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previous: Gus BMP -- 12/4/2000 6:17 PM View Thread

some math

12/6/2000 9:20 PM
some math
I worked out the gains for Mark H's "good sounding" BMP with out the 100k. I used the values at JD's site (thanks!)  
The gain worker out to  
1st stage ~= X10  
2nd stage (1st clip) ~= 34  
3rd stage (2nd clip) ~= 36  
4th stage (after the tone section) ~=4.5  
However the 3rd stages collector sitting at about 1.07 volts might get pulled down to .2 volts with a big enought drive in to the base. I would have to look at that on a scope transistors are funny when they get around saturation the "rapid snap" into sat could possable cause a 2nd harmonic.  
Problem with reproducing this is that the Missing 100k makes that stages Q points dependent on Beta and temp. High beta devices should have a lower Collector voltage I believe. I don't think the sound is because of the the gain being 36 instead of 34 but that the collector voltage is close to ground. I worked this all out long hand re at different Ic's etc. For the missing 100k stage you just can not change one thing without the other Q points moving. I am thinking about this. Right now it seems the best way to do this is to find the beta that works the best in that stage an test it at working temp then Beta select transistors at the same temp. I am thinking about a external bias control to dial this in. If one changes the 15k the tone section will change model the tone section with a 15k source then with a higher or lower source.  
I hope this helps