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Transistor Matching

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peterb View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote peterb Quote  Post ReplyReply Direct Link To This Post Posted: 05 Jun 2017 at 1:39pm
Ok, admission of guilt! Don't know if it is senility or the red wine but I my facts were wrong in my first post. 
It is not the quiescent current that was different left and right but the output dc voltages. Big difference!
The q current is indeed set by a trimmer and is to spec, the dc on the outputs are however 60 and 110 mV.
The amp was built and working 18 months ago and I put on my memorised to do list finding the reason for this difference. I thought it could be due to different Vbe of the LTP transistors, and that matching a pair would be the answer.
How 18 months changed that to current in my brain, I don't know why. Apologises everybody.
Peter
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Dual 505-1, Cyrus CD T, DIY 80W MosFet amp and PreAmp, 2xKEF 103.2
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jun 2017 at 11:55am
60mV develops 0.45mW into 8 Ohms. 110mV develops 1.5mW into 8 Ohms. I don't think it of any real consequence.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote peterb Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jun 2017 at 7:47pm
Yes true, I guess I am just being anal to want them in balance.
Peter
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Dual 505-1, Cyrus CD T, DIY 80W MosFet amp and PreAmp, 2xKEF 103.2
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jun 2017 at 9:23pm
It is probably the difference between the VA amp hFEs on each channel that is loading one LPT more than the other. I would have expected another trimmer to null the offset?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote peterb Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jun 2017 at 10:09pm
Sadly no other trimmers.

Peter
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Dual 505-1, Cyrus CD T, DIY 80W MosFet amp and PreAmp, 2xKEF 103.2
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 2017 at 8:42am
T6,7 being a darlington configuration won't be loading the LTP much at all.

Correcting what I said earlier, there is no bias transistor because this is a MOSFET output stage which has a negative temp. co. so it won't go into thermal runaway like a BJT output stage. The BJT uses a bias transistor thermally connected to the output devices (sometimes in a rather particular way) such that temp. co.'s cancel, preventing thermal runaway. This design simply uses the resistance of a trimmer but with zenner protection in case it turns the output devices both on.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JardCrocker Quote  Post ReplyReply Direct Link To This Post Posted: 31 Oct 2017 at 4:24pm
Hi...i am a new user here. As per my knowledge test does not need to be very accurate, just repeatable by using the same tester under the same conditions. You will get the same test result repeatedly with the same device.< style="display: none;" id="__symantecPKIClientMessenger" -install-updates-user-configuration="true" -sups-flavor-configuration="true" -extension-version="0.5.0.161">



Edited by JardCrocker - 22 Dec 2017 at 8:46pm
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