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Any interest in a 32 ohm speaker?

208.98.182.143
Posted on May 2, 2014 at 13:45:47
Duke
Dealer

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Location: Princeton, Texas
Joined: March 31, 2000
 How much interest would there be in a 32 ohm speaker? Of course it would have to meet your other criteria, whatever they may be.

I'm under the impression that the S-30 would deliver roughly double its rated power [correction: 50 watts] into a 32 ohm load, and that the M-60 also would see a considerable increase [77 watts], but not quite double. Any corrections, clarifications, comments, or additional information is welcome.

Thanks!

Duke

Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.

 

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output power, posted on May 2, 2014 at 14:13:51
Ralph
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 The S-30 goes to about 50 watts with 32 ohms.

The M-60 is about 77 watts- puts out 80 in to 16 ohms.

32 ohms would have tremendous advantage in terms of reducing distortion from the amp if the amp is able to drive the load; Fisher used to make an amp with 32 ohm taps on its output transformers.

In out amps the output tubes will also run cooler and last longer- the amp will actually draw less power from the wall as well. The speaker cable would be really non-critical too.

BTW we gotta guitar amp chassis that could do with a 32 ohm load- two 10" in an open back cabinet would be nice.

 

RE: Any interest in a 32 ohm speaker?, posted on May 2, 2014 at 14:33:31
hitsware
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Posts: 2636
Location: N. Calif.
Joined: December 30, 2004
 I'd like 32 Ohm drivers to parallel 4
for an 8 Ohm load.
(8" please) ... :)

 

RE: output power, posted on May 2, 2014 at 14:57:45
Duke
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Joined: March 31, 2000
 Thanks for the corrections and additional information! I went back and edited my original post.

I had thought that the distortion would decrease, but wanted to hear it from you to be sure that was the source of what I've been hearing. In my experience the subjective improvement in going from 4 ohm to 16 ohm speaker hookup (going from parallel to series) was quite substantial even with solid state amps.

But I didn't realize that tube life would be improved and power consumption reduced. Sweet!

What would we expect to see from a 32-ohm load on the MA-1, MA-2, and Novacron?

Also, what is the physical width of your guitar amp chassis? I think you told me once but I've forgotten. Haven't dabbled in guitar cabs for a couple of years, but I do have an idea that might be worth trying out.

Thanks!

Duke

Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.

 

RE: Any interest in a 32 ohm speaker?, posted on May 2, 2014 at 15:08:31
Duke
Dealer

Posts: 4429
Location: Princeton, Texas
Joined: March 31, 2000
 I should have been more clear... I'm talking about a 32-ohm speaker system, not the individual drivers. Eminence makes a 10-inch 32 ohm bass guitar speaker that is meant to be used in multiples, wired in parallel.

If your end goal is an 8-ohm load, is there any particular reason why you prefer four 32-ohm drivers in parallel over four 8-ohm drivers in series-parallel?



Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.

 

RE: Any interest in a 32 ohm speaker?, posted on May 3, 2014 at 11:00:35
hitsware
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 Drivers in series don't really work together.
I've had this demonstrated both aurally and instrumentally.
I.E...
2 drivers with slightly different Fs if paralleled will
measure with Fs that is the average of the 2
BUT ..
The same 2 drivers in series will measure the 2 Fs's separately.

 

RE: Any interest in a 32 ohm speaker?, posted on May 3, 2014 at 13:55:13
Duke
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 "Drivers in series don't really work together."

That has not been my experience. I received a Golden Ear award from Robert E. Greene of The Absolute Sound, who does extensive measurements which are not included in his reviews, for a speaker that had woofers wired in series as well as tweeters wired in series.

"I've had this demonstrated both aurally and instrumentally.
I.E...
2 drivers with slightly different Fs if paralleled will
measure with Fs that is the average of the 2
BUT ..
The same 2 drivers in series will measure the 2 Fs's separately."

I did extensive measurements before using series connection, to find out for sure what happens to the T/S parameters (there are inconsistencies in the information available on the 'net from professional sources). Fs, Qms, and Qes don't change. Re and Vas both double as expected. The slight wiggle in parameters that I observed in my measurements was within the margin of error of the test equipment I was using.

