OPamp + Darlington Will it work? - Page 2

padamiecki · 20th January 2011, 10:31 AM

Quote:

Originally Posted by Yarpen

This is the schematic with your comments and PCB design

wow!
nice star gnd

and power tracks

Yarpen · 20th January 2011, 01:18 PM

I added a voltage regulators ( LM series 18V ) with the filtering and decoupling capacitors.

Yarpen · 20th January 2011, 02:00 PM

Schematic with voltage regulators

Tekko · 20th January 2011, 02:20 PM

I'd move the bias pot to between B and E of the bias transistor.

As you have it now, the amp will go into self destruct if the wiper looses contact.

Yarpen · 20th January 2011, 02:55 PM

Done

Bigun · 20th January 2011, 03:29 PM

Again, without any gnf the PSRR will be relatively low. The regulators will help with the OpAmp if done well but the bias of the output devices feeds directly off the rails - perhaps these bias points should be moved so that they connect to the power rails after the RC filters.

You'll find it handy to have a probe point or connector for measuring the voltage between the emitters of the output devices - to check on the bias current.

cbdb · 20th January 2011, 04:07 PM

Quote:

Originally Posted by padamiecki

this would kill the benefits of the sound quality
do not apply gnfb

So you think more crossover distortion is good for sound quality? Using an opamp means the circuit already uses a lot of feedback (and its global except for the output), including the output in the loop will make it sound worse how? Or are you just regurgitating the no GNFB mantra?

cbdb · 20th January 2011, 04:16 PM

Quote:

Again, without any gnf the PSRR will be relatively low.

As well as a higher output impedance. The catch might be stability.

Yarpen · 20th January 2011, 07:09 PM

Quote:

Originally Posted by Bigun

Again, without any gnf the PSRR will be relatively low. The regulators will help with the OpAmp if done well but the bias of the output devices feeds directly off the rails - perhaps these bias points should be moved so that they connect to the power rails after the RC filters.

You'll find it handy to have a probe point or connector for measuring the voltage between the emitters of the output devices - to check on the bias current.

Does replacing resistors R3 R10( from last schematic ) by current sources solve the problem?

Bigun · 20th January 2011, 08:31 PM

It would certainly improve the PSRR but why not simply include the bias inside the RC filtered part of the power supply ?

You could also bootstrap these resistors from the output, this would also improved PSRR and provide a more constant current through the bias string. You just replace R3 with two resistors and tie a capacitor from their mid point to the output.

20th January 2011, 03:29 PM	#16
Bigun diyAudio Member Join Date: Jan 2009 Blog Entries: 2	Again, without any gnf the PSRR will be relatively low. The regulators will help with the OpAmp if done well but the bias of the output devices feeds directly off the rails - perhaps these bias points should be moved so that they connect to the power rails after the RC filters. You'll find it handy to have a probe point or connector for measuring the voltage between the emitters of the output devices - to check on the bias current. __________________ "The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

20th January 2011, 08:31 PM	#20
Bigun diyAudio Member Join Date: Jan 2009 Blog Entries: 2	It would certainly improve the PSRR but why not simply include the bias inside the RC filtered part of the power supply ? You could also bootstrap these resistors from the output, this would also improved PSRR and provide a more constant current through the bias string. You just replace R3 with two resistors and tie a capacitor from their mid point to the output. __________________ "The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

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20th January 2011, 02:20 PM	#14
Tekko diyAudio Member Join Date: Jan 2005	I'd move the bias pot to between B and E of the bias transistor. As you have it now, the amp will go into self destruct if the wiper looses contact.