True Class A Amps

thesurfer

Just curious, who of you own one,

pitdogg2

3 that I recollect

afterburnt

Emo's must be class A because they burn the front lawn.

BlueFox

Not pure. Class A for the first 20 watts, which is fine for me.

nooshinjohn

retracted.....

motorstereo

I had a pair of Monarchy sm70 monoblocks that were class A. Ran hot as a firecracker, weren't my cup of tea sonically and required putting a grease fitting on the power meter. I did the happy the dance the day they left a few months ago.

mhardy6647

oh, oh, oh! pick me, pick me!

DSC_4280 (2) by Mark Hardy, on Flickr
Single Ended Class A direct heated triode (2A3) JE Labs "Simple 2A3" variant.

Or, if you prefer soiled solid state:
(courtesy of @ALL212 and an anonymous benefactor who actually directed it to me -- which was/is very much appreciated):

DSC_2871 (2) by Mark Hardy, on Flickr
Single Ended Class A "Amp Camp Amp" variant.

Or even vintage?




OK, this one has switchable Class A PP operation (for something like 10 wpc), but it does sound good when so switched, even forty years after it rolled (or whatever) off the assembly line.

There are others here (mostly SE vacuum tube amps, which operate in Class A by definition).

gfong

got one! Unison Research

Gerres26

My Aragon is a class A/B amp but is biased into class A for the first 26 watts...sounds excellent to me

txcoastal1

Owned Threshold Stasis and like Gfong a Unison research

Also owned an XA30.5

heiney9

motorstereo wrote: »

I had a pair of Monarchy sm70 monoblocks that were class A. Ran hot as a firecracker, weren't my cup of tea sonically and required putting a grease fitting on the power meter. I did the happy the dance the day they left a few months ago.

Pure class A is considered "single ended". The Monarchy's are not single ended.

Stasis is not pure class A if you use the above definition, which most do.

My Pass Aleph 30 is pure class A as it is single ended. Simply put the conductors are on 100% all the time.

Research single ended and it will explain. There are opinions on the subject and many feel pure Class A sounds better than a push/pull configuration. But there are lots of variables and lot's of "hybrid" designs (for lack of a better word) that sound great.

H9

motorstereo

^^^^^Thanks for the clarification but I'll take their word for it

mhardy6647

heiney9 wrote: »

motorstereo wrote: »

I had a pair of Monarchy sm70 monoblocks that were class A. Ran hot as a firecracker, weren't my cup of tea sonically and required putting a grease fitting on the power meter. I did the happy the dance the day they left a few months ago.

Pure class A is considered "single ended". The Monarchy's are not single ended.

Stasis is not pure class A if you use the above definition, which most do.

My Pass Aleph 30 is pure class A as it is single ended. Simply put the conductors are on 100% all the time.

Research single ended and it will explain. There are opinions on the subject and many feel pure Class A sounds better than a push/pull configuration. But there are lots of variables and lot's of "hybrid" designs (for lack of a better word) that sound great.

H9

Well, no -- not exactly. It is perfectly possible to bias a push-pull amplifier to operate in Class A (both sets of devices "on" and conducting for the full cycle). This is incredibly inefficient (which is why the aforementioned Yamaha CA-800 runs so hot in Class A bias!). BUT there are push-pull amps (tube or solid state) that operate in Class A. Very inefficient, but can sound very good.

The takeway is this: single-ended (SE) vs. PP (push-pull) doesn't give enough information to "answer the question" -- i.e., "Is the amp operating in Class A, Class B, or Class AB?".

NOW, that being said, all single ended amplifiers are operating in Class A -- they have to, by definition. This is true no matter how many output devices there are in the single-ended amp. E.g., it is also perfectly possible to make a 'single ended parallel' amplifier stage with two output devices (or even more!) operated in parallel -- but the class of operation will still be Class A.

Hope this is helpful!

heiney9

motorstereo wrote: »

^^^^^Thanks for the clarification but I'll take their word for it

It's market speak. Beleive what you want, but I've heard them, my brother had a pair and I've examined them and they don't fit the definition of pure class A.

There are lots of buzz words and you can twist the phrase "pure class A" anyway you want.

To most pure class A = single ended. The Monarchy's are not single ended and they even use an op-amp at the output stage.

I'm out of this one because it's a very misunderstood concept because of the marketing guru's mucking it all up.

Get your learn on if you are truly interested.........then you'll see what I mean.

H9

P.s I thought they sounded wonderful, even with an op-amp in the output.

mhardy6647

Again, all single ended amplifiers operate in Class A but that in no way implies that a push-pull amplifier cannot be operated in Class A. For very practical reasons (heat and power consumption being the major reasons), most push-pull audio amplifiers operate in Class AB. Class B is the most electrically efficient, but suffers from some anti-high-fidelity distortion issues (see below). Class AB is a compromise that permits reasonably
efficient operation and reasonably acceptable distortion behavior for "hifi".

