univox superfuzz expander/ balance functions

[hellwood]

yeah! i just made one of these using the unicord schematics and by the way its a mind-blowing circuit to say the least (its actually the coolest pedal ive ever heard), but since ive never heard a real one, im not sure how the expander and balance pot is supposed to affect the tone. they seem to be functioning as volume pots of some sort and that is leading me believe something isnt right. any suggestions or smart-ass remarks would be much appreciated! thanks!

[Dan N]

Congrads on your build!

Balance = Volume
Expander = Intensity

Check the schematic. You can see how the Expander would also decrease the volume.

Sorry, can't come up with any smart ass comments at the moment...

[Mark Hammer]

A lot of us are used to seeing the word "balance" with reference to the blend of two signal sources, such as the balance between left and right channels on a stereo, or balance between wet and dry on a delay.  Keeping in mind that the Superfuzz dates from an era when there weren't too many effects around, "balance" refers to the volume balance between bypass and effect in this particular instance.

[alex frias]

Exactly!
They was pioneers, now we get standards. The first time I saw the pedal it made think a bit. But after that it was a breeze to use it.
One company here in Brazil made a copy and I bought one. The good thing is it uses a footswitch for bypass and another one for selecting one of the 2 FUZZ sounds.
It sounds really good, a pain to take it out of my pedalboard. But more than 4 fuzzboxes are too much for anyone.
Or not?

[Sir H C]

For extra boost and insanity, put a 10uF or so cap across the emitter resistor on the last transistor in the circuit.  You get a big gain/distortion boost from this.

[Mark Hammer]

The Superfuzz uses a switch to select between two tones, one of them is unfiltered and the other is a heavily mid-scooped tone.  The mid-scoop bleeds a lot of signal to ground in order to achieve that tone.  Consequently, the unscooped version is attenuated (via the 47k/10k network) so as to provide roughly equal volume levels when selecting between the two tones.

As I've discussed in the past (and apparently as some commercially products have happily exploited before it ever dawned on me), if you insert a resistance between the .1uf cap and ground, you limit how much signal is bled off, and can reduce the depth of the scoop.  I've done this on my Shin-Ei FY-2 fuzzes and it works like a charm.  The problem is that as you change the depth of the scoop, you change the volume as well, such that changing one control (scoop) setting forces you to change another (volume).  Ideally, you want the tonal change to be independent of level. 

One way to accomplish this, so as to be able to get tones other than the just the stock two Superfuzz tones, is to simply vary the component values in the mid-scoop filter and get different sorts of tones.  The stock Superfuzz has a 22k and 10k resistor pair in series in the filter, and the .1uf cap to ground is tapped from their junction.  There is no reason why the two resistors couldn't be replaced with a 12k resistor, 10k pot, and 10k resistor in series (in that order), with the pot wiper serving as tie point for the .1uf cap.  This would let you move around the position of the notch a bit to change the flavour.  That, in tandem with a toggle to select between the .001uf bypass cap in the filter, and one or two other possible values, would net you a number of other tonal flavours.

A second approach is to use a variable midscoop depth pot that simultaneously adjusts signal level such that you more or less get constant level as you change the tone.  Note that as you insert more resistance between the .1uf cap and ground you get less scoop and more volume.  If that resistance was provided by a wiper-to-outside-leg of a pot, then the other wiper-to-outside-leg portion would necessarily reduce its resistance at the same time. That reduction in resistance can be used to attenuate the signal to make up for the evel boost that comes from "lifting" the scoop.

The big question is "where and how".  I've found that anything more than about 10-12k of resistance to ground after the .1uf cap results in little change in tone.  Consequently, something like a 25k pot presents a useful starting point.  Imagine there was a 4k7 fixed resistor in series between the diode pair and the midscoop filter.  That resistor goes to the junction of the .001uf cap and 22k resistor.  At the same time, there is a fixed resistor (of an as yet undetermined value) going from THAT junction in the direction of ground.  To GET to ground, it has to go through the other leg of that midscoop pot.  The 4k7 resistor acts like one leg of a voltage divider, and series combination of the undetermined resistor and pot leg act like the other leg of our voltage divider.  As that pot resistance gets smaller, the voltage divider attenuates the signal.  At the same time as the signal is being attenuated, of course, the resistance to ground for themidscoop filter is being increased, which necessarily results in a "throatier" tone with greater volume.  Since the signal seen by the filter is actually being attenuated a bit, this should (in principle) maintain constant signal level as tone is altered.

What is to be determined is:
a) Whether any other fixed resistors need to be installed in parallel with one or both of the two wiper-to-outside pot legs so as to get the ideal taper and amount of attenuation.  My guess is that a 15k fixed resistor in parallel with the .1uf side of the scoop pot (between the wiper/ground connection and the cap) wouldn't hurt.
b) What the mystery resistor's value ought to be (though I imagine something between 2k2 and 8k2 is probably in the ballpark).

[hellwood]

thanks Mark Hammer for the in-depth info. i wish i knew all of this before i ditched the breadboard and committed it to the PC board, but it sounded so insane i was freaking to have it finished in time for a gig. oh well, i guess that gives me something to look forward to in the near future and thanks again!

[Mark Hammer]

Absolutely no reason why you can't deftly cut some traces on the board, drill a couple of holes and retrofit it with an adjustable scoop.  Alternatively, remove the .1uf cap, solder it directly to a pot lug, and run the wire leads from the former capacitor pads on the board to the cap/pot combo.  No need to feel committed to a configuration just because it's on a board.

[mountainking]

You might want to put a small value(1.5k?) resistor between the expander pot and ground so when you turn the expander all the way down it won't completely cut the volume.

[slacker]

If you get rid the notch switch and use the first variable scoop depth mod that Mark mentioned you should be able to  do it simply by rearranging a few components on the layout. No need to cut any tracks or make extra holes in the case the depth knob can go where the notch switch was.
I did this on mine and it works great, gives you the 2 original sounds plus a nice range in between, only problem is the change in volume between scooped and unscooped. That's not really a big deal though unless you're changing sounds all the time. I might have to try the more complicated version.