Bias control problem with n-channel amplifier (Anthony E. Holton)

[Hielke02]

hello audio giants,

I've build the n-channel amplifier from Anthony E. Holton

PDF file schematics:
http://www.enisadesign.nl/n-channel-amp.pdf (I dont know if its ok to put it here?)

The only thing I changed is I replaced Q5-Q10 (IRFP240) by 2x2 IRFP260N and I put some protection circuits from this schematic:

http://www.enisadesign.nl/PA-300.pdf

The mosfets are matched closely.

You can see on the picture that I've build this amplifier tree times in one case.
With the potentiometer from 2K on zero ohm as Anthony write in the manual, everithing sounds ok, the amplifier sounds dead quit with cd player on the input at pause.
Also the protection circuits works ok.

But when I want to trim the potentiometer to adjust the bias, it needs at least 15 turns to see some voltage on my multimeter (measured at R32).
When I put it on 35mV measured on R32 in cold condition, its after 10 minutes not lower as described by Anthony but about 40mV and it keeps slowly getting higher.

I'm afraid to destroy my mosfets and turn the amplifier off after 15 minutes

I also measured the voltage at the gate resistors in cold and warm condition.
At cold condition I measure at the gate resistors from Q8 and Q10 4.19V and at the gate resistors from Q7 and Q9 4.24V.
At warm condition (after 12 minutes) I measure at the gate resistors from Q8 and Q10 4.26V and at the gate resistors from Q7 and Q9 4.16V.

The LM317 is not getting warm, but the 2sc3298 needs a liitle coolvin.
I also measured at E6 capacitor in cold and warm condition at cold condition I measure a almost stabel voltage from 8.48V but after
10 minutes it's jumps a little between 8.23-8.34V.

pictures Amplifier:

http://www.enisadesign.nl/versterker1.JPG
http://www.enisadesign.nl/versterker2.JPG
http://www.enisadesign.nl/versterker3.JPG
http://www.enisadesign.nl/amplifierboard.bmp



Hopefully you guys give me some advise, I dont know where to start to find the bias troubles.
Anybody also build this amplifier succesfully?



Greetings from the Netherlands,


Hielke

 

[The Saint]

Hi Hielke

The IRF260N while a good transistor is not really suitable for this amplifier.
The driver cct will have a hard time driving the gate C.
Anyway to help stablize the bias better with these devices.
You will need to change the source resistors to a higher level
try 0.47 Ohms first and test how stable it is.
f not then increase them to 0.56 ohms.
also on the bottom bank move the resistor into the source leg on each mosfet.

 

[Hielke02]

The IRF260N while a good transistor is not really suitable for this amplifier.
The driver cct will have a hard time driving the gate C.
Anyway to help stablize the bias better with these devices.
You will need to change the source resistors to a higher level
try 0.47 Ohms first and test how stable it is.
f not then increase them to 0.56 ohms.
also on the bottom bank move the resistor into the source leg on each mosfet.

Hi "The Saint",

thanks for your very fast reaction!
I was this weekend double checking if I had made a mistake in my lay out.

Everithing was ok, I must wait on my order of components (the resistors from 0.47 and 0.56 Ohm) to try your advise.


I don't know exactly what you mean by:

"also on the bottom bank move the resistor into the source leg on each mosfet"

It's becourse my English is not perfect I think

you mean that I must make a change in the layout/schematic and move the 5 Watt resistors from Q6, Q7, Q9 from the drain leg to the source leg and make a wirebridge for these mosfets from drain to the output?

And if so, is this only necessary becourse I use IRFP260N or also by IRFP240?


I also find on this forum:

Samuel Jayaraj say's:

"Just as an aside, I have used both IRFP460 and IRFP260 output devices in the erstwhile N-channel amps of Anthony Holton with very good results. While I used a small circuit from the upper rail to raise the gate voltage of the lower bank of output Mosfets which prevents cross-conduction, the IRFP260 with their larger capacitance sound great in the bass frequencies."

I wonder which changes you make in the schematic Samual.

Hmmm you two guys give me hope to find a solution for my problem.


Greetings,

Hielke

 

[masood]

hi hielke02,
please give right /true site.all that what you have mentioned are are not right site.thanking you.
masood

 

[es44]

hi hielke02,
please give right /true site.all that what you have mentioned are are not right site.thanking you.
masood

Look at the date. The post is more than 4 years old.

Best regards
Ebbe