MXR M288 Bass Octave Deluxe [gut shots, tracing]

[Oloorin]

Hello,

I had recently a possibility to take some board pictures of MXR Bass Octave Deluxe.
I also tried to trace as many path as I could.

All photos are here:
https://drive.google.com/open?id=1AE4iM ... sgfFLAAIxi

ICs in the pedal are:
HEF4013BT - flip flop
MC33179DG - quad opamp
MC33178 - dual opamp
MC33174 - quad opamp
TL064C - quad opamp
3578 AM - switching regulator

If anyone could verify the paths I traced or help drawing a schematic, that would be great.

[george giblet]

I also tried to trace as many path as I could.

You have done pretty good here. I traced over a few parts of the circuit by eye and there's some places where some tracks are missing because I couldn't follow where the connection went.

It would probably be useful to also post a pic the top and bottom of the PCB without the blue traces.

Not unexpected, it does has some parts of the circuit common with the Boss OC-2 (and all the copies).

U3 HEF4013BT - flip flop
U5 MC33179DG - quad opamp
U4 MC33178 - dual opamp
U6 MC33174 - quad opamp
U2 TL064C - quad opamp
U1 3578 AM - switching regulator

[Oloorin]

It would probably be useful to also post a pic the top and bottom of the PCB without the blue traces.

Good idea, uploaded to drive

it does has some parts of the circuit common with the Boss OC-2

So I have retraced the envelope follower and flip flop frequency divider of BOD. It's identical to OC-2, with slight different component values. And it does not have the 4027 IC which divides frequency for OCT 2.

[george giblet]

Good idea, uploaded to drive

Thanks!

So I have retraced the envelope follower and flip flop frequency divider of BOD. It's identical to OC-2, with slight different component values. And it does not have the 4027 IC which divides frequency for OCT 2.

Great. I could see that yesterday but I lost the exact connections between the rectifier and the 4013.

Don't worry about the lack of 4027. The MXR Octave works differently. It only produce one octave down. The Girth control is like the Boss OCT1. The difference is the Growl. I'm not 100% sure what it does. From what I can see the Growl signal path has a different filter with a higher low-pass cut-off frequency. I don't know if they just take signal from the phase inverter (the circuit with the JFET) and pass it through a different filter, or, if it is derived from a square-wave and used some sort of envelope shaping. My gut feeling is it's just another filter but I have a small amount of doubt as the Growl signal has virtually no even harmonics.

[george giblet]

I forgot to mention I eyeballed the phase inverter and the low pass filter yesterday and it was very similar to the OC2, same resistor values.

[george giblet]

OK, here's the phase inverter and the Girth Filter.

I've also posted a plot of Growl spectrum vs Girth Spectrum. These were from a similar but not identical input signal so they are not 100% correct. From the spectrum I extracted the peaks. Then from that I extracted a rough estimate of the Growl filter on it's own (magenta). I also plotted the difference between the Girth and Growl. You can see Growl is like a treble boosted Girth *but* the spectrum isn't the same since Growl has some harmonics missing.

[george giblet]

So it looks like the Growl circuit is similar to the Boss OC2 and there is in fact a similar but different circuit for the Girth.

I haven't done the input socket through to the circuit input as many traces disappear.

The Mid+ LED looks a bit odd. Not sure if it lights when the effect is off.

Notice the JFETs on the phase-inverters are P-channel and the pull-up resistor R33 goes to +V. The Boss unit uses N-channel and the resistor goes to 0V.

[Oloorin]

Great work! It looks like both girth and growl have to differences: different low pass filters and probably a different wave shape.
As I red here https://championleccy.files.wordpress.com/2018/01/stompboxology-freq-div-sub-synth.pdf, due to absence of diode before U5, the wave won’t have the same DC offset.

[george giblet]

It looks like both girth and growl have to differences: different low pass filters and probably a different wave shape.
As I red here https://championleccy.files.wordpress.c ... -synth.pdf, due to absence of diode before U5, the wave won’t have the same DC offset.

That's a good article. You can hear the difference between the different methods.

The Bass Octave Deluxe does a bit more to deliberately change the tone on the Girth. If you look at R51 (330k) the V0.1 schematic it's a high value. That means when the JFET (Q10) is off the gain is about 1 but when the JFET is on the gain is about 7. So one part of the waveform gets stretched and clipped, which changes the character of the tone.

If you look at the schematic for the "other" "MXR Bass Octave" (I'm not actually sure what MXR pedal that is yet), you can see the same 330k as the Girth channel. The filter is different.

viewtopic.php?f=1&t=28143

However, it is possible to make the MXR Bass Octave Deluxe with the 3x68k resistors sound the same as the filter in this circuit. Using the same sounding filter is a guess as without sound samples we still don't know if the Octave sound of the "MXR Bass Octave" is the same as the Girth sound on the Bass Octave Deluxe. It would be a good start though. Really, we don't know the Growl filter on the Bass Octave Deluxe is the same as the Boss OC-2 that's really just a good guess too.

[george giblet]

I had a thought about the filters

The Girth circuit seems to follow the EBS Octabass circuit.

The 330k (R51) resistor on the Girth circuit matches the Octabass circuit.

Also, for the middle and lower filter switch settings the Octabass has 3x68k's on the filter.
So a good guess for the cap values on the Octave Deluxe would to choose those.
To match Middle setting: C29 = 33n, C32=47n, C28=3n3
To match Low setting: C29 = 33n, C32=2x47n (~100n), C28=3n3
I have a feeling the Girth setting might match the low setting as
its purpose is for a deeper sound.