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Old 9th February 2003, 12:29 PM   #1
316a is offline 316a  England
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Question Push Pull Output Stage Question

<big>Hello Folks ,</big><br>
I've been busy recently , rather heavy power supply built (it bends my workbench!) , a dual mono thingy with damper diodes , half a ton of oil caps and chokes . Now moving on to the output stage . It consists of triode strapped S11E12 in push pull , aiming for 65mA / -65V / 340V operating point 6.6k p-p . The bias circuit consists of a 470 ohm resistor in series with a variable cascode current sink running from a negative supply , with bias monitor meter . Sink can be varied from 100-150mA , rock solid and nothing on the sink board gets warm , a good start ! Output stage balance is via a pot , balance checking via a centre reading meter between the S11E12 cathodes . My question before I bolt the thing in and commit is : <i>I have used a 470 ohm resistor in series with the sink , mainly to keep dissipation of the sink down (so I can use smaller , higher Hfe transistors) , what are the disadvantages of this ?</i><br><br>

316a<br><br><center><img src='http://service.globalarray.net/user/bigdaddy/outputstage.jpg'>
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Old 9th February 2003, 01:41 PM   #2
SY is offline SY  United States
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A series resistor will limit the CS compliance a bit; Ohm's law will tell you that.
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Old 9th February 2003, 01:54 PM   #3
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In my opinion, BC549 can't work properly at 100...150 mA collector current, and MJE340 can't work properly at low collector voltage (10 V min)

You have to chose lower resistor for the common cathode (220 Ohms) and use something like 2N2222 instead of BC549.

MJE340 need a small heatsink for correct reliability.

Regards, Pierre Lacombe.
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Old 9th February 2003, 05:24 PM   #4
316a is offline 316a  England
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Default BC549

...you're right about the BC549 , I did not realise it was only rated at 100mA , still seems to work ok . I'll shove a BC184 or an MPSA42 in there instead . The MJE340 is already heatsinked , you may be right about the series resistor though , I'll shove a slightly lower value in

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Old 10th February 2003, 01:04 AM   #5
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Default Power long-tailed pair. Way to go!

I like this approach with the single current source. It makes the whole output stage into a long tailed pair. Should be very well balanced. For what it's worth, have a look at the constant current sink cct I made up a while back. I measured a slope resistance somewhat in excess of 400 megohms. http://www.diyaudio.com/forums/showt...7225#post67225
Best-ever T/S parameter spreadsheet.
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