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Increasing bias in amps.
Increasing bias in amps.
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Old 14th September 2006, 08:44 PM   #1
beppe61 is offline beppe61  Italy
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Default Increasing bias in amps.

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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Old 14th September 2006, 08:59 PM   #2
anatech is offline anatech  Canada
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Increasing bias in amps.
Hi beppe61,
Quote:
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.
Quote:
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.
Quote:
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.
Quote:
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
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Old 14th September 2006, 08:59 PM   #3
AndrewT is offline AndrewT  Scotland
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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 .
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Old 14th September 2006, 11:43 PM   #4
quasi is offline quasi  Australia
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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
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Old 15th September 2006, 02:10 AM   #5
EchoWars is offline EchoWars  United States
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Quote:
Originally posted by quasi
Did someone say Quasi?


I see blown amps all the time from guys who thought that 'if a little bias is good, a lot must be better'.
Quote:
If in doubt, follow the manufacturers recommendations.
If you like listening to music rather than replacing blown transistors and burnt resistors, this is the mantra to repeat to yourself.
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Old 15th September 2006, 02:38 AM   #6
Leolabs is offline Leolabs  Malaysia
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Hi beppe61.Take a trip to www.tnt-audio.com on their DIY article regarding increasing bias of the amp.
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Old 15th September 2006, 03:19 AM   #7
sam9 is offline sam9  United States
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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.
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Old 15th September 2006, 05:42 AM   #8
EchoWars is offline EchoWars  United States
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That tnt-audio article is responsible for more dead amplifiers than I care to count.
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Old 15th September 2006, 06:04 AM   #9
quasi is offline quasi  Australia
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Quote:
Originally posted by EchoWars


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
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Old 15th September 2006, 08:42 AM   #10
johnny1 is offline johnny1  Greece
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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!

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?
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