Ditching Class A Amps due to Heat - Sort of a Poll

If they are so worried about climate change why were we forced to use old technology like the combustion engine for over a 100 years.

@phd For the answer to that, read Internal Combustion, by Edwin Black.

Its fascinating reading!

YOU CANNOT UP THE BIAS QUICK ENOUGH TO CATCH THE MOMENT.

I have read the patent and it does not answer this. It cannot. I asked a senior engineer working on my KRS200s how it solves this issue and he could not answer.

@clearthink You can easily change the bias no worries.

I don’t think they want to publish the trade secret which is why its not in the patent. You are correct that you do have to be careful that the timing of everything works out. That means that you have to have the same bandwidth in both the actual audio circuit and the circuit driving the bias so that they are in fact properly time aligned (IOW no phase shift between the two at any audio frequency). You can’t have any timing constants to filter noise! It simply has to be as reactive as the audio circuit itself.

I think Krell did that- its not hype at all. The problem is that this type of circuit by definition has to introduce distortion of its own. That, ultimately, is why we never implemented it in our amps.

in the sense that it does what it’s designed to do, prevent notch distortion (the basic reason we want Class A) while reducing power consumption.

@axo1989 

Just so you know, any class D amp that uses a choke-filtered output (which is nearly all of them) are incapable of notch or crossover distortion.

I read Krell Solo 575 Mono Power Amplifier 575 Watt Amplifier features 'iBias'.  This is said 'to deliver the rich musicality of Class A amplifiers, the uncompromised dynamics of classic Krell amplifiers, and the efficiency and low power consumption of Class G and H amplifiers.'  Sorry.  I don't believe you can get something for nothing.  In their publicity Krell extol the virtues of Class A and pretend they are still building it. 

@clearthinker

Krell has used a sliding class A bias system for years. The way it works is at lower signal levels the bias is also reduced. As the amplifier power increases, the bias linearly increases with it. In this way it can be biased in the A region all the time (in this case meaning both output transistor banks are active throughout the entire signal waveform) at any power level the amp makes. Because music tends to have lots of transients that are short duration, even though the amp might be quite powerful the net result is it will run cooler and draw less power, significantly so.

If you were to run it at full power long enough for it to heat up, you would find it making the same heat and drawing the same power as any class A amp capable of the same power.

My point here is that its not a 'get something for nothing' proposition as you suggest. Seems to me there was a patent issued on this technology back in the 1990s.

Thanks for the feedback.

The D amps are pretty old (maybe 20 years).

@dekay Uh, Yikes! A lot has happened in the class D world since 20 years ago!! I couldn't take them seriously back then.

Anyone else use Class D to run 16 ohm speakers?

@dekay Yes. I run them on my Classic Audio Loudspeakers. They work fine on there- no complaints.

Have you done away with your Class A Amp due to Heat concerns?

Will you be moving away from Class A Amp due to Heat concerns?

Will you never buy a Class A Amp due to Heat concerns?

Thanks for chiming in. With your selection, it is still more than 5 to 1 in favor of Class A over anything else in this thread.

@brianh61 Just to be clear, my response to the above questions are all 'no'. Your response to my post (also above) suggests to me you thought otherwise.

I replaced my class A triode OTL amps because I have a set of amps that sound better.

A big defender of Class D claims that Class A is going away on it’s own and would never become effected by limits placed on electrical consumption or heat output, but just because everyone no longer wants it or wants Class D or similar.

I’ve been running class A tube amps for over 50 years. I’ve been manufacturing them for nearly that long as well. I’ve never liked the heat, so it was really nice to find that after 5 years of R&D that we had a class D product that sounds better than our tube amps. I run them at home now and don’t miss the tubes at all.

(FWIW Our OTLs have gotten very nice reviews and awards in the high end press, best sound at show, stuff like that.)

What makes an amp sound a certain way is its distortion signature, unless its output impedance is high enough that you run into frequency response colorations as well. Class D amps can have a very low output impedance, eliminating the latter issue and it is possible to have them have similar distortion as tube amps make, meaning they can be smooth through the mids and highs, which is also why people like class A amplifiers in general.

Its very likely that I’m the person @brianh61 is referring to with the comment above.

I went from class A to class D on account of the sound, nothing else. Class A is doomed at this point; ten years from now class A offerings will be vastly reduced and it will be by market forces and not some ’green’ agenda (which isn’t a thing when it comes to amplifier classes of operation- no one is out to regulate that).