[jc]

Hi Everyone,

As I am new to this forum I will probably be covering ground that has been discussed before.
It's good to see that there are guys out there prepared to build their own amps etc. and people like White Noise there to provide kits and support. I built my amp some years ago (too long to remember). It started out as a stereo mosfet job but I have rebuilt it over the years and now is in the form of a pair of monoblocks.
I was interested to see one of the postings referring to the quiescent current setting and how this can effect the performance of the amp. Well I must concurr the fact that I also found that this is a critical factor in sound quality but as I am not an electronics expert and I have no idea how far you can go in jacking up this bias current. My particular amp kit supplier recommended 155mA but I have since bumped it up to 275mA but now I see that figures of 500/600mA are being quoted. I realize that you have to have the heat sinking and or fans to keep the thing from thermal overload but is there any other factor which one should be aware of before going up to these sort of figures as I would like to try mine at that level to achieve Class A  operation.
Any comments or advice would be gratefully accepted/

jc    

[Luis MC]

Hi jc,

     I believe that you are right, first be careful with thermal disipation, with proper heatsinking at output transistors.
     With lateral mosfets there is positive thermal coeficient, but vertical mosfet have negative thermal coeficient and must be cotrol temperature, generaly by putting a transistor in thermal contact in heatsink. With lateral mosfets is more easy to implement.
     Another problem is the power supply. In simple power supplies (unregulated) is easy to raise quiescent current, but active or regulated PSUs is some difficult and heatsinking and/or unwanted voltage drops can occur.

     Regards,
     Luis.

[davidw]

With mosfets there is no critical value of quiescent current that gives minimum distortion, as there is with bjt power amps. So mosfets, heatsinks, and power supply permitting you can raise the quiescent current as high as you like. I've monitored the distortion spectrum of a MOS125 as a function of quiescent current and found that the third harmonic drops off much more rapidly than the second with increasing current. After about 300mA of quiescent current, with moderate outputs, only second harmonic distortion was visible with no third harmonic at all.

David

[jc]

With mosfets there is no critical value of quiescent current that gives minimum distortion, as there is with bjt power amps. So mosfets, heatsinks, and power supply permitting you can raise the quiescent current as high as you like. I've monitored the distortion spectrum of a MOS125 as a function of quiescent current and found that the third harmonic drops off much more rapidly than the second with increasing current. After about 300mA of quiescent current, with moderate outputs, only second harmonic distortion was visible with no third harmonic at all.

David

Thanks for your response. I am using Hitachi mosfets type 2SK176 and 2SJ56 two of each per channel. My power supply is a 1 kV/A transformer feeding 44000 uF of capacitors per channel (monoblocks). My heatsinks are from a large industrial robot and are quite massive. At a quiescent current of 250mA the heatsinks get quite toasty. I have added fans to keep them cool which I switch off when I do some serious listening. To what level would I need to increase the quiescent current to achieve Class A. I don't want to destroy my amps but I would like to increase the Q/C
what do you think?

Cheers

[davidw]

The rule of thumb that I use for quiescent current is that you keep on increasing it in small increments ( say 25mA ) until you can't keep a finger on the heatsink for more than a second or two about an hour after you've set the quiescent current. If you have a thermometer adjust the quiescent current as above for a heatsink temperature of 50 deg C. Your amp will always operate in class A for small inputs and  the width of the class A region will increase with quiescent current. With my power amps you need a quiescent current of around 500mA for most normal listening to be within the class A region. This means big heatsinks, at least 0.2 deg C per watt, and I always add a smaller heatsink across the fronts of the output mosfets too ( big screw mounting TO220 heatsinks are ideal ).