Posted by Bill Lamb on November 13, 2003 at 21:39:53:
In Reply to: Re: Re: ESL Direct Drive Amplifiers......? posted by CV on November 11, 2003 at 04:34:12:
Has anyone considered an OTL design for this application? The Atma-sphere OTL design is well developed, stable and sounds exceptionally good on Sound Labs even through the crossover and transformers. According to this e-mail exchange with Mark Gilmore from Atma-sphere, it CAN be done.
Dear Bill,
I am hoping you could answer a quick question for me here. A group of
> people at the Sound Lab Owners Group (SLOG) are interested in developing
> an amplifier to directly drive Sound Lab electrostats. Would it be
> possible to modify Ralph's designs to produce the voltage swings needed
> to do this? The speakers should ideally see about 16000 V from peak to
> peak and next to no current, although it may be possible to drive them
> adequately with half that voltage.
>
It's possible, but not an easy task. It would require some very special
and custom parts, some very high voltage tubes and some advanced
construction techniques to deal with the high-voltage. Safety is
another concern, with voltages that high.
If it isn't possible with 6AS7
> tubes, would it be possible with 211 or 845s?
>
The 6AS7G is only good to about 150V. The 211/845 is good for just over
1 kV, so you would need to think about much higher voltage tubes. These
tubes are expensive, require a lot of support hardware and tend to run
very high power. They are also difficult to keep cool, so ventilation
will be an issue.
I understand that this
> application is similar to RF transmitting amplifiers used to drive radio
> station antennas.
>
This is true.
This is a well-proven circuit design which has had 25 years of continuous development and has reached a very high level of refinement. More than that, there is an entire community of hobbyists who have worked with modifying the circuit (http://www.otlamp.com/). Plus, Ralph Karsten is very helpful towards DIYers. I think this would be an excellent place to start.