Pass ACA Class A Power Amplifier Review

[scott wurcer]

Here's my first approximation to simulating cross-over distortion. I decided to use two parameters, width and height of the cross-over region. Each of these defines a fraction of the -1 to 1 transfer function. As you said, the cross-over distortion is pretty nasty and generates a large number of odd harmonics at high levels. Here's an example using a simple 1kHz sinewave:

View attachment 67765

I zoomed in on the transfer function near zero to show the effect.

And here's what the waveform looks like with a higher level (10x) of crossover distortion to make it easier to see:
View attachment 67762

This is gross and probably does not exist in any amplifier today. Crossover distortion is caused by the transcendental relationship between Vbe and Ie inside a feedback loop there is no simple distortion model without the complex frequency dependent relationship that will relate to any actual power amplifier.

 

[pkane]

This is gross and probably does not exist in any amplifier today. Crossover distortion is caused by the transcendental relationship between Vbe and Ie inside a feedback loop there is no simple distortion model without the complex frequency dependent relationship that will relate to any actual power amplifier.

ah, but there’s also a configurable feedback simulator. And these can be used together, combined with all the other distortion simulators that are already part of DISTORT.

I’m not saying this represents any specific amplifier, by the way.

 

[MakeMineVinyl]

Here's my first approximation to simulating cross-over distortion. I decided to use two parameters, width and height of the cross-over region. Each of these defines a fraction of the -1 to 1 transfer function. As you said, the cross-over distortion is pretty nasty and generates a large number of odd harmonics at high levels. Here's an example using a simple 1kHz sinewave:

View attachment 67765

I zoomed in on the transfer function near zero to show the effect.

And here's what the waveform looks like with a higher level (10x) of crossover distortion to make it easier to see:
View attachment 67762

That's the general idea. As others alluded to the actual mechanism is incredibly complex. The zero crossing going positive and zero crossing going negative are not typically identical. Crossover distortion of the magnitude you show tends to sound, at least with lower frequencies, like a voice coil rub if you've ever had a woofer that had that problem.

Don't know what amp this is, but there's some serious problems here.

 

[Sal1950]

View attachment 67785

There is nothing wrong with your television set. Do not attempt to adjust the picture. We are controlling transmission. For the next hour we will control all that you see and hear.
You are about to experience the awe and mystery which reaches from the inner mind to the outer limits.

 

[solderdude]

Back in the days when Technics added new Class-A, I also retrofitted the idea in my regular class-AB amps.
Could't hear any differences though (I could switch it on and off), but used it with rather inefficient own design DIY stats.

The idea was neat and based on the fact that the issues are caused by the transistors switching completely off. When they have to switch on again (after reverse polarity had been applied to BE) there is some time lag.
The 'fun' part of the new Class-A was that the output devices always remained on (a little) when otherwise the BE would be below the conduction level.
Later they added 'computer drive' where they could vary idle currents based on some parameters.

later they also built another type of amp (which I also built for myself) where a small output voltage class-A amp. There was just a few volts over the output devices but had 2Amp idle current (or more) and thus didn't become very hot. That amplifier was fed with a class AB amp that 'modulated' the whole class A amp.

Later on they experimented with a bridge circuit (class AA), somewhat different to Quad as I don't work at Technics service dept anymore.

 

[anmpr1]

Back in the days when Technics added new Class-A, I also retrofitted the idea in my regular class-AB amps. Could't hear any differencese.

Related anecdote: many years ago when my ears were younger than yesterday (and presumably working better) I owned a Yamaha integrated amplifier. On the front panel was a switch that would run the circuit in class A. Knocked the power down from 100+ watts/ch to something like 20. I sat for an hour switching back and forth, waiting to uncover the magic. Couldn't tell a bit of difference.

It was sort of like the highway to nowhere. But the amp itself was lovely, as far as these things go.

 

[mhardy6647]

Related anecdote: many years ago when my ears were younger than yesterday (and presumably working better) I owned a Yamaha integrated amplifier. On the front panel was a switch that would run the circuit in class A. Knocked the power down from 100+ watts/ch to something like 20. I sat for an hour switching back and forth, waiting to uncover the magic. Couldn't tell a bit of difference.

