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Old 27th November 2009, 09:21 PM   #1
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Default Crest Pro Series Bias? 10001

Anyone have any information on the Bias procedure for the big Crest pro Series amps???


I am working on a 10001 and while i managed to get this beast running again i really need to know how to set the bias.



Zc
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Old 28th November 2009, 09:51 AM   #2
ontoaba is offline ontoaba  Indonesia
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1.Set volume and listen the speaker in minimum sound.
2.Turn the resistor until crossover distortion heard.
3.Turn back very close to the distortion disapeared.
(do it when the amp not so warm)
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Old 28th November 2009, 03:07 PM   #3
djk is offline djk
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I would try 10mV across any of the emitter resistors for the low rail output transistors.
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Old 28th November 2009, 03:10 PM   #4
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Thanks DJK I will give that a try.
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Old 28th November 2009, 06:25 PM   #5
ontoaba is offline ontoaba  Indonesia
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10mV each .33ohm means 30mA current flow each low rail transistor. It waste so much power when the temperature gets hot.
The resistor are too small, it should be 0.47 ohm for better thermal runaway maintenance.
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Old 28th November 2009, 09:05 PM   #6
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The lower you can set the bias the less chance of thermal runaway.

However the amp should have the bias transistor on the main heatsink for thermal feedback.

I usually go for 5mV across a 0R22 and this is sufficient with the amp warmed up to get rid of cross over distortion.
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Old 29th November 2009, 02:50 AM   #7
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by onto aban View Post
10mV each .33ohm means 30mA current flow each low rail transistor. It waste so much power when the temperature gets hot.
The resistor are too small, it should be 0.47 ohm for better thermal runaway maintenance.
I shoot for 1 to 3 mV (when warm) across each emitter resistor in a pro PA amp where there are 3 or more outputs in parallel. It is NOT the optimum bias for a class AB amp, but it will get distortion down to levels where you can't fault the power amps.
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Old 29th November 2009, 04:14 AM   #8
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This is a BIG pro amp. 5kw per channel at 1 ohm, 14 pairs of bipolar output devices with 8 pairs of mosfet step switch devices. rails are +/-80 and +/-160V and it is used for running many sets of subwoofers so it gets the crap beat out of it.

I don't have a way to run it much past 400-500 watts. My load bank will go to 800 watts but I don't have AC power service in my shop that can handle it. My shop's AC is shared with the office next door unfortunately. (the down side of cheap rent.) So I have to error on the side of caution. I need it fixed and fixed right one time.

I trust DJK's information. I will start there and see how it goes.


Zc
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Old 29th November 2009, 01:21 PM   #9
wg_ski is offline wg_ski  United States
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Originally Posted by Zero Cool View Post
I don't have a way to run it much past 400-500 watts. My load bank will go to 800 watts but I don't have AC power service in my shop that can handle it.
Zc
Pink-noise test it (instead of sine wave) at high power. Use an 8 ohm dummy load one channel at a time. That will keep the AC demand under control and still let you test it to clipping. With most of these amps, the bias tracking is intentionally overcompensated (when it runs hot the idle bias drops a bit). "Light " load at high output will test the bias stability, clipping behavior, and proper operation of the rail switches.
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Old 4th December 2009, 01:57 PM   #10
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With the exception of one bad LM339 comparater on one pre-amp board. I have both output stages up and running again. I set Bias to 30mv for starters. and WOW this baby is rock solid on the bias. if you set it to .30mv it doesn't even flicker a 10th! But i haven't got to do any testing at that bias yet as tracking down the issue with the pre-amp board has taken a lot of time.

One curious thing about this amp that i have never seen another amp do. with bias set, the AC Line idle current is very low. I have not measured it with a digital meter but the analog current meter on my variac is less then 1 amp which is the first mark on the meter and im sure the meters resolution down that low isn't very good...anyway..

With no speaker load connected to the amp but a DVM connected to the output. a sine wave signal feeding the input of the amp. if you vary the input gain. or the gain of the signal generator. As the output of the amp approaches .8vrms output, the amp starts to draw current from the ac line. it seems to have peak current draw around 1-1.5vrms. at this point the amp is drawing 4-5 amps of current from the AC line. scope shows a clean clean sine wave. no oscillations or visible distortion. once you go past this point, the current drops back down to idle levels and you can run the amp up to the point of clipping without any current drawn.

Both channels exhibit this behavior.

I need to repeat this test with a load connected and see if it does the same thing? but i thought it odd. without a load it shouldn't be drawing any current at any output level.



Zc

Last edited by Zero Cool; 4th December 2009 at 02:03 PM.
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