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Class A amps question

stevec67 said:
runs at 50 deg C and 200W *on standby*
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Pretty standard for old school 80s Class A amps like Krell etc Even my old MF A370 would get hot enough that it was uncomfortable to leave your fingers on the heatsinks after it had been on for an hour or so. It would literally warm the room, so I can only imagine how much power it was consuming. It was officially rated at 2x 185W but was known to be capable of comfortably more than 2x 200W (220W was a figure I once saw). Judging by the picture of the CJ 350 my MF had twice the heat sink at least, so can only imagine how hot the CJ will get if it is of a similar topology to my MF.
 
There is something about the ratio of idle power consumption to rated output when judging whether an amp is "truly Class A" or just biased into it.

The A21SE lists an idle of 300W, for an output of 30W, while my memory is that the Pass Aleph consumed 250W for an output of around 25. 10:1 is normal for a "true" Class A
 
gez said:
Pretty standard for old school 80s Class A amps like Krell etc Even my old MF A370 would get hot enough that it was uncomfortable to leave your fingers on the heatsinks after it had been on for an hour or so. It would literally warm the room, so I can only imagine how much power it was consuming. It was officially rated at 2x 185W but was known to be capable of comfortably more than 2x 200W (220W was a figure I once saw). Judging by the picture of the CJ 350 my MF had twice the heat sink at least, so can only imagine how hot the CJ will get if it is of a similar topology to my MF.
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Yes indeed, I'd expect it from a Krell or an A370, it's just that this one is described as Class AB and still eats 200W +
Tim Jones said:
There is something about the ratio of idle power consumption to rated output when judging whether an amp is "truly Class A" or just biased into it.

The A21SE lists an idle of 300W, for an output of 30W, while my memory is that the Pass Aleph consumed 250W for an output of around 25. 10:1 is normal for a "true" Class A
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I've heard similar. I thought that the rule of thumb was that for "true" Class A it was 3x or 4x nominal. This across 2 channels of course, so multiply by 2. So your figures suggest 5x nominal x 2 channels. I suppose a bit gets consumed in the pre section but these are generally as cool as a penguin's bottom so that can't be much.
 
If you don't need the power, consider a single ended tube amp. Class A and very little heat - just the tubes. No heatsinks needed,
 
les24preludes said:
If you don't need the power, consider a single ended tube amp. Class A and very little heat - just the tubes. No heatsinks needed,
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I have a push-pull EL34 tube amp that is alleged to be Class A (it's not, the power consumption goes up and down with the volume) but it runs hotter than a snake's ass in a wagon rut. No heatsinks other than the shiny metal case but the whole thing will fry eggs.
 
hertz said:
I have been eyeing the Sugden A21SE for some time now. I know many of you may have spent time with this amplifier and may have also considered and auditioned more expensive class A amps from the likes of Luxman or Accuphase. From a first hand experience, is there so much difference between these amplifiers ? Price wise, they are so different. In this scenario, I would like to assume the loudspeaker in question will be sensitive enough to be driven very well by the Sugden. I know higher powered Class A will drive difficult speakers better so I want to take that variable out of the equation.
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Wow you certainly have got a number of widely-varying responses! 😮

OK ... so the Sugden A21SE seems to be 30w into 8 ohms and 40w into 4 ohms. So relatively low power.

So the million dollar question is ... can your spks be driven well by this amount of power? I would suggest if your spkrs are "hard to drive" - ie. drop to low impedance and have wild phase swings ... then NO! :(

Of course, unfortunately Class A carries a substantial heat penalty. I have 2-way, active spkrs with 4 ohm drivers and drive them with a pair of stereo amps from the same designer (AKSA); these are:

* in winter ... Class A, 45w into 4 ohms (all the watts are Class A). These two stereo amps output 500w of heat into the room - so, alas, are just not comfortable in the Aussie summer. :(

* in summer ... Class AB, 110w into 4 ohms. These amps never get more than just warm ... however, they just don't sound as good as the Class A amps. :(
 
les24preludes said:
If you don't need the power, consider a single ended tube amp. Class A and very little heat - just the tubes. No heatsinks needed,
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I had one. A really good one. 18 watts. It sounds brilliant with high sensitivity speakers. My Fyne 501sp is sensitive but not really designed for low powered tube SETs. With music with low recording levels, the amp struggles a bit. With audionote or blumenhofer, the amp worked well with all sort of music.
 
artoly said:
It's a Conrad Johnson Premier 350 (350W into 8 and over 600 into 4). I just found the email from C/J where they advised it is a little less wattage than my memory suggested, but still 180W at idle. I also have an ARC valve amp and a thermometer sitting on top shows almost exactly the same temperature. I don't leave either of them on when not in use.
The Naim 250 that I'm listening to right now at work is barely warm.
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From this information I conclude that the output stage is configured in class AB. If this is a stereo amp then each channel is biased to around 20W or so. This means that for up to ~20W the amp is working just in class A but for larger signal swings temporarily enters class B for the extra umph.

