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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: Re: I fixed a bias problem in my Mosfet follower (experiment #1 continued) posted by 13th Duke of Wymbourne on July 1, 2003 at 14:05:05:
These are a lot of good questions and I am fumbling my way through this as I learn why things that are simple are sometimes just too simple.>Is the assymetry you see due to amplitude limiting or due to current limiting on the negative cycle, where the pull-down is trying to sink current from the load? Do you have access to a 'scope?
I am using my scope to look at the problem with this topology. A simple resistor-loaded follower into a low impedance load is an interesting animal to deal with. If the resistance is too low, there is not enough current from the follower to drive it positive and there is a low rounded flattened response on positive swings, if the resistance is too high then it will not pull down the negative swings and it clips too early on negative swings.
>How to you set the output commom-mode point?<I was just setting the bias on the gate side of the follower and fiddling with what gives "best" performance on the scope. The output is AC coupled through a 100uF Black Gate capacitor, so I can set the bias point wherever it likes. I am using cap coupling for the time being because it simply is safer on the speakers in case of accidents. The speakers are just being operated from 150 Hz on up and so a moderate capacitance will work. Bass is separate.
From playing around with this circuit I see the important need for one of two things: a constant current source or a choke in the load. This will lower distortion and make it easier to set a point where both positive and negative swings are evened up. I am seriously considering the choke because I actually have a pair that will work - 0.8H that is broadband and can take 1.6A of DC current. I still have to use resistors (bypassed) to set up the bias, but the choke can even up the swings by its energy storage in inductance.Now the amp is progressing from a little proto board to a wooden proto box with mixed parts all over it and a new power supply. I might still be able to do it with my junk parts and make a decent amp.
My 2SK1058's arrived and I found out of course I don't have the mounting insulators for that size. Oops.
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Follow Ups
- Re: I fixed a bias problem in my Mosfet follower (experiment #1 continued) - kurt s 22:19:06 07/01/03 (0)
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