Right, the next step is to load a big 8 ohm resistor. I have a 25 watt 8 ohm resistor on a headsink.
I hope you have a function-generator. In a pinch, it is also possible to generate sinus and rectangle with the computer and a programm.
First you measure with sinus, clipping and so on ... (the poor dummy
)
Then very careful test with rectangle, 1kHz and 10kHz for example.
I hope you have a function-generator. In a pinch, it is also possible to generate sinus and rectangle with the computer and a programm.
First you measure with sinus, clipping and so on ... (the poor dummy
)Then very careful test with rectangle, 1kHz and 10kHz for example.
I actualy do have a function generator .
I measured the currents with input shorted and no output load.
I put the measurement on the original schematic so the values can be easy seen and compared to original. I also marked the changes I made to original circuit with red color. I removed the coupling capacitors- there is no diference with or without them. I'll put them back if they are needed.
One thing bothers me on my measurements- the current through R22 is 53.22mA! Is this normal?
The measurements:
.I measured the currents with input shorted and no output load.
I put the measurement on the original schematic so the values can be easy seen and compared to original. I also marked the changes I made to original circuit with red color. I removed the coupling capacitors- there is no diference with or without them. I'll put them back if they are needed.
One thing bothers me on my measurements- the current through R22 is 53.22mA! Is this normal?
The measurements:
An externally hosted image should be here but it was not working when we last tested it.
Before changing the values of several components in a proven amp design it would be very important to go through and verify/measure every resistor (diode, etc) and really make sure that you haven't put a wrong value in by accident (check and double check!). Then go through and verify that the transistors have been inserted properly (beware of pin out!). Then go through Quasi's reply and make sure you have done everything he suggested. Also check your soldering and look for bad joints or solder bridges. This amp design has been shown to work well, but you have to make sure that little 'accidents' haven't happened during assembly.
Steve.
Steve.
Hi,...forget it
let me tell you short story about QUASI NMOS350 amp
before 1 year have build my first NMOS350, NMOS 350 is working from first time start since today, ...everyday without any problems, great sound !!!
1 month later have design new doublesided NMOS PCB, after first start catastrophic failure, reversed VBE...amp is burning up, have replace wit new one components but another devices had failure to....did later this pcb in trash and make new one
The new one is working since today everyday
sometimes you can measure components and it looks ok,...but under load conditions... devices is defective
it can be you have counterfait devices, where did you buy ?
and again
sometimes you can measure components and it looks ok,...but under load conditions... counterfait devices fails
Finally: NMOS is working very stable with amazing Sound, if your NMOS is working you will love this sound
If you are interested...can send you gerber from my doublesided NMOS PCB
size 250 x 80
I actualy do have a function generator
I measured the currents with input shorted and no output load.
I put the measurement on the original schematic so the values can be easy seen and compared to original. I also marked the changes I made to original circuit with red color. I removed the coupling capacitors- there is no diference with or without them. I'll put them back if they are needed.
One thing bothers me on my measurements- the current through R22 is 53.22mA! Is this normal?
The measurements:
An externally hosted image should be here but it was not working when we last tested it.
Hi ID_,
If you have 53mA over R22 (220Ω), that equals almost 12V Vgs over the lower Mosfets, indicating something is wrong. Make sure the value of R22 and measure voltage drop over source resistors R30, R32 and R34 respectively, not jus one. Measure drop over R29, R31, R33 as well.
Voltage drop over R20 and R22 should be about the same, 4V.
Do not change R21 unless it's not a 100Ω resistor.
It is possible, tha I got counterfeit devices. I ordered the fets from farnell.com, resistors capacitors and transistors are from locall store. If I'll be unable to solve the oscilation problem, then I'll go and buy new components! NMOS, yes I would like to see your design!
I made some tests with load now. I have made 7.2 ohm 68W resistor out of 4 17W ceramic resistors.
The first picture is 1kHz sine wave (amplitude about 30mV)
The second image is the 1kHz square wave (amplitude approx. 30mV p-p)
And now comes the interesting part.
When I increased the input amplitude The whole story is different
Sine wave 1kHz (amplitude approx. 300mV p-p)
And the square wave 1kHz (amplitude approx. 150mV p-p)
Then I increased the frequency to 10kHz and made the same measurements
Sine 10kHz (amplitude approx. 40mV p-p)
Square 10kHz (amplitude approx. 30mV p-p)
Sine 10kHz (amplitude approx. 250mV p-p)
Square 10kHz (amplitude approx. 160mV p-p)
I made some tests with load now. I have made 7.2 ohm 68W resistor out of 4 17W ceramic resistors.
The first picture is 1kHz sine wave (amplitude about 30mV)
An externally hosted image should be here but it was not working when we last tested it.