The problem with any fixed bias operating point that switches the active devices (tubes or transistors) off for any part of the waveform is a rather unpleasant form of nonlinearity referred to as crossover distortion. In pure Class B, the crossover distortion occurs at exactly zero volts (the "crossover point" from negative to positive) of the signal waveform, and will be at its worse. Class AB bias points, which allow some overlap of activity between the "push" and "pull" output devices, can decrease the crossover distortion, as can negative feedback.

Class A push-pull amplifiers have the harmonic distortion behavior of all push-pull amplifiers (i.e., cancellation of even harmonic distortion products) but they will lack crossover distortion.

Nelson Pass' "F5" is an example of a Class A push-pull hifi amplifier; you can build one yourself.

https://www.audioxpress.com/article/you-can-diy-the-f5-power-amplifier
http://www.firstwatt.com/

It's not related to amplifier operation "class", but there is a profound difference in the distortion spectra of single-ended vs. push-pull amplifiers. That's a different topic entirely, but the difference between single-ended and push-pull probably has far more real audible impact in a good hifi amplifier than does Class A vs. AB bias (in a push pull amplifier).

The problem with marketing hyperbole comes in with a plethora of "pseudo-Class A" amplifier bias schemes that are claimed to give the best of both worlds. My advice, FWIW, view such claims skeptically but use the ears to evaluate the results (rather than reading ad copy, magazine reviews, or internet pontification -- like mine! ).

mhardy6647

Oh, while I am on the soap box
Op amps.

1) It is perfectly possible (indeed, very common) to operate an operational amplifier (op amp) single ended/Class A. Modern audio equipment is chock full of them.

Remember that any unbalanced audio stage (e.g., a line level preamp that uses "RCA" plugs and jacks for interconnection) is single-ended/Class A.

2) The knock against op amps is generally directed at inexpensive integrated circuit (IC) 'chip' op amps. It is perfectly possible to make op amps with discrete components. Indeed, it's perfectly possible to make discrete, vacuum tube op amps. They were very common in the days of analog computers, e.g.

Here are two examples of high quality, discrete op amps designed for audio use.
https://www.bursonaudio.com/products/supreme-sound-opamp-v5/
https://sparkoslabs.com/discrete-op-amps/

mhardy6647

Here's a Heathkit analog computer. That row of vacuum tube modules at the top across the back is a phalanx of discrete, vacuum tube op amps. I count fifteen.


Heathkit analog computer by Mark Hardy, on Flickr

As an aside, I actually had one of these Heathkit computers! Wish I still did -- it was very big, very heavy, and rather difficult to 'program' it to do useful things.

DoneandGone

There’s a pair of SM 70 Pros up for sale here in BC for $1,200 CAD. Are they worth a look (listen)? Are they sonically not that good motorstereo?

motorstereo

My impressions are at first listen I thought they sounded very nice but after living with them for a while I began to think differently. They run hot (130-140 F) so that means they're cold weather amps. They also will drive up your electric bill if you leave them on all day like I used to. We all hear differently and you just may like their sound. Mine sounded their best on a pair smooth sounding ads 1530's and I didn't care for them on a pair of more revealing khorns where I thought they were harsh sounding. The khorns were the speakers I wanted to pair them with and unfortunately they just didn't have synergy to my ears.

DoneandGone

Interesting observations, thanks. They are now sold, but the guy did say there was a intermittent problem with one of the power switches. Running so hot, turning on/off frequently because of the overheating explains that issue now. Cheers

mhardy6647

True Class A amplifiers will always, always, run hot -- the output devices are always dissipating full power, all the time, even at idle (zero signal).

If you can't cook on top of it (whether vacuum tube or solid state), it's probably not really Class A!

motorstereo

Yes they sure do run hot. I had a Yamaha R9 receiver that had a class A switch and it would run luke warm until that switch was engaged. It also would instantly start drawing more current according the power conditioner I had it plugged into. I never could hear a difference in the sound though.

BlueFox

After I got my amps 4-5 years ago for final three weeks in July, I went crazy when I got the electric bill, since I have to run the AC when I play the stereo. I immediately had solar installed, and while it doesn't quite cover the summer electric bill, it is manageable now.

Beta

I believe I fall into this camp. CAD-805RS Monoblocks. Yes. They put out a lot of heat.

SCompRacer

mhardy6647 wrote: »

If you can't cook on top of it (whether vacuum tube or solid state), it's probably not really Class A!

Do s'more count?

DoneandGone

Beta, is the heat from your amps bowing the shelf above? I'm sure it has nothing to do with 150lb+ of PS Audio equipment

Beta

DoneandGone wrote: »

Beta, is the heat from your amps bowing the shelf above? I'm sure it has nothing to do with 150lb+ of PS Audio equipment

That photograph makes the shelf look like it's sagging. It's not. It's straight. Out of curiosity, I just placed a level on it. It's dead even.

DoneandGone

Haha, visual distortion due to poor optics of your portable electronic image capturing device?
Out of interest, does the Powerbase influence the P20? if nothing else, it looks super cool.

jayman_1975

I own 2. PS audio 200c and 200cx. They run smokin hot and sound amazing.