little gem buffer

[tomwarrior]

i built a little gem this weekend and tried adding a buffer for it using the big daddy schematic. im getting alot of noise osilation,ect. my gem is powered by a regulated 12 volt power supply that i built and this is the voltage that is going into the 15k resistor that feeds the drain(pin 1) on the MPF102 FET. do i need to change this value to run it off the 12 volts? also, im using the .01 input cap since i couldnt locate a .0068 and its feeding pin 2(-) instead of pin 3. it was working before i made these changes and any help would be greatly appreciated. thanks.---tom

[tomwarrior]

heres a schematic of what i did;
http://www.img4free.com/images/tomwarrior/bigdaddy2.jpg

[B Tremblay]

It seems like you may have mixed up the FET buffer used in the Grace circuit and the FET gain stage of the Big Daddy.

The oscillation is probably due to improper wiring of the buffer.  Check out the Ruby project for the proper implementation of a buffer.  You can ignore the 10k Volume pot; the adjacent output cap would connect directly to pin 2 of the 386.

[tomwarrior]

B Tremblay,
thanks for the quick reply. i actualy wanted to use the FET gain stage of the Big Daddy in front of the gem.(i should have been more clear, sorry, my bad). can i just switch the connections between pins 2 and 3? are there any sound samples of the ruby? thanks again.---tom

[B Tremblay]

Do a forum search for Ruby sound clips, there's a bunch of them.

Yes, you can use the alternate input as long as the unused one is grounded.

[tomwarrior]

well, i added the ruby buffer to my gem amp and it works . thanks again to  B Tremblay.
i have one more question though;
on the ruby, what purpose does the cap from pin 7 to ground serve? thanks again in advance.---tom

[B Tremblay]

on the ruby, what purpose does the cap from pin 7 to ground serve?

I'm glad to hear that it is working.  That cap from pin 7 to ground is there for added stability (reducing the chance of oscillation).  It's not always necessary and we never did a full study of what circumstances would require it, but decided to err on the side of caution by including it.

[tomwarrior]

 my little gem has been transformed into a ruby with a master volume. this has definetly been a change for the better for me as far as the sound goes.
after listening to the sounds of the grace overdive im thinking that is more my flavor.
can this be used as an amp? can i just change the resistors on pins 2 and 3 along with the input cap or do i have to change the output section too(25 ohm reostat to 100k, ect) ?
thanks again.---tom