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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
 
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?)
invalid
1,073 posts
 
Because a current drive amp is only useful with a speaker that has a flat impedance. Otherwise the amp will act as a tone control and follow the dips and peaks of the speaker's impedance curve. This can hardly be called "accurate"!

How useful would a current drive amp be with a 1 ohm speaker that has a flat impedance?
atmasphere's avatar
atmasphere
11,566 posts
 
Julius Futterman solved the tube amp feedback problem by eliminating the output transformer. His OTL amps can have 60 db feedback with low output impedance and low harmonic/IM distortion. Yet remain 100% stabile into complex loads!
This statement is false. No Futterman amp ever had anything like 60dB of feedback!! The most any had was more like 20dB, and because the amp had very wide bandwidth, oscillation (caused by its phase margin being exceeded by the feedback) was sometimes an issue. So to your closing comment here- they are well-known to **not** be stable into complex loads.


If you doubt any of what I'm saying, just for the record I've been manufacturing OTLs longer than anyone else on the planet (over 46 years); I'm not being hyperbolic.
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