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From: FCFIFRAGA%CIUC2.UC.RCCN.PT@CUNYVM.CUNY.EDU (FRANCISCO AMARAL F FRAGA)
Newsgroups: rec.audio.high-end
Subject: OTL amplifiers
Message-ID: <7211@uwm.edu>
Date: 25 Oct 90 12:50:19 GMT
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Approved: tjk@csd4.csd.uwm.edu


     I have done some research about valve OTL/OCL amplifiers
during the last weeks. (OTL means output transformerless and OCL
stands for output "condenserless"). My main sources were two
articles from L'Audiophile, an excellent french magazine about
audio with a strong bias towards antiques (they love everything
that dates from the 50's and 60's), written by Jean Hiraga, and
some old RCA manuals.
     Due to their inability to supply high output currents, OTL
amplifiers use is limited to electrostatics and some well behaved
coil loudspeakers. Our listening tests were carried mainly with
Quad ELS 63.
     I managed to hear only two amplifiers : a Futterman OTL 3
and a home made copy of the Technics 20A, the first audio
amplifier made by Technics in the sixties.
     These beasts are large, heavy and hot, but their sound is
something special. I am referring to the Technics copy. The
purity of the sound is not believable. Bass is not extended, but
is firm. A friend who owns a pair of genuine Futterman's has
recently acquired a pair of subwoofers and is very pleased. High
frequences are crystalline and sweet. Stage depth and
localization is very good and the Quads disappear completely. The
Audio Research D70II seems flat compared to them. Solid sate
amplifiers seem opaque and lazy. With the Technics 20A you remove
another layer between the music and you. May be fluidity is a
good keyword to class these amplifiers.
     Because they have no output transformers they are ideal to
the home builder. All you need are the power transformers, large
value high voltage capacitors (they are used in switching
supplies and are easily available), valve sockets, valves and a
few components.
     This weekend we are going to hear the OTL's driving
Soundlabs A4x. May be next week I will begin building a pair. If
some one is interested in these matters, please E-mail me.




........................................................
Francisco A. F. Fraga
Dept. Physics
Univ. Coimbra
3000 Coimbra PORTUGAL tel. 351-39-34668 fax 351-39-29158
FCFIFRAGA@CIUC2.UC.RCCN.PT