A little help needed, PLH no Bias ! - diyAudio
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Old 29th October 2012, 12:36 AM   #1
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Default A little help needed, PLH Low Bias ! UPDATE still not working

hi all, any help greatly appreciated..
I've built a pair of PLH monoblocks (well just finished 1 so far)
and during setup i can wind up the power supply to 40v, but with almost no current draw/Bias,
using P3 I can get the DC to around 22 at the output with the pot fully anticlockwise,
but as i try to increase the bias i get nothing,
being careful I've varied P1 between 500ohm and ~250ohm and the bias stays at pretty much nothing (0.0x volts),
checking the voltages given on the circuit i get 7.2v instead of 8v at the gate of Q1 and 3.8v at the gate of Q2/4 this varies a little with P1 but such a tiny amount i don't believe i can get to 4.6v even with P1 at minimum.

should i expect the bias to rise over the first 1/2 travel of P1? or is all the action in the last few turns (10 turn pot)

the only differences to the schematic are,
Z1 is made up from 2x 10V zenner as i had these to hand,
Q1 is a IRF610A marked SEC, (is the issue because i used the A suffix version?)
I have double/triple checked the wiring,
the input has a shorted plug inserted for testing.

any help greatly appreciated.

Last edited by gbullimore; 30th October 2012 at 01:19 PM.
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Old 29th October 2012, 01:27 PM   #2
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Update!
i braved up a bit and winding the bias pot to minimum impedance gives 0.45v Bias on each of the 4 source resistors.
I still have to max out P3 and only get to 23v at the output.
this is with 40v DC supply.
should i decrease the value of R6, maybe parallel another 220R across it?
i'm a little concerned that whilst this may work it might not give such good results/performance.
or should i swap out the IRF610A for an IRF610.

any advice greatly appreciated.
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Old 29th October 2012, 01:48 PM   #3
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this sch ?
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File Type: gif PLH.gif (50.7 KB, 95 views)
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Old 29th October 2012, 01:55 PM   #4
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The later one with 2 pairs of output devices. but basically yes.
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Old 29th October 2012, 02:01 PM   #5
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this one
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Old 29th October 2012, 02:15 PM   #6
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Quote:
Originally Posted by gbullimore View Post
Update!
i braved up a bit and winding the bias pot to minimum impedance gives 0.45v Bias on each of the 4 source resistors.
I still have to max out P3 and only get to 23v at the output.
this is with 40v DC supply.
should i decrease the value of R6, maybe parallel another 220R across it?
i'm a little concerned that whilst this may work it might not give such good results/performance.
or should i swap out the IRF610A for an IRF610.

any advice greatly appreciated.
bias issue - decrease R6 to 100R ( yup - you can parallel another 220R )

output node voltage level - increase R7 to , say , 3K3 or 4k7

difference between 610 and 610A is irrelevant for this circ
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Old 29th October 2012, 02:21 PM   #7
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ok will try that,
do you think that these changes will affect performance/ sound quality?
also interested as to why the suggested values don't give enough bias, with the parts i have?
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Old 29th October 2012, 02:23 PM   #8
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do not worry - it will work as intended

difference - your parts are enough different from parts Pa had in drawer

with mosfets you must count on these things
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Old 29th October 2012, 02:28 PM   #9
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Ok cool, thanks for the good advice.
I just realized its taken me 5 years to go from cases nearly finished to powering up 1 amp!!!!
I did change jobs and move from UK to China in that time too !!
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Old 30th October 2012, 01:22 PM   #10
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ok,
i dropped another 220R across R6 so now R6=110R,
still can only get 0.6 Bias at 40v supply.
increasing supply to 42-43 volts gives 0.8v bias, but using the wall voltage here i only get 38v supply, and bias about 0.5v.

i measured voltages in circuit and are as follows
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