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-   -   Difference in performance between these 2 circuits? (http://www.diyaudio.com/forums/solid-state/81648-difference-performance-between-these-2-circuits.html)

fluckscapacitor 17th June 2006 10:15 AM

Difference in performance between these 2 circuits?
 
2 Attachment(s)
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!:smash:

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.


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