Are SC3953/SA1538 unsuitable as drivers - diyAudio
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Old 29th November 2008, 04:28 PM   #1
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Default Are SC3953/SA1538 unsuitable as drivers

I'm using them as output transistors in a diamond buffer output stage of a headphone amp. They don't sound worse than the BD137/138 they replaced, maybe better, but they distort whith heavy loads and the BD's did not. The bias current is 20 - 25 mA and supply is only 10V. Their Hfe values are about 150 and the BD's about 100.

I had to change the input transistors emitter resistors to 56R. Should I play with the output transistors emitter resistors instead?
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Old 30th November 2008, 10:21 AM   #2
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Nelson,
this pair is one of those astonishingly nice video devices from Sanyo, optimized for high-voltage low-current voltage amplification. Generally, video transistors lose linearity rapidly with increasing current. In fact, according to the datasheet, these particular types should be linear at 30 mA, nevertheless, are not meant for use in current stages, requiring different properties.
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Should I play with the output transistors emitter resistors instead?
I would try to find dedicated types.
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Old 30th November 2008, 10:51 AM   #3
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Thanks for your comments. I think you're right. After I've listened to them for a while, I find them too smooth. The BD137/138 from ONSemi were better. I hope I can use them in another amp later on.
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Old 30th November 2008, 10:59 AM   #4
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These are great transistors to swap for to-92 devices. They have ultra-low capacitances, high ft and reasonable beta, but I wouldn't drive them close to maximum current.
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Old 30th November 2008, 12:22 PM   #5
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http://www.classiccmp.org/rtellason/...ta/2sa1380.pdf
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Old 30th November 2008, 12:46 PM   #6
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Jacco,
that`s another beautiful type for VAS. For CAS something like 2SC3421/2SA1358, even the good old 2SD669/2SB649 are better than the BD fellows.
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Old 30th November 2008, 02:09 PM   #7
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Just an example that video transistors are little good for delivering current.
Toshiba 2SA1184/2SC2824 are nice and pretty cheap in the EC.
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Old 1st December 2008, 02:38 AM   #8
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Jacco,
what is the difference between 2SC3421/2SA1358 and 2SC2824/2SA1184? The price maybe?
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Old 1st December 2008, 11:11 AM   #9
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Same same, the lower numbers handle a bit more power.
Better derated SOA at higher rail/bias levels, not interesting for 10Vdc rails and biased at 25mA.
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Old 1st December 2008, 07:01 PM   #10
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I've lowered the bias currents to 5 mA. Some of the softness and glaze is gone. It's drier and less colored. Even though this is a failure, it's interesting to learn things.
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