And subjectively I and an associate whose ears are better than mine have both preferred series over parallel connection when we had a preference. With one beefy Class A solid state amp that he tried, to his ears the difference was inconsequential, as I recall. I think it was a Plinius. To the best of my recollection, every other test either of us have done where the amp wasn't driven into clipping, we have preferred the series connection.

Duke


Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.

 

Hi Duke!, posted on May 3, 2014 at 14:48:06
Lew
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 Nice to hear from you.
I'd love to hear such a speaker, and ideally it would be so efficient that whether or not the Atma amps would double their nominal power outputs would be irrelevant. I am talking 90db or better, not "horn" levels of efficiency.

 

RE: Any interest in a 32 ohm speaker?, posted on May 3, 2014 at 15:22:58
hitsware
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 You're not the first to refute my 'knowledge' on
this subject, though perhaps the best qualified :)

I myself (on more than one occasion), using a
'woofer tester', have observed the 'double hump'
impedance response as opposed to the 'averaging'
response of the same drivers paralleled.

I have also heard a sort of 'phasing' effect of
series drivers 'supposedly' caused by the 2 resonating
at different frequencies and beating.

Perhaps you have used drivers better matched than I.
Well matched drivers take away the double hump for sure
and thusly perhaps the phasing.

As a 'purist' I prefer not to use series-parallel, but
as a realist (should I really need the array) I will ...

 

RE: Any interest in a 32 ohm speaker?, posted on May 3, 2014 at 16:44:48
Duke
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 Well our experiences have been different, so we've come to different conclusions... like that never happens!! I will look for that difference in resonant frequency impedance bumps that you observed next time I run measurements though - I hadn't noticed it, but that may well be because I wasn't looking for it.

Unfortunately there are some significant technical hurdles when it comes to building a practical 32 ohm woofer: When we increase the voice coil's DC resistance, the Qes will go up proportionately... UNLESS we also increase the BL.

There are two ways that we can increase the BL: We can make the magnet more powerful (which is expensive and has practical limits); or we can increase the number of turns of the voice coil inside the magnetic gap.

If we increase the number of turns in the magnetic gap by making the wire thinner, thermal power handling goes down rapidly. If we increase the number of turns in the magnetic gap by making it a multi-layer voice coil, the voice coil inductance and moving mass both go up, the former inevitably rolling off the top end, and the latter trading off efficiency but gaining low-end extension in return.

My point being, like everything else in speaker design, high impedance woofer design is a juggling of tradeoffs. The best we can hope for is to be informed jugglers, and/or catch our mistakes early!



Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.

 

Hi Lew!, posted on May 3, 2014 at 17:16:20
Duke
Dealer

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Location: Princeton, Texas
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 Likewise, nice to hear from you!

The design I'm contemplating would have an efficiency in the 93 dB ballpark, so even with the S-30 we'd theoretically be able hit 110 dB. Thermal power handling would be good (400 watts), so thermal compression - or more precisely thermal modulation - should be negligible at normal listening levels.

Which reminds me of an under-appreciated advantage of a Power Paradigm amplifier: As the voice coil heats up and its resistance rises, the wattage that the amplifier delivers stays essentially constant, thus negating the major source of thermal modulation. A Voltage Paradigm amplifier (typical solid state amp), on the other hand, delivers reduced wattage as the voice coil heats up (constant voltage into higher impedance = reduced wattage), so thermal modulation can become significant on peaks, reducing the power delivered to the voice coil on such peaks, thus robbing the music of dynamic contrast, and dynamic contrast = emotion.

I'm using the term "thermal modulation" to describe the effects of the virtually instantaneous heating of the voice coil when it gets hit with a lot of power. Considering that a woofer's efficiency is perhaps around 1%, a 100-watt peak is like touching it with a 99-watt soldering iron, and so it heats up immediately (and cools down more slowly). This rapid-onset thermal modulation is not well documented to the best of my knowledge, but smarter people than me believe that it is audibly significant (Earl Geddes for one).

