AE237Week-7:
The schematic below was last updated 25 September '09. Click to download a higher resolution PDF version.
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.JUMPER OPTIONS
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 RT101.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).
CROSS-POLLINATON
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!
