What an OTL is, and why you should care

DonH50

Member Sponsor & WBF Technical Expert
Jun 23, 2010
3,694
76
225
Monument, CO
#41
Just a note to thank Ralph for this thread, cool stuff!

Disclaimer: I owned and/or worked on a number of tube amps through the years, including Futterman (to me sound was "ok" and reliability, well, good for a college kid working as a tech ;) ), and did build a couple of OTL amplifiers based upon a push-pull approach with feedback to get a reasonable (to me) output impedance and a servo circuit to control DC on the outputs. The latter, along with a matching preamp, was a differential circuit so complicated as all get-out. It measured and drove speakers better than any of the other ARC or CJ amps I had around but I never went further with it. Sold the prototypes, but the main feedback I got was "too SS" -- it was much cleaner than the tube amps of the day but that was not what people wanted to hear. I'd love to hear an Atma-Sphere some day.

IME tubes (non-OTL amps) on ESLs are gorgeous in the midrange but have trouble controlling the bass and get harsh in the highs as the impedance drops out of the "good" region of the output transformer. The latter is also a problem for many SS amps IME but most did better than tubes -- if they could handle the low impedance!

FWIWFM - Don
 

kach22i

WBF Founding Member
Apr 21, 2010
1,155
64
210
Ann Arbor, Michigan
www.kachadoorian.com
#42
Yes. They are using an outboard device called the ZERO (see www.zeroimpedance.com).
ML speakers are built with a low impedance in an attempt to make them more compatible with solid state amps. The problem is that ESLs in general are incompatible with solid state unless a bit of current feedback is employed in the amp. This is because the impedance curve of an ESL is not also a map of its efficiency, unlike a box speaker.
From one of the reviews in that link:

http://www.6moons.com/audioreviews/zero/zero.html
Ultimately, speaker manufacturers should install impedance-matching autoformers in their crossovers if design parameters enforce the use of 4-ohm drivers and even lower impedance sags. Such idealism of course is not for sale yet. Paul Speltz originally envisioned the Zeros as a solution for OTL amps.
I guess the question that first popped into my head is answered, and that is: could a impedance-matching autoformers be part of a speaker, or should it be part of the amp.
 

Gregadd

WBF Founding Member
Apr 20, 2010
6,881
117
280
Metro DC
#43
My dream amp would be a direct heated triode otl. I always viewed as wildly expensive and impractical given the dominance of low efficiency box speakers.
 

Atmasphere

[Industry Expert]
May 4, 2010
856
63
180
St. Paul, MN
www.atma-sphere.com
#44
My dream amp would be a direct heated triode otl. I always viewed as wildly expensive and impractical given the dominance of low efficiency box speakers.
We built one of those back in the 1990s using four 6300bs (a beefy 300b with a graphite plate). It made about 7 watts. Essentially, DHTs aren't practical as there aren't any with a low enough plate resistance. But since we were able to compare using this rather impractical amp, it didn't offer anything that the regular OTLs didn't. I'm really not convince the DHTs offer anything special other than they are conveniently sized power triodes. IOW an indirectly heated triode would sound just as good if it otherwise had the same parameters as a DHT. But you can't find very many such triodes so that OTL we made was one of the very few ways to make any sort of comparison.
 

Gregadd

WBF Founding Member
Apr 20, 2010
6,881
117
280
Metro DC
#45
Ralph I did hear your amp CAF driving the Cllassic horns. Very neutral
 

Gregadd

WBF Founding Member
Apr 20, 2010
6,881
117
280
Metro DC
#46
Ralph
Since we have your attention what are the advantages of point to point wiring over a gold trace circuit board.
 

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