Neoctavia Build Report

[petemoore]

Tungngruv mentioned this circuit, I had to build it, I like Octaves.
 We have designers of Octave effects to thank for them, in this case A. Coleman. A good octave [unlike distortion which is very easily 'stumbled upon'] isn't something I'd have stumbled upon like finding distortion.
 Circuit sounds great, a very nice Octave sound.
 mod and a hacks...I found for my tests using SC Pickups, Adding a parallel input cap equal to the shown value worked well [maybe I'll put a sw on one of them], I used 15k divider resistors, and a 10uf output cap in socket because of no 22uf's here.
 Also I found it too harsh with the guitar all the way up, an input gain control would take care of that.

[Aharon]

Hey petermoore,at last someone besides me built the little critter,hahahahaha.
If you take away the diodes and transformer it makes a nice clean full range booster too.

Try it with another pedal,maybe a fuzz.

I have a couple of new designs that I have to proto but I have to get my face off tube amps for long enough......maybe this summer...........
Aharon.

[petemoore]

Well...I just had a nice long session with the Neoctavia after a Voodoo OD...[tried it the other way around first].
 The octave is very pronounced, well defined, throaty. Solid third note sustains well and can be 'bounced' around easily/alot as a result.
 It performs very well and I can get a nice 'nondialed' transition from OD to OD+Octave by just adding OD.
 I have and have had many Octaves, this makes a very nice Octave.
 I was setting the guitar volume [SC Pickups] at around 8 for the most part.

[jmusser]

Well, as you know, I'm always ready for another good up octave, even though I've already built several. I had never really looked at that circuit to see what it was. I still have the Tycho to build one of these days too. Next on the agenda is Scott Shwartz's Octave Screamer. I have had that transformer for a month of Sundays, and it's time to finally put it in a circuit.

[petemoore]

The Octave Screamer is a TS type circuit driving a transformer and diodes to make octave. I tried it, and well...
 the last part of the circuit, transformer and diodes, can be put on the end of a Fuzz Circuit...pick your Fuzz Circuit...so if you have a TS, you could probably just tag that part on the end...i prefer the 'add on' type octaves that work well with a Fuzz after or before them... generally I like to go from Fuzz to Octave...so leaving the Fuzz on and only having to activate one switch to get [a 'dialed in'] octave is what I'd be toting or cabling up.
 Here'es the octaves I have..
 Octave up Sick Box [Nice One]
 Tychobrahe Octavia [Octa-ringmod] does neat / foul  trix
 Univox SuperFuzz [nasty Octave and generally heavy OctaFuzz]
 Fox Tone Machine [Hard Edged pronounced Octave over wide range, I don't prefer the tone of it...opinions vary]
 Green Ringer [very cool for a no knobber]
 Bobtavia   Very Cool sounding ]
 Neoctavia ...  ... Excellent Octave effect.
 They all work great...just a matter of taste !!!

[jmusser]

So far, I've built the Bobtavia, Sick Box, Simple Octave Up, Octup Blender, Digital Octaver Fuzz, Psychtar, Pushme Pullyou, Octup and Titan Octave. The SOU is the kind screamer fuzz, with the Bobtavia running a close second. Octup Blender is very fuzzy all over the fretboard, with down octave on lower strings, and up octave on higher strings. Digital Octaver Fuzz, Frighteningly huge Octave up fuzz, with lots of growl. Psychtar, the nicest and most intense clean octave up I've made. Push me Pull you, fair octave box. Octup, no octave, just over drive, as built. Titan, nice boost and fair up octave. I would guess that would be the category that the Octave Screamer fits into, also. Oh, I forgot the Sick Box. Has a lot of weird background stuff going on for character, and a clucky sound on each pick attack. It's still not as extreme as my SOU. I believe it's tone heads in the direction of the Tycho, but it's its own sound.

[Aharon]

Hey peter,one of the things you (or anyone else) can try is the following,I personally haven't had time to do it but it was my intention from the beginning.
Try to split the signal from before the transformer/diodes and somehow set up a "blend" pot of some sort to have boost/octave dialed in individually.
That's such a versatile little circuit.I have more mods in mind but let's do it in steps.
I wish I could breadboard it now but I can't,instead I pass along the mods and maybe somebody else can.
Aharon

[onboard]

I've been futzing with the neoctavia quite a bit latey. Try changing the first opamp to a buffer, and use a 1M in the feedback loop of the second. This seems to bring out the octave over a much wider area of the fretboard.

