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Old 2nd October 2007, 12:10 AM   #1
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Default Crowhurst's Twin Coupled Amplifier

I'm beginning to miss the smell of burning insulation. I need to build something, and the twin coupled amp is what I have in mind. See:

http://www.audiofaidate.it/articoli/...wincoupled.pdf

and

http://www.audioxpress.com/magsdirx/...0crowhurst.pdf

I can't find any OPTs even close to what is required for the EL34s, though, so I'm thinking of using the Avel dual primary toroids from Parts Express. However, my experience with these, and knowledge of transformer design in general, is nil.

Obviously, I'd need to series connect the primaries, to get a center tap. But should secondaries be paralleled or series connected for best performance? Here, I'm wondering about capacitance & inductance. Am I right in thinking a higher VA would extend low end? Any help would be appreciated.
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Old 2nd October 2007, 12:21 AM   #2
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Stack of donuts... I like it! The smaller ones have more inductance - and as far as low end, I'd expect them to saturate at about 300V RMS at 60 Hz, regardless of size. Efficiency will be better with a larger one, of course.

Secondaries will probably end up in parallel to get the ratio you want - but I think coupling will be about the same either way.

Digi-Key has a series of AC power toroids as well - don't recall who makes them.
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Old 2nd October 2007, 03:10 AM   #3
mach1 is offline mach1  Australia
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Audioxpress published a modern update on the Crowhurst twin coupled amp using 6LU8 tubes, Hammond 125E trannies and other easily available components.

Have a look through the back issue directory. If you can't find it I have a copy at home somewhere.
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Old 2nd October 2007, 03:15 PM   #4
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Thanks for the info! The Digi-Key trannys are Amveco (made in India); I used them in a build of Steve Bench's matrix amp. Not too successful, one of my first forays into solder slinging, so I can't comment on quality. Pricey little devils, though. I was serious about the "burning insulation" part!! Don't want to put no Tangos in this puppy!!

I subscribed to Audioxpress for only one year. It expired April, 2005. I'm guessing that "Crowhurst's Twin-Coupled Amp Revisited" in September, 2005 is what I want. 6LU8??? OK...did a bit of research, seems a little anemic for 98db of Santana.

CD is on the way. It'll be interesting to see what they did. EL34s are still looking good, though.
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Old 1st January 2008, 06:31 AM   #5
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Default Re: Crowhurst's Twin Coupled Amplifier

Hate to resurrect an old thread, but I too am interested in this amplifier. I can try to answer some questions, but I also have some of my own....

Quote:
Originally posted by terrydaniel
Obviously, I'd need to series connect the primaries, to get a center tap. But should secondaries be paralleled or series connected for best performance?
All the articles I've read show the output transformer secondaries connected in parallel, such that the resultant impedance matches your speakers. If you're trying to match a 4-ohm speaker, you need to parallel the 8-ohm taps.

Quote:
Originally posted by mach1
Audioxpress published a modern update on the Crowhurst twin coupled amp using 6LU8 tubes, Hammond 125E trannies and other easily available components.
This was published in AudioXpress August 2004 by John Stewart, which I've read. A follow-up article was published in the same magazine in September 2005, which I haven't read.


Quote:
Originally posted by terrydaniel
.......6LU8??? OK...did a bit of research, seems a little anemic for 98db of Santana.
Stewart describes this in the first article. The 6LU8 is rated at 14W, but for audio can be pushed harder. He got 37W peak in a PP configuration with OPT's rated at 15W each. Stewart claims that the amp can be built for $200 per channel. At that price, it's tempting to consider paralleling outputs for more power.

Finally, here are my questions:
- I purchased but cannot find the AudioXpress CD for 2005, and therefore have not been able to read the article. Can someone post the main points of Stewart from this September 2005 article?
- Is there anything inherently flawed with the twin-coupled amp?
- I was thinking of using the Hammond 1620 rather than the 125E OPT, as the former is wider bandwidth and costs about $20 more per OPT. The 1620 appears to match the published tube parameters for this application, but I suspect that Stewart chose a different operating point for the 6LU8 and therefore the 1620 impedance won't work well. Can anyone comment?
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Old 1st January 2008, 06:44 AM   #6
Zen Mod is offline Zen Mod  Serbia
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Default Re: Re: Crowhurst's Twin Coupled Amplifier

Quote:
Originally posted by weinstro
..............



All the articles I've read show the output transformer secondaries connected in parallel, such that the resultant impedance matches your speakers. If you're trying to match a 4-ohm speaker, you need to parallel the 8-ohm taps.



...........

and you still have 8 ohm tap .

re-read basics, or you prefer burning tubes for nothing?
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Old 1st January 2008, 11:53 AM   #7
jane is offline jane  Norway
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In fact, the Crowhurst's Twin Coupled Amplifier is a unity couplet amplifier ala McIntosh [1]. The variation is that Crowhurst is using two output transformers instead of an expensive bifilar wound transformer. As a result there will be lack of magnetic coupling and Crowhurst had to insert a capacitor between each primaries. Else, he is using most of the tricks that McIntosh did, bootstrapped drivers etc, plus that he has applied some inner feedback.
A true unity coupled output stage has some remarkable characteristics and, if proper designed, will measure and sound very good. The secret is the tight coupling between the primaries, as published by Professor Pen-Tung Sah in his paper “Quasi transients in class B audio-frequency push pull amplifiers” back in 1936. [2]

Jan E Veiset

[1] http://www.veiset.net/tech/Mac1949.pdf
[2] http://www.veiset.net/tech/Mac1936.pdf
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Old 1st January 2008, 12:49 PM   #8
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Jan,

Those links are VERY interesting documents. I am always amazed how often we are struggling to re-invent wheels that have been invented often more than half a century ago. Makes you humble in this so called 'hi-tech' era.

I am intrigued by that second article by Pen-Tung Sah. It seems a scan of book pages. What book would that be?

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Old 1st January 2008, 01:19 PM   #9
jane is offline jane  Norway
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Quote:
Originally posted by janneman
I am intrigued by that second article by Pen-Tung Sah. It seems a scan of book pages. What book would that be?
Proceedings of the IRE (Institute of Radio Engineers), Nov. 1936, Volume: 24, Issue: 11.

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Old 1st January 2008, 07:25 PM   #10
kevinkr is offline kevinkr  United States
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Given what the Amveco transformers cost perhaps you could talk to Jack Elliano over at Electra-Print to see whether or not he could make some unity coupled transformers for you and what they would cost.

He made custom headphone output transformers for me last year for a very reasonable price and which performed extremely well.

I believe dsavitsk has had a similar experience and can corroborate..
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