The better-known thermal issue is thermal compression, which includes not only the increase in the voice coil's DC resistance as it heats up, but also the eventual heating of the entire magnet, which results in a loss of magnetic flux and corresponding loss of efficiency. I see that phenomenon in prosound a lot, where the musicians feel like their amps have run out of power on the last song of the set, but then everything is fine when they start the next set. It's not their amps losing power; it's the speaker magnets in their cabs getting hot and temporarily losing magnetic flux. It's probably rare for speakers to be pushed so hard in a home audio setting that the magnets heat up and lose flux, so I think that thermal modulation from the heat-induced rise in voice coil resistance is the primary culprit at home. Which Ralph's amps neatly deal with!


Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.

 

I've been using..., posted on May 4, 2014 at 06:00:46
sk
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Joined: April 22, 2000
 the Zero Auto Transformers with my Atmasphere M60's and or Novacrons making these amps see a 12 ohm load from a 6 ohm speaker (AZ Crescendos) with great success. I've tried the 18 ohm load or 24 ohm load taps but the 12 ohm load setting sounds the best.

(Dealer disclaimer)

 

RE: output power, posted on May 4, 2014 at 18:38:24
hitsware
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Location: N. Calif.
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 What's the output Z of your amps ?

 

RE: I've been using..., posted on May 4, 2014 at 21:16:59
Duke
Dealer

Posts: 4429
Location: Princeton, Texas
Joined: March 31, 2000
 Thanks for posting... I went to your website, and that's a very nice lineup of companies you represent. Needless to say, I really like your taste in OTL amps!!

Regarding the superiority of the impedance-doubling taps of the Zeros with your M-60 and Novacron amps on your 6-ohm Acoustic Zens (very nice speakers)... do you think that's because the amps sound their best into the resulting 12-ohm load, or because the Zero sounds its best when only asked to step up the impedance by a factor of two, or some combination thereof? I realize that question may be impossible to answer, but ask it just in case.

Thanks!

Duke




Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.

 

RE: I've been using..., posted on May 5, 2014 at 02:06:37
sk
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 Hi Duke ~ for me that's impossible to answer it's just what I hear however I had an EE over a few months back who is rebuilding an older Novacron and he says the Novacrons like to see a 12 ohm load because of (and then he started talking above my pay grade!!).......
Maybe Ralph can chime in and give us his thoughts.

(Dealer disclaimer)

 

RE: output power, posted on May 5, 2014 at 08:09:08
Ralph
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 Yes, even with solid state amps the distortion is lower, so they sound smoother and more detailed.

With our larger amps the output power drops a little and the distortion also decreases.

The guitar amps chassis is 20.125" wide.

 

output impedance, posted on May 5, 2014 at 14:28:40
Ralph
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 The term is a bit tricky and there are two theories about how to measure it.

Do you want to know the impedance of the output section? Or the servo gain of the feedback mechanism? Both are expressed in ohms.

 

Woofer tester?, posted on May 5, 2014 at 15:06:56
Ralph
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 What kind of woofer tester?

I'm also curious about this: a sort of 'phasing' effect of
series drivers 'supposedly' caused by the 2 resonating
at different frequencies and beating.

The term 'beating' sounds like you are suggesting intermodulation. Is this the case?

 

12 ohms is not a special number for the Novacron, posted on May 5, 2014 at 15:27:08
Ralph
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 12 is good, 16 is even better. So my guess is that the ZERO has its own limitations. Generally, the less you make it do, the less sonic footprint it has.

 

RE: Woofer tester?, posted on May 5, 2014 at 16:00:25
hitsware
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 > What kind of woofer tester?

The Dayton DATS now, but started with WT-1

> The term 'beating' sounds like you
> are suggesting intermodulation.
> Is this the case?

Right. The differance frequency of the 2
Fs's. Too low to be heard as a tone, but
discernable as slight phasing effect .

 

RE: output impedance, posted on May 5, 2014 at 16:09:42
hitsware
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 What resistance accross the output
halves the open loop voltage ?
Say @ 1 kHz .........

 

where 'open loop' means unloaded ..., posted on May 5, 2014 at 20:36:00
hitsware
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Posts: 2636
Location: N. Calif.
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 ...

 

RE: Woofer tester?, posted on May 6, 2014 at 09:18:10
Ralph
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 Do you have any idea why that would not happen when the drivers were in parallel? Wouldn't the two midrange drivers express their own individual resonances anyway?

