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mikee55 14th February 2006 12:39 AM

Can I change Bias?
Hi all,Just been asked a question.An article on the TNT website about increasing an amplifier's bias current,gives you more Class A.I have posted this at the Audio Asylum,ages ago.What do you lot think about this?My amp is Class AB and if I increased my bias from 45ma to 120ma,say,do we get more Class A? Yes it runs hotter,but I've been told of increased distortion,somthing about harmonic distortion(I must buy a scope).The original article "Put a Tiger in your amp"gives a few numbers,but I havn't a clue about whats being calculated,and I've always assumed a Class A amp uses one power transistor.I would like to know how you would know if your listening in class A or AB.Am I to assume you would need sensitive speakers and the volume to be low,and wouldn't a transient signal send it into AB?

Can anyone explain this? I'd like an LED to show me when I'm listening in Class A.Do you think I'm barking up the wrong tree?At the end of the day,some sensitive speakers and an indicator and I've got me a Class A amp,right????

Please,can someone clear this up for me.Maths is not my strong point.

pinkmouse 14th February 2006 12:46 AM

Mikee, for a commercial amp, in my opinion, it's probably not worth the bother. Class A running stresses the amp quite heavily, and unless it's designed for it, will greatly increase the risk of failure. If you want Class A, then build Class A, don't just muck around with the bias on an AB amp.

lineup 14th February 2006 12:59 AM

It is possible to put in an indicator circuit
that shows when an Class AB amp is running in A or B.
But it is not as clearly how the limit is to be defined.

At very low level a high bias Class AB can run most of sound in class A.
But some peaks in music would be in class B.
It is true, that with more sensitive loudspeakers
you will get more of the sound produced in the Class A domain.

Some Class AB can get better performance with increased bias.
Other may sound worse.
It is not that simple, that increasing bias always makes things better.

For a pure 100% class A, you wont need an indicator,
because you will hear when a class A amp goes into clipping.
Some class A amps have a RED LED indicator,
that give a warning when amp is -3dB from full power.
For a 30 Watt Class A amplifier, this is at 15 Watt output. (50% of power)

sam9 14th February 2006 02:37 AM

In my opinion that particular article is one of the more notorious bits of audio info on the web. You could probably get away with it in the case of MOSFET output devices. In the case og BJT's, you are a lot more likely to harm the sound or even the amplifier itself. As I recall, the article doesn't even really consider that you need to know some specifics out the type of devices and the circuit configuration. Nor does it consider the heatsinking in the particular unit -- not a minor concern.

In general, for BJT's there is a more or less optimal bias that is hard enough to determine with a 'scope and/or distortion analyseer plus other equipment, let alone by a rule of thumb.

anatech 14th February 2006 02:54 AM

Actually, the same factors are in effect with Mosfets. It's just that some devices, like IR, sound awful at low bias currents. Depends heavily on the circuit too.

My advice, don't do that. Most amps do not sound better when you turn up the bias. The problem is, when you get a group of people in a room trying things, there is a danger of opinion going away from reality.


Lee1234 14th February 2006 09:44 AM

I had an Adcom 5300 that sounded rather shrill until I pushed up the bias. It only required an increase from about 50 -60 ma to 130 an then sounded decent. Increasing beyond that really made no change in the sound.

sam9 14th February 2006 12:23 PM


Actually, the same factors are in effect with Mosfets.
I should have said "may" rather than "prrobably".

ilimzn 14th February 2006 12:27 PM

I'm not sure I would trust an audio site that calls itself TNT. TNT goes BOOOM, I wouldn't want my components to do that :)

mikee55 14th February 2006 01:26 PM

Thanks Y'all
Thanks for the replies,I have looked into building a Class A amp and have not been able to make my mind up as to building one.A few years ago I did make one,sort of,I used a TIP 141 and biased it into Class A and added another TIP 141 as an Emitter Follower I ran it of a 2.5amp transformer and although it sounded better than my AB amp,a JVC AX-2,it just burnt itself out.It hummed big time,I used a 4700uf cap on the speaker output to remove a big DC offset that pushed the drivers cone right out,OUCH,I guess my calculations were off.The transformer heated up,the transistors heated up,which in turn must have heated the transformer up some more.I had to heatsinks bolted together and blasted them with a 9" office fan.I figured that when I calculated the emitter current I forgot that I had to take into account the current drawn by the rest of the circuit,like base bias current used by both transistors.WHOOPS.I used BJT's because I couldn't figure out how to use mosfets.Maybe I'll have ago again one day.Yes TNT should go Boom ha!


mikee55 14th February 2006 01:34 PM

A pic
Trying to send a circuit diagram I based it on.

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