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Old 17th February 2007, 03:08 PM   #1
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Question Modern transistors for CFP output for preamp

I want to upgrade the output stage in my line amp. It's a Marshal Leach Wide Bandwidth Preamp I built in the 80s that was published in Audio Magazine. It uses a push pull emitter follower totem pole (2-n-p-n) output stage that operates Class A. I was looking at D. Self's book and web site and was wondering if I could change this to a CFP (complementary feedback pair). It uses MPS8099 and MPS8599 transistors. I matched these transistors. I was also wondering if there were better transistors from Toshiba or Hitachi like the BC546 and BC556 or the 2sc1324. I would like complementary transistors that can be heat sinked back to back with an aluminum plate between to ward off thermal drift. I also feed the need to leave a space for the base resistor to compensate for self oscillation. Any other leads would be most helpful. Thanks a whole heap! Ray Hughes
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Old 17th February 2007, 04:54 PM   #2
AndrewT is online now AndrewT  Scotland
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Hi,
you might add 2sa872/c1775 to the list. Only 50mA & 300mW.
2sa970/c2240 maybe they're complements.
2sc1845, don't know it's complement.
2sa1145/c2705, 50mA
2sa1349/c3381. 200mW
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Old 17th February 2007, 05:04 PM   #3
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Default Thanks! Looking for 60v c-e,>625mw

Thought about the mje171 and mje181 but more modern. Thanks Ray
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Old 17th February 2007, 05:26 PM   #4
AndrewT is online now AndrewT  Scotland
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60Vce allows up to +-30V supply giving a maximum output of 20Vac (+28dbu).
What are you driving?
That is some overhead margin.

Will the transistors ever see much more than half supply voltage?
625mW will allow upto 5mA bias current but they are going to run hot with 30Vce & 5mA.

Is small signal what you need? or small power?
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Old 17th February 2007, 06:34 PM   #5
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Default 2 CFPs one above the other, totem pole

Using + and -24 volts power supply. I think it was called quasi-complementary. Would like to drive long lines to the power amp >25 feet but still operating Class A with no chance of it ever going AB. D. Self on his web site gives an approximation of the topology;http://www.dself.dsl.pipex.com/ampins/discrete/cfp.htm
down at the bottom of the page for a 1K6 load. Thanks, Ray Hughes
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Old 17th February 2007, 06:55 PM   #6
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Default Seems like they need to be TO220 mounts

where I can mount the two transistors of the CFP back to back on one heat sink but I need to know how large a heat sink to use. Thanks Ray
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Old 17th February 2007, 08:18 PM   #7
AndrewT is online now AndrewT  Scotland
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Hi,
the first Q can be bc550, it's only passing 0.2mA Pd=5mW
Don't bother with cooling.
Re the other two, how about Zetex Eline with 1W dissipation and can be fitted with a flag cooler or a glued copper foil to each face.
6mA and 24V gives Pd=140mW. Ipk into the load will be about 2k resistive and a bit higher 3 or 4k with some capacitance and stay in ClassA.
Unfortunately Zetex don't show the hFE below 10mA for the ztx650/750 pair, maybe they are embarassed to show how low gain becomes for these high current devices.
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Old 17th February 2007, 08:37 PM   #8
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Default Thanks, I'll try it!

Thanks! Ray
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Old 18th February 2007, 01:18 PM   #9
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Default Andrew, One more question, please?

In D. Self's Fig 13 on his web page, I would assume that those two diodes biasing the base of the bottom n-p-n transistor are 1n4148??? Or should I use 1n6263? Any hint as to what the capacitor from the emitter of the top p-n-p to the base of the bottom n-p-n would be-lead or lag compensation??? Start with 10 pf and work up?? Thanks again, Ray Hughes
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Old 18th February 2007, 01:27 PM   #10
beppe61 is offline beppe61  Italy
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Default Re: Modern transistors for CFP output for preamp

Quote:
Originally posted by grhughes
I want to upgrade the output stage in my line amp.
It's a Marshal Leach Wide Bandwidth Preamp I built in the 80s that was published in Audio Magazine.
I was looking at D. Self's book and web site and was wondering if I could change this to a CFP (complementary feedback pair). It uses MPS8099 and MPS8599 transistors. I matched these transistors. I was also wondering if there were better transistors from Toshiba or Hitachi like the BC546 and BC556 or the 2sc1324. I would like complementary transistors that can be heat sinked back to back with an aluminum plate between to ward off thermal drift.
I also feed the need to leave a space for the base resistor to compensate for self oscillation.
Any other leads would be most helpful.
Thanks a whole heap!
Ray Hughes
Dear Sir,
please excuse me if I cannot be of any help but I have a question.

It uses a push pull emitter follower totem pole (2-n-p-n) output stage that operates Class A .... I was wondering if I could change this to a CFP (complementary feedback pair)

May I ask you why ?
I am extremely interested, since a long time, about CFP (complementary feedback pair)
Do you have any experience with them ?
How do they sound ?
Do you have any tested schematic that it could be used to build a nice and simple line stage with very small voltage gain ?

Thank you very much and kind regards,

beppe
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