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Old 23rd October 2002, 10:10 PM   #1
cm961 is offline cm961  Canada
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Question Carvin DCA 800, only NPN?

Hey guys,

I just bought a non-working Carvin DCA 800 from ebay. Anyways, its a really neat amp. One channel has like no gain, pretty much what goes in comes out, and the other channel blows the fuse. I measured the resistance between the +ve supply and ground on the amp channel and it was only 30 ohms or so, easy to see why the fuse blows.

I'm getting somewhere with these boards, but I have a big question. I'm familiar with class A, class B, or at least I like to think so. Now these amp channels each use 6 MJ15003 transistors per channel... That is, they're all NPN. So does this mean the amplifier doesn't use a complementary pair?!? Isn't that strange? Can someone point me to some schematics of amps using non-complementary pairs so I can inspect them.

Any help would be great guys!

Pete
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Old 23rd October 2002, 11:08 PM   #2
cm961 is offline cm961  Canada
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Hey again,

I just wanted to add a few things. I was looking in Randy Slone's book (High power audio amplifier construction manual) (my bible). And out of list of availible output stages, I'd have to say this must be class A single ended. Is there such thing as class B single ended? I really doubt it since it would only output half the signal! Is there such thing as a push/pull (complementary) stage (of whatever class!) using all the same transistor? I don't think so. Man this is a tough one.

Pete
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Old 23rd October 2002, 11:19 PM   #3
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If you have Slone's book - have a look at his diagrams for the BJT Quasi-Complementary output stage.

Your OPS most likely has a PNP driving an NPN (Complementary feedback pair) and an NPN driving an NPN (Emitter follower, full darlington connection) on the opposite rail.

It is indeed possible to have a push-pull class B output stage with only NPN transistors.

Consider the fact that tubes only come in "N" type devices, and they've had push-pull class B tube output stages since before I was alive.
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Old 23rd October 2002, 11:27 PM   #4
cm961 is offline cm961  Canada
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Ok I hate to be a mega poster but...

I just read about the Quasi-complmentary OPS. This appears to be a class B output stage using NPN transistors only! It requires complementary predriver transistors, and it appears that my amp uses RCA1C13/RCA1C12 for that (although there's also TIP31C and TIP32Cs on the board).

Neat!
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Old 24th October 2002, 12:50 AM   #5
djk is offline djk
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http://www.diyaudio.com/forums/attac...p?postid=71431

If the 100F cap in the feedback loop is open the amplifier will have unity gain.
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Old 24th October 2002, 03:34 AM   #6
skaara is offline skaara  Slovenia
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Its note necessarily class B it can be class AB.
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