leigh
Well-known member
This is a design I've breadboarded and been happy with so far, on the bench. In testing, I'm getting between 12 and 16 dB of reduction at max, and it's sounding good. Would like to get feedback on it before building it into a box.
The inspiration for the design comes from discussion about the Manley ELOP. I have not seen the Manley schematic, however, and I imagine that the similarity ends with "driving a pair of vactrols with an LM386". Regardless, that basic idea stayed stuck in my head for months, and I finally had to give it a go.
As you can see, there is no makeup stage yet, or metering. I've got an idea sketched out for VU GR metering, and I'll post that in a bit. Also, for the power supply, I've just been using a 12 VDC bench supply.
Here is the parts list:
R1 = 5k
R2 = 100k audio taper pot
R3 = 680 ohms
R4 = 332 ohms
C1 = 10 µF
C2 = 2 µF
C3 = 1 µF
S1 = SP3T
D1, D2 = 1N34a
U1 = LM386
U2, U3 = VTL5C2 vactrols
D1 and D2 are in series with the vactrol LEDs, because the vactrol LEDs have a low reverse breakdown voltage (3 volts). I used 1N34a's that have a low forward drop (0.25 V), but other small signal diodes could be used without much change.
The value of R4 (current-limiting resistor) was determined so that the vactrol LEDs would see a max of 20mA, even if the 386 chip was swinging rail-to-rail on 12 volts (single-ended supply, so 6 volts per side). This might be a bit conservative, since it won't actually swing rail-to-rail, and 40mA is the listed "absolute maximum" on the datasheet - but I was getting enough gain reduction out of it, and hitting the LEDs with less current will help their longevity. NOTE: See my next post, below, for more about the actual value of R4.
Thanks for your feedback.
Leigh