In the SIM, biasing is pretty easy, yes. VAS resistors 33R, pot 500R at mid gives 600mA bias current (I kept the resistors in the FET input stage at 2k).
I always run chassis plug SPK-GND direktly from PSU-GND. But I see I need to do something abouy the different GNDs now that I have two PSUs to handle.
Originally I hoped for one PSU only... 😎
I am not so sure, verticals have thermal issues like transistors, I speak under correction, I have never used these for linear applications only switching.
But is it thermally stable? Vertical mosfets have a negative tempco, unlikely Laterals. I suspect you'll end up with thermal runaway.
With verticals, you probably need a transistor based bias generator that can mount on the heatsink or onto the back of one of the MOSFETs to sense its temp and compensate.
I think this is the case. Like Nico, I've never designed with them. But I suspect they'll behave like BJTs in this regard.
You might very well be right. I see @quasi seems to have used that thermal coupling in the amp developed here: https://www.diyaudio.com/community/threads/power-amp-under-development.43331/
Will check if I can easily borrow that.
Thanks for the info, @Nico Ras and @brian92fs
Will check if I can easily borrow that.
Thanks for the info, @Nico Ras and @brian92fs
Brian, exactly my opinion. You need a bias spreader as they say that is in thermal contact with your heat sink. I would not do it.
You are ready to go to market. As my wife says don't scratch were it does not itch!
You are ready to go to market. As my wife says don't scratch were it does not itch!
You may be able to use a PTC or is it an NTC of the correct temp range simply bolted onto the heat sink.
Not sure if that was to me... I have a couple amps in the build pipeline which will use the Exicons (of which I have some laying around already). This with the IRF verticals showing so nice in the SIM would be a great alternative. That is why I am following that path too.
This is not the case... exactly. Both start with a negative tempco. But Laterals transition to positive around 125mA. This inflection point is much higher for vericals. It's so high that we can simple say they have a negative tempco from a practical view.
But wouldn't that mean the lateral IRFs were less prone to thermal runaway than the Exicons?
I wish I remembered all of the details, but 4 decades ago I designed an amplifier with complementary mosfet output stage with T0-220 devices which I believe were IRF520 and IRF9520. They definitely needed thermal compensation and I used a bias transistor in a TO-126 package for the purpose, the operating point ultimately stabilized at around 100mA but I remember it took a while for it to get stable - in early experimentation without the heat sink mounted transistor the quiescent current continued to trend upward until I lost my nerve and shut it off - it did not appear like it was going to plateau. (My target was 40W into 8 ohms which was achieved, supply voltage was around +/-32V regulated. Overall it was not a great sounding amplifier, but it did result in my redirection into tube audio. (until recently when I started "dabbling" new jfet/bjt/lateral mosfet designs.)
Is this what would be a compensation circuit in the bias part (approach "stolen" from Quasi's above thread)? With my limited knowledge, I would think so. What do the experts say?
Works fine in the SIM. I guess the question is: if the BD139 is mounted on the same heatsink, will it lower bias as much as vertical MOSFETs getting higher will raise it?
Works fine in the SIM. I guess the question is: if the BD139 is mounted on the same heatsink, will it lower bias as much as vertical MOSFETs getting higher will raise it?
They sell the "S version" as being closely matched. Not sure they advice this eliminates the need for source resistors.
From what I read, matched parallel devices will result in lower THD in general.
Not quite sure what you mean, but I found some designs by Borbely, amongst others, that looked promising (read Simple).
So here is one of Borbely's RIAAs followed by my copy-paste attempt.
Borbely RIAA
Could it be this easy?
Maybe I did not really follow... The original design by lineup is relatively simple (which is a compliment). What exactly was the purpose of the change to making more complex by using the cascodes?