200 watt Class A Amp bias recomendations

[bwaslo]

No, I'm saying that a push-pull amp that biases at 0.5A can only increase in either direction (+ or -) by another 0.5A (so that the total=1A peak output) if its other polarity device set (which can also only go to 1A peak) is to stay linear and not cut off (i.e., to be in true "Class A"). If the amp biases at 0.5A, doesn't use an output transformer (to be operating at a higher impedance internally), then it can only output up to 1A peak at 8 ohms. A sinewave with a 1A peak is I=0.707A rms, and output power is I^2*R=(0.707^2)*8 = 4 Watts. The amp will be in class AB at any time it produces more than 4W output power levels into 8 ohms.

The power at zero output (not at anything else) is what has to be at least double the maximum output power in class A (which as a maximum efficiency of 50%, but can't ever quite get there).

Where are my basics wrong?

 

[erik777]

Best CLASS A I've heard to date and I've auditioned alot the past couple of years.
Check out ExtremeA 100W into 8 ohms by Bruno Putzey. Recent Mods and stability and heat dissipation fixes have launched this amp into league all of its own.

 

[Bonsai]

If you have 1 amp of class A bias with no input signal, you have about 2A PEAK output in a push pull output class A. Single ended totem pole OPS are a bit different and you have a few additional considerations.

I've built (and a few other guys on the forum) a 15W RMS class A amp that delivers about 28W PEAK class A into the load - i.e. both halves of the OPS transistors are still in conduction at 28W peak output.

Obviously I don't know the size etc of your amp, but its probably peak class A power they are talking about.

Secondly, on the class A I mentioned earlier (sx-Amp), it transitions to class B if driven hard enough into lower impedance loads, and will deliver about 50W into 4 Ohms in class AB.

So, I suspect with your amp that it might be a ~40 to 50W class A amp (and that is a big amp make no mistake) that delivers 80-100W peak class A and then into lower impedances, or when it transitions out of class A and into class B, up to 200W peak.

For reference, the sx-Amp has each of two output pairs per channel biased at 700mA for a total of 1.4A per channel. with +-22V rails that is ~61W standing power dissipation or just over 30W for each half of the OPS.

Nevertheless, I read some very good reviews on this amp, so I'd fix it and enjoy it!

 

[AndrewT]

I know it's a very old post but if anybody can benefit from my post then it's great. The Usher R1.5 is a stunning sounding amp and it benefits from a great pre amp that might cost twice the Ushers price the Usher is a steal at the price it comes ,manufacturers or Hiend has lot of overcharging factor,really a lot. Now coming to the point the Usher's default BIAS is 500mA and it generate 38/42 watts of class A . Can be safely Bias ed till 750mA. Give it a good pre and speakers it will kill Amps costing thrice as much.

You need to get your basics right. Just saying. That amp has a total withdraw of 720 watts of power and it outs only 105 watts x2 in that process and 38/42 watts of class A accordingly.

if the output stage bias current is set to 500mA then the peak ClassA output current of this push-pull output stage is 1Apk.
Maximum ClassA power = 1Apk²*8ohms / 2 = 4W ClassA into 8ohms.

To get 38W of ClassA into 8ohms the bias would need to be set to >1.54A
and for 42W of ClassA set to >1.62A bias.

 

[Bonsai]

Yes - total bias per channel will need to be 1.5A or more to get 38 W and higher of course for the greater powers.

Here's the review http://www.usheraudio.cz/data/test/16-usher-r1-5-hifi-world-05.pdf