Increasing bias in amps.

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I read that increasing the bias (provided that the amp does not get too hot of course) often is beneficial for the sound.
Some effects can be noted like low end tightening up, crossover and low level signals distortion reduction, ecc.
Is this true?
My amp is solid state and cheap but I have notice that it sounds perceptibly better when it reaches a warmer temperature.
The problem is that with no signal applied at the input the amp remains basically cold.
The heatsink is quite large and I think it could dissipate the more heat generated by the output devices for the bias increasing.
I read that most of cheap commercial amps are biased exaggerately low for safer operation and maybe to protect weak components.
Typical idle bias current in a class AB cheap commercial amp can be as low as 6mA.
I would like to increase the bias current just a bit, in order to reach a higher but steady temperature on the heatsink.
I have already read that this practice is not such uncommon.
I wonder what should be a normal value for the bias current.
The output devices are 15A Toshibas, 2 pairs/channel.
Any suggestion would be extremely helpful.
If this practice can provide beneficial effects I am willing to take some risks.
Thank you very much.
Kind regards,
beppe
 
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Hi beppe61,
I read that increasing the bias (provided that the amp does not get too hot of course) often is beneficial for the sound.
This is not always true. It depends greatly on the circuit.
Typical idle bias current in a class AB cheap commercial amp can be as low as 6mA.
Actually, typical bias is on the order of 15 ~ 30 mA per device. If you measure 6 mA it may be due to a special circuit requirement, or the bias is wrong. Just plain misadjusted.
I read that most of cheap commercial amps are biased exaggerately low for safer operation and maybe to protect weak components.
Entirely possible, but not always. Sometimes the distortion may increase at higher bias levels.
I wonder what should be a normal value for the bias current.
There are many other factors. What is the make and model? What are your supply voltages?

Do not believe everything you read. If in doubt, follow the manufacturers recommendations.

-Chris
 
Hi,
just to confirm, you have 8 Toshiba 15A devices in your stereo amplifier output stage?

What is Iq just now?
What are your Vrail voltages?
Is it an EF driver and output stage?
or
a CFP driver and output stage?
Is it complementary?
or
Quasi?

If you can keep the sink temperature below 35degC then you could try raising Iq but this is not to gain any benefit from increased temperature but to ensure the output stage does not unbias itself and cause low bias induced distortion .
 
Did someone say Quasi?

If you have a schematic of the power amplifier we can probably give better advice. Higher biasing can help reduce distortion especially at lower volumes but there is no point in trying to get it from say 0.01% to 0.005%.

If you double the bias you will double the heat and double the bias will also increase the residual ripple on the power supply (especially on consumer grade amps).

Post a schematic if you can.

Cheers
Quasi
 
I seen that article. IMO one of the more irresponsible audio posts I've seen. As was mentioned above the details of the output stage more than a little important. As I recall the the article addresses this little or not at all.

I suspect that there may be a basis for the article in dealing with MOSFET output devices. I'm not sure but I think what happens is increasing the bias moves them closer to class A and move the cross over notches (or "half-notches') away from the zero crossing point.

My personal experience trying to set optimal bias on EF output sections is that: A- "optimal" is elusive and requires a lot of fiddling with thermal tracking schemes, B-tyhere is quite a bit more latitude than a lot books and articles would lead you to believe, C- a practical solution is get approximately close to optimal (assuming that's even determinable) and then reduce the bias as much as you can without bringing on audible deterioration -- cooler is better so long as it doesn't adversely affect what you hear.
 
EchoWars said:
:D

I see blown amps all the time from guys who thought that 'if a little bias is good, a lot must be better'. If you like listening to music rather than replacing blown transistors and burnt resistors, this is the mantra to repeat to yourself.

I have fiddled with bias before without any problems. But (and I mean but), with circuits and applications that I understand. So from my part if you truly know what you are doing and understand completely what you're doing it to, then by all means play. By the way I have sometimes turned the bias down a bit ...again according to application.

Cheers
 
Have in mind that manufacturers go for low bias currents because they don't know the ventilation in your rack, or if you drive small bokself speakers or Maggies!

That is the main reason for the low bias current.
My rules of thumb when messing with quiescent current are the following:

1. Don't increase the current with steps higher than 5-10mA

2. Always check the temperature of the heat sinks

3. Heat sinks for B or AB class amps shoud never exceed 50C (empirical rule, based on thermal stability and sound quality observations).

