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|13th May 2002, 08:27 AM||#7|
Join Date: May 2001
Location: Bangalore, India
I have made Anthony Holton's N-channel amplifier. Search the Solid State Forum using the keywords 'N-channel amp' and you will find quite a few posts on this amp among others.
You will also find a picture of the complete populated board of the amplifier for which I used my own PCB layout.
I have elsewhere made my observations about the sonic qualities of this amp. But for everybody's benefit I would state the following:
The sound quality of this amp is second to none as Anthony Holton claims. In fact, the sound of this amp can be compared to any of the top Hi-End contenders. It is absolutely fantastic. The resolution is near ultimate and low level signals come alive. Not the last word in musicality, but very much there.
However, the following caution is cited from my experience. I used IRFP250 devices while the rest of the components were identical to what is in the schematic. I used a trimpot on the input pair to iron out output offset and used a biploar cap in the feedback network bypassed with an MKT. I used 1E 1Watt x 3 resistors for the output devices.
With the above setup an amplifier which has been powered by + -50 volt rails is working for the last eight months without a single failure. There were two other boards that I powered up successively, using a 1.1kVa transformer and + -84 volt rails. Both the boards worked very well in Public Address performances for about two hours but then gave up the ghost. Thermal considerations is not an issue here since I had more than adequate heatsink and a fan to cool. I guess the dissipation exceeded the SOAR limits of the devices used with the higher rails. Hence, I plan to use about + -75 volt rails (off load) and power up two boards in a stereo setup expecting some voltage drop resulting in on-load voltages close to the specified 70 volts. The recommended off-load voltage is + -72 volts but I can't get my transformer to give this voltage even after modifying it.
Alternately it might be safer to use IRFP260 devices since they have a much higher Pd rating. I am also wondering if using the IRF610 for tracking temperature is wise? I used BD681 in the board working off + -50 volts and it still works fine. Hence, it could be either that the voltage is what caused failure OR maybe the temperature tracking proposed by Anthony now (IRF610) as opposed to what was earlier in the schematic (BD681) could be the problem. I will confirm on one of these as causes in the coming week after successfully testing the stereo version with lower rail voltages.
P.S. Refer to Anthony's updates on the N-channel amp to get an understanding of the changes he incorporated in April last year.
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