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A Streamlined Futterman Design
The Futterman OTL design has been around for many years. It has principally been used for power amplifiers. In these arrangements, the output stage uses a bipolar power supply so that the output can be directly coupled to the speakers without a coupling capacitor or transformer.
An interesting application of the Futterman concept for a headphone amplifier is Rudy van Stratum's amplifier. This Futterman amplifier has some nice features, but as van Stratum noted in his article it was not able to hold a consistent 0VDC at the output, necessitating using a coupling capacitor anyway. Also the power supply arrangement, with so many separate supplies for the various bias voltages and bipolar supply, makes building the amp a fairly complex endeavor.
Nevertheless, as I was designing and building White Cathode Follower based headphone amplifiers, my thinking kept returning to the Futterman design. One negative feature of the WCF output stage is that its push-pull balance depends on the load. A properly designed Futterman does not lose push-pull balance for any load.
But, I kept getting stymied by the need for so many independent supply voltages and the attending complexity of the amplifier. Finally, I set a design goal to design a Futterman type headphone amplifier using just one high voltage secondary and one heater secondary. This would mean simplifying the typical Futterman supply from eight independent power supplies to just two. A second design goal was to use only current production tubes (all of my previous designs had use NOS tubes somewhere). At first the task seemed impossible, but as I played with various biasing schemes, it eventually became quite simple. And with that the Bijou Headphone amplifier was born.
WARNING: This amplifier uses potentially LETHAL HIGH VOLTAGES, more than 300V in certain places. Construction and testing of this amplifier should only be done by someone who is thoroughly familiar with safety precautions around high voltages.
- Cathode resistors for Regal's mod. See Regal's Mods.
- Modifications by Amphead to eliminate buzz/noise under certain conditions. See Amphead's Mods.
- Modifications by Regal to increase the quiescent current in the input/phase splitter stages. See Regal's Mods and EZ81 Power Supply. Substituting the EZ81 is recommended for 6N6P installations and other conditions where current draw on the PS pulls the regulator below 250V.
- How to wire the NFB pot. Both possible configurations are shown. See Wiring and Ground at the bottom of the page.
- Wiring the Hammond 370DAX. See Tweaks.