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Old 11th June 2005, 06:26 PM   #1
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Question Quiescent current question? thanks

i've read a few posts stating that 20 to 25 millivolts across emmiter resistors is a pretty good average. ok, my power amp has 2 mjl4281a/mjl4302 pairs for outputs correct me if i have wrong numbers hehe. i have one dual .22 ohm per channel. my question is, when i measure from mjl4281 emmiter to mjl4302 emmiter for same channel, it's the same as measuring across across the outer legs of the dual .22 ohm resistor for that channel correct? so, measuring across both .22 ohm resistors would be .44 ohm. should pot be set for 40 to 50 millivolts across the .44 ohm instead of 20 to 25 millivolt across one .22 ohm? i have about 23 millivolts across each dual emmiter resistor now .44 ohm both outside legs. this is an Onkyo M-282 2 channel 100 watt per channel into 8 ohm with pretty hefty heat sink. if i develop a lot of transformer hum, am i going to high? even though heat sinks are within normal heat range? thanks, crippledchicken
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Old 11th June 2005, 06:53 PM   #2
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Yes you are measuring correctly and your assumptions are correct.

23mV across 0.44 ohms gives an Iq of 52mA, about spot on for most transistor output stages.
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Old 11th June 2005, 06:58 PM   #3
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Quote:
Originally posted by richie00boy
Yes you are measuring correctly and your assumptions are correct.

23mV across 0.44 ohms gives an Iq of 52mA, about spot on for most transistor output stages.
hi and thanks, could i gain anything by raising the current a little? or mainly circuit dependent. i replaced the pots with 15 turn sealed pot as the others were old and dusty. thanks again
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Old 11th June 2005, 10:10 PM   #4
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Hi Crippledchicken,

You could vary it between 25mA and 75mA and note any perceived audible differences, but it is largely the role of NFB to correct things beyond the nominal Iq setting. The thermal capacity of the heatsink is then not compromised.

Greg
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Old 12th June 2005, 01:23 AM   #5
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Quote:
Originally posted by amplifierguru
Hi Crippledchicken,

You could vary it between 25mA and 75mA and note any perceived audible differences, but it is largely the role of NFB to correct things beyond the nominal Iq setting. The thermal capacity of the heatsink is then not compromised.

Greg
HEY THANKS FOR THE INFO. i was reading the article put a tiger in your amp where they said anything less than 100 ma. wasn't enough and over 150 ma. wouldn't be any improvement. and wanted to get more opinions. thanks again, crippledchicken


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Old 12th June 2005, 10:05 AM   #6
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Quote:
Originally posted by crippledchicken
i was reading the article put a tiger in your amp where they said anything less than 100 ma. wasn't enough and over 150 ma. wouldn't be any improvement.
Ill-informed article, and responsible for a lot of dead and bad-sounding amps out there. A disservice to to audio community.
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Old 12th June 2005, 11:55 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
"responsible for a lot of dead and bad-sounding amps"

The Iq will determine the heatsink and transistor temperatures when no signal is output.
Raising Iq will raise the transistor case temperature (Tc) and this will impact on the safe operating area (SOA) of the output transistors.
The designer of a commercial amp will try to optimise the whole package to maximise profit for his employer. This will often result in little margin for running an amp at increased Iq.

However if you are in a cold climate and NEVER experience high temperatures in your listening room then you may have a little leaway for experimentation (but keep a very careful check on Tc and/or heatsink temp). At your level of Iq for BJTs you are unlikely to notice much (any) difference in reduced xover distortion since the designer seems to have chosen a highish Iq already.
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Old 12th June 2005, 12:33 PM   #8
anatech is offline anatech  Canada
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I've said it before. Depends on the circuit design. Increasing bias current may not bring any improvement at all. If you want to check, then try by all means. Use a THD meter or better, and your ears. Don't get caught in the trap where you have a few people imagining differences. Like outlets, power cords etc ... That kind of stuff only improves poorly designed equipment at best.

But, if the amp is running too hot, what's better. Getting sound every time you turn it on, or smoke now and again? Don't make your amp unreliable.

-Chris
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Old 12th June 2005, 02:26 PM   #9
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hi fellas, and thanks for all the experienced advice sounds like you know your stuff! i have raised it from 23 mv. across the outer legs of the dual emmiter resistors .44 ohm total to about 30 mv. which i figured to be about 68 to 69 ma. per channel. no excess heat but not much difference in sound though. should i drop back to 23 mv. or should it be safe enough still? also the original drivers have an ft of 8 but, the ones which are suppose to go with the mjl4281a/mjl4302 pairs has an ft of 30 which i have on hand. would this extra bandwidth cause oscillation problems in the driver stages? or, should they be designed to limit the bandwidth maybe? don't know if they would be an improvment soundwise, but i know their suppose to match the output transistors. don't remember the part# on original drivers but do remember the ft of 8. if i do change drivers, i guess i'll have to rebias again correct? thanks again, crippledchicken
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Old 12th June 2005, 02:38 PM   #10
Mr Evil is offline Mr Evil  United Kingdom
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If you don't notice any improvement in sound from raising bias then I would recommend setting it back down, for reduced power consumption and improved reliability.

I would expect faster driver transistors to be ok, since the output stage speed is limited by the output transistors.
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