Do not use voltage droppers in the supply when setting up the bias current.
No bulb limiter, no resistors in lieu of fuses.
Use the Vre method.
No bulb limiter, no resistors in lieu of fuses.
Use the Vre method.
Thanks Andrew,
I hear what you say, I was just following the build instructions of Quasi. I had three IR 460 fets on the one rail and three Thompson fets on the other rail which I have replaced with three IR fets. The VD over the source resistors are not all the same, so I should use the highest reading and adjust bias. I had some music on while running through the 10 ohm safety rail resistors and it didn't play bad, but when I put the fuses back, the music started distorting. The positive rail is still about 40mv higher than the negative rail. Really don't know what is going on. I use 62VDC rails and other guy's had even used 81VDC on this amp. Or I can use a lower voltage transformer, but it should not be like this.🙂
I hear what you say, I was just following the build instructions of Quasi. I had three IR 460 fets on the one rail and three Thompson fets on the other rail which I have replaced with three IR fets. The VD over the source resistors are not all the same, so I should use the highest reading and adjust bias. I had some music on while running through the 10 ohm safety rail resistors and it didn't play bad, but when I put the fuses back, the music started distorting. The positive rail is still about 40mv higher than the negative rail. Really don't know what is going on. I use 62VDC rails and other guy's had even used 81VDC on this amp. Or I can use a lower voltage transformer, but it should not be like this.🙂
If you have a single pair output stage then the current in the upper NFET and Rs equals the current in the lower NFET and Rs.
The very slight error is current escaping to the NFB resistor, which should be very tiny.
If you have multiple pairs then you should be matching the parallel transistors and matching the parallel resistors.
Once you have done this you will find that the total Irs upper = total Irs lower.
When the resistors are all the same you will also find that the total Vrs upper equals the total Vrs lower.
If these totals are not equal, then something is broken.
The very slight error is current escaping to the NFB resistor, which should be very tiny.
If you have multiple pairs then you should be matching the parallel transistors and matching the parallel resistors.
Once you have done this you will find that the total Irs upper = total Irs lower.
When the resistors are all the same you will also find that the total Vrs upper equals the total Vrs lower.
If these totals are not equal, then something is broken.
Hallo all,
I have finished building a channel of the quasi nmos350 amp. The bias over the source 0.33ohm resistors is between 7,7 and 12 mv. I did not match the IRFp460 fets.Rail voltage is 67 +-. The sound is not bad, definision and bass is good, but is a bit sharp overall , I am used to the more laid back sound of Nad amps. Is there perhaps a way to get it to sound more laid back ? I have already changed the input cap from 1 uf poly to 10 uf electro and it did improve a bit. Is there perhaps anything else that I can try ? I have not have it on a scope yet, I don't know if it can be oscillation causing it to sound so sharp. 🙂
Sorry, here is circuit.
https://sites.google.com/site/quasisdiyaudiosite/nmos-series/nmos350-500
I have finished building a channel of the quasi nmos350 amp. The bias over the source 0.33ohm resistors is between 7,7 and 12 mv. I did not match the IRFp460 fets.Rail voltage is 67 +-. The sound is not bad, definision and bass is good, but is a bit sharp overall , I am used to the more laid back sound of Nad amps. Is there perhaps a way to get it to sound more laid back ? I have already changed the input cap from 1 uf poly to 10 uf electro and it did improve a bit. Is there perhaps anything else that I can try ? I have not have it on a scope yet, I don't know if it can be oscillation causing it to sound so sharp. 🙂
Sorry, here is circuit.
https://sites.google.com/site/quasisdiyaudiosite/nmos-series/nmos350-500
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Exaggeration of transients result in "sharpness" and extra apparent detail.
This exaggeration is due to the NFB not reacting quickly enough to control the gain of the amp for fast signal changes. The transients get over-amplified. This can be checked with a squarewave test signal and a scope. The difficult bit is identifying where to add, or adjust, compensation to get stability margins back to acceptable.
This exaggeration is due to the NFB not reacting quickly enough to control the gain of the amp for fast signal changes. The transients get over-amplified. This can be checked with a squarewave test signal and a scope. The difficult bit is identifying where to add, or adjust, compensation to get stability margins back to acceptable.
