Split Feedback

Well, for one thing, a "non-inductive resistor" is hardly a good stand-in for most real world loudspeakers. You could start a prolonged argument as to whether the guy's logic is flawed on this point. I'm just noting this in passing.
You're losing power when you remove a pair of output devices. In the case of one pair, when you add a pair to act as proxies, you're going to load the power supply down; possibly with dire consequences.
I'd question some amplifiers' stability when faced with modifications to the output stage.
Assuming that you can get the thing to work (for this, I'd recommend designing an amp from the ground up, rather than modifying an existing one), I have no doubt that it sounds different...but does it sound better?
Sounds like sophistry to me.

Grey
 
A very interesting idea. He is actually compensating for non-linearities in the output stage (with feedback) but the reactive nature of the load will not affect the the feedback loop.

Might be a good, idea the only problem I see is the output stage (not in the feedback loop) has to be large to maintain a low output impedence.

This might make a good comparison to a circuit with nested feedback loops.

Jam
 
I looked at it, I wouldn't say interesting, rather weird. Look at the schematic under 'this is it':
He uses feedback to linearize the output of the block 10. Due to the nature of feedback, this will result in the input to '10' to be heavily distorted. Then he uses this heavily distorted node to feed the stage that drives the speaker. And claims it sounds much better....
As I said, weird.
 
I've been mulling this over and it might be interesting to pursue for drivers that present a purely resistive load, i.e. ribbons, planars, etc. I still don't like the idea of modifying an existing circuit, but a from-the-ground-up design might be interesting.
Hmmm...wonder if it's too late to stick this into the Aleph-X...

Grey
 
Nelson,

It surely will work better IF the two output channels on the SPIF are matched, AND if the dummy load on the feedback-output stage completely and at all times tracks the characteristics of the speaker load at the other output stage. Because only then can you guarantee that the signal at both output nodes (the dummy load and the speaker) is identical at all times. But, if that is the case, you can just connect the two output nodes together, right? And if you do THAT, you may as well delete one of the output nodes and the dummy load, right? Guess what, it just looks like a normal feedback amp now.......

Janneman
 
Janneman,

The idea is to attempt to correct for the nonlinearities in the amplifier, not necessarily linearize the output (into a reactive load). As such, you really don't want a highly reactive load at the feedback node...I would guess you'd want a resistance of similar impedance, or a less reactive simulation of your target load.

Whether or not this is a good idea is another thing entirely. To me it seems as though this really is a no feedback design that happens to be extremely linear (into a resistive load). Most low-distortion, no feedback designs I am familiar with are stages driving high-impedance resistive loads--not the case with loudspeakers. I think you'd have to build it to really be able to say much about it as a concept.
 
tiroth said:
Janneman,

The idea is to attempt to correct for the nonlinearities in the amplifier, not necessarily linearize the output (into a reactive load). As such, you really don't want a highly reactive load at the feedback node...I would guess you'd want a resistance of similar impedance, or a less reactive simulation of your target load.

Whether or not this is a good idea is another thing entirely. To me it seems as though this really is a no feedback design that happens to be extremely linear (into a resistive load). Most low-distortion, no feedback designs I am familiar with are stages driving high-impedance resistive loads--not the case with loudspeakers. I think you'd have to build it to really be able to say much about it as a concept.

Tiroth, Im not sure I fully understand you're saying. But this idea is wrong in principle. Either your goal is to get a distortion free signal at the load, and then you end up with simulating the load at the output stage where the feedback is attached, and 'two identical output stages as I described above, and that's a no-go.
Or your goal is to get a distortion free signal at the output of the output stage where the feedback is attached, loaded by a linear load or at least a load different from the speaker. In that case, you really have no idea what the signal at the speaker is doing. Again, a no-go. Sorry.
 
Yup...as I read it, Altman's philosophy is "deliver (what would be) a linear signal to the load, it knows what to do with it"; the assumption being that whatever nonlinearities result from the difference between the actual and simulated load are less objectionable than the application of global NFB.

Please don't think I'm gung-ho on this idea, I just think that it is possible that it would produce acceptable quality, especially with a load with a large resistive component.
 
Hi Jam & Tiroth,

Well, it's all very nice, but this guys makes money from it, that gives me the right to voice my criticism & opinion, isn't it?
You guys talk about 'he is trying to get rid of, sorta...', well, he may be trying, but isn't. The point is that having a linear or distortion free signal at the resistive load doesn't do diddly for the signal driving the speaker, UNLESS both output stages are identical (may be done for double the cost) and the speaker is resistive. Yes, probably there are some speakers with a large resistive component, in part of the freq range. But, be honest, knowing what you know, would you gamble on it with your present speakers? I wouldn't. But, of course, if I missed something, I'm sure Mr Altman will explain what it is.

Cheers,
Janneman
 

djk

R.I.P
2001-02-04 4:23 am
USA
Consider the ESP 03,
http://sound.westhost.com/project03.htm
If we move the feedback connection to the center of the two 1N4000 diodes we can run the output stage open loop. Moving the 0R22 resistors up to the emitters takes care of the errors here. Funny, this now looks just like the amplifiers in the Nakamichi Stasis receivers. Those sounded very good too. Hmmm, didn't Nelson Pass have something to do with those?