I'm pretty sure that Thorsten was *not* referring to any differences in SPL mag and phase response, rather he relates to the differences *once* SPL mag/phase has been made equal to the same target, no matter what complex impedance profiles the amp outputs had.
This also implies that first order small signal impulse responses of transducer voltage, and current resp, are the same (as they must be because of identical SPL impulse in a causal system), yet large signal behaviour and the "fine print" can and will be different (measured HD/IMD shows part of that). How the "microphonic" velocity signal generated by the transducer will be handled is what makes the difference, which is not always better with higher impedance drive. Velocity-controlled operation (instead of force-controlled) still has it's benefits and applications. Similar to amplifier circuits, when you choose to apply feedback (in the transducer) then use lots of it, and never use it to "fix" a mediocre transfer function -- Baxandall strikes back.
BTW it was a post of Thorsten over a decade or so in some forum that first made me aware of this interface mechanism and rethink the common view. I owe him something...
An electrodynamic transducer itself is a feedback system under voltage drive, degenerative feedback that is. When you change the feedback factor (as a scalar, or even frequency dependent), all else equal, there are more or less subtle changes in sound, depending on how nonlinearities/noise and reflections get "recycled" by the feedback, if any.
- Klaus
Or, like "What causes raise of harmonics with increased signal level: first tube, phase splitter, or output stage". Or, "How phase will be modulated by the stage on 20 Khz, let's check applying 300 KHz for better visibility". Or, "How crossover distortions will be modulated by signal envelope", and so on.
Or, "How beta of output transistors depend on current". Lots of measurements, depending on topology. In order to find optimal topology, parts, regimes.
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Probably the best example of white box testing one can do for most "commercial" amps with NFB is open loop testing.
It is always most interesting to test open loop distortion, and also gain. Then also gain /phase margins which gives a big measure of inherent stability.
I have read many well heeled amp designers speak of getting the design "right" with no feedback, i.e. get to the lowest distortion, best response, etc, and then closing the loop with moderate feedback to lower output impedance, etc. This makes sense, to me at least.
Hard to read minds, I can only go by what's written. In any case, the engineering of current drive systems is well-established, with several JAES papers and at least one book working out the principles according to conventional engineering. And experiment indeed accords with theory. The differences beyond SPL mag/phase are not exactly gross, but certainly measurable and understandable.
I believe the stated reason is large amounts of NFB are not needed as much if the original open loop design is inherently low in distortion. I think many of the Pass DIY amps like the F5 use this sort of feedback philosophy.
Versus giving it 100dB of open loop gain, and then feeding back 70-80dB or more to "fix everything". Not really saying one is right or wrong, but it's interesting to look at it in a white box fashion.
For instance, most op-amps have over 100dB of open loop gain. Versus a 6SN7 class A tube stage with no FB and perhaps 20dB gain open loop. Two very different topologies.
Two different topologies, and both seem to work when properly engineered and used.
The right amount of feedback is not necessarily 'moderate' or 'modest' or 'massive' or 'none' but the right amount for that circuit in that application. Getting open loop behaviour correct before closing the loop is obviously right, but 'correct' does not necessarily mean 'OK as an audio amp without feedback' although this may indeed be the situation with some circuits.
The right amount of feedback is not necessarily 'moderate' or 'modest' or 'massive' or 'none' but the right amount for that circuit in that application. Getting open loop behaviour correct before closing the loop is obviously right, but 'correct' does not necessarily mean 'OK as an audio amp without feedback' although this may indeed be the situation with some circuits.
Not difficult, possibly too easy.
I think most don't like the idea of 'good enough'. That is, It does it's job and doesn't add anything noticable. Then it's an amplifier. If it does add something to the sound, like a tube amp that adds "warmth" (whatever that means?), it's not just an amp, it's an amp+effect box.
I think most don't like the idea of 'good enough'. That is, It does it's job and doesn't add anything noticable. Then it's an amplifier. If it does add something to the sound, like a tube amp that adds "warmth" (whatever that means?), it's not just an amp, it's an amp+effect box.
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Good, that Quad-405 was mentioned. 
The amp can't work at all with open feedback loops.
I myself designed similar amp (similar in the class A+B approach, but realization was quite different), that relied on nested feedback loops.
It is more complex than to take any amp, open it's global feedback loop, measure and compare
The amp can't work at all with open feedback loops.
I myself designed similar amp (similar in the class A+B approach, but realization was quite different), that relied on nested feedback loops.
It is more complex than to take any amp, open it's global feedback loop, measure and compare
Too true. A few nights ago, I had a long conversation with a recording engineer about my mike preamp. He just couldn't "get" the notion of a box of gain that simply amplified without coloring or why in the world someone would want to build such a thing.
Heh-heh!
Did you try my Pyramid amps?
That add warmth, like any tube amps. But you can't measure it as some kind of added distortions. They are as low as transistor amps have, that have no tube warmth.
Any amp is "Good Enough", no perfect amps exist. The amp is the result of optimization of multiple variables, and criteria of optimization are different. Some designers know standards what and how to optimize, some designers know secretes what and how to optimize, but in any case going for better one parameter you inevitably make worse some other parameters.
You don't speak his language, obviously. Let him decide how good is sound of the mike preamp that adds nothing you don't like, and both will be happy.
Absence of colorations usually is perceived by great ears as soft, clean, warm, lovely sound. I gave up my previous attempts to convince them that I can't add such colorations. Instead I say, "Yes, sir!: and remove everything odd to get clean sound.
I have unfinished solid state (well, with tube driver) project in my living room that will add 1.4 KW of warmth, mostly by solid state devices.
Of course not, that's not how recordings (or even live) work. Mics and preamps are chosen for their "sound". They are tools to achieve and end, an artistic end. If that were not the case, all you'd ever need would be a pair of B&K measurement mics. Record anything and everything with those.
Photographers use filters, odd exposures, Photoshop and printing tweaks to achieve the look they want. Straight forward photos are often pretty boring.
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