Push Pull Output Stage Question

316a · 9th February 2003, 01:29 PM

<big>Hello Folks ,</big> 
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 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 ? 

316a <center><img src='http://service.globalarray.net/user/bigdaddy/outputstage.jpg'>

SY · 9th February 2003, 02:41 PM

A series resistor will limit the CS compliance a bit; Ohm's law will tell you that.

P.Lacombe · 9th February 2003, 02:54 PM

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.

316a · 9th February 2003, 06:24 PM

...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

316a

Circlotron · 10th February 2003, 02:04 AM

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

9th February 2003, 01:29 PM	#1
316a diyAudio Member Join Date: Dec 2002 Location: West London	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'>

9th February 2003, 06:24 PM	#4
316a diyAudio Member Join Date: Dec 2002 Location: West London	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 316a

10th February 2003, 02:04 AM	#5
Circlotron diyAudio Member Join Date: Jun 2002 Location: Melbourne, Australia	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. http://www.diyaudio.com/forums/multi...tml#post353269

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9th February 2003, 02:41 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	A series resistor will limit the CS compliance a bit; Ohm's law will tell you that.

9th February 2003, 02:54 PM	#3
P.Lacombe diyAudio Member Join Date: Jul 2001 Location: Orleans, France	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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