Difference in performance between these 2 circuits?

fluckscapacitor · 17th June 2006, 10:15 AM

Can anyone offer an explanation as to what the difference in performance, if any, would be between these 2 circuits?

Thanks =)

bear · 17th June 2006, 04:11 PM

I guess the performance difference might arise if the two transistors being biased are not set up identically. Which, they are probably not.

The circuit at A permits different bias points.

Run it in a PSPICE and see what you see??

_-_-bear

PS. I don't like LEDs for biasing things...

AndrewT · 17th June 2006, 04:40 PM

Hi,
circuit B is a cheapskate version that has poor control of the constant current function on the right hand half.

D.Self did a similar thing and then wrote a chapter about his failure to equalise the slew rate in the positive and negative directions. He never published the result of doing the CCSs properly and may have avoided the need to alter his circuit as he ended up doing, although it did give him a chance to show dependance on a cap from base of control transistor to Vas collector, he called the effect feedthrough.
Many others copy circuit B to save a resistor. Can't understand why when they cost little board space and parts of a cent.

Circuit A gives equal priority to both CCSs and since the VAS usually has more Ic it deserves good control.

BTW. most designers recommend degenerating the emitters. You also need a collector load to drive the VAS.

mikeks · 17th June 2006, 05:58 PM

Quote:

Originally posted by AndrewT
....Can't understand why when they cost little board space and parts of a cent.

Yeah!

forr · 17th June 2006, 06:46 PM

In case of failure of the diode, there is no current flowing in circuit B and then no defined DC at the output. In case of failure of either diode in circuit A, there is a DC output almost equal to the voltage of one of the rails.

djk · 18th June 2006, 07:27 AM

Circuit B is to be preferred.

The single resistor and LED produce the reference voltage, the independent output currents are trimmed by their respective emitter resistors.

The base resistor for the right hand current source prevents the reference voltage from collapsing on a negative clip (recovers from clipping cleaner).

MikeB · 18th June 2006, 10:32 AM

Quote:

Originally posted by djk
The base resistor for the right hand current source prevents the reference voltage from collapsing on a negative clip (recovers from clipping cleaner).

Without this base resistor it's even possible that the amp does not recover at all from negative clipping, means it keeps stuck to v-.
I paid with a speaker for this experience...

Mike

fluckscapacitor · 18th June 2006, 12:19 PM

Quote:

Originally posted by bear
PS. I don't like LEDs for biasing things...

Can you explain why?

bear · 18th June 2006, 05:20 PM

They're all over the map in terms of parameters.

They modulate with light... admittedly not a big issue if you are a) in the dark or b) always in a closed chassis.

Spectral ran into this problem big time when they used them in their DMC-10 preamp, which was very pretty inside. They supplied it to their showrooms with a plexi cover to show off the guts. It took a while to figure out where the 60cycle hum was coming from... since none of the units back at the factory seemed to have that problem!

Ymmv.

_-_-bear

djk · 18th June 2006, 11:06 PM

"They modulate with light... "

And so do most glass diodes, at work we paint 'em black.

17th June 2006, 04:11 PM	#2
bear diyAudio Member Join Date: Apr 2002 Location: New York State USA	I guess the performance difference might arise if the two transistors being biased are not set up identically. Which, they are probably not. The circuit at A permits different bias points. Run it in a PSPICE and see what you see?? _-_-bear PS. I don't like LEDs for biasing things... __________________ _-_-bear http://www.bearlabs.com ...ur feeback please - like/dislike my what I have written? PM/email tnx. --

17th June 2006, 04:40 PM	#3
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, circuit B is a cheapskate version that has poor control of the constant current function on the right hand half. D.Self did a similar thing and then wrote a chapter about his failure to equalise the slew rate in the positive and negative directions. He never published the result of doing the CCSs properly and may have avoided the need to alter his circuit as he ended up doing, although it did give him a chance to show dependance on a cap from base of control transistor to Vas collector, he called the effect feedthrough. Many others copy circuit B to save a resistor. Can't understand why when they cost little board space and parts of a cent. Circuit A gives equal priority to both CCSs and since the VAS usually has more Ic it deserves good control. BTW. most designers recommend degenerating the emitters. You also need a collector load to drive the VAS. __________________ regards Andrew T.

18th June 2006, 05:20 PM	#9
bear diyAudio Member Join Date: Apr 2002 Location: New York State USA	They're all over the map in terms of parameters. They modulate with light... admittedly not a big issue if you are a) in the dark or b) always in a closed chassis. Spectral ran into this problem big time when they used them in their DMC-10 preamp, which was very pretty inside. They supplied it to their showrooms with a plexi cover to show off the guts. It took a while to figure out where the 60cycle hum was coming from... since none of the units back at the factory seemed to have that problem! Ymmv. _-_-bear __________________ _-_-bear http://www.bearlabs.com ...ur feeback please - like/dislike my what I have written? PM/email tnx. --

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17th June 2006, 06:46 PM	#5
forr diyAudio Member Join Date: Dec 2004 Location: Next door	In case of failure of the diode, there is no current flowing in circuit B and then no defined DC at the output. In case of failure of either diode in circuit A, there is a DC output almost equal to the voltage of one of the rails.

18th June 2006, 07:27 AM	#6
djk diyAudio Member Join Date: Feb 2001 Location: USA	Circuit B is to be preferred. The single resistor and LED produce the reference voltage, the independent output currents are trimmed by their respective emitter resistors. The base resistor for the right hand current source prevents the reference voltage from collapsing on a negative clip (recovers from clipping cleaner).

18th June 2006, 11:06 PM	#10
djk diyAudio Member Join Date: Feb 2001 Location: USA	"They modulate with light... " And so do most glass diodes, at work we paint 'em black.

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