4. An amp designed for 15mA QC will never work reliably with 120mA QC

5. follow rules 1,2,3 and 4!:D

I was in the electronic service bussiness and have seen many blown amps and fried woofers caused by silly tweaks.

Your BJT's are probably the 2SC5200/2SA1943 or the
2SC3281/2SA1302.

Please post a schematic or make/type of the amp

What QC do you have now?
 
Hi,
main reason for the low bias current
I suspect the main reason for low bias current is to achieve low production cost. Even to the detriment of sound quality.

BUT, it does not follow that increasing the bias current will increase the sound quality.

If designed properly the amp should leave the assembly line adjusted to best Iq for that set of design compromises.
 
I have had several phone calls from people wanting to increase the bias in their amps just lately. TNT to blame eh:whazzat:
I'm sick of trying to explain to numptys that they should leave well alone unless they know as much about the design as the manufacturer does!....it's like that for a reason:eek::rolleyes:
The last one to phone me about this wouldn't listen.....he rang back a few days later to say that if it hadn't been for the thermal cutout....you can guess the rest!
A good start would be to read Doug Self's book on this subject= wise words;)
 
AndrewT said:
BUT, it does not follow that increasing the bias current will increase the sound quality.


Well, that depends on the design.
For example, the toshiba BJT's mentioned above have linear hfe curve for Iq>100mA.
If an amp with those toshibas is running at 15mA, it will benefit from the Iq increase.
edit:--> if the rest of circuit + heat sink can support that!

This is the reason that made Onsemi to introduce the MJL 0281/MJL 0302 which are a great choice for low Iq and low power consumer amps.
 
I thank you all Sirs for the extremely kind and valuable reply.
I understand that increasing bias is a very risky move and moreover it is not a guarantee of a better sound (!).
The amp in question is a Behringer A500.
I asked Behringer for a service manual and I got a no.
But what I wanted to know above all other things is if there is a sure correlation between bias level and quality of sound.
Now I know that high bias does not guarantee high quality sound.
It depends on the actual design.
This is very fundamental to me.
Now I wonder if there are, on the contrary, low bias high sound quality amps.

Thank you very much indeed.
Kind regards,

beppe
 
Doug Self's amp design book goes into some detail about ideal bias on an A/B amp.

High bias just shifts the distortion points away from the zero crossing point. Effectively giving you two places where distortion occurs rather than just 1.

Doug's Class XD does reduce this effect. I did have a link to the tech details of class XD but I've lost it. Sorry!

There is some seriously questionable advice on TNT audio, oh well, there will always be sites like that.
 
Hi,
low bias high sound quality amps
are unlikely to be FET output stage.
Much more likely to be BJT.

Crimson have done a low bias quasi for decades that sounds pretty good using one pair of 2n3773.
They have moved on to complementary recently but I have no info on bias but suspect it is also low bias (just to maintain reliability with the plastic packaged devices).
 
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johnny1 said:
[snip]For example, the toshiba BJT's mentioned above have linear hfe curve for Iq>100mA.
If an amp with those toshibas is running at 15mA, it will benefit from the Iq increase.[snip


Not true. The final sound quality with different bias currents has NOTHING whatsoever to do with the Ft at that particular bias point.

The bias point is just that, a bias point. From there on, the amp starts to work with varying currents and voltages and frequencies. The Ft, and its variations, while working with all those different frequencies and currents and voltages, have an impact on open loop gain versus frequency, and that, together with the feedback factor, *may* influence the sound.

Jan Didden
 
johnny1 said:

Well, that depends on the design.
...

Very interesting and kind reply Sir. Thank you.
Let me quote some your sentences.
For example, the toshiba BJT's mentioned above have linear hfe curve for Iq>100mA.
If an amp with those toshibas is running at 15mA, it will benefit from the Iq increase.

If I understand well given a certain device it has an optimal working point where it is most linear, for instance.
So more than on design it depends on the specific device.
Am I wrong?
The Toshi bjt mentioned working at 15 mA it just does not work optimally and the design that makes the Toshi working like that is not optimal.

edit:--> if the rest of circuit + heat sink can support that!
I agree completely on this.
Actually I understand the importance of a large heatsink surface.

This is the reason that made Onsemi to introduce the MJL 0281/MJL 0302 which are a great choice for low Iq and low power consumer amps.
That is very interesting.
So low bias (i.e. consumption) power amps are possible.

Thank you very much again. Interesting topic this for me.
Kind regards,

beppe
 
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