Question about Pass a40 and speaker loads

[Eric]

So that's what Nelson meant when the said the design cannot easily be modified for higher power...

It looks like a good compromise might be to design for 40w into 4 ohms as well. Thanks for all of your help! I do appreciate it!
Eric

[Eric]

Thanks for all of your help and patience with me! Is this why this type of design referred to as a "constant current" ? It seems that the bias current does not change under varying loads, so you're stuck with making the best of the available current, no matter what the speaker load is.

[Geoff]

Eric

The A40 is not a 'constant current' amp as it has a push-pull output that will continue to operate outside the Class-A limits. The supply/load current will vary from zero to twice the quiescent current in Class-A mode and even higher when driven into Class-AB. The power availability is limited by the supply rail voltage and by the capabilities of the power supply. Taking the previous figures of 30V rails and an Iq of 2.25A, the limits will be: 40W into 8ohm (Class-A) and 40W into 4ohm (Class-A) or about 80W into 4ohm (Class-AB). Even higher power would be available into a 2ohm load, the limit being set by the power supply and by the output transistor capability/protection circuit.

A 'constant current' amp has a single ended output stage which will provide no more current into low impedance loads than that catered for in the design (i.e. Iq or sometimes up to twice the Iq for special designs). Single ended amps are limited to Class-A operation and cannot move into Class-AB if presented with a low impedance load (unlike most push-pull output stages).

Geoff

[Eric]

Thanks for the explanations, Geoff! My mentality started off with "just follow the instructions, and you'll have a nice amp". I'm quickly moving into the "gotta know more!" phase.

I've read Randy Sloan's book, but there are many topics here that he does not address. Most of his designs seem to lean toward Class-B stuff.

Eric

[Edited by Eric on 04-24-2001 at 08:13 AM]

[Geoff]

Eric

If you want more information on Class-A working, have a look at the Class-A Amplifier article (amongst others) at the ESP Audio Pages ( http://www.sound.au.com ). I assume you have read the article on Class-A amps at the Passlabs site since you have been considering the A40.

Geoff