MJL subtitutions

[alkyleda]

Hi guys..... this is going to such a noob question but here goes...
I am building a pair of 500W amps designed by the guys at "Silicon Chip" magazine (August 1997) and they spec 12 X MJL21193/4 output devices +/- 80V supply rails and use the same MJL as the drivers too. I have been looking around and found the MJL1302/MJL3281 are almost identical specs. SOA is the same at 50V but 1302/3281 are slightly less at 80V, collector current 15A vs 16A, Hfe is quite different 4MHz vs 30MHz.
Anyhoo..........the 1302/3281 are $2 a device cheaper ( big saving when you need 28 devices) or am I being a tight *** and should buy the 2113/4??🙄

 

[Ian Finch]

At such high power and total expense, you don't take stability risks with the design, based as it is on those mighty semis. Though it's not his personal design, Nicholas Vinen at Silicon Chip Magazine (try his email) is the best to contact and get advice, since he took up the reins of audio design quite some years back. His predecessor as editor, Leo Simpson who developed the amplifier, has since retired so not sure he can help.

 

[alkyleda]

Thanks Ian........🙂 I was going to try and use 2sa1302/2SC3281 as I have a pile of genuine ones from days gone past, but they are only 150W and might become an exercise in frustration. I will probably ask more mad questions as I find I learn better from peeps than reading stuff I don't understand

 

[JMFahey]

Datasheet power dissipation is not *that* important by itself because its conditions are unrealistic (transistor case held at 25C which is much harder than it looks), but since original spec´d transistors are available, go for them.
$2 per device pales compared to transformer, filter caps, chassis, etc. $$$$$ anyway and bring peace of mind.

 

[alkyleda]

I sent a query thru to silicon chip and got a response.................


Hi Leon,
While the MJL21193/94 transistors from ON Semiconductor are recommended, you could use the ON Semiconductor NJW21193/94 transistors instead.

MJL1302/MJL3281 could be used. As you mention they have a higher cut off frequency, however, this shouldn't be an issue.
According to the circuit you would need seven of each type.

Regards

John Clarke

😀😀

I miss calculated what I was doing and have ordered 25 of each pair so now I have lots of spares to play with.😉

 

[wg_ski]

You are supposed to order then in batches of 25. When you order full rail quantities you get them all from one batch, and matching becomes easier. Match closest groups for paralleling and use outliers for drivers.