Analog Octavers (OC-2/MXR Blue Box)

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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redbagy
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Post by redbagy »

I built both the OC-2 and MXR Blue Box. Both seem to use a similar way of producing the lower octaves (D Flip Flop using the CD4013). However I noticed that on guitar especially, the OC-2 has a much cleaner octave and has less glitches than the MXR Blue Box. I think that it has something to do with the input circuitry which 'prepares' the signal for the CD4013. For example on the MXR Blue Box I noticed the glitches decrease with the tone rolled off.

Thoughts?

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Post by Nocentelli »

(this thread probably needs moving to a different section)

I think the blue box schematic is quite simple / crude way of getting a sub-octave and is prone to the octave leaping up and down. The boss OC-2, DOD octoplus, Pearl OC-07 and others of that era have an extra envelope follower section ahead of the 4013 that tracks the dry guitar peaks to trigger the sub-octave
vanessa wrote: 20 Aug 2007, 17:51 You can't arrest someone for reverse engineering something.

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Post by plush »

redbagy wrote: 19 Jul 2021, 20:41 I think that it has something to do with the input circuitry which 'prepares' the signal for the CD4013. For example on the MXR Blue Box I noticed the glitches decrease with the tone rolled off.

Thoughts?
Yep, OC-2 has more precise peak detector circuitry and overall is way better engineered.

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Post by redbagy »

I agree that the OC-2 is better engineered and I personally prefer it's less predictable glitch unlike the MXR Blue Box (but this is just my opinion, it depends on what you're playing etc.).

Thank you for the heads up re the peak detector circuitry prior to the 4013. In the case of the Blue Box, the peak detector circuitry prior to the 4013 is the Schmitt Trigger right? I was thinking of analyzing this stage better and try to improve it if possible?

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Post by Nocentelli »

redbagy wrote: 20 Jul 2021, 05:23 In the case of the Blue Box, the peak detector circuitry prior to the 4013 is the Schmitt Trigger right? I was thinking of analyzing this stage better and try to improve it if possible?
"peak detector" in the blue box is a pair of diodes and an electrolytic cap:

https://www.diystompboxes.com/smfforum/ ... c=105991.0
R.G. wrote:"The first opamp both amplifies and filters the signal to produce a much larger signal with some of the high frequency grak removes. The second opamp is a Schmitt trigger comparator - it has some hysteresis to produce a snap action comparison. This is where the square waves are produced.

The amplified, filtered input is passed through C7,D3,D2,R15,C8 which makes a DC (ish) level out of it. The DC level is the "envelope" of the input signal, and this is chopped by the action of Q2 and Q3 into the output signal. The square waves at the output of the Schmitt drives Q1, which provides both the input to the non-divided signal at Q2 and the input to the divider network in IC2. IC2 divides it by 4, then feeds Q3 for the divided-down bass signal. R20 feeds a mix of 1x frequency from Q2 and 1/4 F frequency from Q3 to the output."
vanessa wrote: 20 Aug 2007, 17:51 You can't arrest someone for reverse engineering something.

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Post by redbagy »

"peak detector" in the blue box is a pair of diodes and an electrolytic cap:
I agree with this, however that peak detector sets the output level for both the fuzz and octave right? To have a difference in output volume when playing "soft" vs "hard"?

I think that the glitchyness in the sub-octave comes from the schmitt trigger no? Since it is the one driving the CD4013. I am trying to pin point the source of the glitchyness.

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Post by stolen »

Hi!

We love subharmonizers! Yes, the glitchyness comes from the schmitt trigger. In essence, the OC-2 and other clean down octavers (most of them are more or less the same topologically) use what could be described as an adaptive schmitt trigger: The state changes when the signal reaches say 90% of its peak value. This means low amplitude overtones are rather effectively cancelled. The blue box uses a fixed threshold, there is an amplitude where it does just the same, and an external hard limiter can keep the signal just there.

Modern octavers generally have some sort of high (~3) order filter in front of the tracker circuit to filter out overtones, but these contribute less to stable tracking than the adaptive threshold, and in fact going too wild with these might make the tracker unresponsive to high notes, so the filters should preferably inactive above ~1.5kHz. However, depending on your application (for example, bass in standard tuning only) you might find that modding the filter can give you even more stable tracking.

