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Old 10th March 2009, 07:35 PM   #1
sanju21 is offline sanju21  India
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Unhappy Help needed! strange sound from nmos quasi amp

i recently assembled one channel of nmos500 quasi mosfet (irfp240 x 10) amp.
http://www.adam.com.au/cgpap/QuasiWe...0schematic.pdf


rail voltage +- 80 v,dc at speaker outlet 0.03 volt.

there is one strange problem .at very low volume the sound in speaker is clear,but as i try to increase the volume there is a repeating loud thump sound in the speaker & it's frequency increases with the increase in volume.

when i switch off the amplifier & the supply voltage starts collapsing ,then at some point the thump noise vanishes while i could hear the music clearly.

one more thing , while checking the bias current the probe of multimeter slipped and accidentaly shorted the drain & source of one mosfet at positive rail side.(before i hooked up the speaker).
can somebody tell what might be wrong ? i did not installed R35,C12,C5,C6 component.will installing these compoents solve my problem?

tomorrow i will change the drivers and capacitors on the pcb.Is there anything else which i should look for ? help me guys
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Old 10th March 2009, 07:41 PM   #2
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sanju baba C12 & R35 is must, its called zobel network which helps in stability of amplifier from HF oscillations.
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Old 10th March 2009, 08:15 PM   #3
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That sounds like 'motorboating'

* Positive feedback instead of negative feedback i.e. usually caused by the Output Transformer secondary being wired the wrong way around and often determined by experiment/accident.
* Poor de-coupling capacitors, generally not enough capacity or have "dried out" over a long period thereby reducing capacity.

* Excessive interstage coupling capacity .
* Incipient instability, where the amplifier goes into motorboating after a large transient like a bass drum.

hope that helps..
OS
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Old 11th March 2009, 03:12 PM   #4
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Isnīt there a missing resistor in parallel to L1 ?
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Old 11th March 2009, 03:25 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
try putting back the output Thiele network.
Add 4r7 // to the 4uH inductor.
Change C5 and C6 to 100uF low ESR 100V electrolytic.

Measure the voltage across R2 while this is happening/not happening.
What is the RC of the PSU? RC = speaker ohms * smoothing capacitance (F)
The input RC = 1*10^-6 * 33k = 33*10^-3 seconds, i.e. 33ms.
The NFB RC = 100ms.
Changing C7 to 47uF will be far cheaper than adding extra smoothing capacitance.

Finally consider adding a third part to the output Thiele Network. 5r6+47nF across the speaker terminals.

The three parts give an effective 5r load at very high frequency and have almost no effect (very high impedance) at audio frequencies.
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Old 11th March 2009, 08:00 PM   #6
sanju21 is offline sanju21  India
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when i was testing the circuit with +- 35 volts.i discover that when i raise the volume ,wih every thump noise the Vbe of t9 & t10 collapses below 0.63 volts & also there is no bias current in output mosfet.
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Old 11th March 2009, 08:01 PM   #7
sanju21 is offline sanju21  India
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Join Date: Aug 2005
Location: Gurgaon,Haryana
i am so sorry guys,that was my mistake

i did not biased(set vr2) properly,thats why i wasn't getting any bias current.i didn't knew that for mosfet Vgs is arond 4v.

Only when i saw the datasheets, i realised where i was wrong.i increased the Vgs & guess what, thumping noise vanished.

i am working with mosfet for the first time.Six months earlier i didn't even knew ,what is a bias current!

i want to add zobel network but i'm having difficulty in finding 0.068uf cap.

This is my second amplifier.All thanks to you guys & Rode.
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Old 12th March 2009, 06:25 AM   #8
fjr is offline fjr
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Then use 0.047u or 0.1u as Zobel cap. Won't make any difference.
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Old 24th April 2012, 04:47 AM   #9
masood is offline masood  India
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Quote:
Originally Posted by sanju21 View Post
i recently assembled one channel of nmos500 quasi mosfet (irfp240 x 10) amp.
http://www.adam.com.au/cgpap/QuasiWe...0schematic.pdf


rail voltage +- 80 v,dc at speaker outlet 0.03 volt.

there is one strange problem .at very low volume the sound in speaker is clear,but as i try to increase the volume there is a repeating loud thump sound in the speaker & it's frequency increases with the increase in volume.

when i switch off the amplifier & the supply voltage starts collapsing ,then at some point the thump noise vanishes while i could hear the music clearly.

one more thing , while checking the bias current the probe of multimeter slipped and accidentaly shorted the drain & source of one mosfet at positive rail side.(before i hooked up the speaker).
can somebody tell what might be wrong ? i did not installed R35,C12,C5,C6 component.will installing these compoents solve my problem?

tomorrow i will change the drivers and capacitors on the pcb.Is there anything else which i should look for ? help me guys
dear sanju,
the site above u have mentioned is not opening can you share this amplifier details with me .this type of another circuit discription i have there are seven mosfet irfp240 in each channel http://www.adam.com.au/cgpap/QuasiWe...0schematic.pdf

Last edited by masood; 24th April 2012 at 04:55 AM.
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Old 24th April 2012, 05:54 AM   #10
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The post is 3 years ago. Sites come and go. Perhaps you should try contacting the member or OP.
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Last edited by Ian Finch; 24th April 2012 at 06:09 AM.
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