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In Reply to: Bedini 150 MK II Amplifier (SS) posted by Ozzy on April 9, 2006 at 07:51:46:
When your amp produces a wave form it has both a positive and a negative sign wave. This sign wave should exactly match the incoming signal. However when you strap the amp up to say a 12 inch woofer, the amp does a good job of moving that piston forward, but rarely can the amplifier control the back wave energy of the driver so the negative sign wave contorts and no longer looks anything like the original wave.So many engineers build in a negative feedback circuit to correct the non linear signals as described above.
Ideally you would want to read the sign wave right at the woofer itself, and then adjust the appropriate negative feedback necessary to best create an acurate sign wave. I had an old Kenwood M1 amp that had a second set of wires you were to run out to the speakers to accomplish just this. It provided a dampening factor of 1000!
John Bedini understands that a perfectly matched negative feedback loop would exactly cancel the noise in the sign wave. But in real life negative feedback circuits do not exactly cancel, so you are feeding a contorted signal that bleeds right into the positive signal that is not contorted. So you are in effect hoping you get it right and settle for a bunch of inject noise to make your amp measure better.
By utilizing positive feedback to the circuit the idea is not to eliminate the noise of the negative leg of the sign wave but to reinforce the positive wave so it produces more energy to drive the woofer forward, and now the suspension of the woofer goes to work to provide the smoothest possible wave form.
This is like adding boost to the positive sign wave instead of adding noise to cancel noise. You end up with a higher THD and ID signature, but without adding the grungy noise of the negative feedback.
Overall i feel the sound is more life like and natural. Believe me when I tell you I have run away from much more expensive Class A amps that sound so sterile and analytical that the music seem constipated instead of natural.
A Bedini positive feedback amp will often measure at .1 thd and .1 imd, when a negative feedback amp will measure at .02 and .03 for instance.
John made amps with negative feedback as well. It is really just what the engineer sets as the design center for the product. There is a bunch of pressure out there to build amps with really good measurements. This is what the magazines report as being the important aspect of electronic sound reproduction.
Hell my old SET amps put out 1.0% THD and still sounded way better than any solid state amp at .02 THD IMHO. Of course the bass was not a solid or detailed piece of that puzzle, but I bi-amp so I can get the best of each amplifier I use.
I hope some people with more techical backgrounds than mine can help clarify and point out any inaccuracies in my oversimplification.
Cheers!
Cheers!
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Follow Ups
- I bet I butcher and oversimplify this explanation..... - tubesforever 04/9/0612:48:26 04/9/06 (0)
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