Power amp under development

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#3,322
Hi have sent to all today

who email me NMOS Power Board (with DC detect and Turn on Delay) and NMOS Amplifier X Gerber Files with Schematics and PCBs Layout

Questions or improvements are welcome

I think about forthcoming to add CLASS H with SMPS Power Board for NMOS Amp....

Does somebody know low cost PCB Supplier in Europe or in the world for small order,

in Germany local PCB Suppliers charge us with 200 EUR equal. to 260 USD for 2 green plastic Board sample PCB ?
 
#3,325
Hi everyone

I am building one more Nmos500 with original Quasi board. I just want to ask one thing on the amp board can I use 470UF 100V caps instead of 470UF 200?
I am going to run it with +/- 67VDC rail. With +/-67VDC what power should I get from Nmos 500 in 8ohm?

Thanks
Regards
 
#3,326
#3,329
here's one pic
 

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#3,334
strange DC output

Hi all,

I've been using this amp (Nmos 500) for a while. At the start I had trouble with fake fet's, which caused a big bang (there are pictures of it somewhere on here).

Anyway, since then i have re-done the amp and have been running it nicely for over a year. I decided to turn the amp into an instrument amp, and installed a valve preamp before the amp. However this caused the amp to have a very strange 1.5V dc on the output. Sometimes the DC voltage gets up to 2.5V and trips off the DC protection relay.

So I spent ages trying to find out why the preamp would put 1.5V dc on the output. The output of the preamp is 0.09 Vdc, so its not from that. I am stumped. When I disconnect the preamp from the amp inputs the 1.5V dc goes away.

Also, after about 10 min of testing at very very low volumes (hardly audible, and with the preamp plugged in) the heat sink got really warm (not hot). Soon after i found that out one of the 27R resistors smoked, and i turned the amp off as quick as i could.

Any ideas?? im at a loss, been trying to work it out all day... 🙁


Cheers.
 
#3,335
Most probably the amp is oscillating as soon as you connect the preamp.
Perhaps you have to increase the Vas miller capacitor, if you have one. Try that first, increase the value until you reach about 100 pf.
If you do not have results increase the driver compenstation capacitors.
Maybe it helps!
 
#3,338
thanks for the quick reply.

The Nmos500 does have input filters (hi and low pass). This should also explain why there should not be any DC input into the diff amp stage. If there were no input filters then 33 gain would explain the 2.7 volts.

also NFB loop? i'm not familiar with that term

VAS miller capacitor is 39pF. i might try increasing that and see if that helps.

Also i should say i have tried changing VR1 (output offset voltage) but it doesn't make any difference to the output at all.
 
#3,339
if I have found the correct schematic then
C1 =DC blocking capacitor and with R3 forms the high pass input filter
C2 = RF and with R1 forms the low pass filter
C7 = DC blocking capacitor in the NFB loop.

Are all three fitted?
What are the values?

is C8, 10pF fitted?
 
#3,340
yes all those capacitors are fitted, and their values are the same as those on the schematic (you seem to have found the same one I am using).
C1 is a 1uF greencap, C2 is a 330pF ceramic, C7 is a 100uF 63v electrolytic, and C8 is installed, and it was marked as a 10pF ceramic.

I was thinking, it could have been the amp oscillating. When i hooked the output of the amp up to the cro i could see there was something, but it was too HF to see on my cro in detail. could it be the output impedance of the preamp is making the amp oscillate?

I plan to get some new capacitors today, im going to change C4 up to something higher and see if that helps.

thanks 🙂
 
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