Lowering the bias of Forte 1a?

needtubes · 19th August 2008, 06:35 PM

Hello all-

I am trying to set up a really good sounding 2-channel "office" system for my apartment (home away from home theater

). I have been looking for a good integrated/power amp at an affordable price (grad student here), but it seems most "good" amps start at around $300, which is more than I want to spend.

I do, however, have my Forte 1a, which I had first decided not to use since it is 50WPC Class A and my apartment is "energy efficient" and the power bill is the only utility not included in the rent. But, I have no other option, really...

I recall the bias of the 1a being about 89W (120mV across .47-ohm emitter resistors) per side (from a post by NP). Could I safely lower the bias to around 0.5mV across the resistors? This gives me (I think) around 38W bias per side, so maybe 20WPC in class A? This is still much more than I need, though, but would hopefully be nicer on the power bill...

Another question about class A amps in general... If an amp is biased for 50WPC in class A, does this mean it can push 50WPC and then it totally runs out of steam? Or, does it push 50WPC in class A and then many more watts in class A/B, if necessary?

Thanks all

Nelson Pass · 19th August 2008, 08:36 PM

Quote:

Originally posted by needtubes
Could I safely lower the bias to around 0.5mV across the resistors?

Another question about class A amps in general... If an amp is biased for 50WPC in class A, does this mean it can push 50WPC and then it totally runs out of steam? Or, does it push 50WPC in class A and then many more watts in class A/B, if necessary?

A) Yes, absolutely

B) Depends on the design. Most push-pull Class A amps will just make their
rating as Class A into 8 ohms, and will not typically deliver more as AB unless
they are operating at a lower impedance, for example, 50 watts Class A
into 8 ohms becomes 25W Class A into 4 ohms and runs AB to 100 watts.

needtubes · 19th August 2008, 08:55 PM

Thank you!

I will get to work re-biasing and see how it goes...

needtubes · 21st August 2008, 01:18 PM

Ok, I re-biased the amp so that it idles around 50mV across the resistors. I guess that probably leaves me, still, with about 15WPC in Class A, which is more than enough. From my intial listening tests, everything sounds fine- nothing too obvious appears to have been lost in the re-bias.

Thanks!

19th August 2008, 06:35 PM	#1
needtubes diyAudio Member Join Date: May 2003 Location: USA	Lowering the bias of Forte 1a? Hello all- I am trying to set up a really good sounding 2-channel "office" system for my apartment (home away from home theater ). I have been looking for a good integrated/power amp at an affordable price (grad student here), but it seems most "good" amps start at around $300, which is more than I want to spend. I do, however, have my Forte 1a, which I had first decided not to use since it is 50WPC Class A and my apartment is "energy efficient" and the power bill is the only utility not included in the rent. But, I have no other option, really... I recall the bias of the 1a being about 89W (120mV across .47-ohm emitter resistors) per side (from a post by NP). Could I safely lower the bias to around 0.5mV across the resistors? This gives me (I think) around 38W bias per side, so maybe 20WPC in class A? This is still much more than I need, though, but would hopefully be nicer on the power bill... Another question about class A amps in general... If an amp is biased for 50WPC in class A, does this mean it can push 50WPC and then it totally runs out of steam? Or, does it push 50WPC in class A and then many more watts in class A/B, if necessary? Thanks all

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19th August 2008, 08:55 PM	#3
needtubes diyAudio Member Join Date: May 2003 Location: USA	Thank you! I will get to work re-biasing and see how it goes...

21st August 2008, 01:18 PM	#4
needtubes diyAudio Member Join Date: May 2003 Location: USA	Ok, I re-biased the amp so that it idles around 50mV across the resistors. I guess that probably leaves me, still, with about 15WPC in Class A, which is more than enough. From my intial listening tests, everything sounds fine- nothing too obvious appears to have been lost in the re-bias. Thanks!