hi.
i have a problem with testing my krell clone:
i use 8 x MJE3281a and 8 x MJE1302A for outputs per side (should be biased with 300mA).
Q1, Q2 are 2sc2240
Q3, Q4 are 2SA970
Q9, Q11 are 2SA968
Q10, Q12 are 2SC2238
Q13, Q14, Q16 are MJE15030
Q15 Q17 are MJE15031
when i power up slowly even with 10V DC per rail the bias is raising (>300mA) too high.
i checked everything but i couldn´t find the fault.
did anybody have the same behaviour and can help out?
thank you in advance.
slavko
i have a problem with testing my krell clone:
i use 8 x MJE3281a and 8 x MJE1302A for outputs per side (should be biased with 300mA).
Q1, Q2 are 2sc2240
Q3, Q4 are 2SA970
Q9, Q11 are 2SA968
Q10, Q12 are 2SC2238
Q13, Q14, Q16 are MJE15030
Q15 Q17 are MJE15031
when i power up slowly even with 10V DC per rail the bias is raising (>300mA) too high.
i checked everything but i couldn´t find the fault.
did anybody have the same behaviour and can help out?
thank you in advance.
slavko
Is the Vbe multiplier thermally coupled to the output stage?
The KSA100 should be biased to ~2.6A
That gives a maximum ClassA output of ~5Apk and that equates to 100W of ClassA into 8r0 dummy load.
For an 8pair output stage, each pair should be biased to ~325mA +-tolerance, so expect your bias to be >300mA on any single emitter resistor.
The KSA100 should be biased to ~2.6A
That gives a maximum ClassA output of ~5Apk and that equates to 100W of ClassA into 8r0 dummy load.
For an 8pair output stage, each pair should be biased to ~325mA +-tolerance, so expect your bias to be >300mA on any single emitter resistor.
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if the potentiometer is wired in correctly, then a high value gives a low bias voltage.
Have you set the bias voltage correctly?
Have you wired in the pot correctly?
I doubt your heat sinks will support the full bias from the looks of it.
You might get away with half their value.
I would set the trim pot to half rotation and run it up with a Variac. Many boards are opposite from the way you think they should work and this allows you see which way increases bias
Immediately turn up the Variac about a 1/3rd to get the transistors to switch and go from there. You should be alright but seriously think the sinks are not enough surface area.
David
You might get away with half their value.
I would set the trim pot to half rotation and run it up with a Variac. Many boards are opposite from the way you think they should work and this allows you see which way increases bias
Immediately turn up the Variac about a 1/3rd to get the transistors to switch and go from there. You should be alright but seriously think the sinks are not enough surface area.
David
thats what i tried. i powered it with a variac with about 1/5 from the net. (europe 230V)
and with trimpot on minimum i had more than 300mV (300mA). then i stopped the test.
this should be different i think with just 1/5 from mainpower.
don't want to destroy any transistors just to see that it doesn't work...
regards
slavko
and with trimpot on minimum i had more than 300mV (300mA). then i stopped the test.
this should be different i think with just 1/5 from mainpower.
don't want to destroy any transistors just to see that it doesn't work...
regards
slavko
If you have not done so, you can and should start the board without connecting the output devices. You can even connect a speaker and play it at low volume. No need to put all those expensive transistors at risk.