It was sort of like the highway to nowhere. But the amp itself was lovely, as far as these things go.

View attachment 67831

While my own experience with running these dowagers in Class A doesn't mimic yours -- yeah, they were, and (I would opine still are) exceptionally nice consumer/massmarket integrated amps.

Apropos of nothing -- I have her big sister here
(sorry for the ultra-wide angle lens distortion in this photo)

DSC_6689_z by Mark Hardy, on Flickr

 

[DSJR]

I remember having loads of fun with my short lived Yamaha CA1000mk2 with 'Tone Jump' bypassing the tone controls and so on and also aforementioned Class A switch. To hear any difference with this latter switch, you didn't switch quickly back and forth. Rather, you play music with some reverb or venue atmosphere in it, five minutes or so of each setting. After twenty seconds or so in Class A at moderate levels, you could usually hear the sense of reverb increasing. Thing is, this particular amp was rather veiled from the preamp outputs, so any advantage in the power amp section was swamped really by the bottleneck behind. It's many years now since I sold it.

 

[anmpr1]

To hear any difference with this latter switch, you didn't switch quickly back and forth. Rather, you play music with some reverb or venue atmosphere in it, five minutes or so of each setting. After twenty seconds or so in Class A at moderate levels, you could usually hear the sense of reverb increasing. Thing is, this particular amp was rather veiled from the preamp outputs...

Playing at 'moderate' levels could, depending on loudspeaker sensitivity and/or how moderate is moderate, drive the amp to clipping or otherwise increase distortion since, in Class A, power dropped to 18 watts 8 ohm/ch from 90 per channel in AB (according to the spec sheet).

As far as having to play music with some reverb in order to notice a sonic difference? That's a new one on me. I'm reminded of a 1977 quip from Peter Aczel about a largely forgotten but back then highly touted (in some quarters) Bravura preamplifier. He was told that in order to listen, identify and understand its sonic benefits it had to be used in a certain way (V15 III G cartridge and Fulton J loudspeaker). Peter wrote how this "...led us to speculate about a preamplifier that must only be used for listening to Tibetan music while sitting on yackback."

One thing to note, at least in some later day Yamaha integrated amplifiers: David Rich found output levels in 'direct' mode were a bit higher than in full preamp mode, which in itself could result in a perceived 'subjective' improvement. For my part I never recognized any loudness difference going from Class A to A/B on the CA-1010. I did notice that it didn't play as loud or dynamically, which possibly might be expected going from 180 watts down to 36. In any case, from listening at low levels (regardless of how long the session) I can't claim there was a difference. If it was there it was extremely subtle. As with all of these things, YMMV.

If I had to characterize the feature it I'd file it under 'marketing'. Those were days of a lot of Class A action: Levinson, Lux, Stax, Bedini, Rappaport, Threshold, and even Pioneer (the Series 20 'plate' amp) come to mind.

About the preamp section being 'rather veiled'? I can't comment on that aspect as I never took apart the two stages, although it was easy enough to do with the junction on the back panel. I never really understood the practical purpose of the junction, unless once was attempting to biamp, or insert some kind of processor. Most integrated amps from that day sported the feature.

As a final word, I notice that Lux still makes those old style integrated amps. Very classy look and feel. But if you want Class A it's a dedicated proposition with Lux. No handy switch. And very expensive. But that's Luxman for you.

 

[pjug]

The goal is to be able to test the amplifier against itself.

Hi - sorry that it was not clear but that comment was for @amirm just for testing of Class AB amplifiers going forward. But I do have a question for you:

I have read speculation that the Outlaw M2200 and NHT A1 are designed by ATI. Do you know if this is true? Would you even be allowed to disclose it if it were true?

 

[amirm]

Related anecdote: many years ago when my ears were younger than yesterday (and presumably working better) I owned a Yamaha integrated amplifier. On the front panel was a switch that would run the circuit in class A. Knocked the power down from 100+ watts/ch to something like 20. I sat for an hour switching back and forth, waiting to uncover the magic. Couldn't tell a bit of difference.

I repaired one of those with a ton of stuff blown up and no schematic! This was in early 1980s. Took so much effort to fix it. Like you, once I got it working, I was so anxious to hear a difference but none was there.

