FCFIFRAGA%CIUC2.UC.RCCN.PT@CUNYVM.CUNY.EDU (FRANCISCO AMARAL F FRAGA) (10/25/90)
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
FCFIFRAGA%CIUC2.UC.RCCN.PT@CUNYVM.CUNY.EDU (FRANCISCO AMARAL F FRAGA) (11/01/90)
I received some feedback about OTL amplifiers, which I tried to acknowledge, but surely due to the mailing system features and my poor knowledge of networks and addressing systems some of the replies were returned to me as undeliverable. I am still trying to fix it. In a future posting I will summarize the messages I received. All the feedback I got was about the Futterman's, no reference was made to the Technics 20A. I am still choosing the model I will built. The Futterman OTL3 is coupled through a 2200 uF capacitor in parallel with some decoupling film capacitors and the Technics is DC coupled ( the power supply is symmetrical), and besides the components for the Technics are cheaper and easier to obtain in Europe (if one can accept replacing the output 50HB26 valves with EL519's). I have heard both, but conditions were so different that I am not able to make a choice based on this auditions only. I think that the Futterman had a fuller and more brilliant sound and the Technics had more speed and better bass. If some one knows something about the Technics 20A, please Email me, as his views will be welcome. ........................................................ Francisco A. F. Fraga Dept. Physics Univ. Coimbra 3000 Coimbra PORTUGAL tel. 351-39-34668 fax 351-39-29158 FCFIFRAGA@CIUC2.UC.RCCN.PT