise, the current amp would increase the drive. That was the idea, no?

Jan Didden
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Old 9th January 2008, 07:22 AM   #8
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Quote:
Originally posted by janneman
I don't understand your comment?
:^)

I was actually commenting on the post before yours... i wasn't expecting you to be writing a post at the same time i was, so i didn't bothrer to quote. I've fixed that.

dave

PS: how is tomorrow morning in BE?
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Old 9th January 2008, 07:40 AM   #9
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Tomorrow morning is drab, as is fitting for the season..... Just had a global-warming-blamed storm. I was hoping to lose a few trees that are too close to the house but they didn't give in

Jan Didden
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Old 9th January 2008, 07:42 AM   #10
CLS is offline CLS  Taiwan
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Thanks for all your comments

agent.5, I found that article, and studying it, thanks a lot. (but I always have some problems to convert those circuits in the articles into reality )


janneman, you should be a teacher, and you can be a good one I understand you clearly and I'll follow that for my own calculations.


plante10, Actually, I once tried pulling out the feedback resistor of this amp and had a listen. (virtually no NFB)

It was very interesting, almost funny, and really scaring. I just hooked it up to my bass box (not an OB back then), and the sound was heeeuuuuge, vague, overly expanded, and almost no control at all. (And the gain is just too much)

Maybe I'll try it again with OB to see if any good
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