2. amps-preamps
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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
aball
2,018 posts
 
Be careful not to confuse global and local feedback. The comment:

"Feedback circuits have been with us since the 1920s and we are now just elliminating this basic design feature in modern amps and preamps"

is quite incorrect. There are designs with no global (aka "overall") feedback which does have some benefits - and thus some disadvantages too - but they definately still use feedback in the circuit. It is just applied around the transistors instead of around the circuit.

The marketing gurus have done a great job of convincing audiophiles that zero feedback is the way to go but no functional amplifier design actually exists without any feedback.
keis OP
176 posts
 
Aball, the Ayre products have no local or global feedback. That was my point. They now do exist and I don' believe Ayre will be alone for long. If you don't believe me go to Ayre.com (no I'm not an Ayre rep but I do own a K1xe.)

Ayre has zero feedback designs in their video circuits also.
ar_t
165 posts
 
The Ayre amps use local feedback, sometimes called degeneration, and no loop feedback.

The Stasis amps had no feedback from the speaker output to the signal input, but instead had feedback loops applied to "building blocks".

The new generation of so-called "digital" amps (the self-oscillating varieties) do have overall feedback loops from the output to the modulator. Some sense current, some voltage, some both.
herman
2,259 posts
 
Keis, I believe it is impossible to build a practical amp using discrete fets that doesn't use some type of feedback. I'm going to need more proof than the fact their web site uses the term "zero feedback." I sent an email to Ayre asking them to clarify. I'll let you know if they respond.

Aball, my functional amp uses zero feedback. That is the beauty of the triode vacuum tube. They are linear enough to use without any feedback.
gregm
3,408 posts
 
SS uses feedback to stabilise the circuit. In the more exotic designs this f/back is local. OTOH tube circuits can be stabilised w/out any type of feedback (even there it's not plain sailing -- it requires effort).

Objective measurements: let me insist that measurements are useful and can be reliable. Depends what you're measuring FOR, i.e. it's not always for sound reproduction.

If for marketing, then you go one way. If for guessing the precision of amplified signal, you go another way (i.e. checking into amp-speaker interface matters, et alia).
A Norwegian called R. Lian had looked extensively into this matter. To a certain extend so have we all: if I'm to check for amp performance, WHAT (and how) should I measure?? Cheers
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