Since the octave is more pronounced with the guitar tone control rolled all the way back, try a .047uf cap (or your favorite flavor) to ground at the input. Not much need for a variable control here due to the circuit. Switchable would be flexible enough.

When you're tapping the output before the transformer, an Si diode to ground added at the final output smoothes things out. A .001uf cap to ground can help too.

Blending the tapped opamp output with the transformer output keeps the resonant quality in the tone -  lifting the connection to the transformer after the 100R resistor when using the tapped fuzz lets twice as much bass through.

One last thing I tinkered with so far -  a 100k or so series resistor before the output. This can increase the resonant quality of the tone. But how would it affect the output impedance? Or is that still set by the 100k level pot?  

What a great circuit! Excellent octave. Although my ears are killing me after demoing it while tinkering :roll:

[Aharon]

Wow onboard,great post,lots of mod info.
Aharon

[jmusser]

On Board, How do I change the first op amp to a buffer? I'm embarrased to say this, but I don't know what a buffer does! :oops: I know I've heard that term a hundred times. I figured that "buffering" meant some sort of transition from one section of a circuit to the next to try to prevent noise, or to maintain a certain level of amplification from one section to the other. I may be way off though. The feedback loop I believe is the 220pf and 620K resistor from the output to the input of the second op amp, correct? So, the 1 meg would replace the 620K? Would the .047 input cap be switchable, so that it would take the place of the .1 coming in, or would it be paralleled with the 1meg resistor to ground? The Si diode would be oriented, with the catode to ground after the 100 ohm resistor? Did your build include the 250K pot in the firstt feedback loop? I believe the higher that value (to a point) gives you more gain, right? Lastly, would there be any improvement in the octave up effect, by using a bridge diode set up on the output, instead of the full wave? I know in power supplies, if you use a bridge, it will double the amount of "humps" that are on top of the reference. I would expect that to enhance the octave, but I don't know. It is still hard for me to see an audio signal as AC :? In power supplies, you're only half the way there with a bridge rectifier, because you still have the ripple to filter into as straight of a line as you can (DC). In audio, you've made an up octave, but in power supplies without filtering, you've just made a mess! I'm am a card carrying up octave whore, so I'm always on the look out for the next screamer. Modded or not, it'll have to be intense, to out do the SOU.

[onboard]

Sorry I didn't back to this sooner - from time to time I'm reminded there are other things going on besides pedal building and browsing this forum :roll:

On Board, How do I change the first op amp to a buffer? I'm embarrased to say this, but I don't know what a buffer does!

Don't be embarassed - someone else please jump on this if I get something wrong. I guess the first opamp already is a buffer, so my wording was wrong. A buffer is a stage that provides high input imedance, low output impedance, and a gain of 1, or unity gain.

I did a forum search for ocatve circuits and read virtually eveything. One thing that came up was the fact that your signal gets flattened out very quickly from having been squared by the gain stages and then folded over by the diodes. That's part of why there's not much appreciable sustain, which I thought might be changed (I think) by tweaking overall gain.

As it stands, the first opamp is an inverting unity gain stage. I thought to try a non-inverting unity gain stage to see if that would make any difference, though now I don't know why it would. There *seemed* to be octave over a wider range of the fretboard. Tie the ouptut to the negative input, and supply the positive input with the signal through a coupling cap and Vref through a 1M resistor.

The feedback loop I believe is the 220pf and 620K resistor from the output to the input of the second op amp, correct? So, the 1 meg would replace the 620K?

Right. I went as low as 220k and still had distortion with octave, although the overall response of the circuit changes. Using 1M would probably negate the "less gain" approach now that I think about it.    

Would the .047 input cap be switchable, so that it would take the place of the .1 coming in, or would it be paralleled with the 1meg resistor to ground?

I was using that like a guitar tone control rolled all the way back - so cap to ground at the input, parallel to the 1M pulldown resistor. It could be hardwired, hard switchable, or in series with a 250k or 500k pot wired as a variable resistor to act exactly the guitar's tone control.

