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Old 14th March 2010, 04:48 PM   #11
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Bias can be setup on 100mA, who want go over better to use A class design.
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Old 14th March 2010, 05:09 PM   #12
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Originally Posted by russel View Post
OK, thats quite low for a FET. Lateral FETS usually need 150mA and Vertical FETS are run at 400mA. If you look at the transfer characteristics in fig 3 on the link: http://www.datasheetcatalog.org/data...rf/irfp250.pdf

You can see if you ran it at 400mA there is chance of thermal runaway.
I find I can run my IRFP240/9240 amps at 10mA bias and there is no crossover distortion.
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Old 14th March 2010, 07:32 PM   #13
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I find I can run my IRFP240/9240 amps at 10mA bias and there is no crossover distortion.

Is it a current dumping design?
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Old 14th March 2010, 07:35 PM   #14
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Is it a current dumping design?
Its just a bog standard classAB--- ltp, vas, bias and complimentary output transistors.

I just wind up the bias with a sine wave applied and a speaker connected until the crossover disotrtion goes on the scope.
It sounds good so the low bias must be fine.

I read somewhere Peavey do this as well, keeping the bias as low as possible.

It does make sense as all extra bias is doing is heating up the heatsinks.
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Old 14th March 2010, 07:42 PM   #15
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Originally Posted by nigelwright7557 View Post
I find I can run my IRFP240/9240 amps at 10mA bias and there is no crossover distortion.

Is it a current dumping design?
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Old 14th March 2010, 07:51 PM   #16
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Quote:
Originally Posted by russel View Post
Is it a current dumping design?
No it's not a current dumping amp.
It is unusual however, and not easy to analyse "in your head".
The bias arrangement is odd... how stable is it I wonder.
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Old 14th March 2010, 07:54 PM   #17
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Quote:
Originally Posted by nigelwright7557 View Post
Its just a bog standard classAB--- ltp, vas, bias and complimentary output transistors.

I just wind up the bias with a sine wave applied and a speaker connected until the crossover disotrtion goes on the scope.
It sounds good so the low bias must be fine.

I read somewhere Peavey do this as well, keeping the bias as low as possible.

It does make sense as all extra bias is doing is heating up the heatsinks.
Crossover and switching distortion that is audible may not be visible on a scope measurement you really need a distortion analyser of some sort. Switching and crossover distortion does not seem to affect some peoples enjoyment of music. For others, me included, it sounds like scratching glass which is why there is a valve and class A contingent in this conf :-).


As far as I am aware Peavey amps are used for guitars so will not have much HF content and if they did the bass unit would not reproduce it anyway so there is no point in trying to reduce it.


As far as increasing the bias current it does make sense if you are one of the lucky or unlucky few, the cost of getting rid of those distortions is high!, that finds switching and crossover distortion annoying. As the bias current increases the switching and crossover point occurs at higher power levels and the louder the music the less sensitive the ear is to distortion.
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Old 14th March 2010, 08:13 PM   #18
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Human ears cant hear THD<1%
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Old 14th March 2010, 08:52 PM   #19
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Human ears cant hear THD<1%
As far as I am aware if its the second harmonic then you are right, as musical instruments produce large amounts of second harmonics the ear does not find it objectionable. In the case of higher order harmonics produced by switching distortion and to a lesser extent crossover distortion then the ear is sensitive at much lower levels.
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Old 14th March 2010, 09:09 PM   #20
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BIAS 100mA
THD< 0,05%; TIM< 0,07%
F 20Hz-20kHz +/-0,1dB; 2Hz-250kHz +0/-3dB
Damping >200
S/N >108dB

Last edited by apexaudio; 14th March 2010 at 09:13 PM.
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