i recently assembled one channel of nmos500 quasi mosfet (irfp240 x 10) amp.
http://www.adam.com.au/cgpap/QuasiWeb/pdf's/Nnmos350-500 schematic.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
http://www.adam.com.au/cgpap/QuasiWeb/pdf's/Nnmos350-500 schematic.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
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
* 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
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.
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.
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.
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.
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/QuasiWeb/pdf's/Nnmos350-500 schematic.pdf
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.