My thinking here was to have the octave switchable, and eliminate the need for keeping track of "Ok, I switched the octave on, roll the guitar tone back. Ok, switched off the octave, roll the guitar tone forward". This tidbit about getting more octave with the highs rolled off comes straight from Roger Mayer.

The Si diode would be oriented, with the catode to ground after the 100 ohm resistor?

I tried a 1n4148 (cathode to ground) immediately before the 100k output pot just tinkering to see what might do what. It's hard to hear what's going on with respect to the added clipping, but I thought there was a slightly longer note decay. Adding it after the second opamp would be cool to try. It's weird because I though a shunt Si diode should be buzzy, but at the end of this circuit it does the opposite and smoothes off the edge.

Did your build include the 250K pot in the firstt feedback loop? I believe the higher that value (to a point) gives you more gain, right?

Right, the higher the value of the feedback resistor (Rfb), the more gain. Assuming the input resistor (Rin) is a lower value since inverting gain = Rfb/Rin. I left out the pot in the first opamp's feedback loop, since I was trying a non-inverting buffer.

Lastly, would there be any improvement in the octave up effect, by using a bridge diode set up on the output, instead of the full wave?

That I don't know - I'm still learning about half/full wave rectification. If you take a look at John Hollis' Titan Boost he includes a Titan Octave mod that involves adding a transformer and diode bridge to the tail end.

It is still hard for me to see an audio signal as AC

Me too. I have a vague intuitve view of it as far as electric string instruments are concerned - the strings cutting the coil's electromagnetic field do so in an altrnating manner, therfore the current produced must also be alternating. That might be the completely wrong way to look at it :? I suppose there's alot more to it than that when you consider all of audio electronics in general.

I'm am a card carrying up octave whore, so I'm always on the look out for the next screamer. Modded or not, it'll have to be intense, to out do the SOU.

Wellsir, havn't tried the SOU, so I don't know. The Neoctavia is definately capable of some very high pitched screaming. I'm curious to hear the difference since the neo and the SOU are so similar.

Sorry for the long post, but I wanted to cover everything and try to make sense.

[jmusser]

Hey Onboard, Thanks for the in depth response! If I get one of those round-to-its, I'll pull the SOU apart, and add a couple more diodes like the John Hollis Boost/Octave, just to see if there is a marked change in the octave effect. That's where I got the idea from in the first place, because I just built it last week sometime. I'll make it switchable while I'm messing with it, to see if there is a marked difference between the two. I have an idea that with the rectification only being driven by a boost, that John may have had to go to a bridge rectifier instead of the standard full wave to get the octave to be as pronounced as it is. Like I said on the Boost/Octave review, the up octave is not horribly pronounced, but, I believe it really fits well with the boost. You get boosted octave, instead of fuzzed octave, so it's a smooth transition when I flip the switch from one to the other. It gives a general warming to the tone all over the finger board, and of course, your octave really starts showing up somewhere around the 10th fret and up. I believe it's there somewhat for the "warming" (trebling) effect of the lower strings too, but, not as intense. I would have to go back and read the post again, but I believe that this is the root of the second octave up tone of the ZVex, "Johnny Octave", going from full to bridge rectification.

[petemoore]

Yupp this one's really good one.
 I let more bass in, and rolled treble off a bit at the output, so it's voiced a bit 'lower'...this is working out very well for me, takes the harsh edge off the top end and fills out the tone for my subjective tests.

[jmusser]

Pete, just wondering if you've breadboarded or perboarded this beast? I thought if you had only breadboarded it, I would impose on you, to go ahead and try bridge rectification on it for grins, to see if there was much of a difference. I've got mine crammed into a light fixture box, and kind of hate to pull that mess out of there! I'm glad you like it. How is it stacking up to your other octave boxes?

[petemoore]

Stacks up tall and of course differently than my others.
 It does a realy PH, which is my 1rst test of Octave, second is "Machine Gun" and it works very well for that.
 I like the tone of it, some of the others seem to get more of the thicker ring mod tones with rollback of input, like the Tycho.
 I have it perfed and made a nice little 2 piece steel box for it.

[jmusser]

Great! I have been talking it up a lot, and it's always good to know that what I'm saying isn't all hot air (I know it's about a 95% to 5% ratio though . Hopefully I'll get a chance to do the bridge mod this weekend. It's just one of those things I want to know, and there's only one sure way.