 

RE: output impedance, posted on May 6, 2014 at 09:59:52
Ralph
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 That is a common means that works fine for a high impedance source but in a power amp seems to be a flawed means. Wouldn't you want to see that it makes half Power rather than half voltage? That way the impedance of the circuit under test will be equal to the known impedance of the load; where half the power generated by the output section is dissipated in the load and the other half in the output section itself.

Here are the numbers from the two methodologies (the Voltage and Power paradigms).

For the M-60, the impedance of the output section is about 4 ohms, as predicted by the formula and confirmed by measurement. The Voltage method gets more like about 16-17 ohms.

The M-60 makes its maximum power into 16-17 ohms, which suggests that its actual impedance must be lower.



 

RE: Woofer tester?, posted on May 6, 2014 at 10:16:24
hitsware
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 > Do you have any idea why that would not
> happen when the drivers were in parallel?

Sorta metaphysical, but evedently because the
signal 'wave' travels (in the case of series)
accross the drivers one at a time, while crossing
both at once in parallel ?

 

RE: output impedance, posted on May 6, 2014 at 10:56:13
hitsware
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Posts: 2636
Location: N. Calif.
Joined: December 30, 2004
 Gnd-----Volts-----OutputZ-----Measure-----LoadZ------Gnd

It seems obvious that Measure will be Volts/2 when
LoadZ = OutputZ
Also that the same Power will be dissapated in both Load
and Output
Perhaps my schooling was bunk ? .... :)
(unless there is a powerful reactive component somewheres)

 

RE: Woofer tester?, posted on May 6, 2014 at 12:10:51
Ralph
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 Actually I was referring to the midrange drivers with the intermodulation.

But with regards to the woofers if we are talking about a resonance, it is mechanical in nature and so would seem to manifest either way. That is not your experience?

I can tell you that I have seen a number of very successful speakers using a series woofer arrangement.

 

RE: output impedance, posted on May 6, 2014 at 12:20:27
Ralph
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 The technique is quite solid when no current is involved.

I have to assume though that if there is a resistance that is used as a load that the voltage can only be there due to the fact that current is flowing. So power is involved.

Now if that is the case, you can't have power with no load, right? So you would start with some very high impedance well outside the circuit's range, and decrease the impedance until a peak in power is observed. Now we know the maximum power the circuit can produce. To sort out the output impedance, it will be equal to the load impedance when equal amounts of power are dissipated by the load as is by the output circuit. This point is of course when the load impedance equals that of the internal impedance that is dissipating the other half of the power.

This is how it was done in the old days. If you were educated in audio after the 1970s, its likely that the method you proposed would have been taught. You might want to look at this link to see what is going on.

The Voltage Paradigm has a number of 'charged terms' which don't quite mean the same thing as output impedance. I can give you a wonderful example of how this can affect your viewpoint.

If you were taught this method, you were probably also taught that applying loop negative feedback will lower the output impedance. But have you ever considered that such would violate Kirchoff's Law?

 

RE: Woofer tester?, posted on May 6, 2014 at 14:59:04
hitsware
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 > That is not your experience?

Yes. That is not my experience.

> I can tell you that I have seen
> a number of very successful speakers
> using a series woofer arrangement.

No doubt .

 

RE: output impedance, posted on May 6, 2014 at 15:34:21
hitsware
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Location: N. Calif.
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3.125 Watts = 25 Volts / 8 Ohm so peak power is where Zout = Zload
and Vload = Vnoload / 2

 

Looks like you are on the Voltage Paradigm, posted on May 7, 2014 at 09:23:27
Ralph
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 Which is neither a good or bad thing.

The reason I use 'paradigm' is such exists as a body of thought, outside of which everything else appears incorrect or even blasphemous.

FWIW I did not create the Power Paradigm- it existed before I was born. Google 'Fisher A-55 amplifier' and look at the first hit, which is a YouTube image. The setting at 12 noon on the control is 'Constant Power', at fully CCW it is 'Constant Voltage'. At fully CW the control is marked 'constant current' but that never developed as a common method of driving loudspeakers.

Amplifiers on the voltage paradigm tend to have very low output impedances, those on the power paradigm tend to have output impedances in the range of or just below that of the loudspeaker impedance, constant current amplifiers tend to have output impedances that are some multiple of the impedance of the load.

