transformers/output impedance

Gentlemen, what Ralph appears to be referring to regarding the relation between feedback and output impedance was discussed extensively in this thread, beginning with the post that the link opens at.

Best regards,
-- Al

P.S. to my last post. Was thinking to myself that if zero negative feedback is so disadvantageous, why would top flight companies like ARC use it???

This is just a guess ... so here goes. Perhaps simply stated ... NF is used to reduce the output impedance and increase DF for **marketing** purposes.

As Ralph and Al have explained many times already, the world of tube friendly, true Power Paradigm speakers is not as large as Voltage Paradigm friendly speakers. So .. using NF to reduce output impedance and increase DF increases the playing field of speakers that may be compatible. That's my thesis.

Perhaps a good case in point is the match with my amp and speakers. Technically speaking, my speakers are poster-child "SS-amp" friendly speakers because they have rough impedance and negative phase angle functions. Yet my ARC amp manages to muscle its way through the tough spots with a modicum of aplomb.

I am somewhat dubious that a true Power Paradigm amp having a "high'ish" output impedance would do as good a job. Not because of design, build or sonic deficiencies. But just because of the "high'ish" output impedance. At the very minimum, all other factors being equal, there will be a tipping point where acoustic coloration will outweigh reduced NF induced distortion (e.g., TIM and odd-ordered harmonics).

I don't know how most Power Paradigm amps address power supply issues. I suspect that access to lots of joules can only be a good thing when the amp is being tasked to deliver current when called upon.

This post is just a bunch of guesses. So please don't come down hard on poor little me. Perhaps Ralph who is in the business can speak to the bona fides of my surmises.

Cheers,

Hi Bifwynne, first you are correct that the cathode cross-coupling thing you see in ARC amplifiers is a form of negative feedback. I've used this a lot when modifying Dyna ST-70s. I think the oldest amp I have seen with that was employed in a Leslie organ speaker made in the 1950s.

To your second point- the highish output impedance of a Power Paradigm amplifier (SETs being a good example) may or may not be a problem, based entirely on what speaker you are using. You may recall in that article I wrote on the subject that I mentioned that the two design approaches can't be mixed or else you wind up with tonal anomalies (due to the voltage response of the amp on the speaker).

This is why horns had such a bad reputation for so long- their crossovers were designed for amps with a higher output impedance, and so the older ones in particular can sound 'honky' as drivers in the system can be trying to operate outside of the passband the designer had in mind.

Conversely, a choke in series with a woofer is supposed to cause the woofer response to fall off at a certain frequency, but if the amplifier output impedance is high, the amplifier power will not drop off in the way that it is supposed to, causing the woofer to play higher frequencies than the designer had in mind (this would be an example of a Power Paradigm amp being used with a Voltage Paradigm speaker).

Regarding the output impedance (for those that did not follow the link Al dropped), its pretty simple. Amps with a lower output impedance can drive lower impedance loads with greater ease than amps with a higher output impedance. I think we all understand that simple fact.

So if we add feedback, will that cause the amplifier to make more power into lower impedances? The answer is 'no'. You get more power into lower impedances by having larger heatsinks, larger power transformers, larger output transformers, more tubes, etc. You can't get more power out of an amp by adding feedback- that would violate a fundamental rule of electronics known as Kirchoff's Law.

IOW, the term 'output impedance' as we generally understand it in audio is a 'charged term' in that it has a meaning that only works inside the Voltage Paradigm in audio. Anywhere else the term refers to the internal impedance of the output circuit, whatever that might be.

Al and Ralph, I reread Ralph's explanations in the other thread. I understand a little bit better why using the term "output impedance" is a bit of a misnomer. Maybe the industry could sex-it-up by referring to the attribute as "synthetic" or "apparent" or "hypothetical" output impedance.

I think the point that Ralph was trying to make is that using a meter to measure an amp's impedance/resistance at the output taps is **not** what the term "output impedance" is speaking to. Instead, it's referring to the amp's operational behavior in delivering current/power to a speaker load **as though** it's output impedance was a specified number.

Where an amp's "apparent" output impedance is close to zero, one could expect that the amp would double down current/power if speaker load is halved, but only if it is acting like a true constant voltage source, and **only if the amp is performing within its operational limits.**

And ... as Ralph said, using NF only permits the amp to perform in such a fashion. Ergo why the use of the term "output impedance" without an additional adjective is a bit of a misnomer, almost misleading.

I guess in a goofy kind of way, if a designer kept adding NF to the circuit, at some point, the NF could be greater than the current/power being delivered by the main output circuit. This may be what Ralph meant when he said that NF could also be viewed as a voltage source.

Well, maybe I still don't get all of it, but maybe a little bit more than I did before. This still makes amp/speaker matching a tricky business.

Output impedance and ability to drive low impedance are two different things. My amplifier has DF=4000 at low frequencies but cannot drive speakers below 3 ohms.

I cannot speak of tube amps but in SS amp feedback always reduces output impedance. I made this small example a while ago as a proof:

Let’s take amplifier that has gain of 30 (31.6dB). When input voltage is 1V output voltage is 30V. Output voltage drops (for whatever reason) 1V under 1A load to 29V. That's 1ohm output impedance (DF=8).

Now, let's build this amp with gain of 300 but feed 3% of the output voltage back to the input in opposite phase. As a result amplifier’s output is the same 30V as before but input is the difference between 1V and 3% of 30V = 0.1V Let’s verify (1V-0.03*30V)*300=30V

Let’s load this amplifier with 1A. Our voltage drop inside is still 1V under 1A load, but output voltage will be higher than 29V because we subtract less from the input. Output voltage will be 29.9V and output impedance will be 0.1V/1A=0.1ohm (DF=80). Let’s verify. (1V-0.03*29.9V)*300-1Vdrop=29.9V.

Output impedance dropped 10 times. Expression 1+B*Aol is known as “Improvement Factor”. In our case B (“Feedback Factor”) = 0.03 (3%), Aol (“Open Loop Gain”) = 300 thus Improvement Factor = 1+0.03*300=10

Adding NFB WILL add more power to lower load impedance because output impedance is lower. I'm sure Ralph has maximum power in mind.

It is important to realize that it doesn't matter in above example why voltage initially dropped by 1V at 1A load. It could be output impedance or bandwidth limit (or anything else). NFB will reduce output impedance, increase bandwidth, improve linearity - hence THD and IMD.