Yamaha class A switching

[jseifert1984]

Hello all. I want to start off by saying that AK has been a great source of information for me. I've spent many hundreds of hours browsing this site. Thank you all for your contributions.

Now for my first post (I think). I have a Yamaha A-1000 integrated stereo amplifier. It is rated for Class A up to 10 watts @ 8 ohms, then switches to AB. My question is, how do you tell when the amp (or any yamaha with class A) switches to AB? I've tried to determine this based on how hot the amplifier gets to no avail. I have the A-1020 manual (predacessor to the A-1000) but it gives no mention of HOW to tell when it switches from A to AB. Another question I have is do you get "more watts" of class A with a 4 or 6 ohm load? Any help would be appreciated. Thanks.

 

[sreten]

Hi,

Class A depends on the amount of standing current in the output stage.

If peak output current is less than this then the amplifier remains in class A.

Consequently all such amplifiers produce more Class A watts into 8 ohm
than 4 ohm, about twice as much, and twice as much again into 16 ohms.

If you want more class A you increase the standing current, but the amp
will run hotter, junction temperatures will depend on the effectiveness
of the heatsinking and the thermal interfaces of the output devices.

10W class A needs about 1A of standing current. for a 100W amplifier
with say 45V-0-45V rails this = a standing dissipation of 90W, upping
the current will increase this, but FWIW a good test of the heatsink
temperature is "painful to simply touch = too hot". Dropping the
voltage rails (not easy) will allow more thermal class A.

rgds, sreten.

 

[jhoyt]

Consider that "normal" listening volumes are generally under 5 watts* (unless your name happens to be Similost ). So the amp would only cross into A/B in transients and peaks. If the amp is designed well, which Yamaha is, you would never really "know" when it is operating in A/B. They have reduced "crossover distortion" down to negligible levels.

* Using very inefficient speakers, or loud listening in general could blow this away.

 

[Rex Aeterna]

i thought power doubles lower the impedance meaning class A should have more headroom as well right? it's rated at 8ohms,yes,but wouldn't the given power headroom double in class A? i have a Yamaha R-9 Class A reciever and it gives class A up to 30w but i noticed class A to be very effective in low impedance driving as well. it ran hot even with 6ohms nominal speakers but class A ran cooler when driving 3ohm nominal loads i had it doing. guessing cause less dissipating current as heat? i even ran 2ohm nominal loads on it with absolutely no issues. i recently got it serviced and it sounds better then ever.

 

[sreten]

i thought power doubles lower the impedance meaning class A should have more headroom as well right? it's rated at 8ohms,yes,but wouldn't the given power headroom double in class A? i have a Yamaha R-9 Class A reciever and it gives class A up to 30w but i noticed class A to be very effective in low impedance driving as well. it ran hot even with 6ohms nominal speakers but class A ran cooler when driving 3ohm nominal loads i had it doing. guessing cause less dissipating current as heat? i even ran 2ohm nominal loads on it with absolutely no issues. i recently got it serviced and it sounds better then ever.

Hi,

Whilst an enriched class A class AB amplifier can drive low loads, it
moves into class B depending on the output current, so it will move
into B at lower powers with low impedance loads.

You don't get more class A with low loads, you get less. 10W class A
into to 8 ohms, is 5W class A into 4 ohms and 20W class A into 16 ohms.

No way can you bias a 30W into 8 ohms class A to remain in class A
for 2 ohms, it would need 4 times the standing current and dissipate
4 times the heat, needing far bigger heatsinking.

A pure class A only amplifier* (doesn't fall back to class B) is very
poor at driving low impedance loads, it runs out of current easily.

rgds, sreten.

*single ended with a fixed current source active load.

 

[Jon_Logan]

This is a bit off topic, but Yamaha a very interesting amp that (to me) solved the Class-A vs. dissipation problem.

I saw it at a Yamaha Service Seminar in about 1985, or so. I don't recall the model number. This amp had a floating/bridged output. One of the outputs was a standard Yamaha A+A/B conventional design. The input signal was compared to the A/B amplifiers output and this error signal was fed to a low power amp that drove the other side of the load (inverted, or course). This other smaller amp was full Class A but with low voltage rails. The smaller amp only needed rail voltages that had to be large enough to neutralize the distortion signal, and in full Class A.

Genius. Of course some people had to speak up and say "well you, the loop speed and all......". I think for audio frequencies, it was still genius.