Having read the Pavel Macura (MOSFET) power follower article and some subsequent variations, there are some common themes that come up when people look at substitute parts. Similarly, the PASS design builders also note that certain alternate MOSFET's are of the wrong type of geometry and don't sound sound right.
I have all kinds of large MOSFET's, but they are mostly switching converter and motor drive types...
I can't help but be curious what the non-linear application parts do to audio...even for less critical applications...
And among the 'wrong' ones, are some worse than others? ...one can see significant variation in transconductance among a variety of part numbers...I've seen some with yfs = 7 S and some with 45 S.
Are they just not agreeable to sine waves ;O)?
I have some APT 47N60BC3's. Horrendously high input capacitance... >7 nF, but 600 V 47A, 0.07 ohm Rds(on), 417W.
Others like 2SK1544 are impressively large...but ill-pedigreed as you find them in SMPS, motor drive, and other PWM applications.
Current source for a more linear device?
Just seems a shame to not make use of such things.
Even a low-fi garage band amplifier?
Thank you
I have all kinds of large MOSFET's, but they are mostly switching converter and motor drive types...
I can't help but be curious what the non-linear application parts do to audio...even for less critical applications...
And among the 'wrong' ones, are some worse than others? ...one can see significant variation in transconductance among a variety of part numbers...I've seen some with yfs = 7 S and some with 45 S.
Are they just not agreeable to sine waves ;O)?
I have some APT 47N60BC3's. Horrendously high input capacitance... >7 nF, but 600 V 47A, 0.07 ohm Rds(on), 417W.
Others like 2SK1544 are impressively large...but ill-pedigreed as you find them in SMPS, motor drive, and other PWM applications.
Current source for a more linear device?
Just seems a shame to not make use of such things.
Even a low-fi garage band amplifier?
Thank you
The input capacitance gets a substantial bootstrap in a source follower configuration - what's normally the capacitance to pay attention to is the Crss (feedback capacitance). Its highly voltage dependent.
I'm using some Toshiba FETs (TK13E25D) in a source follower amp - sounds great. I put up a few pics of the amp in this thread : Mosfet amplifiers including Susan Parker Zeus amplifer - Tir Na HiFi. I chose Toshiba parts because of all the ones I looked at, these had the best transconductance per unit capacitance.
2SK1544 would've been a good choice over a decade ago but looks to be beaten by TK50J30D in this application (even though the input capacitance is about double) due to the reduced Crss and higher Yfs.
I'm using some Toshiba FETs (TK13E25D) in a source follower amp - sounds great. I put up a few pics of the amp in this thread : Mosfet amplifiers including Susan Parker Zeus amplifer - Tir Na HiFi. I chose Toshiba parts because of all the ones I looked at, these had the best transconductance per unit capacitance.
2SK1544 would've been a good choice over a decade ago but looks to be beaten by TK50J30D in this application (even though the input capacitance is about double) due to the reduced Crss and higher Yfs.
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> some with yfs = 7 S and some with 45 S.
At what current?
All active devices go to zero Gm at zero current, higher at normal current, and most keep rising until something smokes.
For a given construction, a device of greater area will be greater Gm. The 45S device could be seven 7.5S devices made in one piece.
As abraxalito says, transconductance per unit capacitance is an important figure at the top of the audio band, it tells how hard you have to work to swing the Gates.
All MOSFETs are prone to a sharp "cut-off" at low Gate voltage. Some sharper than others. Most logic-oriented models do not cover this range well.
At what current?
All active devices go to zero Gm at zero current, higher at normal current, and most keep rising until something smokes.
For a given construction, a device of greater area will be greater Gm. The 45S device could be seven 7.5S devices made in one piece.
As abraxalito says, transconductance per unit capacitance is an important figure at the top of the audio band, it tells how hard you have to work to swing the Gates.
All MOSFETs are prone to a sharp "cut-off" at low Gate voltage. Some sharper than others. Most logic-oriented models do not cover this range well.
In the case of MOSFETs the bigger devices allow higher current and this is what allows them to get to higher gm. Down at the bottom end, the best MOSFETs (Toshibas mostly) achieve a gm about half what a bipolar transistor does, but that doesn't extend down to sub-uA levels due to the 'cut-off' effect as mentioned. The 'headline' figures in the datasheets for gm are normally at unworldly high drain currents.
Thanks, PRR & Abraxalito.
I have a need for a 10A DC heater current regulator to make a weird salvage store purchase completely functional, put it on eBay, and make better use of the small space it frees up, and pocket the change.
Weird lab instrument that used some other heater means that got separated and is thus incomplete.
That sounds like a better use for the largest MOSFET’s.
Back to triodes and pentodes for now, before I’m distracted again.
I have a need for a 10A DC heater current regulator to make a weird salvage store purchase completely functional, put it on eBay, and make better use of the small space it frees up, and pocket the change.
Weird lab instrument that used some other heater means that got separated and is thus incomplete.
That sounds like a better use for the largest MOSFET’s.
Back to triodes and pentodes for now, before I’m distracted again.
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