Historical look at amps

Hi Ar_t, I'm not trying to get in an argument here, I'm just looking for a bit of clarification on what you meant by your last post. I'm sure the answer to your question is obvious to you but not to me. Either you think we all realize how difficult it is OR you think we are incapable of understanding this.

In either case, does your statement refer to the need to use feedback to overcome the limitations of SS devices such as their nonlinearity in order to have a practical circuit, OR the feedback paths inherent in the devices themselves?

I will defend Ayre in two regards on this issue. First off he has tried to educate us all. In Stereophiles Ayre Factory Tour article Hansen discusses zero feedback at length.

On a separate note I guess I am guilty of emphasizing the wrong thing. My real reason for starting the thread in the first place was to point out that amp circuits have been getting better and better and I believe one major reason is our understanding of what feedback does to sound. I think the argueements have been enlightening but I'd summerize them as an arguement about whether zero feedback is local or global and whether it's evolutionary or revolutionary in terms of a company's claim to having a unique design.

I think Ayre equipment sounds wonderful and jumped to a conclusion about it being the next evolutionary rung on the ladder to sonic perfection.

I know you aren't trying to argue. But it does get frustrating to us technical types when it seems like we speak, but no one listens. I know........maybe we don't put it in terms the layman can grasp. But all too often the layman's response comes across to us as it is us who is the one that "no capisce".

OK.......let us look at some very elementary building blocks, and you will understand what I mean. I hope.

Almost all SS amps use emitter-follower outputs. Even ones that use what is called a "complementary feedback pair", like Rowland and Threshold/Forte used in the 80s, still have an emitter follower at its core.

The emitter follower, but its very nature, has 100% local, degenerative feedback. Simple as that. Followers are all over the place in a typical amp. Even ones with ICs, use lots of them. A very handy building block. Sometimes it used to amplify current (output stage), sometimes to isolate stages or even just to shift voltage levels. So, any circuit with one in it anywhere has feedback.

Obviously.......it is local, not loop. That is the point that the guy from Ayre was trying to make.

Another example:

Take a single 1 transistor circuit......make an amplifier with it. Well, to do so, you need to stick a resistor in the emitter leg in order to bias it on. Guess what........more local feedback.

You say that you don't want feedback there.......ok......here are your options:

Take the resistor out. Connect the emitter to ground. Great, now you have something that won't bias on in a linear manner. You have what is called a Class C amp. Great for RF, useless for audio.

Bypass the resistor with a capacitor. Fine, except that it won't look like 0 ohms at all (or really any) frequency. So, there will always be a small amount of local feedback.

Come up with some bias scheme that allows the emitter to be at ground wrt AC, but not DC. You need 2 supplies......and what eventually ends up is you take the easy way out and make a differential amplifier. Ok......great.....now you have done away with the local feedback, but to make something functional, you have to apply loop feedback if want it to incorporate it into an amp design. Or apply local degeneration to make it work without overall loop feedback.

Now.....if you really want to get confused.......get a couple of amp designers with very different views and ask them if they prefer voltage feedback or current feedback. One guy will claim that current feedback is a made up term, that what the other guy calls current feedback is really voltage feedback. No, the other will claim that current feedback does exist, and that it is something that is used in conjunction with a certain topology that is characterised by bandwidth that does not change with gain......etc., blah, blah........enough squabbling to drive even me mad.

In that case, I would have to side with the layman and tell both of them to shut up. But I would not suggest that either try to run for political office. If they ran against each other, we would have to find some way to void the election, since one would have to win.

(Amps using "current feedback" have been made. Analog Devices has a paper called the "Alexander Amplifier". You can find it on their site. Rowland made an amp using that scheme.......Model 8(?), maybe. I made a CD player that had a current feedback circuit inside. The dealers hated it, and I almost lost all of them.....a story for some other day.)

But back to the original subject.......historical look at amps. You would need to include "current feedback" types to have a complete perspective. Another subject of discussion could be bandwidth........how much does an amp really need, and does it help? Some amps......Spectral........have tons of bandwidth.......and some will say that they sound bright as a result.

