krell kma amplifiers adjusting bias anyone know anything ?

zuki · 15th February 2005, 02:47 AM

i have the krell kma 100 mono block amps.

bought them new.

had them adjusted maybe 5 / 6 times since then ,by krell.

the other day i noticed one of the boards inside was leaning.

so i bent it up to match the other 3 boards.

now one amp runs hot.

each amp has 2 small flat screw driver/blue box pots inside.

with a dab of glue holding them at their adjusted point.

can i turn these to reduce the amplifier heat possibly ?

i wrote krell 3 weeks ago, and have yet to get a reply.

i don't know how to post a picture or i could show better.

any help will do.

p.s.
i leaned the board back down to where it was,
but amp still gets hot.

zuki · 15th February 2005, 11:18 PM

this is confusing for me.

make sense to others ?

http://audioworld.com/sw/Forum1/HTML/004170.html

i will get out my multi meter now.

zuki · 16th February 2005, 01:58 PM

would turning a pot counter clockwise be a reduction in bias ?

or it doesn't always work this way ?

sorry i don't know how to build amplifiers,

i do " really" like audio.

just don't know how to build the gear.

Nelson Pass · 16th February 2005, 07:33 PM

You turn the pot in whichever way the designer intended. Most
will help you out by increasing the bias with clockwise movement.

Biasing an amp isn't all that tough, but it does require very
careful procedure, as it's easy to accidentally over-bias and
have the amp blow up. Of primary importance is to monitor
the current draw at all times and watch it for drift over a good
period of time, like an hour. Most circuits drift upward with
temperature, so watch out for that.

If you don't know the desired bias setting, I recommend that
you go for a specific heat sink temperature of 45 to 50 deg
C. This is a typical figure, and is a temperature range that you
can easily judge by tough. The 10 second rule: If you can
put your hand on the sink for 10 seconds, then the temperature
is 45 to 50 deg. C.

zuki · 16th February 2005, 08:33 PM

i found out 1 pot is a dc offset and the other is the bias

but do not know which is which or the millivolt value or temp to set the amp at.

i am thinking that the board i "leaned" is the one with the pot that requires turning.(since i had no trouble prior)

but if i turn the pot counter clockwise/left.

will this reduce temp. and if so, how much of a turn accomplishes
the temp reduction.?

a hair of a turn or 1/4 turn ?

thank you so much for the help.

Nelson Pass · 16th February 2005, 09:26 PM

Don't do anything without measuring the current draw. You
will have to use a multimeter at least. If you want, find the
DC voltage across the power Emitter resistors of the output
devices on the working channel as your guide. Again, watch
the current, as this is where people usually smoke their
amps if they're not careful.

AndrewT · 16th February 2005, 10:57 PM

Hi,
when you said leaning and then bent it up. Did you mean you physically moved the PCB to a new position?
Could this have broken a solder joint and/or copper track somewhere?

zuki · 17th February 2005, 01:50 PM

i will look.

but i doubt it

thanks

zuki · 18th February 2005, 05:41 PM

i looked .

nothing is wrong with any of the leads

thank you

zuki · 20th February 2005, 02:25 AM

what would happen if i turned

dc offset counterclockwise a small amount

without any multi meter connected ?

15th February 2005, 02:47 AM	#1
zuki diyAudio Member Join Date: Feb 2004 Location: earth	krell kma amplifiers adjusting bias anyone know anything ? i have the krell kma 100 mono block amps. bought them new. had them adjusted maybe 5 / 6 times since then ,by krell. the other day i noticed one of the boards inside was leaning. so i bent it up to match the other 3 boards. now one amp runs hot. each amp has 2 small flat screw driver/blue box pots inside. with a dab of glue holding them at their adjusted point. can i turn these to reduce the amplifier heat possibly ? i wrote krell 3 weeks ago, and have yet to get a reply. i don't know how to post a picture or i could show better. any help will do. p.s. i leaned the board back down to where it was, but amp still gets hot.

16th February 2005, 10:57 PM	#7
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, when you said leaning and then bent it up. Did you mean you physically moved the PCB to a new position? Could this have broken a solder joint and/or copper track somewhere? __________________ regards Andrew T.

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15th February 2005, 11:18 PM	#2
zuki diyAudio Member Join Date: Feb 2004 Location: earth	this is confusing for me. make sense to others ? http://audioworld.com/sw/Forum1/HTML/004170.html i will get out my multi meter now.

16th February 2005, 01:58 PM	#3
zuki diyAudio Member Join Date: Feb 2004 Location: earth	would turning a pot counter clockwise be a reduction in bias ? or it doesn't always work this way ? sorry i don't know how to build amplifiers, i do " really" like audio. just don't know how to build the gear.

16th February 2005, 07:33 PM	#4
Nelson Pass The one and only Join Date: Mar 2001	You turn the pot in whichever way the designer intended. Most will help you out by increasing the bias with clockwise movement. Biasing an amp isn't all that tough, but it does require very careful procedure, as it's easy to accidentally over-bias and have the amp blow up. Of primary importance is to monitor the current draw at all times and watch it for drift over a good period of time, like an hour. Most circuits drift upward with temperature, so watch out for that. If you don't know the desired bias setting, I recommend that you go for a specific heat sink temperature of 45 to 50 deg C. This is a typical figure, and is a temperature range that you can easily judge by tough. The 10 second rule: If you can put your hand on the sink for 10 seconds, then the temperature is 45 to 50 deg. C.

16th February 2005, 08:33 PM	#5
zuki diyAudio Member Join Date: Feb 2004 Location: earth	i found out 1 pot is a dc offset and the other is the bias but do not know which is which or the millivolt value or temp to set the amp at. i am thinking that the board i "leaned" is the one with the pot that requires turning.(since i had no trouble prior) but if i turn the pot counter clockwise/left. will this reduce temp. and if so, how much of a turn accomplishes the temp reduction.? a hair of a turn or 1/4 turn ? thank you so much for the help.

16th February 2005, 09:26 PM	#6
Nelson Pass The one and only Join Date: Mar 2001	Don't do anything without measuring the current draw. You will have to use a multimeter at least. If you want, find the DC voltage across the power Emitter resistors of the output devices on the working channel as your guide. Again, watch the current, as this is where people usually smoke their amps if they're not careful.

17th February 2005, 01:50 PM	#8
zuki diyAudio Member Join Date: Feb 2004 Location: earth	i will look. but i doubt it thanks

18th February 2005, 05:41 PM	#9
zuki diyAudio Member Join Date: Feb 2004 Location: earth	i looked . nothing is wrong with any of the leads thank you

20th February 2005, 02:25 AM	#10
zuki diyAudio Member Join Date: Feb 2004 Location: earth	what would happen if i turned dc offset counterclockwise a small amount without any multi meter connected ?

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