What ohm to set amp

I took a look at the literature on the MC205 at the McIntosh site. It is a solid state amplifier that has neither output transformers nor autoformers nor multiple output taps. It has a rear panel switch for selection of either 4 or 8 ohm speaker impedances. Apparently the function of the switch is to select higher internal voltage rails (the DC power supply voltages that are provided to the output stage) when an 8 ohm speaker is connected, compared to when a 4 ohm speaker is connected. The manual recommends using the 4 ohm setting if a 6 ohm speaker is being used. The power rating of the amplifier is 200 watts into both 4 and 8 ohms, assuming that the load impedance corresponds to the switch setting.

Kalbi, what specific models are your Sonus Faber speakers? Knowing that would hopefully allow us to look up their impedance curves. In any event, if the dips into the 4.1 to 4.7 ohm range occur in the bass region, where a lot of energy is typically required, it seems clear that you SHOULD use the 4 ohm setting, as Schubert indicated. And likewise for the 6.2 ohm center speaker, although the selection is presumably less critical with respect to that impedance, and in any event the amplifier just provides one overall selection between 4 and 8 ohms, not independent selections for each of its channels.

Bruce (Bifwynne), I don't know what a glarff is, but I'll comment on it anyway :-). Both this amplifier and the Mc solid state amplifiers that have autoformers have output impedances that, as with most solid state amplifiers, are a tiny fraction of an ohm regardless of tap or setting. So they will act like typical solid state amplifiers with respect to the interaction of their output impedance with the impedance vs. frequency variation of the speaker.

As to why using a tap on a tube amplifier that is intended for an 8 ohm load can sometimes be preferable to using a tap intended for a 4 ohm load, as you realize some speakers conform to what Ralph refers to as the power paradigm, and are designed to sound their best when used with an amplifier having relatively high output impedance. Also, a mismatch between the actual load impedance and the impedance a given amplifier output tap is designed to drive can, if severe enough, degrade the sonic performance of the amplifier, as Ralph indicated above.

Best regards,
-- Al

Thanks Bruce; I've learned a new word today :-)

As I indicated, the MC205, as well as the MC solid state amplifiers that use autoformers (which the MC205 does not use), have negligibly small output impedances. The MC205 is spec'd as having a damping factor of 140, corresponding to an output impedance of 8/140 = 0.06 ohms. So it will act as a voltage source with respect to the interaction of its output impedance with the speaker's impedance vs. frequency variations.

Which means that for a given signal level at the input to the amplifier it will output twice as much power at frequencies for which the speaker's impedance is 4 ohms as it would deliver at frequencies for which the speaker's impedance is 8 ohms, and half as much power into 16 ohms as into 8 ohms.

PROVIDED, however, that the maximum limits of its power capability are not exceeded. What will NOT double into 4 ohms, compared to 8 ohms, is the MAXIMUM amount of power it can put out. And I suspect that if the rear panel switch were set to 8 ohms and the impedance of the speaker is close to 4 ohms in most of the bass region, its MAXIMUM power capability into that 4 ohms would be significantly LESS than the 200W it could deliver into 8 ohms at that setting.

Basically, what the switch is apparently doing is optimizing internal voltage, current, and thermal operating conditions for whichever of the two load impedances is selected, such that a maximum of 200W can be delivered into either impedance.

Best,
-- Al

Does that permit the inference that a SS amp that operates as a Voltage Paradigm amp would be expected to use NF of some type (local or global) to regulate actual power output into loads which vary as a function of FR in order to maintain output power that corresponds to the signal presented at the amp's inputs??

Usually voltage paradigm characteristics (i.e., maintaining constant voltage, not power, into varying load impedances) go hand in hand with the use of some amount of feedback. But that is not always the case. For example many and perhaps all of the Ayre amplifiers use zero feedback, yet have output impedances of a small fraction of an ohm. As a consequence of that low output impedance they will deliver essentially the same voltage and very close to twice as much power into 4 ohms as into 8 ohms when operated within their maximum power capabilities. And, in addition, those maximum power capabilities double into 4 ohms relative to 8 ohms.

Best,
-- Al

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

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

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