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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
 
Julius used hollow ferrite rods on the wiring of his amps - at least the one's I've seen. 
bdp24's avatar
bdp24
9,865 posts
 
I seem to remember Modejeski discussing that Futterman 60dB feedback figure in one of the threads on the Music Reference AudioCircle Forum (though dormant, all posts are still viewable), and maybe in one of his three seminars at The Burning Amp Festival, which I encourage everyone to watch on YouTube. All contain a wealth of free information and wisdom, as does the MR AC Forum.
clio09
3,360 posts
 
@jasonbourne52 - It's been a couple years so I would have to look at that Harvard H3 again. I seem to recall 1" long beads, but they could have been rods. They were all around the power tube sockets.

@bdp24 - I'll see if I can dig something up to verify. I keep forgetting those AC circles are still accessible although AC banned Roger.

@atmasphere - I have about 3 hours (cassette tape) of Roger interviewing Julius Futterman (also interviews with Saul Marantz, Sid Smith, and Brian DePalma) that he recorded when visiting Julius in NY years ago. Upon opening the door to his shop Julius asked Roger if he could come back in an hour as he was about to get started eating his lunch (which he brought to work everyday in a brown paper sack). BTW - if you are interested in dissecting an H3 I have two of them. I'll send one off to you if you like. Perhaps we can find out if he was able to pull off that 60 dB of feedback.
atmasphere's avatar
atmasphere
11,859 posts
 
@clio09 I'd love to see it!
imhififan's avatar
imhififan
2,208 posts
 
United States Patent Office. 2,773,136. Patented Dec. 4, 1956. 2,773,136. AMPLIFIER. Julius Futterman, New York, N. Y.

This invention relates to amplifiers, and more particularly to audio-frequency power amplifiers employing large amounts of negative feedback to reduce distortion, and is therefore useful in the reproduction of speech and music...
One object of my invention is to provide a low cost power amplifier that does not use an output transformer, and is capable of Supplying large amounts of undistorted power directly to a low impedance load of the order of 16 ohms, such as the voice coil of a conventional loud speaker. Another object of my invention is to provide a power amplifier useful over the audio range of 20 cycles to 20 kc. and capable of utilizing large amounts of negative feedback, of the order of 60 db, without instability.
United States Patent Office. 2,773,136.
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