Historical look at amps


The amplifier evolution thread reminded me of the history of amplifier circuits that has occured over the last 20 years. Lots of changes but the one that stuck in my mind was the change in feedback circuits. In the early 1980s a good amp like Crown, McIntosh, Phase Linear etc all had large amounts of feedback and distortion levels of 0.00001% IM and THD. These amps sounded bad and the question was raised (and still is) why objective measurement didn't jib with listening tests. A Finnish engineer (OTTELA) came up with a new measurement called Transient IM Distortion (TIM). I wont go into the details but it did show that large amounts of feedback which made static IM and THD measurements good, made music waveforms bad. The result has been today's amps with low levels of global and local feedback, and better sound but with IM distortion levels of only 0.01% (and of course tube amps with more even then odd distortion harmonics). Just recently Ayre, and probably other companys are offering zero feedback designs. Feedback circuits have been with us since the 1920s and we are now just elliminating this basic design feature in modern amps and preamps.
keis

Showing 1 response by bigtee

We may have come full circle but that circle has some waves. For example, one of the more respected SS amps that has received great reviews is the Parasound JC-1's. Well, guess what, it has 30db of GLOBAL feedback! (and to me, it sounds like it---it's soundstaging is relatively 2 deminsional) Global feedback is being used more than you think. Ayre, Theta, some Pass and maybe another or so don't use global feedback. It's still fairly uncommon. These amps have higher distortion values and are generally lighter in the bass than higher FB amps. They act somewhat like a tube amp and clip similarly.
I agree completely with Bombaywalla. Marketers try to influence you with BS but it is up to the individual to educate themselves. All designs have tradeoffs. As one remains in the hobby, your tastes with change---usually with age. There is no doubt in this "Old fart's" mind that no global feedback amps offer a higher level of sonic purity (and notice here I didn't say anything about sounding better---that's for you to decide.) If you listen to voices and instruments, you'll find a higher level of natural sound.
I played sax for 30 years and I can tell you without hesitation that the low feedback (0 GFB) amps sound more realistic (they may not meet the dynamic criteria or impress with a "Balls to the wall" sound some favor.) They are certainly not for everyone. I can also say GOOD tube amps are even closer. They offer that special bloom but not all. Some are noticably soft and bloated. I think the perfect amp would be tube mids and highs with SS bass.
I always thought the "High end" was about accurate portrayal of a source. If it's not and it's about "Good sound" then I need to get out because it all becomes a moot point. There's a huge difference between what offers good sound and what is an accurate portrayal of the recorded venue. Some stuff just doesn't sound good and shouldn't sound good.
So much equipment is offered because of the tastes of the individual. However, some of it is wrong and a little of it is right. It does take years to figure a lot of this out. I've been at it 35+ years and it is amazing how my tastes have changed since the beginning.
BUT---to each his own! Everyone knows what they like.
With all that said, I will add one last thing. It is my held belief that some of the differences in sound of no feedback and feedback amps is do to time domain distortions created by out of phase global feedback---but this is a discussion for another day.