Yes, the 470 ohm gate resistors are a bit high - 270 should do it.
Get the oscillation under control, and then you can tweak to fine tune the performance.
Get the oscillation under control, and then you can tweak to fine tune the performance.
Hi!
Thank you all for the interest in my little problem.
You’re right with the gate stopper resistors. They are wrong and I'm gone recalculate to find the optimal value. But I do not agree regarding the resistors R39-R47. This is lateral MOSFETs and they do not need source resistors when paralleled. R39-R47s only function is current monitoring.
Bonsai you are definitely into something. I will try to make some calculation around the NFB network to make the amplifier more stable.
But there is something “fishy” here. I just remembered that I think I built a prototype board with this amplifier 5 years ago, and I can’t recall I had any problems back then. This is an old project I started 5 Years ago, but other projects and lack of time suffocated my surround amplifier project. But now I am gone finish what I started.
I think there must be something picking up noise (internally in the amp), but I think this is positive because it revealed a design weakness before any speaker where connected. I’m currently building a monoblock casing and I’m gone solder together another PCB and test the original design under more optimal conditions just for fun, before I make the design modification discussed above.
If someone is interested I can post the revised design schematics when I am finished.
Thank you all for the interest in my little problem.
You’re right with the gate stopper resistors. They are wrong and I'm gone recalculate to find the optimal value. But I do not agree regarding the resistors R39-R47. This is lateral MOSFETs and they do not need source resistors when paralleled. R39-R47s only function is current monitoring.
Bonsai you are definitely into something. I will try to make some calculation around the NFB network to make the amplifier more stable.
But there is something “fishy” here. I just remembered that I think I built a prototype board with this amplifier 5 years ago, and I can’t recall I had any problems back then. This is an old project I started 5 Years ago, but other projects and lack of time suffocated my surround amplifier project. But now I am gone finish what I started.
I think there must be something picking up noise (internally in the amp), but I think this is positive because it revealed a design weakness before any speaker where connected. I’m currently building a monoblock casing and I’m gone solder together another PCB and test the original design under more optimal conditions just for fun, before I make the design modification discussed above.
If someone is interested I can post the revised design schematics when I am finished.
It's important that the junction of the feedback resistors and where they meet the inverting input are kept as close as possible to each other. If you have any long tracks you will pick up problems.
Your gate resistor values look quite typical for a lateral MOSFET amplifier and you are correct that lateral MOSFETs can be used without source resistors. But I see IRF(9)540 printed in the schematic and those are not lateral fets. Vertical MOSFETs like those have much higher gate-drain capacitance and it inreases even more when the output nears the rails and this can lead to instability as the output stage slows down. 🙂
Oooh ****. You are right. I remember now, this is a bit awkward; but will certainly explain a couple of thinks. Like I said earlier this is an old project and originally I hadn’t written down on the schematics witch output transistors I intended to use, I just assumed the IRFs because it was laying 50 of each in my project box. Actually I think I bought the IRFs for a RC H-bridge speed ctrl project 😱 I have found a couple of 2sk1058/2sj162 in the same box… They are probably leftovers from the prototyping. But since I’ve already started with the IRF(9)540 and have a bunch of them laying around, I probably have to make some modifications, add source resistors and put them to good use J.
I feel like an idiot, and will post revised schematics when I’m done.
Thanks everyone. 🙂
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