Hi,
a) I want to use the FA-1 booster circuit, but with a symmetrical power supply (+/-9V). What changes are needed?
Sure, omit the voltage divider etc... But what about the biasing of the input transistor?
(https://i.postimg.cc/dkNZ0gxR/FA-1.jpg) (https://postimg.cc/dkNZ0gxR)
b) That schematic isn`t exact. The original has one more 10uF cap, C5. What difference does that make?
https://www.freestompboxes.org/download/file.php?id=40244&sid=9282ad3dfacaddc6c62cf90e0275c98d&mode=view
The FET is only unity gain, you won't lose anything by keeping it on the single rail positive supply as it shouldn't need more headroom. You won't get rid of the 4.5v bias as it's also where the FET gate bias comes from.
There's nothing to stop you from hanging the T1 buffer and bias circuit between the +/- rails but the resistor values will probably need a tweak. Basically, ground connections to the right of the input cap become -V supply connections. I think it's simpler to keep all of that as is and only change the opamp supplies.
If the opamps have a negative supply, the 10uF coupling cap C4 should probably become non-polarized (could argue it should be NP for single rail too).
Quote from: anotherjim on October 08, 2022, 02:05:48 PM
Basically, ground connections to the right of the input cap become -V supply connections....
... and the +4.5v bias on op amp pins 3 & 5 becomes a ground bias (halfway between +9 and -9 supplies.)
May I ask the reason for "FET amp" designation..??
Believe it or not, I think it is because of the transistor, which is a FET. I used to think that the op-amp also had FET inputs but the datasheet for the HA 1457 W doesn't seem to indicate that.
Was this a bit of marketing by BOSS?
Quote from: anotherjim on October 08, 2022, 02:05:48 PM.
There's nothing to stop you from hanging the T1 buffer and bias circuit between the +/- rails but the resistor values will probably need a tweak.
Can someone help me with the math? How do I calculate those values?
And is it possible to connect the 3m3 resistor to the neg. supply rail?
https://www.diystompboxes.com/smfforum/index.php?topic=124847.0 (https://www.diystompboxes.com/smfforum/index.php?topic=124847.0)
Configuration (g) in PRR's last post is your case.. :icon_wink:
(although I think it's worth reading the whole thread..)
Also https://sound-au.com/articles/followers.html (https://sound-au.com/articles/followers.html) 6 - FET Buffers