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Old 20th September 2010, 07:54 PM   #1
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Default what's a good average bias setting

I have a cheap, 4000w 4 channel amp that was having severe crossover distortion. I turned up the bias from its setting of .888v to 1.200v and the sound cleaned right up. but now i am wondering if i need to turn it down a bit. It's a Technical Pro lz4001, and there is no service manual anywhere that i can find. so i am wondering what is a typical setting i might try.
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Old 20th September 2010, 08:27 PM   #2
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If you dont have the manual what I normally do is turn the bias right down.
Apply a sine wave and attach a speaker to the amp.
Turn the bias up slowly until crossover distortion goes.

Any more and you waste power as heat, any less and you get distortion.
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Old 20th September 2010, 08:54 PM   #3
lineup is offline lineup  Sweden
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0.888 to 1.200 Volt across emitter resistor sounds very high.
How much current does it take to get away with crossover distortion?
I would say for bipolar power transistors, anything from 30 to 60 mA.
As we do not know what value your big emitter resistors have
we can not say how much voltage it takes to get 30-60 mA idle current.
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Old 20th September 2010, 09:18 PM   #4
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John claims from experience that Re should drop about 25mV to be optimally biased.
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Old 21st September 2010, 01:30 AM   #5
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Ear to screw driver approach most scientific indeed.

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Old 21st September 2010, 07:18 AM   #6
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Crossover distortion is very hearable on piano trebles, listening at very low level.
It sounds like a cracked crystal glass.
This can be used for bias tuning.
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Old 21st September 2010, 11:45 AM   #7
DF96 is offline DF96  England
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Optimum quiescent current range depends on the output stage configuration. If it is BJT Darlington then 100-200mA is typical. If it is BJT complementary pair then 10-20mA, unless you want gm doubling for small signals.
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Old 21st September 2010, 01:25 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

Optimum bias is a voltage, the current it causes varies with emitter resistor
values, the lower the Re's, the higher is the correct bias current caused.

rgds, sreten.
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Old 21st September 2010, 02:25 PM   #9
lineup is offline lineup  Sweden
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The optimal bias is a voltage.

This may be true.
But within certain limits of current to bias into Class AB.
So the current level plays a role.

It is not like you can take 10 Ohm and put a very small current.
Or take 0.001 Ohm and put an enormous current across resistor.

Most people advocate a resistance of 0.100 Ohm to 0.470 Ohm
Depending on the specific amplifier this of course can be different.
Above and below the 0.100-0.470 Ohm.
A special case is when we have parallell transistors.
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Old 22nd September 2010, 04:27 PM   #10
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by lineup View Post
The optimal bias is a voltage.

It is not like you can take 10 Ohm and put a very small current.
Hi,

Yes you can if optimally biasing the output stage of a low power amplifier.
(Though here you may prefer AB biasing to aB, poosibly whacking up the
A biasing to be full Class A for normal loads, only going into B for high loads.)
The important parameter is the voltage, not the current. Practicalities
rule out very low emittter resistors, 0.1R being the lowest I've seen.
But consider a tripled parallel output stage with 0.1R per transistor,
Re effective is 0.033R, bias current is triple, bias voltage the same.

For a given topology there is one optimum bias voltage. Details
of the topology determine the current related to this voltage.
Its not true each topology has an optimum current.

rgds, sreten.

Last edited by sreten; 22nd September 2010 at 04:35 PM.
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