This all has to do with how the speaker is damped; in the old days there was not a set formula for that; so matching speakers and amps was done differently. That is why older speakers (EV, Altec, JBL and other horns) usually have midrange and treble controls. They are not there to adjust the speaker to the room, they are there to adjust the speaker to accommodate the voltage response of the amp.

(Loudspeakers that operate with open baffle designs are best driven by amps that provide little or no damping and might require only 1:10 as a damping factor in the amp)

EV and MacIntosh led the way to create the idea of making the amplifier into a voltage source; in this way speaker response would be predictable with greater plug and play. This worked quite well with box speakers (although the Acoustic Research AR-1, the world's first acoustic suspension loudspeaker, was designed for an amplifier with a 7-ohm output impedance and was an example of a Power Paradigm device), although not so well with horns, ESLs and planar loudspeakers whose impedance curve does not derive from the resonance of a driver in a box.

However the voltage paradigm pretty well took over in the 1960s and 70s, with transistor amps being more common and with which it was easier to create a constant voltage response. Along with it our methodology of measurement changed- as in output impedance here, also we see the spec of speaker Sensitivity as opposed to Efficiency (which are the same thing only when the speaker is 8 ohms).

FWIW no loudspeaker needs a damping factor of over 20:1 and most are best damped with something less. A link to the article written by the head engineer of EV is below.

 

RE: Looks like you are on the Voltage Paradigm, posted on May 7, 2014 at 12:00:58
hitsware
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Location: N. Calif.
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 I guess so as calculation
BUT I don't subscribe to super
low Z out amplifiers. My main
circuit is ~ 1 Ohm out (channel
resistance of mosfet followers)
(no global feedback)
I'm working on a floating supply
circuit with ~ 80 Ohm out, (sans
feedback), but so far can't get
the hum out :( (I hate when that
happens)

 

RE: output impedance, posted on May 7, 2014 at 12:25:39
cpotl
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 Your argument seems sound to me. It is reasonable to model the output of the amplifier as an ideal (zero-impedance) voltage source in series with a resistor R0. Here R0 is what one would call the "output impedance."

This should be a good approximation as long as the amplifier is not being asked to deliver more current than it is capable of supplying. (If more current than that is asked for, then the amplifier will be overloading and clipping horribly, and so discussions of output impedance then become essentially irrelevant and academic.) So in any measurement, it should always be ensured that the amplifier is not pushed beyond clipping.

There are various ways that R0 in the above model could be measured. One way would be as you said, to connect a load R and adjust R until the measured output voltage is half of the open-circuit output voltage. Another way would be to adjust R until the power dissipated in R is maximised (at fixed output level). The answer in either case will be the same, namely when R is equal to R0. (And the power dissipated in R will equal the power dissipated in R0, if R is set equal to R0.) And so whether judged by voltage or by power, the conclusion would be the same, that the output impedance is R0.

If the M-60 delivers maximum power into 16-17 ohms, then that seems to support the argument that the output impedance is 16-17 ohms.

Chris

 

RE: output impedance, posted on May 7, 2014 at 15:29:58
Duke
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 Having designed speakers on Ralph's amps, my measurements are consistent with his claims for output impedance. For instance I can look at the bass response curves with a high-damping-factor solid state amp, and then with his amp, and see that the change is consistent with the Qes modification predicted by Ralph's output impedance claims.

I'm not EE enough to debate the theoretical validity of his technique, but I'm speaker designer enough to confirm its results via measurements.



Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.

 

RE: I've been using..., posted on May 7, 2014 at 16:03:27
Catastrofe
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 I have MA-1s paired with Daedalus Ulysses, and use the Speltz autoformers in the 4X mode. It just sounds best this way. . .around 24 ohms.

 

RE: I've been using..., posted on May 7, 2014 at 16:32:05
Duke
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 Nice combination!

And thanks for letting me know what impedance setting works best with your MA1 + Zero + Ulysses system. Very interesting that such a high impedance is what you've found to work the best. This is quite encouraging for what I'm hoping to do.

Duke


Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.

 

RE: output impedance, posted on May 8, 2014 at 04:48:07
cpotl
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 "For the M-60, the impedance of the output section is about 4 ohms, as predicted by the formula and confirmed by measurement. The Voltage method gets more like about 16-17 ohms."