 

[DSJR]

So what was I thinking then (no need to answer!). The thing certainly ran much hotter in 'Class A mode' and I apologise if my subjective vibe on the difference were actually nothing, but to me it was better reproduction of 'reverb tails' after a few seconds*! If anyone has access to one, it'd be interesting to try it again with modern (better) sources.

*Someone will come along and tell me that switching up the bias like this should make an instant difference if it was going to at all, so apologies again if I've crapped on the thread. Maybe it was the audible effect of soft clipping adding compression?

 

[anmpr1]

If anyone has access to one, it'd be interesting to try it again with modern (better) sources.

I repaired one of those with a ton of stuff blown up and no schematic!

I too would be interested in a test of something like this. As far as repair--this was a time when Yamaha was touting their V-FETs. I think Sony had them, too. I have no idea about the availability of replacement parts anymore. I read somewhere (I think I'm not making this up) that Nelson Pass scored the last of a batch of Sony sourced V-FETs for some of his DIY designs.

Say what you will about the old toob gear--If my Dyna goes south I can still get parts and have a reasonable chance to repair it!

 

[tomchr]

View attachment 67785

Don't know what amp this is, but there's some serious problems here.

Not necessarily. The sine wave is the output. The other trace is the distortion residual. It shows crossover distortion at the zero crossing plus some harmonic distortion. No surprises there. Since we don't know the scale (V/div) of the distortion trace, we can't say anything about whether this is reasonable or seriously broken. Even if the crossover distortion is too high, it may be that the solution is to nudge the bias adjustment pot a little. That's not exactly "fundamentally broken, redo from start" territory.

Tom

 

[MakeMineVinyl]

Not necessarily. The sine wave is the output. The other trace is the distortion residual. It shows crossover distortion at the zero crossing plus some harmonic distortion. No surprises there. Since we don't know the scale (V/div) of the distortion trace, we can't say anything about whether this is reasonable or seriously broken. Even if the crossover distortion is too high, it may be that the solution is to nudge the bias adjustment pot a little. That's not exactly "fundamentally broken, redo from start" territory.

Tom

I was being a bit tongue in cheek with that comment. Still, with all the amps I see measurements of (I work in an amp factory after all ) this example is a worse than usual. I do know it wasn't one of ours, but some off brand unit.

 

[mhardy6647]

Playing at 'moderate' levels could, depending on loudspeaker sensitivity and/or how moderate is moderate, drive the amp to clipping or otherwise increase distortion since, in Class A, power dropped to 18 watts 8 ohm/ch from 90 per channel in AB (according to the spec sheet).

As far as having to play music with some reverb in order to notice a sonic difference? That's a new one on me. I'm reminded of a 1977 quip from Peter Aczel about a largely forgotten but back then highly touted (in some quarters) Bravura preamplifier. He was told that in order to listen, identify and understand its sonic benefits it had to be used in a certain way (V15 III G cartridge and Fulton J loudspeaker). Peter wrote how this "...led us to speculate about a preamplifier that must only be used for listening to Tibetan music while sitting on yackback."

One thing to note, at least in some later day Yamaha integrated amplifiers: David Rich found output levels in 'direct' mode were a bit higher than in full preamp mode, which in itself could result in a perceived 'subjective' improvement. For my part I never recognized any loudness difference going from Class A to A/B on the CA-1010. I did notice that it didn't play as loud or dynamically, which possibly might be expected going from 180 watts down to 36. In any case, from listening at low levels (regardless of how long the session) I can't claim there was a difference. If it was there it was extremely subtle. As with all of these things, YMMV.

If I had to characterize the feature it I'd file it under 'marketing'. Those were days of a lot of Class A action: Levinson, Lux, Stax, Bedini, Rappaport, Threshold, and even Pioneer (the Series 20 'plate' amp) come to mind.

About the preamp section being 'rather veiled'? I can't comment on that aspect as I never took apart the two stages, although it was easy enough to do with the junction on the back panel. I never really understood the practical purpose of the junction, unless once was attempting to biamp, or insert some kind of processor. Most integrated amps from that day sported the feature.

