With a 3.07 ohms resistor + 1.02 mH inductor I get a 4 ohms load at 400 Hz, with 40 degree phase angle.
Now, if I put this at the amplifier output, the maximum V*I product is at 15V*2A. This is exactly the SOA of D44/45H. So it is too risky, I think ?
But the MJE15030/31's have SOA of 4A at 15V ! Much better.
Can I assume that this 4 ohms / 40* phase is the worst scenario for the output power transistors ?
I have a pair of speakers made by me long time ago, with PA-800 and PCT-300. They have LR4 pasive filters with 2.9KHz crossover. I tried to simulate the impedance and phase that my amplifier will see. With the TS parameters from their datasheets I calculate an equivalent model, using the instructions from this page. And the impedance and phase plot is like this... As you can see the minimum impedance is about 4 ohms at 400Hz. But the speakers ar rated as 8 ohms ! I don't know if it's an error in mai calculations or that is a normal behavior... And then, those 12 ohms at 2KHz, are strage too... The phase is 60* at 100Hz/8R.
Edit: Hmm... I simulated without the filters and now the lowest impedance is the bass speaker DC resistance, 6.5 ohms. So it means that the filters lower it to 4 ohms...
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No, I don’t think the D44 is too risky, with it exactly at the S/B limited SOA. You will get a little lift even with a 100 ms pulse, and will be enough to cover temperature rise. But this experiment is EXACTLY why you don’t raise the voltage over +/-18 with those parts. The supply will likely dip under load anyway. At +/-25V you are squarely in the red, and that is risky.
Ok, now that we're done with transistors, let's move on to something else: the Zobel network and the output coil. The idea is to miniaturize these as well, using SMD components.
For Zobel I thought of using two 47nF capacitors in parallel, in 1206 package, C0G type, 50V.
And a 1206, 0.5W resistor. A full power sine wave of 20 KHz will dissipate 90mW into this resistor. So I selected a 500mW package to stay cold...
The damping resistor in parallel with the coil is also 0.5W 1206 package. Its dissipation is 30mW...
So far, so good... But now comes the hardest part: The Coil !
I know that here is always recommended to use a simple air wire-wound coil. But this is to big for what I want here...
Is there any chance that I can use a SMD coil made on ferite core ? Like this one: SM7850-1R0MT ?
It saturates at 7A and the maximum current throug it is 3A in my amplifier. It also has high Self Resonating Frequency of 145 MHz (I understand that this is good). But I don't know if there are other aspects that matter here, that such a coil doesn't meet...
For Zobel I thought of using two 47nF capacitors in parallel, in 1206 package, C0G type, 50V.
And a 1206, 0.5W resistor. A full power sine wave of 20 KHz will dissipate 90mW into this resistor. So I selected a 500mW package to stay cold...
The damping resistor in parallel with the coil is also 0.5W 1206 package. Its dissipation is 30mW...
So far, so good... But now comes the hardest part: The Coil !
I know that here is always recommended to use a simple air wire-wound coil. But this is to big for what I want here...
Is there any chance that I can use a SMD coil made on ferite core ? Like this one: SM7850-1R0MT ?
It saturates at 7A and the maximum current throug it is 3A in my amplifier. It also has high Self Resonating Frequency of 145 MHz (I understand that this is good). But I don't know if there are other aspects that matter here, that such a coil doesn't meet...
