In the first part of this two-part blog post I talked about one of my first "up-close-and-personal" experiences with tube-based audio electronics - a home-built hybrid solid-state/tube electrostatic loudspeaker amplifier. While I didn't build it, I did perform the occasional repair or modification on it during the several years I owned it and became comfortable with the idea of working with the higher-voltage circuits and components that tube designs usually entailed.
So when space constraints eventually forced me to a smaller system, another tube amp was a natural choice, although I specifically looked to avoid anything that might deliver the infamous "tube sound." (Did I want some euphonic coloration along with my music? No thanks!) I settled on a small upgraded Dynaco tube amp that I used for driving - through an additional transformer - a pair of electrostatic headphones.
While this set-up sounded fine, neither the audiophile nor the engineer part of me was ever completely satisfied with the idea of multiple transformers in the signal path. So eventually I designed and built a solid-state direct-drive amplifier that replaced both the tube amp and the separate headphone transformer.
I managed to survive tubeless for several years after that, until I found myself intrigued by a DIY tube DAC project that used 6DJ8 triodes for its output stages. The design was based around the highly regarded Burr Brown PCM63 20-bit DAC and stretched simplicity to its limits by using a simple resistor at the output of the DAC chip rather than the usual op amp to perform current-to-voltage (i/v) conversion.
Having auditioned and compared many different DAC units over the years - both commercial and home built - I'd come to the conclusion that most of the sound differences I'd heard were probably due to differences in the analog output stages. So I was skeptical of how well this unit - with its simple-to-the-extreme (and inevitably higher-distortion) output stage design - would hold up.
As it turned out I was pleasantly surprised, and even felt that it was audibly more neutral than some high-end solid-state commercial units I'd heard at that time. I still use that tube DAC in my secondary audio system - the one I use with headphones when I'm at the computer. (Which means it gets listened to a lot!)
A couple of my other forays into tube circuits have included a Class-A guitar amplifier built from scratch from a schematic for a musician friend, and a yet-to-be built direct-drive electrostatic headphone amplifier with an EL34-based output stage. Also, somewhere along the way I picked up a few tube-related design books, including Build Your Own Audio Valve Amplifiers by Rainer zur Linde, Valve Amplifiers by Morgan Jones, and Beginner's Guide to Tube Audio Design by Bruce Rozenblit.
Other resources I've found useful include the respective tube forums at diyAudio and Audio Asylum, which generate what seems to be an inexhaustible amount of information (and opinion) on just about anything related to audio. And finally, I'd be more than a little remiss if I didn't also mention John Broskie's Tube CAD Journal - a great online resource for anyone interested in tube audio design.
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