The schematic below was last
updated 25 September '09. Click to download a higher resolution PDF
The circuit board for this
circuit was originally designed to be modified, the circuit as described
below is the 3rd revision a.k.a JFET-9.
There were several goals
for this circuit, let's look at what it does...
INPUT to OUTPUT
From INPUT connector J-1,
let's follow the sine wave at pin-3 as it travels to GATE of Q1, the 2N5457
JFET. The output at the DRAIN is larger and inverted - 20 dB
Voltage Gain - which then feeds a capcitor inserted into the former
Q3 position, where it is then routed via jumper to the input of the second
stage. Note the junction of the 1k5 resistor (R3) and the Q1 JFET
source is the 'negative feedback input.' On the circuit board, this
conection is made at the jumper between one leg of the former RT101 and
the former Q7.
In this version, Jumper
J4 is not used, but J5 and J6 are connected. Note the same value
source resistor is now in parallel with the 470uF cap, and that combo is
in series with a 47 ohm resistors. This is Scott Hampton's original
JFET circuit. The 47-ohm resistor is in place of variable resistor
Notice the sine wave at the
DRAIN of Q2 has again reversed polarity - the resulte of more voltage gain
- and then feeds the base of the MPS-A14 Darlington Q4 for current gain,
the output of which is no larger in amplitude, but now has more current
driving ability - good for long cables).
The circuit with no feedback
has about 46 dB of gain. A jumper is added from "C6," which is not
a capacitor but a 510-ohm resistor. The jumper terminates into the
former output of the first stage (C4), so it can drive R13 and the limiter
Threshold pot located on the front panel (FP). Also on the FP is
the Sensitivity pot that used to bne a simple voltage divider, but it is
now being used in the feedback lop in parallel with the opto. Gain
is at max when the pot is 'open,' at 500k, unless the Limiter Threshold
pot is full up, then the photo resistor reduces gain. Notice the
two pairs LEDs that comprise DL-1 / R301 and DL-2 / R302, one pair
is visible and the other pair is inside its respective opto-coupler, an
encapsulated LED and photo-resistor, Silonex
and VACTROL are two manufacturers.
MORE TO COME
While there is much more
to write, the main thing is to note how much fun there is to be had - so
may options, which one is best for what we want to do? Just like
MIXING, devolping and tweaking a circuit is an ongoing process, but at
some point, it becomes time to lock everything in, bounce the mix to a
file and master it!