DV
 
Can we, for once and for all, stop referring to the Sugden A21as if it were a sort of A21a or that either of these 2 had any sort of resemblance with the A21SE. True, they are by the same company but so are a Hitachi tele with a dirty great dockside crane. Oh yes, and the Toyota Corolla is the world’s biggest selling car - just look at the original and its latest guise.
 
Chris said:
Can we, for once and for all, stop referring to the Sugden A21as if it were a sort of A21a or that either of these 2 had any sort of resemblance with the A21SE. True, they are by the same company but so are a Hitachi tele with a dirty great dockside crane. Oh yes, and the Toyota Corolla is the world’s biggest selling car - just look at the original and its latest guise.
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That's a poor analogy, a crane and a TV are not similar. Class A amplifiers from the same company, albeit designed by different people are more closely related. I do agree though that the SE is very different sounding amp to it's predecessors.
 
Interesting thread, but many mentions of various A21s but not Sugden's higher output amps (40 wpc?) Why?

Also, mention of valved SE amps (lowish output) but what about the many other push-pull valved jobbies up to 100wpc? My 100 watters use 180VA (each) according to the manual. I guess they're class A (but I'm guessing).
 
Mike Reed said:
Interesting thread, but many mentions of various A21s but not Sugden's higher output amps (40 wpc?) Why?

Also, mention of valved SE amps (lowish output) but what about the many other push-pull valved jobbies up to 100wpc? My 100 watters use 180VA (each) according to the manual. I guess they're class A (but I'm guessing).
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Probably because the Ia4 integrated and the Masterclass pre/power combos are a bit OTT pricewise for many of us. The A21SE Signature must be getting close to twice the price of the A21a. Much cheaper than Luxman or Accuphase but more than enough for most of us. A bit of a sweet spot, perhaps.
 
Tim Jones said:
There is something about the ratio of idle power consumption to rated output when judging whether an amp is "truly Class A" or just biased into it.

The A21SE lists an idle of 300W, for an output of 30W, while my memory is that the Pass Aleph consumed 250W for an output of around 25. 10:1 is normal for a "true" Class A
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Single-ended, class A (which these two examples, are)- in theory you can squeeze that to 12.5% for a single-ended, Class-A output stage- but the front-end of course requires a little current to run at all. So your 10% looks right.

Amps running push-pull wholly within Class A can approach 25%.
 
andyr said:
Wow you certainly have got a number of widely-varying responses! 😮

OK ... so the Sugden A21SE seems to be 30w into 8 ohms and 40w into 4 ohms. So relatively low power.

So the million dollar question is ... can your spks be driven well by this amount of power? I would suggest if your spkrs are "hard to drive" - ie. drop to low impedance and have wild phase swings ... then NO! :(
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Well, my Accuphase E-800 promises 50W Class A into 8 ohms, but 200W into 2 ohms, continuous. And 300W into 1 ohm, music signal. So it may depend on the actual amp.
 
Erm, it might make 50w into 8 ohms in class A - that's about 1.25A standing bias, quite reasonable, if hot.

But 300w into 1ohm within Class A - would require a ludicrous 8.6A standing bias.

It might well make the power numbers, no dispute - but its long, long, not in Class A at that point. and that , is a perfectly reasonable trade-off to make.
 
awkwardbydesign said:
Well, my Accuphase E-800 promises 50W Class A into 8 ohms, but 200W into 2 ohms, continuous. And 300W into 1 ohm, music signal. So it may depend on the actual amp.
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Again that looks like a class AB output stage thats biased into 50W class A. If so I'd expect the standing (quiescent?) power at around 400W+ for two channels.

Just found the manual - 390W power consumption.

DV
 
martin clark said:
Single-ended, class A (which these two examples, are)- in theory you can squeeze that to 12.5% for a single-ended, Class-A output stage- but the front-end of course requires a little current to run at all. So your 10% looks right.

Amps running push-pull wholly within Class A can approach 25%.
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Something that you can probably clear up for me: I thought single ended class A consisted of a single pair of output devices - someone suggested that the Aleph 5 (60wpc I believe) is single ended - I don't understand how this is possible

My lack of understanding extends much further than this, however, so possibly this is a really stupid question
 
awkwardbydesign said:
Well, my Accuphase E-800 promises 50W Class A into 8 ohms, but 200W into 2 ohms, continuous. And 300W into 1 ohm, music signal. So it may depend on the actual amp.
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Of course it does depend on the actual (design of the) amp. Your E-800 is a good amp with an excellent PS (doubling down into half the load)!

Contrast to the Sugden which was rated at 30w into 8 ohms and only 40w into 4 ohms.
 
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