Audio amp design: Do you tube?
By Rich Pell
Audio DesignLine
(08/06/2007 9:58 PM EDT)
>>"Probably like most people who learned engineering after the 1970s I was
never formally taught anything about vacuum tubes. They'd long since been
relegated to the electronics history books.
But I had been studying, designing and building all manner of
do-it-yourself (DIY) audio projects long before I ever set foot in an
engineering class. So although I hadn't actually built anything with tubes
at that point, I'd seen my share of schematics s****ting those recognizable
but still mysterious - and intriguing - symbols.
And it had been one of the projects I had built - a DIY electrostatic
loudspeaker (ESL) published in Audio Amateur - that had led to my first
piece of tube audio equipment. It was a home-built hybrid solid-state/tube
direct-drive electrostatic loudspeaker amplifier with an 8068 beam pentode
output stage, and I obtained it from the author of the ESL article
himself, who had since switched to using a conventional solid-state audio
amplifier with step-up transformers to drive his ESLs.
It was huge, weighed over 50 lbs, and was an excellent apartment heater.
Nonetheless, it and the ESLs it drove produced some great (although highly
directional) sound, with an equally great soundstage. Symphonic music and
Pink Floyd's The Wall never sounded so good!
Still, while I enjoyed the amp's sound and the glow of its tubes in the
dark, I'm sure I would have enjoyed it just as much had it been all solid
state. After all, the use of tubes in that ESL amplifier was mostly a
matter of necessity based on its direct-drive design requirements - not a
conscious decision on the part of the designer to choose a valve design
over a readily available solid-state solution.
In fact I might easily have managed to avoid the lure of further tube
audio projects altogether, except for the fact that my interest in sound
and music reproduction extended to the niche market of high-end audio. And
like any good card-carrying audiophile, I knew that those glowing
electron-emitting relics of the past were (and still are) hot commodities
in that rarified realm.
Part 2 of this post (to be posted shortly) concludes with a few more of my
tube adventures, including a DIY tube DAC, and some tube audio resources.
Comments, questions or suggestions? Email me at rich.pell@[EMAIL PROTECTED]
amp design: Do you tube? (Part 2)
By Rich Pell
Audio DesignLine
(08/08/2007 4:26 PM EDT)
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.
Comments, questions or suggestions? Email me at rich.pell@[EMAIL PROTECTED]
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