(BTW.....if you do build amps with overall loop feedback, the more bandwidth, the better. It just gets very hard to increase it beyond a certain point. No sure how Spectral does what they do, except the designer is a sharp dude.)

I bring this part up because I have found that Class D amps, with the same bandwidth as a typical SS amp, will sound bright to almost all listeners. You have to lower the bandwidth to make it "sound" the same. Honestly, I am not sure why. On one hand it is interesting, the other frustrating as hell for an amp designer.

The next rung??? Don't know that I would characterise it that way. I would say it is just another branch on the tree, and trust me, I have climbed all over it.

OK.....at this point someone scratches their head and goes "Doesn't this guy make digital amps now? Did he abandon "zero feedback?"

Yes, and no.

Making "digital" as a way to segue into the HT market. "Zero feedback" designs are not going to go far there. At least not for amps. Small, light, efficient, powerful, lots of "oomph"; Class D is the way to go.

As for anything else that we make.......still has "zero feedback" inside. In fact, a lot of "zero feedback" thinking in the auxillary parts of the "digital" amps.

One thing that has been missing so far is the role of transisor evolution in all of this. Most of it is way too technical, but there are a few major developments in transistor design that have led us to where are. And a few side branches, like MOSFETS. Not only do they allow for kilowatt Class D amps, but different processes led to amps like the Acoustat line (and others) which found favor with electrostat owners. The Trans Nova series comes to mind.

Just one comment re. Ar_t's prev post:
"You say that you don't want feedback there.......ok......here are your options:

Take the resistor out. Connect the emitter to ground. Great, now you have something that won't bias on in a linear manner. You have what is called a Class C amp. Great for RF, useless for audio."

even when that resistor in the emitter leg is removed there IS local feedback in that device. It's the intrinsic emitter resistance called "little re" + any package lead resistance. The issue is that it's really very small & of no practical consequence i.e. not large enough to bias the transistor in its linear region.
========================================================

IMHO, there are several reasons that amps s.s. amps (or sand amps as I like to call them) sound better:
* firstly, the transistor devices themselves have gotten radically better than they were 5, 10, 20 years ago. Ar_t did cite this as well. The distortions from these newer devices is much lower than what it used to be so amps using them can play louder for longer. Tighter manuf tolerances also makes it easier to match them. Many manuf like Pass, Rowland, Symphonic Line & a whole host of others use several of them in parallel for high current outputs.
* 2ndly, the s.s. amp designers themselves have grown in their skill set to design these amps. I bet that Ar_t can testify to this! :-) It takes a while & several generations of products to understand how the semiconductor devices *really* works & how to coax the best from it.
You can see this in the realm of CD music too. When was the CD 1st introduced? If anyone of you has a CD, say, from 1985 or so & you compare it to a recent re-issue of the same music, you can tell that the people re-mastering the CD have a vastly better understanding of the whole process.
The growth of the individual amp designers to skillfully use the semiconducting device to its inherent strengths is what I consider a more important reason for better sounding amps. You can have the best BJT or MOS but if the implementation is poor, you'll still get bad sound.
* 3rdly, *more* s.s. amp designers understand & believe within themselves that there is no or very little correlation between THD, TIM measurements & sonic character of the amp. Thus, applying large amounts of negative feedback to ensure that the amp measured superbly in some reviewer's lab is not a top priority any more. There have always been s.s. amp designers thru the yester years that believed in less global feedback & we consumers have voted w/ our money by owning these products.
* 4thly, there is a lot of admission from the s.s. amp design camp that the vacuum tube, tho old & to some unreliable, was & remains a really fantastic amplification device for audio where lack of harmonic & inter-mod distortion is king. You'll often find people seeking a "tubey sounding s.s. amp" - the forums are littered w/ such posts. Why are these people seeking such an amp?
Also, note that some of the best s.s. amps sound like a good tube amp. Anytime I have ever tried to find an adjective to describe a good sounding s.s. amp I've mostly come up with "sounds like a tube amp!". Fancy that!!! I wrote this in my orig post (which Unsound picked up on). The vacuum tube might be a real old fart but it remains the most linear amplification device that we can work w/ practically. Any world class system will have it somewhere in the chain. Real good s.s. amps only approach that quality of sound.
FWIW. IMHO. YMMV.