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Old 13th January 2011, 02:05 PM   #1
gruni is offline gruni  Germany
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Default Will it work? Grounded Source + Darlington

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!
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Old 13th January 2011, 02:08 PM   #2
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Default 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.
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Old 13th January 2011, 02:20 PM   #3
gruni is offline gruni  Germany
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oh, you're right. will 33 ohms be adequate?
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Old 13th January 2011, 02:24 PM   #4
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47ohms
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Old 13th January 2011, 02:55 PM   #5
gruni is offline gruni  Germany
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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.
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Old 13th January 2011, 02:57 PM   #6
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I cant see how this circuit could possibly work with the output grounded and the centertap of the filter caps made the output.
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Old 13th January 2011, 03:17 PM   #7
gruni is offline gruni  Germany
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Quote:
Originally Posted by Tekko View Post
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.
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Old 13th January 2011, 03:28 PM   #8
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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.
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Old 13th January 2011, 03:31 PM   #9
gruni is offline gruni  Germany
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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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Old 13th January 2011, 03:43 PM   #10
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Quote:
Originally Posted by gruni View Post
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 View Post
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.
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