Modding a Dr Quack for bass

[Mark Hammer]

I started writing this before lunch, and when I got back I guess it had timed out...BAH!!

Okay, V2.0

I'm putting together some pedals in exchange for a Leslie a fellow gave me.  He has a small (all-analog) recording studio, but I know he plays bass so I figured I'd make at least one of the pedals bass-oriented.  I had an unused Dr Quack sitting around, semi-installed, so I figured I'd adapt it for bass.

First thing I did was to drop the sweep range down an octave by increasing the value of the filter capacitors from .0047 to .01uf.  I may increase them a little more, but at the moment, it provides a nice compromise that straddles bass and guitar.

Since the filter setting that sweeps way too high is useless for bass, I hard-wired it to the "bass" mode, and saved myself some grief and panel space.  A couple of jumpers on the board and you're done.

I installed a 1k attack pot that has a SPST switch on the back, so what I did was wire it up so that clockwise rotation decreases attack time (i.e., it's a "faster" control).  When rotated fully counter-clockwise, it sweeps slower, and when you click it "off" you kick in another cap in parallel with the existing one for an even slower decay.

One of the difficulties with swept bandpass filters is that they lose bottom as they sweep upwards.  One solution is to have several staggered bandpass filters for a fuller sound, as in the E-H Baseballs and Anderton Bi-Filter Follower, however that is pretty close to a redesign, and since I already had a populated GEOFEX-layout board, I wasn't about to do anything quite that major.  A second solution is to mix in some of the dry signal with the filter so that there is always some bottom present.  This is what is used in the Lovetone Meatball and some other higher-end swept filters.  This is essentially a blend panpot, and it is what I went for.  

I use a passive mixing arrangement, similar to what you find on the Small Stone, Univibe, and Zombie Chorus.  In those arrangements two signals are fed through resistors to a common junction at the output.  The relative balance of the two signals depends on the value of the resistors.  If you make one resistance larger than the other, the signal with the lesser resistance will dominate the other.

The GEOFEX board has a .047uf cap and 470k resistor-to-ground tied to the pad for the output lead.  To do the mod, we want to stick a variable resistance between the cap and 470k resistor, so what I did was just remove the 470k from the board.  The output lead now goes to a pot.

The wiper lug of the pot (500k) is now going to serve as a substitute output pad, so the 470k resistor from the board is resoldered to the wiper lug and from there to the pot housing (assuming it is grounded to the pedal chassis).  The lead from the board goes to one outside lug of the pot (linear or log), so that you now have up to 500k resistance in between the .047 output cap and the terminating resistor.  As you increase this resistance, you attenuate the signal from the filter.

To get the dry signal I simply ran a 100k resistor from the input lug of the sensitivity control to the other outside lug of the added mix/blend pot.  This lug is connected to the 10uf cap between the input buffer and filter so it is a perfect spot to tap things from.  The 100k + pot means that you can have up to 600k of resistance impeding the dry signal, resulting in considerable attenuation, relative to the filter signal, when combined with the 470k terminating resistor.

Unfortunately, the filter is inverting, which means that at some settings, what comes out of the filter and what comes from the dry feed cancel each other out, lowering the overall output level.  But that is not true of all blend settings, and the retention of the bass end as the filter sweeps ought to help that out a bit.  In an ideal world, there would be an inverting unity-gain stage either after the filter or for the dry signal so that the two signals were truly additive.  On the other hand, this mod requires no changes to the existing board layout, merely a repositioning of components, so it's nice that way.

The accessing of the dry signal via the sensitivity pot also meant that I could simply place the sensitivity and blend control adjacent to each other on the chassis and run the resistor between the two outside lugs.  If you locate the blend pot to the right of the sensitivity pot that will make matters easier.

All in all, it sounds nice and delivers a broader range of tones.  With the filter mixed in the background (and it never quite disappears), and a slow attack and decay, you get a nice wobbly animation to the sound, nicely suited to bass.  What Zappa used to call "pillowy".  I haven't tried it out with bass yet, but it gets most of the standard DQ sounds anyways so whatever it lacks in perfection for bass, it more than makes up for by at least trying to do right by the instrument.

A worthwhile mod.  I'm pleased.