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Old 3rd April 2014, 04:13 PM   #1
Mihkus is offline Mihkus  Estonia
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Default What can cause noise in this design?

I built an amplifier, just for fun...

Used 2 2SC5200 and one 2SC.. something Should be driverstage NPN
The circuit:

Click the image to open in full size.

I know its ultra simple only 9 components... But when i adjust feedback more than 500 ohms it starts humming...
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Old 3rd April 2014, 10:25 PM   #2
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The circuit is not properly designed. It can not achieve a stable quiescent current. It have no feedback divider. Collector load from emitter is against voltage amplification logic. Hum comes probably from supply .

You have to measure few voltage points to see that it can't work.
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Old 3rd April 2014, 10:31 PM   #3
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Look up class AB amplifiers for some good examples.
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Old 4th April 2014, 02:50 AM   #4
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Quote:
Originally Posted by Mihkus View Post
I built an amplifier, just for fun...

Used 2 2SC5200 and one 2SC.. something Should be driverstage NPN
The circuit:

Click the image to open in full size.

I know its ultra simple only 9 components... But when i adjust feedback more than 500 ohms it starts humming...
Although it has some design issues. What happens and what do you get audio wise with feedback less than 500 ohms ?

Cheers / Chris
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Old 4th April 2014, 02:32 PM   #5
Mihkus is offline Mihkus  Estonia
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That "VAS" is 2SC1826 and damn, after few modifications it sounds totally fine.
Even if it has 280R input resistance... lol
Added 100R resistor to 2SC1826's emitter, used fixed resistor on feedback
And changed that 2,2k trimmer to 1k and turned it on almost half way and theres 8 volts across the output capacitor and a 33pF capacitor across feedback resistor without a reason...
That laptop charger i used had that noise, I dont know after adding 2,2R and very low resistance capacitor after it it starts up and the weird humming noise is gone...
Cant set gain too high it picks up everything going thru the air...

Last edited by Mihkus; 4th April 2014 at 02:34 PM.
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