The Fat ‘n’ Pretty is a two channel optical compressor/overdrive that sounds great. It’s based on a circuit that Johan Blomdahl dubbed the LA-Light, because he designed to mimick the functional blocks of the well known LA2A Leveling Amplifier (compressor). Johan’s design is best suited to line level input signals, and he mostly uses it for vocals. Here is what Johan’s original design looks like:
My goal was to modify the circuit to make it useful for low level signals (bass, guitar, etc.) while retaining the original functionality for line level inputs. I also wanted to minimize the amount of wiring to keep the assembly as simple as possible. So, I designed a layout that incorporates PCB mounted potentiometers.
Along the way, Johan pointed out that the LEDs in the sidechain were in a typical clipping orientation, and suggested the posibillity of getting a second, distorted output from the LEDs. Well, of course, he was right. So, after some tinkering, re-arranging, and modifying, this is what I came up with:
The second stompswitch selects between either the compressor (Fat) signal, or the overdrive (Pretty) signal for the main output. There are two pannel mounted bi-color LEDs. One is the on/off status indicator. The other is the threshold indicator, indicating both the amount of signal attenuation in the (Fat) compressor section, and the degree of clipping that is occurring in the sidechain (Pretty) overdrive section. When the Fat compressor output is selected, both LEDs are green and the green knob sets the output level. When the Pretty overdrive output is selected, both LEDs are red and the red knob sets the output level. The two knobs can be set so that the output level sounds the same when switching between Fat and Pretty, or so that there is a boost when switching from one channel to the other.
The Fat ‘n’ Pretty has a variable gain input buffer that allows the response, sensitivity to be adjusted to suit different level inputs such as line/low level or instruments with active/passive electronics. There is also enough gain to overdrive the circuit in either Fat or Pretty mode. The threshold and ratio controls, which do what you’d expect them to do for a compressor, allow a very wide range of different character overdrive sounds to be produced. It’s lots of fun to play with.
The Fat compressor output is both very tweakable and transparent at the same time. It does warm up the input a bit, in a very nice way. But, there is a range of tame compression settings that you might not notice until you turn it off. At the other end of the spectrum, it’ll certainly do some pumping and breathing if you want to take it over the edge.
The Brite/Warm switch that shunts one of the capacitors in the output stage doesn’t seem to have much effect on the sound. Although, it sounded good on my breadboard, I’m not sure why. I really like everything else about this pedal a lot. So, for now, I’m not going to mess with it.
This circuit requires a bi-polar power supply. I wanted to have a lot of headroom, so I went with +/-15V. The power supply circuit I used is shown on the schematic. I use a 18VDC wall wart to power it. If you build this power supply, make sure to keep the power ground separate from the circuit ground, and put the regulator on a heatsink.