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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

Showing 5 responses by bifwynne

bifwynne OP
2,206 posts
 
Thank you Kirkus for your cogent and thoughtful response. Hopefully, we will read more posts that address the very narrow issue about using NF to compensate for "varying impedance-vs-frequency characteristic[s]" of speakers.

You also said that "[i]f the amplifier's output impedance differs significantly from that which the speaker designer used for evaluation, then the response of the speaker will be different from what the designer intended."

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.

Therefore, the amount of acoustic deviation between the actual versus intended presentation presumably will be affected by how flat (or "tube friendly") the speaker's impedance curve is, and how much NF is being using to reduce the output impedance of the amp, be it SS or tube. I surmise that if a tube amp uses NF to reduce its output impedance, it will behave somewhat like a "Voltage Paradigm" amp. In other words, the acoustic deviation between what the designer intended and actual performance may be mitigated (i.e., reduced). Is this correct?

So Kirkus, the follow up to my Q was how do so called zero NF amps manage to compensated for changing speaker impedances?? Is the bottom line objective simply to reduce the amp's output impedance by whatever means is possible, i.e., through NF or otherwise?

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. Is that all that is needed, low output impedances and the amp will be a champ??

Thanks again.

BIF
bifwynne OP
2,206 posts
 
Quick follow-up to my last post. As I recall, some time back, one or more of our EE audio tech members commented that a DF of more than 20 DF doesn't add much acoustic benefit. I think a DF of 20 correlates to an output impedance of .4 ohms.

In late 2011 or sometime in 2012, ARC released a new Referance tube amp, the Ref 150. This baby has a 1000 joule power supply, which I assume is a lot. In addition, the ARCDB web site says the Ref 150 uses 14 db of NF and has a DF of 17, the later stat getting close to the DF factor of 20 mentioned above.

Synthesizing all of this at a high level, what I understand is that lowering output impedance much beyond .4 ohms may yield little benefit. However, at .4 ohms or lower, the amp (be it tube or SS) will behave like a Voltage Paradigm amp and be better able to compensate for the changing impedances values of a speaker in a way the designer intended. Hence, using the Ref 150 as an example, can one expect that it might do a fair job handling a speaker that was designed to be driven by a tube amp, even if the speaker does not have tube friendly impedance curves and negative phase angles??

I suspect I'm probably mixing and matching concepts here, but hopefully you can untangle what I just wrote in this post.

Thanks again.
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.
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. ;>')
bifwynne OP
2,206 posts
 
Marakanetz, based on what littleI do get, I think Kirkus is spot on. I doubt the outcome would be diffent though. I think both would advise that in order to make the best decision, you gotta listen to the gear.

However, as others have also written, auditioning gear can be challenging because there aren't many hi-end B&M retail stores around. And even if you're close to a good hi-end outlet, it may not have all the gear you're intersting in.

So, what are you left with?? Go with your gut, research as much as you can, read the Forum to see if others have been where you are going??? And, as I mentioned above, sometimes it's just trial and error. I've done a lot of that. So far I've been lucky because I haven't lost too much on the flips.
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