Andrew,
Thanks for the explanation, I have used a ceramic or two in places where a poly cap should be, I'll try to get some and put it on a scope and see what happens.
Thanks for the explanation, I have used a ceramic or two in places where a poly cap should be, I'll try to get some and put it on a scope and see what happens.
I'm not at all sure the problem can be thrown at the use of ceramics.
Maybe an audible improvement to try an NP0 if not already used.
Maybe an audible improvement to try an NP0 if not already used.
Andrew thanks,
Just to refresh, what is a NP0 ?
What would you think about altering the feedback circuit, Increasing R17 to 36K( is 33K now) , Decreasing R18 to 800E( is 1K now) or increasing the value of C7 slightly higher, is 100uf now.
Just to refresh, what is a NP0 ?
What would you think about altering the feedback circuit, Increasing R17 to 36K( is 33K now) , Decreasing R18 to 800E( is 1K now) or increasing the value of C7 slightly higher, is 100uf now.
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Hello, Andrew!
What do you think, if one has only a (clear signal) sinewave generator with changeable frequency? Would a PC-(Soundcard-)scope do the job? Or is this too slow for analysis?
Greets,
Fred
What do you think, if one has only a (clear signal) sinewave generator with changeable frequency? Would a PC-(Soundcard-)scope do the job? Or is this too slow for analysis?
Greets,
Fred
I have replaced the 1uf nonpolar capacitor on the input with a 10uf electrolytic, the positive of the cap goes to the T1 transistor base (NPN) and the negative to source input. Is this correct or should the caps leads be reversed . Will it make a difference ? 😕
https://sites.google.com/site/quasisdiyaudiosite/nmos-series/nmos350-500
https://sites.google.com/site/quasisdiyaudiosite/nmos-series/nmos350-500
Measure the voltage across the electrolytic.
It should be about identical to the voltage across the 1uF.
If you read the cap report you will see that a series pair of 22uF bi-polar electrolytics perform better than any other 10uF electrolytic. Except for size and cost and weight and volume and ......
It should be about identical to the voltage across the 1uF.
If you read the cap report you will see that a series pair of 22uF bi-polar electrolytics perform better than any other 10uF electrolytic. Except for size and cost and weight and volume and ......
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hi guys !
this is my first post in this forum 😱
i want to build nmos200-TO247 version but before building i have a few questions :
1.can i use 2 pairs of irfp260 fets in this amp instead of one pair ? (for driving 4Ohm speaker) how should i connect them ?
2.what was the final decision about using an amber LED instead of r21 in this circuit ? does it improve sound quality ?
3.can i use 2sd669/2sb649 instead of mje340/350?
4.can i use 2n5401 instead of bc556 ?
forgive me for my poor english ..
cheers
this is my first post in this forum 😱
i want to build nmos200-TO247 version but before building i have a few questions :
1.can i use 2 pairs of irfp260 fets in this amp instead of one pair ? (for driving 4Ohm speaker) how should i connect them ?
2.what was the final decision about using an amber LED instead of r21 in this circuit ? does it improve sound quality ?
3.can i use 2sd669/2sb649 instead of mje340/350?
4.can i use 2n5401 instead of bc556 ?
forgive me for my poor english ..
cheers
hi sina8
How is it going on ? did you ready have answer for your question?
build it up and test it get the job and funny...
How is it going on ? did you ready have answer for your question?
build it up and test it get the job and funny...
Here`s my Quasi NMOS 350 with 2 pair of fets per chanel. The voltage is ~ =/-68v dc and i used the case from Kenwood KA-880SD. The amp isn`t ready, i have to upgrade with two 60x60x10mm fans, power supply for them and output connectors for speakers.
Why do you think you need fans?
The heatsink on each 2pair amplifier looks OK.
What dissipation do you expect at your intended output bias current?
The heatsink on each 2pair amplifier looks OK.
What dissipation do you expect at your intended output bias current?
In time, I`ll add the third pair per module and I think that 2 fans are always welcome. It`s possibly to add a small on/off switch or temperature control.