Will it work? Grounded Source + Darlington

gruni · 13th January 2011, 02:05 PM

Hello!
I'd like to build an amplifier out of junk parts i have lying around. This is the design i ended up with. Will it work? I'm a bit worried about oscillation caused by the Darlington power transistors.

Hope you can help!

Workhorse · 13th January 2011, 02:08 PM

Copying QSC is easy , but you forget to place collector to rail resistor at both driver transistor collectors.

Watch out for VCE excursion of output devices.

gruni · 13th January 2011, 02:20 PM

oh, you're right. will 33 ohms be adequate?

Workhorse · 13th January 2011, 02:24 PM

47ohms

gruni · 13th January 2011, 02:55 PM

okay, great.
i want to run the amp at about 45V DC, which should be ok for the transistors.

is there any benefit of driving the opamps by a transformer winding instead of the zener diode voltage regulation? the transformer i want to use has two unused 9-0-9 windings i could use.

Tekko · 13th January 2011, 02:57 PM

I cant see how this circuit could possibly work with the output grounded and the centertap of the filter caps made the output.

gruni · 13th January 2011, 03:17 PM

Quote:

Originally Posted by Tekko

I cant see how this circuit could possibly work with the output grounded and the centertap of the filter caps made the output.

this is answered quite easily:
instead of fixed supply voltages in respect to ground and a modulated output signal, with this amp the supply voltages are floating and therefore the virtual ground (center tap between filter caps) is modulated with the supply voltage.

Tekko · 13th January 2011, 03:28 PM

Weird design. So if the output devices go south this circuit doesent blow the speaker since it essentially is a capacitor coupled output.

Thats about as far as i can see the advantage, after that i only see the disadvantages of capacitor coupled output, reduced low end response, lower audio quality due to lytics in the signal path and that stuff.

gruni · 13th January 2011, 03:31 PM

right, speakers are protected from dc voltage that way. as the design is a pa amp, influence in audio quality by the electrolytics isnt important. reduced low end response by the caps can be compensated by negative feedback attached to the center tap.

Workhorse · 13th January 2011, 03:43 PM

Quote:

Originally Posted by gruni

okay, great.
i want to run the amp at about 45V DC, which should be ok for the transistors.

is there any benefit of driving the opamps by a transformer winding instead of the zener diode voltage regulation? the transformer i want to use has two unused 9-0-9 windings i could use.

Quote:

Originally Posted by Tekko

Weird design. So if the output devices go south this circuit doesent blow the speaker since it essentially is a capacitor coupled output.

Thats about as far as i can see the advantage, after that i only see the disadvantages of capacitor coupled output, reduced low end response, lower audio quality due to lytics in the signal path and that stuff.

If +/- 40DC then you can use TIP142/147 Darlingtons.
Better use that 9-0-9 for best performance than zener regulation.

@ Tekko, You can get DC coupling by using a center tapped trafo, in that way no more LF thru elcos.

13th January 2011, 02:08 PM	#2
Workhorse diyAudio Member Join Date: Dec 2004	Its Grounded Collector not Source Copying QSC is easy , but you forget to place collector to rail resistor at both driver transistor collectors. Watch out for VCE excursion of output devices. __________________ It's a fruitless endeavor to try and educate a fool that rejoices in ignorance

13th January 2011, 02:24 PM	#4
Workhorse diyAudio Member Join Date: Dec 2004	47ohms __________________ It's a fruitless endeavor to try and educate a fool that rejoices in ignorance

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13th January 2011, 02:20 PM	#3
gruni diyAudio Member Join Date: Feb 2008	oh, you're right. will 33 ohms be adequate?

13th January 2011, 02:55 PM	#5
gruni diyAudio Member Join Date: Feb 2008	okay, great. i want to run the amp at about 45V DC, which should be ok for the transistors. is there any benefit of driving the opamps by a transformer winding instead of the zener diode voltage regulation? the transformer i want to use has two unused 9-0-9 windings i could use.

13th January 2011, 02:57 PM	#6
Tekko diyAudio Member Join Date: Jan 2005	I cant see how this circuit could possibly work with the output grounded and the centertap of the filter caps made the output.

13th January 2011, 03:28 PM	#8
Tekko diyAudio Member Join Date: Jan 2005	Weird design. So if the output devices go south this circuit doesent blow the speaker since it essentially is a capacitor coupled output. Thats about as far as i can see the advantage, after that i only see the disadvantages of capacitor coupled output, reduced low end response, lower audio quality due to lytics in the signal path and that stuff.

13th January 2011, 03:31 PM	#9
gruni diyAudio Member Join Date: Feb 2008	right, speakers are protected from dc voltage that way. as the design is a pa amp, influence in audio quality by the electrolytics isnt important. reduced low end response by the caps can be compensated by negative feedback attached to the center tap.

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