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Old 27th June 2002, 12:29 AM   #21
Duo is offline Duo  Canada
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Okay, I see what you're saying... So then it would be more feasible to make a class ab using my other transistors, I do however need info on how to work with long tailed pairs and global feedback, since they seem so important... Even if someone could help with my diagrams and numbers...
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Old 27th June 2002, 12:58 AM   #22
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If you use your 3055's, you'll need to go single ended, and I don't know much about that topology (but I'm sure someone out there does), but I do know that in class AB mode, complementary stages are better (actually, there may be some controversy there), so it would be more feasible to either use your Japanese transistors or get MJ2955s to go with your 2N3055s.

Just like in an integrated opamp, you need global negative feedback coming back into your long tailed pair (diff. amp), or the circuit becomes a comparator. It would take all day to go into all the details, Geoff and I told you some of the the basics in your other thread. With that info, you can design simple amps. I think you can learn alot by studying that schematic and others, as long as you know what everything does, you can use ohm's law to see what's going on in the circuit and what values are practical. Slone's book does a much better job of explaining all this, it tells you about compensation, current mirrors, and all that fun stuff, but it's got over 400 7" by 9" pages. It's worth the read though.
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Old 27th June 2002, 01:10 AM   #23
Geoff is offline Geoff  United Kingdom
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Quote:

"If you use your 3055's, you'll need to go single ended, ..."

Why? What is wrong with a quasi-complementary output stage operating in push-pull?
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Old 27th June 2002, 01:14 AM   #24
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If you want to go Class AB, you must go push/pull you have no choice.

If you want to use the 3055's in the output, you'd have to use a quasi complementary output stage.

EDIT:

Geoff, I see you beat me too it
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Old 27th June 2002, 01:56 AM   #25
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You would use either single ended (class A) or quasi-comp stage. Complementary beats them all for what you want to do. That's what I mean.
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Old 27th June 2002, 02:10 AM   #26
Duo is offline Duo  Canada
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Okay, well, I've decided to do complimentary, since I myself have a better understanding of the topic and I don't know much about quasi comp circuitry. I know I could get a good lot of power out of the japanese transistors so I'll use them... I see what you mean bi big comparator, I've had that problem before but couldn't figure it out. Only thing is, I need to know how to implement a longtailed pair, I do understand that it's a diff amplifier and what it's doing on a basic level. But I have troubles with the math of that part. Also, I'm not sure how to work with bias for direct coupling of a push pull pair... I was thinking the usual two diodes forward biased between the bases of the O/T pair, but I guess that's not a very good idea is it?
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Old 27th June 2002, 06:13 AM   #27
Duo is offline Duo  Canada
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Does anyone have ideas on how to calculate values for long tailed pairs?
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Old 27th June 2002, 06:43 AM   #28
e96mlo is offline e96mlo  Sweden
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You could use quasi complementary and bias it high enough so that it runs in class A for low level listening. When the need for power kicks in then it will automatically change to class AB.

You should use a current generator i the diff amp. Once that is done you know the current through the pair and you have only to use Ohm's law.

/Marcus
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Old 27th June 2002, 07:38 AM   #29
Duo is offline Duo  Canada
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Thanks for the info, I think automatic level switching quasi and constant current sink for the long tailed pair sounds great, that's what I'll try... Though, I think I might just use my japanese matched complimentary pairs for the output, but I think it would still be able to switch to a lower bias when run hard, don't you?
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Old 27th June 2002, 07:44 AM   #30
e96mlo is offline e96mlo  Sweden
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Well, you don't switch bias. If you set the bias current in the output pair to, say, 1 A then it will always be 1 A. So if at low listening levels the current drawn by the speaker is below 1 A then you have a class A amp. But when you turn up the volume the current drawn will be greater than 1 A and you are now in class AB.

All class AB amps work like this. The only difference is that they usually have only some mA in bias current to keep the efficiency as high as possible.

Hope this helps.

/Marcus
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