Glowing Hybrid Amp

Posted on Feb 6, 2014

This odd little amp was something I designed and built for a shootout at the European Triode Festival in 2007. The topic was `White Light` - amplifiers that use thoriated tungsten tubes. Some great photos of the event can be found here and here. Since it`s impractical to tote an enormous tube amp from the US to Europe, I decided on an approach th

Glowing Hybrid Amp
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at uses a solid-state power output stage and a small thoriated tube as a voltage amp. I tried out just about all the small tubes of this type that I could find, and wound up using the 703A. The 703A is a small UFO-shaped triode designed for radar jammer service. I found that it made a pretty decent low-level amp when run with a B+ of 48V and a ~1mA CCS plate load (!), so that`s what you find in this circuit. It is coupled to TI (Burr-Brown) OPA548 power op-amps, which drive the speakers. Power is +/-24V, generated from some Vicor DC/DC converter modules. The 1. 15V filament is supplied by another pair of isolated DC/DC converters, running form the +/-24V (48V) DC supply, with significant common mode filtering. I set the amplifier up to sound somewhat like a single-ended 300B amp. It puts out about 20 watts into 8 ohms, and has an artificially high (for a solid-state output stage) output source impedance of 1 ohm, to emulate a tube amp output. It has about 0. 3% - 0. 5% THD at 1 watt output, dominated by 2nd harmonic, as a single-ended tube amp should be. And finally an FFT of the output. Dominated by 2nd harmonic, like a SE tube amp. but there are crossover distortion products from the solid-state output stage that extend all the way to 20kHz, albeit 90dB down. Note the measurement floor is -135dB! The power supply uses Vicor modules: An IAM AC input module (filter and rectifier with some supervisory functions) and two...

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