# Darlington Audio Power Amp

Discussion in 'General Electronics Chat' started by Joster, Dec 2, 2013.

1. ### Joster Thread Starter Member

Jun 12, 2013
95
1
Hi Folks,

I made a short video outlining my troubles getting this circuit going. Any tips would be awesome!!

http://youtu.be/CdUqb_PwcHU

Thanks,

Joe

Troubleshooting Class AB Darlington Audio Power Amp

2. ### crutschow Expert

Mar 14, 2008
16,199
4,329
What is the bias current through the diodes?
What type of diodes are they?
Are you absolutely positive that everything is wired correctly including the transistor pinouts and/or interchanging the NPN and PNP (?) since it really sounds like something is miswired.

3. ### Joster Thread Starter Member

Jun 12, 2013
95
1

I'll have another look at the wiring tonight. I think I'll spread it out on the breadboard so I can make it easier to wire and to show the connections. The diodes are 1N4001. I'll measure the current through the diodes aswell. I would assume it would be the same as the current through R1? i.e. (Vcc-2.8)/R1? I suppose aslong as I measure a 2.8 drop across all four diodes I can assume that the current through them is efficient.

Am I right about setting Vq to 1/2Vcc. If so, how is this done?

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,167
1,188
Diode current is equal to
Id = (Vcc - 4VD)/(R1 + R2) if we ignore Darlington base currents.
And Vq = 1/2Vcc only if R1 = R2

5. ### Joster Thread Starter Member

Jun 12, 2013
95
1
Awesome! Thanks! Can't wait to take another stab at this circuit tonight!

If everything is working correctly what would the standby current draw be. I assume it would be a lot less than the 5 amps I'm drawing currently. Or would it be equal to the current through R1, D1-4, R2 plus yet base currents?

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,167
1,188
To be honest you should never try to build this theoretical circuit in real life.
This circuit is a disaster waiting to happen. Firs add some emitter resistor.
Re1 and Re2 = 0.1Ω ... 1Ω. Next replace D1 with a potentiometer 1KΩ and set the standby current in range of 0.1A.

absf likes this.

Nov 9, 2007
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8. ### crutschow Expert

Mar 14, 2008
16,199
4,329
Adding a pot is a good idea but you want to retain the diode. You need four diodes to help provide temperature compensation for the four transistor base-emitter junctions. In practice these diodes should be thermally glued to the transistor cases (two on each case) to improve this compensation effect.