Hello prasi
Can you send me a PDF file?
Did this project work without problems?
Thank you very much
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DC Servo MOSFET Amplifier
It's already posted post no.707
It worked for me and one other member RickG.
Regards
Prasi
It's already posted post no.707
It worked for me and one other member RickG.
Regards
Prasi
Hello Jan,
I and RickG have posted sound impressions few pages ago.
I think it will be same as earlier 2 pair by apex built by still4given and many others except higher voltage capability.
Reg
Prasi
I and RickG have posted sound impressions few pages ago.
I think it will be same as earlier 2 pair by apex built by still4given and many others except higher voltage capability.
Reg
Prasi
any replacement for bc546 and bc556?
5551 and 5401 can be used there?
BC546 CBE --> 5551 EBC
BC556 CBE --> 5401 EBC
A while back, Prasi sent me some boards for the FX100 3 pair. I populated them right away except for the outputs. I finally got around to installing the outputs and mounting them on some heatsinks. Of course, one board works fine and one has issues. I'll get around to tracing it out tomorrow. The reason I posting this is that I wanted to set this up as a smallish PA amp. I built a couple of the APEX XLR pres so I would be able to use balanced sources but the gain of this amp is too high to use them. They overload the amp. Are there any XLR preamps that are unity gain?
Thanks, Terry
Thanks, Terry
Attachments
Hello Terry,
nice build.
R32= 2R2 ground lifter resistor doesnt work with dc servo as per Mile, so i removed it and ran a wire from input to psu ground.
DC Servo MOSFET Amplifier
Also ceramic compensation caps are rated 2x supply volt.
I think gain of the amp can be reduced by increasing R14, but I am not the right guy to say it. Mr. Mile can only answer.
regards
prasi
nice build.
R32= 2R2 ground lifter resistor doesnt work with dc servo as per Mile, so i removed it and ran a wire from input to psu ground.
DC Servo MOSFET Amplifier
Also ceramic compensation caps are rated 2x supply volt.
I think gain of the amp can be reduced by increasing R14, but I am not the right guy to say it. Mr. Mile can only answer.
regards
prasi
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I don't recall the 2R2 resistor but I did run a ground wire from Signal ground to the PSU so it should be shorted anyway. I don't know which caps you are referring to. I don't have a schematic with part numbers for this board.
By the way, when I first tested the boards, they both worked separately but when I fired them together they blew the V- fuses. Now one board works flawlessly and one has issues. The right channel sets off the protection circuit when the volume is advanced.
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here is the sch DC Servo MOSFET Amplifier
here the pdfs and stuffing guide..
DC Servo MOSFET Amplifier
2R2 is right below the pin 8 of op-amp.
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OK, still having the same issues. Alone each channel plays OK though I see something a little weird on the scope at high output. However, when they are hooked up together they oscillate and blow the fuses. I never had this problem with the 2 pair amps I built so it stands to reason that something has been changed from the original circuit. I'll check things over and see if I can find it.
I compared the two schematics. The only things I see different are the two 22R/.5w resistors added before the VAS, the two 100n caps on the Op amp and some extra filter caps. No idea why it is oscillating. Maybe Mile will have some input.
I looked at the board layout and there is a distinct lack of star grounding, so that the high currents from the rail decoupling capacitors get injected into small signal ground - that's probably enough to risk oscillation.
This is made worse by the ground trace being pretty thin and split two-ways, rather than flood-filled plane - most of the top side is unused and could be used for decent ground.
This is made worse by the ground trace being pretty thin and split two-ways, rather than flood-filled plane - most of the top side is unused and could be used for decent ground.
I found the issue. It was in my PSU. The relay had shorted inside across the outputs so the output from both channels were shorted together. I installed a new relay and now everything plays beautifully. Boy this was a weird journey.
Now I am wondering if this caused the problem with the little MOSFET amp I built a couple of months ago. I cant be sure but I think I used this PSU to test it. I wish now I had kept them.
Now I am wondering if this caused the problem with the little MOSFET amp I built a couple of months ago. I cant be sure but I think I used this PSU to test it. I wish now I had kept them.
Attachments
Hi Terry,
Glad you found the issue. Congratulations. Yes debugging can take lot of patience and but you feel immense satisfaction once you rectify the issue. regarding 22pF caps, Mr. Mile had advised me to use 100V rated ceramic caps, just from memory. I need to find exact post. It was long ago so I dont remember specifics.
regards
prasi
Glad you found the issue. Congratulations. Yes debugging can take lot of patience and but you feel immense satisfaction once you rectify the issue. regarding 22pF caps, Mr. Mile had advised me to use 100V rated ceramic caps, just from memory. I need to find exact post. It was long ago so I dont remember specifics.
regards
prasi
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I forgot how much I liked this amp. It really sounds strong and deep. I am putting this one in a case. Only running it on +-58VDC but it is really loud even at that voltage. Should make a nice little PA amp with the balanced inputs and volume controls.
Very cool!
I have it mounted in a case now. I used balanced 1/4" inputs as well as RCA which feed the XLR Pre's. The transformer is 40-0-40VAC on the high side and 14-0-14vac feeding the low voltage circuit which feeds the XLR pre's. The rails are holding at +- 58V under a full load. The only issue I now have is with the clipping circuits on the PSU. They tend to blow the LED's when clipping occurs so I have chosen not to use them. If Mile can tell me how to safeguard the LED's I will use them since I think it would be useful when used in a PA format. I still need to tidy up some wires but this thing is dead quiet.
At +-58V this 3 pair FX100 is a brute into 4 ohms. Very impressive!
Here's some pics. I know the diamond plate is not for everyone. 😀
At +-58V this 3 pair FX100 is a brute into 4 ohms. Very impressive!
Here's some pics. I know the diamond plate is not for everyone. 😀
