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I fixed a bias problem in my Mosfet follower (experiment #1 continued)

Posted by kurt s on June 30, 2003 at 17:43:49

I discovered the bias was not very optimum for "under load" conditions, acting much differently than unloaded. It was way to asymmetrical than it needed to be. Also, since it isn't driving bass below 150 Hz and since I was not used to this type of asymmetrical hard clipping behavior (I listen to tube amps all the time) the harshness I was listening seems to be caused by two means: 1) excessive clipping from not enough power, 2) I had a bad day from my tinnitus.

Don't laugh too much about number 2. I have tinnitus and some days are simply worse than others but if the signal is clean I can usually tell it's sounding okay. If the signal is not clean it can exacerbate the tinnitus and sound harsher and it's not easily recognizable to me what's the cause, like simple clipping from the amp. Clipping can sound different to me on different days. My ears are often the weak link in my system.

So my amp was tweaked up a little and the problem with it is that I can only get about 100mW cleanly out of it on just battery power and the small heatsink. For this amp I think I want about 1W of power, more than my 300 mW of tube power where the tube amp clipping is very soft. So this is going to require a big heat sink, a bigger power supply, and probably a current source instead of a simple load resistor, or maybe a choke.

Otherwise, the amp sounded pretty good, a little opaque compared the tube amp. Maybe I can get the new lateral MOSFET to put out enough power with just 12V of battery, but I think more like plus-and-minus 12V will be needed to get enough power out of it. Dang, transistors do clip hard and mercilessly. Fortunately you can get high powers from them, but now it's going to be real work with the power supply and hear sinking.

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