Craig Anderton Octave Doubling Fuzz Problem

[fourtwenty]

Hi,

I'm trying to build the ODF but am having nothing but problems.  I'm fairly new to this but managed to make a simple distortion pedal the other dy and have worked with electronics before.

Here is the schematic I used:

http://www.geocities.com/CollegePark/Library/1355/octavedoubling.gif

I had to make a few minor changes because I couldn't find the exact capacitor sizes at my local electronics store.  The changes I made are:

changed the 220pf cap for a 180pf
changed the 10pf cap for a 22pf

Do you think these would cause the effect not work?  Right now all I get out of my amp is a weird spanish radio station.

It also calls for a bipolar 9V power supply.  The first time I built it I didn't realize what this was so I just connected a single 9V battery and assumed that the ground would be picked up by the amp.  When someone explained to me what bipolar meant, I connected two 9V batteries together; the positive from 1 to the negative from the other then both to ground, and then the other positive and negative as shown on the schematic, is this right?  

My other concern is if I blew the op-amps by connecting the battery the wrong way.  I can't find any easy way to test the op-amp though.

Any help/suggestions would be greatly appreciated.

Thanks alot, and may I say that this website is really cool!

[fourtwenty]

Ok, so I think I solved the cap problem.  I just found the section that talks about putting caps in series and parrallel to get different values, so now I have 2 x 22pf in series to give me 11pf and I have the 180pf in parallel with 2 x 22pf to get 224pf, which is pretty close to the 220 I needed.  I'm still getting the weird radio station though and nothing is coming out of the guitar...

[Mark Hammer]

First, welcome.

Second, though it is, in my experience, not the best-sounding octave fuzz out there, in terms of the octave tone, this is still a very flexible unit that can produce many musically useful sounds.  Very easily modifiable too.  Note that since IC2a, IC2b, and IC1b are used in inverting mode, the Channel 2 level control will result in some cancellations at different points in its rotation, and will not simply increase octave intensity as you turn it up.  Far from a weakness, it actually provides some sounds that other units can not achieve.

Third, the capacitor changes you made have NO negative impact.  IN parallel with the 10pf unit, the 220pf cap normally provides a treble rolloff around 692hz, and changing it to 180pf simply moves that up to 837hz.  Similarly, the "always on" 10pf cap provides a default rolloff around 16khz  (way more high end than any fuzz needs or even should have), and the change to 22pf drops that down to a much more civilized 7.2khz.  So, neither of the changes you made will prevent the pedal from working.  If anything, I think these are probably improvements on the original.

Fourth, while I am grateful to Justin for drawing it out (especially with a standard dual 8-pin op-amp pinout instead of the 4136 or 4739 that Craig likely originally used), the drawing is a little confusing, and perhaps that is where you made some errors in your construction.  I am assuming you are using perfboard since I am unaware of any posted PCB layout for it.  Note that the battery power is provided to both chips, through protection diodes and decoupling caps.  The diodes should be oriented properly so that current flows only in the intended direction, and the caps should be oriented properly too.  Perhaps the best place to start your troubleshooting would be to remove the chips and simply verify that +9 and -9 volts is arriving where it should be.  

As advice for future builds, it is unfortunately all too common that schematics focus on the effects circuit itself, and tuck all the power-related information away in a corner or don't even show it (I'm guilty of that myself when drawing things up).  As a consequence, the perf-builder tends to start with the audio part of the circuit and figures they will deal with the power distribution when they get done.  It is wise to plan out your power distribution to semiconductors first, and deal with the audio part of things second.  Personally, I wish I'd follow this advice more often.  Many of the problems I have in getting perfed projects to work can be traced down to the omission of ground connections, and the headstands I have to do to get V+ from over here to over there.

It IS remotely possible you blew the chips by incorrectly connecting power.  Fortunately, these are not very expensive, and there are many possible replacements that fit in the same socket (you did use a socket, right?).

Finally, IMHO the added 220pf cap for IC2a is a wasted effort.  More importantly, the rolloff produced by the extreme treble cut it produce probably impairs the ability of IC1b to produce octaves on higher notes.  A better strategy is to simply leave your 22pf cap in place.  Instead, for changing the tone of the normal fuzz, use the Channel 1 switch to connect a .01 cap to ground.  With a .01uf cap to ground from the 10k/15k junction, the normal fuzz channel will have a rolloff around 1.6khz on top of the one starting around  7.2khz.  That will leave a bit of bite but still mellow it out enough to make a difference.  It will also round out the normal fuzz sound enough to let the octave fuzz channel sound more obvious.  Stick a cap of 47-100pf in parallel with the 220k resistor in the last stage to keep hiss and fizz down to a blessed minimum.