The second image is the 1kHz square wave (amplitude approx. 30mV p-p)
An externally hosted image should be here but it was not working when we last tested it.
And now comes the interesting part.
When I increased the input amplitude The whole story is different
Sine wave 1kHz (amplitude approx. 300mV p-p)
An externally hosted image should be here but it was not working when we last tested it.
And the square wave 1kHz (amplitude approx. 150mV p-p)
An externally hosted image should be here but it was not working when we last tested it.
Then I increased the frequency to 10kHz and made the same measurements
Sine 10kHz (amplitude approx. 40mV p-p)
An externally hosted image should be here but it was not working when we last tested it.
Square 10kHz (amplitude approx. 30mV p-p)
An externally hosted image should be here but it was not working when we last tested it.
Sine 10kHz (amplitude approx. 250mV p-p)
An externally hosted image should be here but it was not working when we last tested it.
Square 10kHz (amplitude approx. 160mV p-p)
An externally hosted image should be here but it was not working when we last tested it.
I made voltage drop measurements over each sourse resistor. The readings are as follows:
(I'll refer to the markings of FETs on layout)
source resistor on FET___current
T11__________________9,6mA
T12__________________8,7mA
T13__________________27,6mA
T14__________________14,6mA
T15__________________33,6mA
T16__________________24,5mA
T15b_________________29,7mA
T16b_________________14,3mA
T15c_________________17,2mA
T16c_________________19,1mA
There are huge differences ant it shouldn't be! The DC offset voltage at output is 0,8mV!
Hi ID_
The issue around R21 was discussed in the thread "Power Amp Under Development". An improvement is made by replacing this with a string of 3 small diodes or with a LED. Alternatively leave this resistor value as 100 ohms. It might be worth reading about this to get the whole picture. http://www.diyaudio.com/forums/solid-state/43331-power-amp-under-development-91.html#post1439353. The idea here is to set a firm voltage drop of 1.8 volts rather than a variable one through the resistor.
The current through the 220 ohm resistors should be around 15-18mA setting the gate turn on voltage to between 3.3 and 3.9 volts (FET dependant).
C8 reduces the overall gain of the amp at high frequencies, so I'm puzzled by its removal. Remember this is linear circuit and not subject to switching artefacts.
C3 should be a polyester capacitor.
I would not be concerned about the differences in bias currents throught the output stage, good current sharing depends on how closely the FETs are matched and even then under signal conditions.
You cannot test the amp accurately into a load with the 100 ohms resistors in place; these are used to set the amp up and to do some fault finding.
For testing into a load you can insert 10 ohm 5 watt resistors but remember to keep the power down. The 10 ohm resistors will limit the current to 8 amps in the event of a fault and will probably be destroyed but that better than losing the output FETs.
Cheers
Quasi
The issue around R21 was discussed in the thread "Power Amp Under Development". An improvement is made by replacing this with a string of 3 small diodes or with a LED. Alternatively leave this resistor value as 100 ohms. It might be worth reading about this to get the whole picture. http://www.diyaudio.com/forums/solid-state/43331-power-amp-under-development-91.html#post1439353. The idea here is to set a firm voltage drop of 1.8 volts rather than a variable one through the resistor.
The current through the 220 ohm resistors should be around 15-18mA setting the gate turn on voltage to between 3.3 and 3.9 volts (FET dependant).
C8 reduces the overall gain of the amp at high frequencies, so I'm puzzled by its removal. Remember this is linear circuit and not subject to switching artefacts.
C3 should be a polyester capacitor.
I would not be concerned about the differences in bias currents throught the output stage, good current sharing depends on how closely the FETs are matched and even then under signal conditions.
You cannot test the amp accurately into a load with the 100 ohms resistors in place; these are used to set the amp up and to do some fault finding.
For testing into a load you can insert 10 ohm 5 watt resistors but remember to keep the power down. The 10 ohm resistors will limit the current to 8 amps in the event of a fault and will probably be destroyed but that better than losing the output FETs.
Cheers
Quasi
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Maybe the problem to oscilation was the C3. I put there ceramic disc type capacitor. When I replaced it with 2.2nF poly type, the amp stabilised. So im thinking to put the C8 back in and check it out.
@Quasi
I'll read the thread and in future do the tests with 10 ohm resistors instead of 100 ohm (I didn'k know that is not good).
@nigelwright7557
The DC offset is 0.8mV! The answer is in post 52!
@Quasi
I'll read the thread and in future do the tests with 10 ohm resistors instead of 100 ohm (I didn'k know that is not good).
@nigelwright7557
The DC offset is 0.8mV! The answer is in post 52!
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