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VCA Mods

Bass Boost Mod

This mod is absolutely essential if you want your x0xb0x to be a bassline machine. It is a really good one to make! Change C21 and C20 from .01uF to .1uF. Some people have found that it gets too boomy if you use your x0xb0x for more of a lead sound, but you can usually roll off the bass on your eq instead.

Muffler like Distortion

The DevilFish has a post VCA soft-clip distortion called "The Muffler" that apparently retains bass response.

Soft clip distortion is fairly easy to set up. You get a triple pole switch, and leave one of the poles disconnected, The other pole should lead to a diode going to ground. The other pole should lead to a pair of diodes going to ground, one of them reverse biased. The character of the distortion would change according to the diodes that you choose. You could change this circuit to double up on the diodes

This also might make a passive filter due to C38.

From C38 ----+---- To VR8 
           O O O
           O O O
             | +-----+
             |    +--+--+
             |    |     |  
             V    V     -  Diodes
             -    -     ^
             |    |     |
            ---  ---   ---
            GND  GND   GND

Choosing Diodes

From the Guitar Effects FAQ:

Silicon is reputed to clip abruptly, causing a harsh sound. Germanium clips at about half the voltage of a silicon diode, but is reputed to turn on more slowly in its smaller range. LEDs turn on at about two silicon diode drops, and are also reputed to be slower turn on, with it and germanium giving more "tube like" sounds. Some units with "tube" in the title use LEDs or germanium for clipping.

Oddly enough, the Marshall JCM800 uses a set of two pairs of silicon diodes back to back as clippers. This seems strange in a Marshall, but it is there. When I looked at the schematic, I first thought it was a protective clamp for a tube grid, but the circuit doesn't work that way. They are there for the distortion.

If you perform this mod, please post your experiences with it!

Another Overdrive Distortion

I think a good overdrive is probably the single best mod for the 303. For a fully adjustable overdrive section I did a circuit based on the same principles as the many stomp box overdrives out there. I just tailored it for the 303. Below is the basic idea of the circuit. The values in the circuit should work well. I took a look at the output level from the VCA on an oscilloscope and found that it's generally about 50mVrms compared to 100mV for the main output of the unit. This is right in there with the levels from guitars etc… so I found the circuit clipped well using values similar to what you find in stompbox overdrive pedals. You could also put the circuit in there post-volume pot and have some control over the drive level (using the 303's volume control). If you were going to do it this way you could obviously just then do it externally like a guitar pedal since it's really just workig on the final output. I spent many hours testing different circuits and different component values and stuff and this was the best case for my tastes. You could bypass the input and output discrete buffer stages (the emmiter followers based around Q1 and Q2). I have them in there becuase I found that for whatever reason I found the sound preferable to what I get when I bypassed them - I like it best with both the input and output buffers.


/home/ladyada/public_html/wiki/data/attic/x0x/vcamods.1175666877.txt.gz · Last modified: 2016/01/28 18:05 (external edit)