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Old 12th November 2012, 09:24 PM   #11
gagnou is offline gagnou  Greece
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Quote:
Originally Posted by djk View Post
An interesting current limiter, sensing the driver current.
It seems to me like a classic current limiter, sensing the voltage developed across the 0.22 Ohm output transistor emitter resistor.
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Old 13th November 2012, 12:05 AM   #12
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When the current in the output stage goes beyond a certain point the Beta will begin to droop and the driver current will go up.
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Old 13th November 2012, 08:14 AM   #13
gagnou is offline gagnou  Greece
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Still don't get it.
The 100 Ohm resistor is part of the darlington, used to quickly turn off the output transistor.
In order to sense the driver's current there should be another resistor from its emitter to the base of the MPSA06.
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Old 13th November 2012, 09:57 AM   #14
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Quote:
Originally Posted by djk View Post
An interesting current limiter, sensing the driver current.
No, its not in any way sensing the driver current.
Quote:
Originally Posted by djk View Post
When the current in the output stage goes beyond a certain point the Beta will begin to droop and the driver current will go up.
Driver current will surely go up, but the sensing is still from the output emitters. The 100ohm B-E resistors are just like in normal crown amps.
Quote:
Originally Posted by gagnou View Post
Still don't get it.
The 100 Ohm resistor is part of the darlington, used to quickly turn off the output transistor.
In order to sense the driver's current there should be another resistor from its emitter to the base of the MPSA06.
Yes, agreed.
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Old 13th November 2012, 09:58 AM   #15
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You're right, I stand corrected.

A second look sees just a voltage divider across the emitter resistor, about a 7A limit?
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Old 13th November 2012, 06:55 PM   #16
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Ian,

I managed to find some BD911 locally, but they are branded "CDIL" (Continental Device India Limited)

Do you think it is good to go or should I prefer the Fairchild BD743 sold on ebay ?

I also see on their site that they have CN653 and CN753 as equivalents to ZTX653/ZTX753,
will these do any good ?

Just have some bad experience ordering parts from Hong Kong (they not always arrive :/ )

Last edited by gilwe; 13th November 2012 at 07:15 PM.
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Old 14th November 2012, 06:09 AM   #17
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No doubt CDIL and a lot of other manufacturers make good BD911 clones. I'm sure they will also work in the application but I can't say that it will be as good as the original.

I can't locate full data sheets on CDIL's product but the specs look to be generic - i.e. not necessarily applying to that device but probably to a group of parts developed to match or better just the leading figures of the original manufacturer's spec. (sound confusing? Yes, generic product is confusing.)

I don't feel confident there without seeing the performance graphs so, if you don't mind buying from Houston, Mouser at least have a qualified product in ST's version. Mospec product looks good too, if you come across it and I forgot to mention that the lower voltage ranks BD907, BD909 are OK too but are obsolete since BD911 is one size that fits all.

I haven't had a supply problem with Ebay HK traders yet but a lot of guys who buy in small quantities and deal with unfamiliar traders seem to. Personally, I wouldn't run risks with any semi purchases since I occasionally sell and repair stuff and can't afford risks. As I suggest, stay with current manufacture, qualified parts and traders where possible.
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Old 14th November 2012, 06:46 AM   #18
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Default substitutes

I forgot to say ZTX 384C is not critical, it's likely whatever was in the parts bin since any small NPN
would likely work here as the bias controller, VBE multiplier or whatever.

Don't substitute the other Zetex ZTXxxx parts. These tend to be pushed to the limit in most NAIM amplifiers.
If you want substitutes, as drivers for the output stage, consider proper driver transistors in a larger, TO126 case.
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Last edited by Ian Finch; 14th November 2012 at 06:48 AM.
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Old 14th November 2012, 07:11 AM   #19
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CDIL parts are worst, don't use them at all. I have used them in past and have got many failures.
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Old 14th November 2012, 07:31 AM   #20
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I'm glad it was you who said it, Workhorse. I didn't want to start a fight.
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