What ohm to set amp

Kalbi, I couldn't find a complete impedance curve for the Liuto, but I did find the following comment in this review:

... a nominal impedance of 6 ohms would be more representative than the quoted 8 ohms.

Measured data was presented indicating a minimum impedance of 4.5 ohms at 124 Hz, a current-hungry phase angle of -54 degrees at 68 Hz, and an "equivalent peak dissipation resistance" of 2.2 ohms.

And the measurements in this review indicate a minimum impedance of 4.1 ohms at 100 Hz, at a phase angle of -23 degrees.

Given that those impedance minima occur in the bass region, where lots of energy is typically required, and given also that the manual for the amplifier specifies that the 4 ohm setting should be used for a 6 ohm speaker, it all seems to add up to the 4 ohm setting being best.

My guesses are that the automatic switching mechanism that was referred to may involve circuitry whose primary function is protection, and that its operation may not be sonically seamless.

Bruce, regarding your question consider the case of a small two-way speaker. The combination of small woofer size and small cabinet size will result in low efficiency in the bass region. It is not uncommon for that kind of speaker to have an impedance of close to 4 ohms in the bass region, and close to 8 ohms at higher frequencies. Generally that kind of speaker will conform to the voltage paradigm, and will be intended for use with a solid state amplifier. The near zero output impedance of the amplifier will result, for a given input signal level, in twice as much current and power being delivered at low frequencies as at high frequencies. That doubling of power at low frequencies will compensate for the low efficiency of the speaker at low frequencies, presumably resulting in a flat overall frequency response. If the amplifier were to deliver the same number of watts to that speaker at high and low frequencies, for a given input level, the response would be decidedly bass-shy.

Best,
-- Al

Thanks Al. Would you kindly clarify my understanding of the Voltage and Power Paradigm area just one more time.

At least with my tube gear, ARC publishes so-called output regulation stats. In the case of both the Ref 150 and VS-115, output regulation off the 8 ohm tap is +/- .8 db and probably 60% of that number off the 4 ohm tap. See Stereophile and Soundstage bench test reports.

Perhaps I am misinterpreting what the term "output regulation" means. So please help me here. Based on the explanations provided in the two magazines, I gather that output regulation relates to how closely a tube amplifier's voltage output will vary so that the amount of power (watts) presented to the speaker will correspond to the magnitude of the input signal, thus compensating for varying speaker impedances which change as a function of FR. In short, if impedance increases, voltage will also increase. This in turn will increase current. All of this is accomplished through NF.

As I understand the Voltage Paradigm, SS amps generally maintain constant voltage and increase or decrease power in response to speaker impedance changes. But here again, if a SS amp is asked to feed current into a high impedance segment of the speaker's FR spectrum, somehow the amp must increase its current output or else power (watts) will decrease. Here again, I would think that NF would throttle up or down as the case may be the amount of the amp's gain in order to compensate for these variations.

I surmise that since Voltage Paradigm amps naturally increase power in low impedance loads, it is easier for the SS amp to deliver current and ultimately power into the bass regions. In contrast, if presented with a high impedance segment of a speaker's FR spectrum, the amp will need to deliver much more current in order to "power" its way through the speaker's impedance mountain, and may choke. Again this is where NF comes into play ... I think.

If I am tracking so far, what I don't understand is how a so called zero NF amp (tube or SS) can properly respond to impedance values which change as a function of FR in order to maintain constant power through a speaker's FR spectrum. I would have guessed that some type of NF, either global or local would be needed. It would seem that the only way to avoid NF is to use a speaker that has extremely flat impedance curves and phase angles.

Btw, I was reading some of the tech data on the Ref 150. Can't say I understand it, but from what I gather, ARC has somehow directly coupled the power tubes to the output trannies in order to regulate power output. I don't know if this is just another term for local feedback, but that's as much as I understand. I recall ARC used the term partial cathode following, or something like that.

Sorry for getting back into the tech stuff again, but all of this touches on the OP's question and the area is confusing, albeit interesting, to this lay person.

