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Are all amps being built wrong?

The power amplifiers that drive our loudspeakers are mostly built as a low impedance voltage source. They have always been ... but why?

Loudspeakers have a (greatly) varying impedance over the frequency range. A current drive amplifier would eliminate the issues that stem from this varying impedance, and at the same time make discussions about esoteric speaker cables that strive for optimal R, C, L superfluous. Although there still would be these un-measurable ’this (very expensive) cable sounds better’ debates and opinions ... and that’s OK, that’s part of the fun. :)

So ... why are amplifiers not built as a high impedance current source?

This is an interesting read: https://www.current-drive.info/
rudyb
jasonbourne52's avatar
jasonbourne52
2,863 posts
 
The solution is to add an autoformer to the Futterman OTL's output. Say, 16 to 32 ohms. This will allow low impedance drive. And a corresponding increase in power/watts from 60 to 120 to 240 from a sextet of 6LF6 output tubes! 
jasonbourne52's avatar
jasonbourne52
2,863 posts
 
I have three Futterman OTL's. Two mono's and a stereo chassis version that Julius made and sold as the Harvard Music H3 in the mid-Sixties. That one got a review in the old High Fidelity mag! The H3 I bought has an added pair of Altec autoformers because the first owner used it to drive a large custom-made pair of panel speakers that had ten oval cone drivers per side! Consequently these presented a low impedance that the stock H3 couldn't drive - hence the Altec autoformers!
jasonbourne52's avatar
jasonbourne52
2,863 posts
 
There is a purity and transparency of the sound of the Futterman's that must be heard to be believed! I first heard a friend's pair driving Quad 57's back in 1978 and was instantly convinced that the Futterman OTL's are world-beater amps! 
jasonbourne52's avatar
jasonbourne52
2,863 posts
 
Miyajima in Japan copied the Futterman circuit and made some mono amps. The late Art Dudley tested and reviewed a pair for Stereophile some years back.
holmz's avatar
holmz
2,328 posts
 
PS, db Volts is calculated this way:

db = 20 x log( V original / Voltage now) 

So doubling the amplifier voltage = 6 dB louder in both voltage and sound pressure level (SPL) so long as everything is not over driven. 

Technically dB(V) is 10log(V)
Since power is V*I… and with a constant impedance is also equivalent to V^2/R…Then we use dB(W) as 20Log(V).
(I am pretty sure of it?)
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