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Electronics 101: Use of Negative Feedback; a Q

The topic of negative feedback (NF) has been beaten to death in other posts. Nevertheless, I have a question that touches on another aspect of NF that I do not recall having been explained. The Q starts in the middle of this OP.

By way of summary, many of our technical members explained why NF is used in amplifier circuits, e.g., extends bandwidth, helps to prevent oscillation, reduces distortion, reduces output impedance and therefore increases DF, and so forth. However, there is a cost.

Many of our technical members also explained that NF creates TIM (temporal intermodular distortion) because of the nano-second of time it takes for the NF circuit to tell the input circuit to make adjustments. TIM distortion results in distortion in odd-ordered harmonics (particularly the 5th, 7th and 9th) which our brains interpret as loudness, and in turn acoustic harshness.

Hopefully, I got the basics down. I'm sure my summary will be "clarified" by our techies.

But here's the Q (or 2). I read that NF also enables the amplifier (SS or tube) to regulate its output power to match a speaker's changing impedance stats as frequency changes. Could the tech members please clarify my understanding about this point.

It's kind of important because even though the subject of "tube friendly" and "tube unfriendly" speakers has also been killed, it would appear that electrical matching concerns between an amp and speakers having fluctuating impedance stats as a function of frequency may be mitigated in whole or part by using NF. Of course, I not suggesting that NF will compensate for a short or an open circuit, but somewhere in between there will be harmony. ;>')

Perhaps the technical members can put some "flesh on the bones" with respect to this Q.

I suppose the Q raises a related Q about amps (SS or tube) that do not use NF because of concerns pertaining to TIM distortion as mentioned. Great . . . the TIM distortion problem is solved if no NF is used. But how does the amp regulate its output power to compensate for fluctuating impedance curves?

Thanks.
bifwynne
kirkus
502 posts
 
I believe that Ayre SS amps do not use NF in the circuit design, yet have relatively high DFs, thus suggested low output impedances. I don't know how Ayre achieves these results without NF, but they do.
No. They use feedback, even if they say they don't, or come up with a reason why their feedback is "different". This is akin to all the "LED Televisions" on the market - virtually all of them are in fact LCD televisions with LEDs used as a backlight. But they needed a new acronym to differentiate them from the previous models, and marketing departments aren't exactly known for their terminological precision. How many times have you seen "solid-state" on the outside of a television who's principal component was a picture tube? "Zero-feedback" is a similar label.

In reality, virtually every audio power amplifier without an output transformer (tube or transistor) uses some sort of unity-gain voltage follower circuit as the final stage to lower its output impedance. This type of circuit has 100% local negative feedback, hence the unity gain.
I assume from a lay person's perspective, in plain English, that means if a speaker was voiced to be driven by a "Voltage Paradigm" amplifer (as described in Ralph Karsten's White Paper, a SS amp), then using a "Power Paradigm" amp (usually a tube amp) to drive the speakers may affect the sonic presentation. This assumes of course, that the tube amp has a relatively high putput impedance if little or no NF is used.
Ralph really seems to favor a binary viewpoint of the subject with these two clearly-defined camps of amplifiers and speakers . . . but it's indeed correct that certain some loudspeakers produce far more variation in response with a high output impedance than others. This is an important subject to him because his circuit design preferences place practical limits on how low his amplifiers' output impedances can be.

As for the subject of how low an amplifier's output impedance *needs* to be, I actually think John Atkinson approaches this subject in a rational and practical manner when he measures an amplifier's response into an IHF speaker load, and the corresponding impedance/phase plots for the loudspeakers he measures. It's also possible that one may prefer a loudspeaker's response to be somewhat different that its designer, and that a higher-output-impedance amplifier may yield a pleasing result.

Personally, I can't say I've found such a modification to a speaker's frequency response to be a pleasing one, except for a few very rare occasions. But your mileage may vary . . .
bifwynne OP
2,206 posts
 
Thanks again Kirkus. Admittedly, I am not familiar with Atkinson's test involving an IHF speaker load. Care to explain what that test is all about?

So, going back to the use of NF to reduce an amp's output impedance, in the case of the ARC Ref 150 which uses 14 db of NF and probably has an output impedance between .4 and 1 ohm (probably closer to .4 ohm because its "rated" and reported DF is 17), can one assume that the Ref 150 will behave "SS-like" if presented with a speaker load that was voiced to be driven by a high current SS amp?

Isn't that the bottom line consideration here? Short of shlepping a 75 pound tube amp to a dealer to properly audition speakers, it's helpful to know (or at least be able to reasonably predict) if a tube amp can "switch-hit" and function in a "SS-like" way if presented with a speaker load which was voiced to be driven by a low-impedance, high DF, high-current SS amp. If the foregoing proposition is technically accurate, is it also fair to consider a tube amp's rated output impedance and correlatively its DF, when matching it to a speaker load? Any back of the hand guidelines?

