LM1875 as transconductance amp for Fostex - Page 3

Antoinel · 6th February 2012, 01:17 AM

The output transistors are operating in the common emitter instead of the customary common collector configuration. The center junction of the 30 Ohm resistors is grounded so as to make their operation common emitter. The collectors are opposed. Since the dedicated +/-15 V supply is a dead short for AC, then the center point of the +/-15 V supply is the output node. Put 2 batteries instead of the +/- 15 V power supply and trace the closed circuit for each of the output transistors. The ground node is the center point of the +/-6 V supply. It is the reference for the input signal and the output signal emanating from the emitters of the output stage

KSTR · 6th February 2012, 09:29 AM

Nice circuit, and yes, that's a "rail shifter" type of amp. The outputs are outside the feedback loop, though, but since it's class-A and Darlington there is no big penalty in distortion from base currents.

Antoinel · 6th February 2012, 11:48 AM

Thank you KSTR. By convention, the base currents; though trivial do flow in the resistors (30 Ohm). Thus they are subject to an (irrelevant) correction by loop feedback. Class A is circumstantial in this design case. The parts at hand dictated this outcome. Still, this transconductance amp was able to step out into Class AB and drive a small loudspeaker (4 Ohm)!

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
ZV9 as transconductance amp	we416b	Pass Labs	15	4th April 2006 09:52 AM
Transconductance.	lndm	Tubes / Valves	16	18th March 2006 09:51 PM
What is Transconductance?	Svetlana	Pass Labs	37	20th November 2003 04:14 PM
transconductance vs. current	RobPhill33	Pass Labs	20	10th February 2003 05:30 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

6th February 2012, 01:17 AM	#21
Antoinel diyAudio Member Join Date: Jan 2012 Location: Pennsylvania	The output transistors are operating in the common emitter instead of the customary common collector configuration. The center junction of the 30 Ohm resistors is grounded so as to make their operation common emitter. The collectors are opposed. Since the dedicated +/-15 V supply is a dead short for AC, then the center point of the +/-15 V supply is the output node. Put 2 batteries instead of the +/- 15 V power supply and trace the closed circuit for each of the output transistors. The ground node is the center point of the +/-6 V supply. It is the reference for the input signal and the output signal emanating from the emitters of the output stage

6th February 2012, 09:29 AM	#22
KSTR diyAudio Member Join Date: Jul 2007 Location: Central Berlin, Germany	Nice circuit, and yes, that's a "rail shifter" type of amp. The outputs are outside the feedback loop, though, but since it's class-A and Darlington there is no big penalty in distortion from base currents.

6th February 2012, 11:48 AM	#23
Antoinel diyAudio Member Join Date: Jan 2012 Location: Pennsylvania	Thank you KSTR. By convention, the base currents; though trivial do flow in the resistors (30 Ohm). Thus they are subject to an (irrelevant) correction by loop feedback. Class A is circumstantial in this design case. The parts at hand dictated this outcome. Still, this transconductance amp was able to step out into Class AB and drive a small loudspeaker (4 Ohm)!

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)