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Old 19th June 2017, 09:18 AM   #4751
Zen Mod is offline Zen Mod  Serbia
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Old Yesterday, 09:40 PM   #4752
Bfpca is offline Bfpca  Canada
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I've just got my mono block F5t amps wired up and am in the testing phase. I am unable to get past about 230mv of bias using some store bought 74/170. They are the b grade but are at the extreme low end of the range.
I am measuring just less than 6ma of current and am unable to get enough voltage across the 1k/5kpot combination to get past 230mv. The bias is slightly over compensated thermally by the thermistors. It actually goes down a bit as the amp warms up. This may be because I am at the high end of the adjustment range on the 5k pot. which makes the voltage across the 1k more sensitive to changes in the thermistor value.
I am thinking of changing out the 1k for 1.2k. Any other suggestions are welcome.
I am also getting offset drifts of maybe 40mv from cold to hot which seems to be fairly common. This is much trickier to adjust than my Aleph J which locked onto it's bias setting and maintains a low offset as it warms up.

Last edited by Bfpca; Yesterday at 09:43 PM.
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Old Yesterday, 10:26 PM   #4753
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Quote:
Originally Posted by Bfpca View Post
......
I am thinking of changing out the 1k for 1.2k. ......
good for start
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Old Yesterday, 11:01 PM   #4754
laverda is offline laverda  United Kingdom
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Quote:
Originally Posted by Bfpca View Post
I've just got my mono block F5t amps wired up and am in the testing phase. I am unable to get past about 230mv of bias using some store bought 74/170. They are the b grade but are at the extreme low end of the range.
I am measuring just less than 6ma of current and am unable to get enough voltage across the 1k/5kpot combination to get past 230mv. The bias is slightly over compensated thermally by the thermistors. It actually goes down a bit as the amp warms up. This may be because I am at the high end of the adjustment range on the 5k pot. which makes the voltage across the 1k more sensitive to changes in the thermistor value.
I am thinking of changing out the 1k for 1.2k. Any other suggestions are welcome.
I am also getting offset drifts of maybe 40mv from cold to hot which seems to be fairly common. This is much trickier to adjust than my Aleph J which locked onto it's bias setting and maintains a low offset as it warms up.
I had the same and ended up with 2.2k/5k pot as that was all I had knocking around which worked well for me. About 10 turns on the 20 turns pot for 250mv bias.

Offset, you should be able to get within just a few mv, even 0mv for a while. Mine runs less than 5mv after about 30-45mins. I ignore the cold mv reading ...
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Old Yesterday, 11:37 PM   #4755
Bfpca is offline Bfpca  Canada
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After I get the bias up to my intended target of 250mv -(2 amps total) I'll start to adjust for low offset. My chassis are not that large, being 150mm high and 400 deep, but they have 10mm thick sinks with a total depth of 50mm versus 8mm and 40 total for the store chassis. At 225mv -.45amps/fet with a 48v supply the hottest temp I can find with an IR thermometer is 110f. The sinks and transistors are not what I would subjectively call hot after 45 minutes but just warm. I'm thinking .5amps/fet is doable , maybe a little more. I am not using the diodes, I just don't think they are worth the trouble for the loads I am going to drive. With 4 sets of outputs/channel the diodes don't get you much extra output with 4-8ohm speakers.
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Old Today, 04:14 AM   #4756
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Quote:
Originally Posted by Bfpca View Post
I am also getting offset drifts of maybe 40mv from cold to hot which seems to be fairly common. This is much trickier to adjust than my Aleph J which locked onto it's bias setting and maintains a low offset as it warms up.
I get an offset drift of about 400mV from cold to hot. So I still quite don't understand from where it is coming....
Maybe i should change R11 / R12 resistors to some other value... But in which direction to go, more or less?
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