Also, is output impedance less critical in the higher frequencies? Many 3-way speakers have an impedance bump at the midrange-tweeter cross-over point. I believe Ralph's White predicts that a "pure" Power Paradigm amp will deliver more power (watts) than a "pure" Voltage Paradigm amp at higher impedance levels. If so, it may matter.

Thanks again.

P.S. check the Ayre web site re the zero NF point. And I do have a very high-level understanding of the difference between local versus global feedback.
kirkus
502 posts
 
Admittedly, I am not familiar with Atkinson's test involving an IHF speaker load. Care to explain what that test is all about?
Start with http://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements and then read the loudspeaker measurement section of some loudspeakers. Please don't take this as my endorsement of Stereophile's reviewers in general . . . but specifically Atkinson's measurement sidebars tend to be very consistent an well-reasoned, and he explains himself well.
in the case of the ARC Ref 150 which uses 14 db of NF and probably has an output impedance between .4 and 1 ohm (probably closer to .4 ohm because its "rated" and reported DF is 17), can one assume that the Ref 150 will behave "SS-like" if presented with a speaker load that was voiced to be driven by a high current SS amp?
In the above link, you'll see that Atkinson actually measured the output impedance as 1 - 1.4 ohms from the 8-ohm output tap, corresponding to a Damping Factor of 8 - 5.7 respectively. This amp actually has a somewhat higher output impedance than most of the high-quality, correctly-operating vintage tube amps that I've measured . . . most of which actually have lower output impedances than many modern "low-feedback" solid-state designs. I think with the ARC Ref 150, the interaction between the amplifier's output impedance and most loudspeakers' impedance curves will be a significant part of its sonic signature.
Short of shlepping a 75 pound tube amp to a dealer to properly audition speakers, it's helpful to know (or at least be able to reasonably predict) if a tube amp can "switch-hit" and function in a "SS-like" way if presented with a speaker load which was voiced to be driven by a low-impedance, high DF, high-current SS amp.
The loudspeaker/amplifier equation is complex, and schlepping an amp to the dealer (or demoing the speakers in your home) is absolutely the best way to be sure you'll be happy with it. After all, can you really study automotive specifications and get an idea whether or not you'll like a car without at least putting forth the effort to take a serious test drive?

If you really don't want to pair up your amplifier with speakers you're considering, and you want to be happy with the way they work together . . . then I feel the most logical solution is to purchase the amplifier, loudspeakers, and loudspeaker cables all together, or purchase active speakers.
bifwynne OP
2,206 posts
 
Fascinating!

I've copied an excerpt from Mr. Atkinson's review of the Ref 150:

"As expected, the Ref 150's output impedance varied according to the transformer tap selected. The 16 ohm tap measured 1.4 ohms at low and middle frequencies, rising to 1.9 ohms at the top of the audioband. The figures for the 8 ohm tap were 1 and 1.4 ohms; for the 4 ohm tap, they were 0.55 and 0.87 ohm. All three taps offer quite a low source impedance for a transformer-coupled design; as a result, the modulation of the amplifier's frequency response, due to the Ohm's Law action between that impedance and that of our standard simulated loudspeaker, was relatively mild. From the 8 ohm tap (fig.1, gray trace), it was ±0.8dB; the 4 ohm tap offered ±0.4dB, the 16 ohm tap ±1dB. Fig.1 indicates that the Ref150 has a wide bandwidth, particularly into loads higher than the nominal tap value, which correlates with a well-defined 10kHz squarewave (fig.2)."

I assume based on the results of these tests that the Ref 150 performed like a Voltage Paradigm amp to a large degree. That is, even though the simulated speaker's source impedance varied with frequency, the amp was able to maintain relatively level amounts of gain notwithstanding. I further assume that gain correlates to power (watts).

In other words, if I have this right, the Ref 150's circuit topology, using NF, was able to appropriately compensate for the changing impedances of the simulated speaker, which change as a function of FR.

Bottom line: the use of judicious amounts of NF in a tube amp enable the amp to perform, to some extent like a tube amp. Hence, the proposition that some speakers are "tube friendly" versus "not tube friendly" should be accepted with circumspection. It may very well be that the tube amp's use of negative feedback is a compensating factor, to some degree, when driving speakers that may otherwise be considered tube unfriendly.

Do I have it??

If I do, I think folks need to take Electronics 101 when mathing speakers and amps. It ain't so simple. ;>')
marakanetz's avatar
marakanetz
5,473 posts
 
WOW fokls It's much more simplier than you're describing:

--No need to get into one or another's review details
--No need to know EE101
--You can be just a trivial consumer

and the rule is

NEVER JUDGE OR VALUE THE AMP BY PRESENSE, ABSENSE, LARGE OR SMALL VALUE OF NEGATIVE FEEDBACK.

For consumer these are only marketting term values.
For engineers it's simply a value to keep an amplification stage stable to prevent self-oscillations that can damage your speakers.
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