As a final word, I notice that Lux still makes those old style integrated amps. Very classy look and feel. But if you want Class A it's a dedicated proposition with Lux. No handy switch. And very expensive. But that's Luxman for you.

Two comments (OK, maybe 3).

1) Note that the CA-610II's "Main Direct" actually cuts level by 6 dB (see the front panel photo I posted above).
2) many (not all!) of the 1970s Yamahas used a switch rather than jumpers to tie the preamp and power amp sections together. The switches will cause some fun of their own at this late date (i.e., they get dirty, of course, but the audible symptoms range all over the map).

source: http://sportsbil.com/yamaha/cr-2020-om.pdf

and maybe 3)

I do hope Peter Aczel wrote "yakback" and not "yackback".

PS The rationale for being able to separate preamp and power amp sections was (as hypothesized) mainly to provide an easy way to add a processor of some sort (typically an equalizer in those halcyon times ) or to permit use of the two sections separately. A friend in college ultimately used his CA-600 integrated amp as a preamp only, adding a GAS Son of Ampzilla power amp for his Polk Audio Monitor Series Model 10 loudspeakers (e.g.).

 

[SIY]

I was being a bit tongue in cheek with that comment. Still, with all the amps I see measurements of (I work in an amp factory after all ) this example is a worse than usual. I do know it wasn't one of ours, but some off brand unit.

Here's what is more typical for a decent AB amp, in this case a 2.83V 1kHz sine wave into 8R (1W), residual x1000. Amp cold, which is usually worst case; it looked pretty much the same the last time I checked it after several hours of use. BTW, this amp (Adcom GFA-555) is more than 35 years old, thousands of hours on it, all original components, bias checked once a year. I could probably drop the hum (not easily audible but measurable) by replacing the power supply caps, but that's a story for a different time...

 

[scott wurcer]

Here's what is more typical for a decent AB amp, in this case a 2.83V 1kHz sine wave into 8R (1W), residual x1000. Amp cold, which is usually worst case; it looked pretty much the same the last time I checked it after several hours of use. BTW, this amp (Adcom GFA-555) is more than 35 years old, thousands of hours on it, all original components, bias checked once a year. I could probably drop the hum (not easily audible but measurable) by replacing the power supply caps, but that's a story for a different time...

View attachment 67932

Pretty benign, I get tired of folks showing cases of horrible behavior as if they are the norm.

 

[restorer-john]

I was being a bit tongue in cheek with that comment. Still, with all the amps I see measurements of (I work in an amp factory after all ) this example is a worse than usual. I do know it wasn't one of ours, but some off brand unit.

It's a blatantly deceptive plot.

Not only do we know the residual is likely 60-80dB down and amplified by the output of the distortion meter for the "scope output jacks" (probably 1000 times at least for that X/over image), we also don't know the second trace's sensitivity (V/Div).

Without signal processing to get rid of the noise, I don't reckon I could get a clean X/over distortion residual even if I deliberately drop the bias right down for a pretty pic, but I'll have a go now I've repaired my distortion meter's self cancelling range switch array. (The most fiddly thing I have done this year)

The Benchmark plot is really no better than the hundreds of "night and day" "with and without" pictures in HiFi brochures over the years.

Pioneer (July 1985)

 

[MakeMineVinyl]

It's a blatantly deceptive plot.

Not only do we know the residual is likely 60-80dB down and amplified by the output of the distortion meter for the "scope output jacks" (probably 1000 times at least for that X/over image), we also don't know the second trace's sensitivity (V/Div).

Without signal processing to get rid of the noise, I don't reckon I could get a clean X/over distortion residual even if I deliberately drop the bias right down for a pretty pic, but I'll have a go now I've repaired my distortion meter's self cancelling range switch array. (The most fiddly thing I have done this year)

The Benchmark plot is really no better than the hundreds of "night and day" "with and without" pictures in HiFi brochures over the years.

Really, folks - there's no sinister intent with that picture! I took it several years back when we had a no-name amp brought back from Asia somewhere and I threw it on the AP. I took a picture of the crossover distortion because it was so bad. The inside construction of this amp was laughable and it clearly was made to be cheap, cheap, cheap. Trust me, it was really that bad. No real amplifiers were harmed in this experiment.