FET buffers...

[Chris Crosskey]

Hi folks...

I'm in the process of cracking out the ladder filter circuit on the Kawai 
S100F and am wondering how to get rid of the discrete FET buffers that are 
on the differential amp that goes to teh feedback and output....It's a pair 
of n-channel JFET's with the Drain (I believe, though it isn't made clear 
on the drawing) going to +15V, the Gate going to the ladder and the source 
going via a 56K resistor to ground, and also to the diff inputs of the 
op-amp via 10k resistors.
	 Problem I've got is that the inputs of the TL0x series op-amps I 
prefer to use wherever possible are p-channel for starters, and also do I 
need to keep the 56K resisotros or can I junk them out of the circuit 
completely....ie can I just run the ladder directly into the TL074 inputs 
via 10 resistors ......If neccessary I'll leave them in (replaced with 
2N3819's I guess) and use half a 1458 for the op-amp the way they do in the 
circuit but I'd rather not.....
	My reason for choosing the S100F for a cracking project is cos I've 
got one (needs fixing ATM) and the filter is lovely, plus it certainly 
runs at a fairly high P-P level (8V) so setting it up to work at 10V P-P 
will be easy too, minor reduction on the front end, minor boost on the 
output and I'm there......I'll be buffering input and output with op-amps 
anyway so I might as well incorporate the level shuffling at that point....

chrisc