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.
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:
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.
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
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?
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?
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
difference - your parts are enough different from parts Pa had in drawer
with mosfets you must count on these things
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 !!
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 !!
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
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
not sure it it's relevant, but i just checked and found i'm using IRFP240 from IR,
could this explain the difference,
when i get back from work i'll try increasing R3, but with these changes will i upset NP's delicate balance in the design?
could this explain the difference,
when i get back from work i'll try increasing R3, but with these changes will i upset NP's delicate balance in the design?
thanks again for the feedback,
tried R3=510R and still no luck,
with V+ at 40v, i can get the Bias to 0.89v.
but i checked again the wall voltage, and when running from the wall the V+ is 38.5
in this condition i can only get 0.68v Bias.
should i change R3 to 730R (the original 390 + 220R) or more like 780R (390+390R),
or altogether more brave at 1k,
or should i be looking at reducing R6 below 110R ?
starting to worry a little at drifting so far from stock values.
would a change in brand / type of IRF610 be better?
although it's be nice to get it up and running quickly without having to order more FET's.
by the way i have not yet changed R7, will this affect the bias much?
i'm seeing ~25v at the output cap,
but i see almost equal bias on all 4 devices
tried R3=510R and still no luck,
with V+ at 40v, i can get the Bias to 0.89v.
but i checked again the wall voltage, and when running from the wall the V+ is 38.5
in this condition i can only get 0.68v Bias.
should i change R3 to 730R (the original 390 + 220R) or more like 780R (390+390R),
or altogether more brave at 1k,
or should i be looking at reducing R6 below 110R ?
starting to worry a little at drifting so far from stock values.
would a change in brand / type of IRF610 be better?
although it's be nice to get it up and running quickly without having to order more FET's.
by the way i have not yet changed R7, will this affect the bias much?
i'm seeing ~25v at the output cap,
but i see almost equal bias on all 4 devices
Update,
I realized i'd been stupid by leaving R7,
I changed it out to 10k, now i can adjust to 23v at the output cap, but now the bias maxes out at 0.97v,
I guess if i change R7 to 47k i might be there, i might even need to undo the R3 or even R6 change.
I realized i'd been stupid by leaving R7,
I changed it out to 10k, now i can adjust to 23v at the output cap, but now the bias maxes out at 0.97v,
I guess if i change R7 to 47k i might be there, i might even need to undo the R3 or even R6 change.
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