Hi, I found this place via Stuart Y. and John Curl. I recently passed my 30th aniversary as IC designer (AD797, AD711/12, many more) and find myself tinkering in audio again for fun. I guess I'm a heretic of sorts, my own preamp is all discreet JFET, no feedback (and I don't have a home theater). Hopefully I'll be able to find and give some useful info here.
scott wurcer said:
Cool. Harvard Square is one of my favorite places (only had an opportunity to take a few evening classes at the two schools--gave me a taste of "heaven on earth"). (And since you mention John Curl, I'd like to throw in that Telegraph Avenue is all right too.)
I'm building Borbely's all JFET Super Buffer for a headphone amplifier. Any JFET recommendations other than 2sk170/2sj74's? I've been waiting for 3 weeks for mine to arrive so that I can start matching them and actually build something. Don't mind waiting as I'm using the time to plan my point to point wiring.
It's interesting that you chose no feedback. And I guess you mean no global feedback. (hmmm, I wonder if I can modify Borbely's circuit to be no global feedback?)
Actually, I thought of a question. I read that it's hard to build both P and N channel JFETs in op-amps (n channel being the hard one). Is this not true now? Also, where could I find the dielectric constant of the plastic material used to encapsulate ICs?
Enjoy your time here.
JF
Same FETS, I bought mine at Akihabara 15 yrs ago and finally got around to using them. If you conside source degeneration feedback then I'm guilty. I'm not religious about this stuff, just trying to see how well utter simplicity works. The D for packages is in the ballpark of published numbers for epoxy. The DA is BAD! So bad that we had to can a product (sample and hold) because the 2pF parasitic in parallel with the 100pF external capacitor had a net DA that could not meet spec.
scott wurcer said:
Thanks, Scott. I was thinking a dielectric constant of 4 and 0.7pF parasitic (based on Google searches and running a calculation of capacitance with guess-timates). Seems like I was in the ballpark. Again, hope you find it of value to spend some time here.
And 15 years ago in Akihabara...then you probably got them for a fraction of what they cost now: $2-$3 a piece. The only hard part for me was deciding how many extra to purchase so that I had enough matches. I created a spreadsheet with a bell curve distribution over the three grades to help me determine how many to purchase. The stickler is I'm trying to match two different parts (2sk170/2sj74). So, I'm hoping for variance. To put numbers on things, I've purchased 24 pairs to try to get about 14 matches. Regardless, it's interesting to me.
JF
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