Another aspect that is often not considered is that clean octavers typically achieve their dynamics via ring modulation, and this affects how glitches are perceived! The flip flop has of course constant output volume, and while restricting the amplitude to a dynamic value is quite possible, most clean octavers typically modulate the flip flop's output with the input signal, sometimes with a simple switchable inverter, sometimes with a sample and hold (which is not really ring modulation in a purist sense, but does almost the same with a slightly different timbre). This modulation result is then heavily filtered. Let's look at the resulting differences:

Let's say we feed a 100Hz signal at the input, and the tracker randomly decides to latch on the 3rd harmonic, resulting in the flip flop's output to be 150Hz. With a blue box-style "direct" octaver the output is just that, however with a ring modulator it's a bit more complicated. We'll ignore the overtones of the square wave and the input signal for simplicity's sake so that the output of the ringmod contains (150-100)Hz = 50Hz as well as (150+100)Hz = 250Hz. Even though the tracker locks on the wrong overtone, the lower octave is still there, just with some enhanced overtone on top! Admittedly we chose an odd harmonic on purpose, and considering the full spectrum of input and flip flop square makes things look a little more chaotic, but the principle is that by their very nature ring modulator type suboctaves have an inert "stability" against tracking errors that pure synth types do not have. If this is not clear we could illustrate it in more detail and add some graphs, however we're just typing this as we're getting ready to get to work so it'll have to wait until later :D.

All the best,
stolen

PS: Can we just say how we really enjoy how this thread doesn't imply that more stable octavers are better? We've had this a lot in the past, especially with guitarists who expect them to replace their bass player, and we for one really enjoy being able to throw off a just-so-kinda-sorta stable tracker with a little bit of dirty playing.

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Post by Nocentelli »

stolen wrote: 20 Jul 2021, 10:22 Can we just say how we really enjoy how this thread doesn't imply that more stable octavers are better?
vanessa wrote: 20 Aug 2007, 17:51 You can't arrest someone for reverse engineering something.

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Post by redbagy »

stolen wrote: 20 Jul 2021, 10:22 Hi!

We love subharmonizers! Yes, the glitchyness comes from the schmitt trigger. In essence, the OC-2 and other clean down octavers (most of them are more or less the same topologically) use what could be described as an adaptive schmitt trigger: The state changes when the signal reaches say 90% of its peak value. This means low amplitude overtones are rather effectively cancelled. The blue box uses a fixed threshold, there is an amplitude where it does just the same, and an external hard limiter can keep the signal just there.
Thank you for this! And I also agree with the reply following yours, glitchy sub-octaves can sound cool :D

Now, in my case I would like to reduce the glitchyness a little in the Blue Box or at least try to as an experiment. Would it be possible to implement an adaptive schmitt trigger? Maybe I can take this section out of the OC-2 and implement it instead of the current schmitt trigger. I think that filtering the input of the schmitt trigger to keep the fundamentals only will also improve the tracking greatly. Thoughts?

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Post by stolen »

Hey! Absolutely! There's basically two approaches:

A) Get two envelope followers running, one for minimum and one for maximum, and route them to be threshold references for the schmitt trigger. The boss-style circuits do this by feeding each (slightly attenuated) envelope and the input signal into their own respective comparator, and then tying those to the set/reset of a flip flop. In essence this is a schmitt trigger. Of course, only using one envelope and resetting on the zero crossing is an option too and still is a significant improvement from pure zero crossing. Instead of the dual comparator + flip flop scheme other adaptive schmitt trigger topologies are possible too, such as using an electronic switch to route each envelope selectively to a standard comparator, which allows for a slightly smaller circuit (unless you need flip flops anyways), but needs a bit of design care to make the transient states behave. Either way, this approach takes some design effort.

B) Normalize the signal before and send it to a regular schmitt trigger by using compression/limiting. This is uncommon in commercial devices; while it can reduce parts count significantly it often costs more and/or needs calib. The disadvantage here is that both sides are treated equally and don't have their own threshold each, but that is not too much of an issue. Ideally you'd want a hard limiter with a peak (as in, not RMS) envelope and drive it hard; however, most sufficiently high-gain compressors should do the trick nicely. You can go for the simplest designs you can find there. The threshold of the schmitt trigger should be slightly below the limiting peak, but in practice your ratio will be finite so a little bit of wiggling room is recommended. Optical and fet designs can be very simple and would do a good job here, however due to their slightly unpredictable components the threshold of the trigger should definitely be trimmable.

Since the parts count for B) may be much lower, we'd recommend that route if space is a concern; also frankly we're a bit bored by the homogeneity of common designs.

Any filters should precede that entire circuitry.

For simple compressors you could check out these mictester designs for starters; you would have to adapt them a little bit to have sufficient clean gain, but they do a good job at demonstrating the complexity.

viewtopic.php?t=8581
viewtopic.php?p=175187

All the best,
stolen

PS: Another theoretically neat version of B) would be a 571 design in the ~forbidden~ configuration (compressor, but with the detector hooked to the input, and maybe two in series to get the desired gain), but since companders have become super expensive and are mostly available from a company we don't wish to support we cannot recommend it here. Although the second half could also be used to apply input dynamics... How do we convince sound semi to reissue them in THT?

Edit, PPS: Generally instead of spending lots of parts on one very nice high gain compressor it might be more efficient to use two of them in series if you find that a low parts count single unit system doesn't quite cut it.

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Post by redbagy »

Thank you. I've got the circuit in LTSpice and plan on tinkering around with the Schmitt Trigger stage (filtering before, and changing the thresholds) and see what I can achieve.

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