[fourtwenty]

Cool, thanks for the detailed response.  I checked and I'm getting -4.3V to pin 4 and +5.5V to pin 8.  Are these what they are supposed to be?  Any other suggestions on where I might look for a problem with this cct?  I don't have nearly as much knowledge about this stuff as you do?

I figure I'll get it working first and then make the mods you suggested to see how it changes the sound.

[fourtwenty]

What do you mean by socket for the op-amps?  I've wired this on a breadboard, I have no intention of soldering this all together until I get it working..

[Mark Hammer]

Cool, thanks for the detailed response.  I checked and I'm getting -4.3V to pin 4 and +5.5V to pin 8.  Are these what they are supposed to be?  Any other suggestions on where I might look for a problem with this cct?  I don't have nearly as much knowledge about this stuff as you do?

I figure I'll get it working first and then make the mods you suggested to see how it changes the sound.

What do you mean by socket for the op-amps? I've wired this on a breadboard, I have no intention of soldering this all together until I get it working..

If it's on breadboard, then I guess it's easier to undo all sources of error, right?  I thought you had built it on perfboard already.  When you DO move it to perfboard, use IC sockets, though.  VERY cheap insurance.

Those voltages you give are wrong for those pins.  If you have a pair of fresh batteries hooked up, you should be getting +9v on pin 8, and -9v on pin 4.   So, either you did something to the batteries earlier on and drained the daylights out of them, or something is shorting.

Here's an idea.  Since it is on breadboard at the moment, lets try rebuilding this a bit at a time.  First, get your power set up right.  Two 10uf caps, with their opposite polarity pins joined together at ground.  Two diodes with the stripe pointed towards the + side of the cap for V+ and AWAY from the '-' side of the other one for V-.  The red lead of the battery snap goes to the non-stripe end of the V+ diode and the black from the other one goes to the stripe end of the V- diode.  The two free ends of the battery snaps go to ground.  Do that and measure what you get at the capacitor ends of the diodes, relative to ground.  Should be just a hair lower than what you get from the batteries themselves on their own.

With your power assured, lets build the first two stages - the input buffer of IC1a and the clipping stage of IC2a - with a single dual op-amp chip.  The schematic shows the first and 3rd stages built from one chip, and the 2nd and 4th from another, but there is absolutely no requirement to do it that way.  Building just these two stages with one dual op-amp will allow you/us to simplify connecting of power (just one set of power connections) and verify that the power is good, the chip works and the audio is coming through as intended.  Fortunately the pin numbers shown are suitable for doing the both on one chip.

Once you have the first and second stages assembled (making sure to have all the electrolytic caps oriented properly and all ground connections verifiably tied together), plug your guitar in, strum, and with your meter (you have one, right?) set to measure small AC voltages (if your meter doesn't autorange, set it to something that will measure 2 volts AC and less) touch your probe to the side of the 100nf input cap *away* from the input pin just to verify you're getting signal to the chip.  It should read somewhere in the 50mv zone when you strum with your guitar volume up full.

Now touch your meter probe to pin 3, and then pin 1 to verify that signal is indeed making its way safely to the input buffer and to the other side (the pin 1 reading should not be lower than the pre-input cap reading).

So far, so good.  Now verify that the first control is working as it should by touching your meter probe to the wiper of that pot, and verifying that the measured signal level decreases as you rotate the pot while strumming.

Okay, we know stage 1 works.  Now for stage 2.  The 2.2uf cap after stage 1 blocks any stray DC voltages from the audio path.  Different op-amps contribute differing amounts of such stray DC.  Without strumming, check pin 7, both for stray AC (if its on a breadboard, there is little shielding hence some hum will leak in and show up at pin 7 even without input signal) and stray DC (set your meter to a range that can measure a couple of volts DC).  If these both check out fine, stick your meter probe on the OTHER side of that 2.2uf cap after pin 7 and measure AC as you gradually turn the overdrive control up.  You should get something in the 400mv-600mv range when it is maxed.

If the sequence of events described here matches what you see, then the first two stages are successfully built and you can simply add the second two stages on.  As mentioned earlier, you really CAN scrap the switchable tone thing on the fuzz stage (stage 2).  It doesn't add much and only complicates matters during preliminary build.

Try out this first part, and I'll check in with you later.