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Old 25th February 2010, 08:17 PM   #11
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Quote:
Originally Posted by apexaudio View Post
Q9 and Q10 run with 30mA, bias of the output tranzistor 200mA set by P1, and bias stay stabil. Maybie must corect after few minutes but that will be small corection.
I found I could get away with 20mA on my design.

I then test by putting a household iron on the heatsink until it goes above 80 degrees c. I found it to be very stable despite not having bias transistor on output heatsink.
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Old 25th February 2010, 09:08 PM   #12
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Actually, if D1-D4 have reasonable thermal contact with the output mosfets then they will stabilize the BIAS current, maybe just being inside the hot case might work. Not particular great, but might work in practice....

Q8 & Q12 need to have a diode, otherwise their intrinsic CB diode could be biased. Although it might not matter that much in PA applications....

And of course, the simple current limit should be made into a proper VI limiter.


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Old 25th February 2010, 10:20 PM   #13
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D1-D4 are 15V zener diode for protection not for bias. Once again I must replay this circuit is no experimental, amplifiers tested and uses over 10 years. Take a good look on this schematics again.
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Old 25th February 2010, 11:16 PM   #14
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PCB and Montage Plan for APEX MOSFET
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Old 25th February 2010, 11:32 PM   #15
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I would like to suggest on that current limit circuit a diode be placed in the collectors of Q12 and Q11. It is also nice to place a diode in the emitter of those same transistors.
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Old 25th February 2010, 11:42 PM   #16
soekris is offline soekris  Denmark
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Quote:
D1-D4 are 15V zener diode for protection not for bias. Once again I must replay this circuit is no experimental, amplifiers tested and uses over 10 years. Take a good look on this schematics again.
You take a look again, Z1-Z4 are the zeners for protection.

D1-D4 are regular diodes, they have a negative temperature coefficient, basically lowering the current though the simple bias resistors when things heat up.... And you should be happy for that because otherwise you WOULD have had thermal runaway.

Again, not very stable or predictive, but apperently good enough....

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Old 25th February 2010, 11:45 PM   #17
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Quote:
Originally Posted by nigelwright7557 View Post
I found I could get away with 20mA on my design.

I then test by putting a household iron on the heatsink until it goes above 80 degrees c. I found it to be very stable despite not having bias transistor on output heatsink.
I think you got it a little wrong , apexaudio is saying his mosfets are biased at 200 mA, the vas current is 30 mA, its quite high but thats why its a cfp vas.
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Old 25th February 2010, 11:59 PM   #18
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I suggest anyone who want make upgrade on this circut is free to do it, and build new version. Original design work great for long time, and even beginer can easy build this amplifier. If you are beginer in DIY use my PCB design for first build.
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Old 26th February 2010, 12:27 AM   #19
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I am sory for mixing diode, my mistake. When amp heating mosfet bias current litle lowering in fact.
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Old 27th February 2010, 02:54 PM   #20
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This is PCB for protection. Use 2x18V AC for power. There are Soft-Start, DC, Overload and Thermal protect on 90deg. Two speed fan control 60deg (12V to 24V) when temperature heatsink is 60deg. Stabilized +/-15V for XLR and bridge input PCB. Temperature senzor is tranzistor BD241C.
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