I gather that output regulation relates to how closely a tube amplifier's voltage output will vary so that the amount of power (watts) presented to the speaker will correspond to the magnitude of the input signal, thus compensating for varying speaker impedances which change as a function of FR. In short, if impedance increases, voltage will also increase. This in turn will increase current. All of this is accomplished through NF.

No, that's not right Bruce. Output regulation in this context refers to how little or how much the amplifier's output voltage will vary as a function of load impedance, for a given input signal. Period. The magazines define it based on the range of impedance variation of a particular simulated speaker load. The ARC specs define it based on a load variation from 8 ohms to infinity ohms (i.e., an open circuit). The tighter the regulation, meaning the smaller the +/- number, the LESS the voltage will increase as load impedance increases, and the LESS the voltage will decrease as load impedance decreases.

An ideal voltage paradigm amplifier will have a regulation of +/- 0.0000 db. Its output voltage will not change at all as load impedance varies. And, per Ohm's Law, for a given output voltage the amount of current drawn by the load will vary inversely with the impedance of the load. So since power is proportional to voltage x current, and equals voltage x current in the case of a purely resistive load, the power delivery of a voltage paradigm amplifier will NOT "correspond to the magnitude of the input signal."

And a voltage paradigm speaker is, by definition, one that is designed based on the expectation that it will be driven by an amplifier which behaves in that manner. Such as the small two-way I described in my previous post.

If a SS amp is asked to feed current into a high impedance segment of the speaker's FR spectrum, somehow the amp must increase its current output or else power (watts) will decrease.

In that situation power WILL decrease, but if the speaker is (correctly) designed with the expectation that it will be used with a solid state amplifier, having negligibly small output impedance, the acoustical output of the speaker will nevertheless be correct. In other words, a voltage paradigm speaker whose impedance varies significantly as a function of frequency will require less power at frequencies for which its impedance is high than at frequencies for which its impedance is low, to produce a given amount of acoustical power.

If I am tracking so far, what I don't understand is how a so called zero NF amp (tube or SS) can properly respond to impedance values which change as a function of FR in order to maintain constant power through a speaker's FR spectrum.

So at this point it should be clear that a voltage paradigm amp does NOT maintain constant power as load impedance varies. That is the behavior that a power paradigm amp, having relatively high output impedance and generally minimal or no feedback, will approximate to some (usually loose) degree.

As far as feedback is concerned, an amplifier whose output impedance is negligibly small in relation to the load impedance, at all frequencies, will behave as a close approximation of an ideal voltage source, maintaining an output voltage that does not vary significantly with load impedance (as long as it is operated within the limits of its voltage, current, power, and thermal capabilities). Given a low enough output impedance (as in the case of the Ayre amplifiers) it will do that to a close approximation regardless of how much feedback it does or does not use.

Two closing points:

1)Feedback, as it is usually applied in this context, does not cause output power to "correspond to the magnitude of the input signal." It causes output VOLTAGE to more closely correspond to the magnitude of the input signal (multiplied by some gain factor).

2)As Ralph has pointed out in the past, tonal balance problems result from using speakers with amplifiers that are not of the same paradigm.

Best,
-- Al

Al, thanks again for the education. So to restate what you said above, "[t]he tighter the regulation, meaning the smaller the +/- number, the LESS the voltage will increase as load impedance increases, and the LESS the voltage will decrease as load impedance decreases."

"An ideal voltage paradigm amplifier will have a regulation of +/- 0.0000 db. Its output voltage will not change at all as load impedance varies. And, per Ohm's Law, for a given output voltage the amount of current drawn by the load will vary inversely with the impedance of the load. So since power is proportional to voltage x current, and equals voltage x current in the case of a purely resistive load, the power delivery of a voltage paradigm amplifier will NOT "correspond to the magnitude of the input signal."

And as to Ralph's comment about tonal balance problems, I suppose using a tube amp that has a high output impedance implicates the mismatch issue more so than using a tube amp that has a lower output impedance.

Last and final point: I think I'll keep my day job.

And Al .... thanks again for the education.

Bruce

As to Ralph's comment about tonal balance problems, I suppose using a tube amp that has a high output impedance implicates the mismatch issue more so than using a tube amp that has a lower output impedance.

You've got it correctly!

Best regards,
-- Al