Sunday, March 2, 2014

Julius Futterman – Father of the Output Transformerless Vacuum Tube Amplifier

Back in the days when high fidelity amplifiers were still made of vacuum tubes, did Julius Futterman inadvertently made the best hi-fi audio amplifier in the world - the Output Transformerless Amplifier?

By: Ringo Bones

Julius Futterman didn’t set out to build the best sounding high fidelity audio power amplifier in the world when he built his output transformerless vacuum tube amplifiers or OTL vacuum tube amplifiers, he was merely trying to lower the cost of hobbyists building their own high fidelity amplifiers for home use by eliminating one of the most expensive components of the hi-fi vacuum tube audio amplifier at the time – the output transformer. But by eliminating the output transformer in his hi-fi audio amp designs, did Futterman inadvertently create the – even until this day – the best sounding vacuum tube audio power amplifier design?

Like Sid Smith of Marantz, Julius Futterman was in the US Army Signal Corps in World War II. After the war, Futterman became involved developing a power amp that did not require an output transformer, which was generally required during the heyday of vacuum tube amplifiers due to the huge impedance mismatch between the typical output stage of a typical vacuum tube amp and the dynamic hi-fi loudspeaker connected to it. A typical vacuum tube amplifier’s output impedance is around 3,000-ohms while recent post World War II era hi-fi dynamic loudspeakers had an impedance rating that hovers between 8 to 16-ohms.

Julius Futterman’s article “An Output Transformerless Power Amplifier” appeared in the October 1954 issue of the Journal of the Audio Engineering Society. Some accounts state that Futterman only sought to reduce the cost of building vacuum tube power amps in kit form sold to DIY hi-fi enthusiasts by eliminating the output transformers since – then as well as now – the output transformers are generally the most expensive single component of a typical vacuum tube audio power amplifier. Whether increased sound quality was the motivation by Futterman or just a happy side-effect – a high fidelity audio miracle was born.

Futterman’s 1954 J.A.E.S. article was followed in 1956 by a piece describing the commercial implementation of his circuit that was the first iteration of the Harvard Electronics Amp featured in the article. It utilized TV sweep vacuum tubes of low cost and long life and was better suited to high impedance loads like the original 1957 Quad Electrostatic Loudspeaker. There was a 500-ohm impedance speaker made by Stephens sold to accompany Futterman’s first commercial OTL tube amp. By the 1960s, Harvard Electronics was gone and the commercial version of the Futterman amp disappeared with it.

Julius Futterman then started making OTL vacuum tube amps by hand, refining the design to a level known as the H3AA. It originally used 6AS7 vacuum tubes. When a hi-fi enthusiast decides to restore the Futterman H3AA these days, he or she can use the Russian military workhorse 6C33C vacuum tubes and the 6336A / B / C are also good choices and a single one of either vacuum tube will comfortably replace a matched pair 6AS7 vacuum tubes. Though many here in the Philippines have experimented replicating the Futterman H3AA OTL amp using PL509 vacuum tubes from junked Radiowealth TV sets during the late 1960s and into the 1970s. 

During the “evolutionary” refinement of the Futterman H3AA OTL amp, Dr. Harvey “Gizmo” Rosenberg -who would later become an advocate for zero feedback single-ended triode amps in the 1990s – was also very instrumental in his suggestive design input in advancing further the sound quality of the Futterman H3AA. And the rest is now history.


Marie Lynne said...

Just read in one of Steve Hoffman's hi-fi forums that the Rogers LS3/5a mini-monitor is the "cheapest" hi-fi loudspeaker that you can connect to the Futterman H3aa OTL amp and still give satisfying results. Given that the Rogers LS3/5a has a nominal impedance of around 14-ohms, there might be some truth to this but given the inherent upper midband dip of the Rogers LS3/5a, it might result in a "soft" sound. Am I right?

Ringo said...

Given the vacuum tubes used in most DIY Futterman H3AA OTL amps have an inherently brighter sound than a typical vacuum tube amp - i.e. horizontal sweep vacuum tubes normally intended for TV use, not audio amplifier use - the "soft-sounding" Rogers LS3/5a mini-monitor could be one of the most cost-effective hi-fi loudspeakers that can be hooked up to a typical Futterman H3AA OTL amp.

Sans Ferdinand said...

Sometimes I wonder if some clever DIY high fidelity audio enthusiast here in the Philippines back in the late 1960s ever tried replicating Julius Futterman's H3AA OTL audio amp circuits using PL509 and 12BH7 vacuum tubes from junked Radiowealth television sets ala MacGyver.