Do you have a reference to the formula you are referring to, that gives 4 ohms? I'd be curious to see how that works.

Chriis

 

Nominal impedance of Daedalus, posted on May 10, 2014 at 13:15:59
Lew
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 I would have thought that the MA1 would require no autotransformer help to drive the Daedalus. Doesn't the Daedalus have a higher than 8-ohm nominal impedance, to begin with?

 

RE: Any interest in a 32 ohm speaker?, posted on May 10, 2014 at 18:23:18
Rod M
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Contributor
  Since: March 1, 1999
 The S-30 surely likes a 16 ohm load. I recall Ralph saying that it'd put out 45 watts at 16 ohm.

-Rod

 

Yes one of the best sounds I've had was, posted on May 10, 2014 at 20:11:21
sk
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 using an S30 to drive a 12" PHY speaker. Lovely!!!!!!

 

RE: Any interest in a 32 ohm speaker?, posted on May 10, 2014 at 20:12:28
Duke
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 "The S-30 surely likes a 16 ohm load."

Yes it does!

I've done four Rocky Mountain shows with the S-30 driving 16-ohm speakers, and imo three of those were my top three best-sounding rooms (the other time I had an ear infection and didn't know it, so the top end of my speakers was mis-calibrated - otherwise my 16-ohm/S-30 rooms would have swept my top four spots).

Last year we had a 16-ohm system in the room that had some serious output capability. On a pipe organ disc that someone brought, the low-end pressurization was enough to modulate voices. Another time we briefly cranked the volume on a Kodo drum disc, and the exhibitor two rooms down thought his subwoofer amp had gone into oscillation. Nope, just forty-five real watts!

And the S-30 offers a free lunch to the speaker designer... roughly 1/3 octave greater low-end extension than the same speaker driven by a solid state amp, assuming we take advantage of the S-30's lower damping factor.

I make a habit of designing with the S-30 in mind, as I think it's a real giant killer of a high-end amplifier. It will be in my room at RMAF again this October, surprise surprise.

Duke


Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.

 

Thank you all for your replies, posted on May 13, 2014 at 01:08:58
Duke
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 I appreciate everyone who's taken the time to post in this thread.

I've been working on a speaker that could easily be configured for 8 ohms, but the possibility also existed of configuring it for 32 ohms, at some additional trouble and parts cost. Ralph had told me years ago that he liked the idea of a 32 ohm speaker, and I kinda liked the idea too, but wanted to get an idea of whether there was potential interest. Well it looks like there is... not overwhelming, but enough that I forged on ahead, and it's been done.

Duke


Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.

 

RE: Nominal impedance of Daedalus, posted on May 15, 2014 at 15:55:51
Catastrofe
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 I believe the nominal impedance is around 6 ohms on the Ulysses.

 

So given the high-ish efficiency,..., posted on May 18, 2014 at 22:14:53
Lew
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 I am surprised that you are apparently less happy with either no Zero or Zero used at a much lower multiple of impedance, like 2X or 2.7X, compared to 4X. I guess you've tried all the other options.

 

RE: output power, posted on July 12, 2014 at 06:42:36
Bromo33333
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Location: Ipswich, MA
Joined: May 4, 2004
 "In out amps the output tubes will also run cooler and last longer- the amp will actually draw less power from the wall as well. The speaker cable would be really non-critical too."

Real world, class A runs hottest when putting out little or nothing. I suspect that the impedance of the speaker with average drive (likely under 1W) won't affect the temperature of the active devices (tubes) very much given the dissipation of class A.
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"You are precisely as big as what you love and precisely as small as what you allow to annoy you." ~ R A Wilson

 

RE: output power, posted on July 14, 2014 at 09:49:47
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 FWIW, load plays a huge role in the operation of a class A amplifier with a higher output impedance. If there is an output transformer, loading the transformer on a tap lower than the tap is set up for (4 ohms on the 8 ohm tap) can cause the amplifier to not be operating in the class A region.

This of course is true of OTLs as well. The M-60 can operate class A, or it might be AB2, depending entirely on the load impedance.

There is also the issue of efficiency. If the load is lower impedance, a portion of the power generated by the output section will not be transferred to the load but instead dissipated by the output section itself. This causes it to run hotter.

 

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