Help improving the RM Octavia (Simple 9V Version)

[Kevin Mitchell]

A couple nights ago I breadboarded the Roger Mayer Octavia hoping to get that unique Hendrix octave fuzz. I've chosen this one due to it's simplicity and at the time of my research I believed this would be the best choice to achieve that tone. But once everything was set up I was a little disappointed.

When I cranked it up a brutal harsh sound was present when playing. I'm all for crazy, ridiculous sounds but this sound isn't usable at all. I understand most fuzz circuits aren't practical when cranked to the max although somehow I'd like to improve the fuzz but not until I fix what's really bothering me. My main concern is the presence of the octave. I can barely hear it especially when playing on the higher notes.

After watching videos of people demoing their pedals and reading reviews/debugs based off of the same schematic I guess it's just how it should sound. I've tried switching out resistor values to bring that octave out a bit more but I'm starting to feel that it just isn't there.

What I'd REALLY like to do is mod the circuit so I can use a pot to adjust the octave's presence. Then after that's fixed up maybe I can make the fuzz a little more pleasant/suited for my taste. My understanding of how a circuit physically works is very poor so I'm hoping anyone here could help me out.

Here's the schematic I followed part for part


Should I have chosen one of the transformer octave fuzz circuits?

[Mark Hammer]

Try adding a couple of kilohms in series with the Drive pot.  My sense is that the original anticipated a less hot signal, and that what you want requires less drive.

Incidentally, I had the same issue with the transformer version, and found better sounds by adding 330R in series with the 1k pot there.

[Johan]

Apologies to any R.M fans out there,  but that one does sound like crap and would need serious rework to be usable.  I got an autographed, "rocket series" one as payment for an amp repair years ago,  thought it was broken, realised it was not an put it away.. Found it when i moved a few years ago, tried it again, thinking i probably didn't know how to use it.  Still crap...put it away. ?. Should probably sell it but it's still down in the basement. .
.. put it down and walk away,  spend your time and money building something else, it won't make you sound like Hendrix.
j

[Kevin Mitchell]

.. put it down and walk away,  spend your time and money building something else, it won't make you sound like Hendrix.

My goal is to make a good yet simple octave pedal. Walking away from a project isn't something I'll consider. At the least I can improve this circuit to be more usable or even "perfect" for what I need it to do.

That being said I will definitely not take your advice due to you receiving a unsatisfactory stompbox that you have not tampered with. Might not even be the same circuit anyways.

Try adding a couple of kilohms in series with the Drive pot.  My sense is that the original anticipated a less hot signal, and that what you want requires less drive.

Incidentally, I had the same issue with the transformer version, and found better sounds by adding 330R in series with the 1k pot there.

I will try this! I read that lowering the base resistors of Q4 brings the octave out a bit more but I found it to be the opposite. Higher resistors worked a little. But not enough to even consider.

[jdub]

Kudos on your determination! If I may make a alternative suggestion, though, the Foxx Tone Machine is a fairly simple project with an outstanding octave effect.  I highly recommend checking it out. 

[Kevin Mitchell]

Kudos on your determination! If I may make a alternative suggestion, though, the Foxx Tone Machine is a fairly simple project with an outstanding octave effect.  I highly recommend checking it out. 

Thanks I definitely will! The 1st project that caught my attention was the Tycobrahe (a transformer octave fuzz) but I'm iffy about working with transformers. I'm more comfortable with simple 9V circuits. But I might step into the deep end soon.

[Keppy]

Kudos on your determination! If I may make a alternative suggestion, though, the Foxx Tone Machine is a fairly simple project with an outstanding octave effect.  I highly recommend checking it out. 

+1. If you're still on the breadboard anyway, the Foxx is another 4 transistor octave fuzz, and in the opinion of many a much better one. There's not a pot to control the octave effect, but there's a switch to turn it on and off, and I bet you could put a pot there to make it variable.

As far as designs that already have variable octave, check out the Ampeg Scrambler. It's got a good octave, but many find the fuzz to be less usable than some others. YMMV.

[Kevin Mitchell]

Kudos on your determination! If I may make a alternative suggestion, though, the Foxx Tone Machine is a fairly simple project with an outstanding octave effect.  I highly recommend checking it out.  

+1. If you're still on the breadboard anyway, the Foxx is another 4 transistor octave fuzz, and in the opinion of many a much better one. There's not a pot to control the octave effect, but there's a switch to turn it on and off, and I bet you could put a pot there to make it variable.

This one?;


I'm pretty sure I could put a rev-pot in place of the switch to get a octave presence control if that's what you're talking about. No clue of what value to use (poor understanding of how these things work) Maybe I could throw resistors in place until the octave signal is completely gone. Or at least that's how I think it'll work.

I'll put the parts on order. Until then I'm still open to making the best of the RM Ovtavia. All positive suggestions are welcome.

[Keppy]

Since fuzzes more or less create square waves, it is very important for a clear octave to reduce treble harmonics to prevent any zero crossings that don't coincide with the fundamental frequency of the signal. This needs to be done before too much gain is applied, otherwise the problem of false zero crossings becomes insurmountable. Here are several ways:

1) Use your neck pickup and roll of your tone control. That goes for every octave fuzz design.

2) Enlarge the .001uF cap off the 220R resistor from Q1 collector. That cap along with the 100k cuts some treble above 1.5k, and enlarging the cap moves the frequency down. You might also get a little more filtering if you move the Q2 base connection to the 220R/.001uF junction.

3) Add a lowpass filter between Q1 & Q2. 10k resistor connecting the two, .01uF cap from Q2 base to ground. Tinker with the values as needed. I think this will be your biggest improvement.

4) Enlarge the .1uF cap on Q2 collector to increase bass gain. You won't get much improvement here, since the current filter cutoff is 40 Hz, but there might be a very small improvement to your lowest notes.

5) Enlarge the .01uF coupling cap between Q2 & Q3. Current cutoff is around 80Hz, so again you will only get so much improvement from here.

6) Match your diodes as closely as possible for forward voltage drop. That's nothing to do with treble but will generally help a little.

7) Enlarge the input cap to .1uF or so. It probably won't help. It definitely won't hurt.


Part of the reason the Foxx has a stronger octave is that the octave effect is done at Q2, before a ton of gain is applied, and there is filtering between Q1 & Q2. The RM Octavia, by contrast, puts all of the gain before the octave effect is applied, and also uses less filtering. This means that treble harmonics have more chance to mess up the operation of the octave.

[Quackzed]

an easy thing to try is to stick a booster or overdrive/distortion in front of it, try to slam it into octaveing by cranking up the volume into the circuit, or conversely, turning down the boosting into it and see if theres a gain range where the octave is better...
you can also pre cut some treble with the overdriver,  if you can get rid of some harshness pre-octavia it helps the octave from getting nasty with higher harmonics...
looking at it, the 2 diodes with the 22uf caps are basically each letting through the top peaks of their individual signals, one of the signals is 'out of phase' with the other, so the tallest peaks get through, also the flipped signals tallest peaks get through. so instead of peak,trough,peak,trough, its peak,peak,peak,peak and theres the octave (twice as many peaks so twice the frequency)... they are matched by being on either side of the same transistor, so as long as the values and connections there are ok, it should octave for ya...
the post above is all good advice +1

[jdub]

Yep, +1 to everything Keppy said...personally, on the neck pickup w/ tone rolled off, my stock Foxx has nice octave from around the 5th fret on up.  By the time I get above the 12th fret, that sucker can cause damage