Aleph With IRFZ44

[Jay]

Dear experts,

I’m very interested to build an Aleph using IRFZ44. This MOSFET is in a smaller case than the IRFP250. The characteristics that got me in the first place was the high transconductance (and low input resistance):

Transconductance (IRFP250 vs IRFZ44): 19 vs 32.6
Input resistance (IRFP250 vs IRFZ44): 0.085 vs 0.024
Input capacitance (IRFP250 vs IRFZ44): 2n vs 1n8
Voltage rating (IRFP250 vs IRFZ44): 200V vs 60V
Power dissipation (IRFP250 vs IRFZ44): 180W vs 126W
Current (IRFP250 vs IRFZ44): 33A vs 50A
Delay On (IRFP250 vs IRFZ44): 18-30ns vs 20-40ns

As you can see, the IRFZ44 has more preferences for a good sounding single ended amplifier than the IRFP250.

One thing about the IRFZ44 is that it is specified to withstand hi temperature (175 degree C?). So, even with the Aleph3 original current source circuit, I cannot see why this IRFZ44 will be damaged. (Aleph3 MOSFET won’t dissipate more than 120W right?)

Before I go to deeply into this project, I need your ideas, comments, or advices to save me from wasting my time doing something stupid. I don’t have electronics background, but I have a strong feeling that this will outperform the Aleph3 with bigger MOSFET.

[Brian Donaldson]

One small concern. The max voltage is 60V.

If you run 35 volt rails, and run the amp to clipping, the fet will be seeing 60 volts. I don't thing I would want to push the limits on the output devises.

I'm not the resident expert, but this is my 2 cents.

Good luck

PS, I like to see you think outside the box. That's the only way to learn. I also explored other fets for my up and coming Aleph 2+ but returned to IRFP240

[Jay]

I don’t understand how the FET may see variable voltage across it’s Drain and Source. When you said “Running the amp to clipping” I got it as cranking the volume up, meaning that we feed the Gate with greater current. Well, let me think. The current source circuit is almost constant, right? So I will picture the other FET. The Source voltage is almost constant (determined by fixed 3W resistor and the current through it). So if I set the output voltage to half the rail (say, 20V) when no signal flowing into the Gate, do you really think the signal may be able to swing the voltage to a large deviation?

3 hours after my post, I completed a mono of this using 30V rail. I think I could hear more music compared to the IRFP, especially in low SPL (it was 2:00 in the morning). In the morning I tested for a high volume, I think the IRFP was better, but may be after matching and paralleling the IRFZ, the situation will be different.

Only a few minutes after that, I replaced the transformer with a 40V rail. The distortion is very very high (I used only my ears)! As if the current is insufficient. With the last setting, I think I will prefer the IRFP. But if you think the Vds were the cause, I will disagree.

Oh no, thank’s for the 2 cents!! If the pins are short due to excessive Vds, the speaker will be destroyed, aren’t they?! Wait, may be I’m wrong… (But I will still keep the 2 cents )

[Brian Donaldson]

I may be wrong, but I thought the max voltage was refering to voltage from source to drain (or vise versa) If the active drivers are pulling the output to negative rail (or 5 volts above in reality) then the current source will see rail to rail voltage - 5 volts across active driver - voltage across source and drain resistors.

Likewise, If the voltage swings to the other side, the active drivers will see same voltages.

Please smack me one if I'm wrong. I'm trying to learn, but I'm hard headed.

[sianturi]

Hello Jay,

Brian was right, the max voltage is only 60V, which is a little low for 35V supply. You're pushing the FET to its limit, I think....

This is all related to transistor Safe Operating Area (SOA). As a rule of thumb, select a transistor with voltage rating at least > 2 x voltage supply. This is to anticipate the parasitic inductance in your circuit. In the case of your circuit/amp... yes, it worked, but you are probably having a reliability problem in your circuit.

Well, at least that's what I read from Randy Slone's Amp book. Hope this helps.

Salam audio dari Indonesia, hehehe....

[Jay]

Thanks! For the moment I’m convinced that I had pushed the MOSFET out of it’s SOA, such that I could clearly hear distortion with higher rail voltage (40V), also that with 30V rail voltage, the performance was decreased with higher SPL (It seemed like the power is drop, but not like clipping).

But because I could hear better performance at low SPL, and because 12V-0-12V transformer is the suitable high power transformer I already have, I will think of this other possibility: using a 15V rail voltage! The Source resistors will be 0.1 Ohm, and I will workout the resistances in the current source. Hmmm, may be I will have a problem in the front end. Anymore help?

[Tazzy]

IRFZ44... I've got quite a bunch of those lying arround doing nothing

[AudioFreak]

Jay,

You could probably build either an Mini-A or an Aleph-X with suitable voltage ratings for those output devices.