Roland Micro Cube Effect Loop add-on

[Thomeeque]

 Hi guys,

as I already stated in my post at pix of Custom built guitar amps/cabs board, to be able to use my Micro Cube with looper pedal, I have designed and built in effect loop circuit. In principle it is nothing special, biggest challenge was to actually get inside my MC (there's a rubber around chassis and it sticked to the cabinet over the years), but it had few specifics. First was that MC guts are quite complex and I could not find schematic. Luckily Cube 30 schematic is available on-line and all the critical parts of the circuitry turned to be almost identical. Second was, that basically all except the most low level functions is handled digitally inside DSP, including Volume control, so add-on also has to feature master volume pot But it is actually a good thing, as I realised later, because the original Volume control is quite useful when using looper, as it allows to adjust levels of different setups when layering loops. Third sort-of-challenge was placing add-on physically to the MC chassis, as there is not that much space, but it was not that bad either. I have found nice place for the Master Volume pot at the top panel (see pics) and there is some space to squeeze send/return jack(s) at the back side. For now to save some drilling I used stereo jack for both signals (requires special cable), but two jacks would fit either.

TL'DR:

Basic topology (blue is MC, green is add-on):



Add-on replaces decoupling capacitor C32 (this capacitor has the same part name on MC and Cube 30 btw. ), power is taken from C107 pads (- is AGND, + is A+8), be careful not to connect audio ground (AGND) with chassis. To be able to easily find right spots, check my pics (wire colors: ⬤ A, ⬤ B, ⬤ A+8, ⬤ AGND, ◐ FX-IN /SEND/, ◐ FX-OUT /RETURN/, ◐ FX-GND).

Add-on schematic:



Circuit is quite simple, could be easily soldered on universal PCB or whatever technique you prefer, but in case you are interested, my PCB layout and Eagle files are also available at my page. Btw. for master volume linear (not audio) taper pot is used. With combination of 10k load at C32- input it works better

Attenuation and Gain are set to 3x (so the TRIM_ATT is set to one third and TRIM_GAIN is adjusted to unity gain of the add-on with POT_MASTER at maximum), but this may yet change in the future, as I was not sure what would be the best value TBH. It works fine with my Boss RC-3 looper, but it would be maybe worthy to try with your pedals before putting your MC back together for good.



Sample Loop

Good luck, T.

[deadastronaut]

brilliant, nice one tomas....great idea making a thread for this i'm sure many others

will want to do this too...

[junor]

Great, I've been searching for instructions how to connect looper to MC!

Didn't quite understand the addon circuit and need for master volume though. What would happed if you just connect send/receive jacks to +/- of C32 and AGND? Full volume without control?

Other question if somebody knows, I'm adding speaker out but understood that there is some bass enchase circuit that would be good to bypass. Any help with this?

-J

[Thomeeque]

Didn't quite understand the addon circuit and need for master volume though. What would happed if you just connect send/receive jacks to +/- of C32 and AGND? Full volume without control?

Yup, connecting send/receive jacks (thru decoupling capacitors at both ends) to +/- of C32 and AGND would be the absolutely most minimalistic working FX loop solution.

Master volume would be controlled either by volume pot of the stompobox plugged into the loop (but not every stompbox has it, mostly it works only if the effect is engaged and it would be also quite clumsy) or by the original MC Volume with some limitations, based on fact that regulation would be happening at the input of connected circuit (signal level may be too small and lead to bad SNR or too big and overload the circuit or even affect it's function if it would be e.g. some envelope follower based FX).

Input impedance at the return input would be 10k, which is also little too low.

Add-on circuit with additional volume pot takes care of proper levels and impedances at both ends (at least principally, the exact values may need some attention yet), provides standard master volume control and also provides some electrical protection to the original MC circuitry (I did not want to expose onboard op-amps directly to the outside world).

About second question, I read the same, I guess it happens somewhere after C32 (as it is probably common for both guitar and AUX input and also signal, I recorded by looper, does not seem to be equalised this way).

T.

[junor]

Yup, connecting send/receive jacks (thru decoupling capacitors at both ends) to +/- of C32 and AGND would be the absolutely most minimalistic working FX loop solution.

About second question, I read the same, I guess it happens somewhere after C32 (as it is probably common for both guitar and AUX input and also signal, I recorded by looper, does not seem to be equalised this way).

T.

If I would test this minimalistic setup, would those decoupling capacitors same as C32 capacitor? I will definitely build also that addon circuit but it takes a while to get to it..

About ext speaker, would filter capacitor be a easy workaround..?

[Thomeeque]

 Hello J, sorry for late answer , did you get somewhere meanwhile?

As you say, caps same (or similar) as C32, not critical for first experiments.

About ext speaker - first I'd try connect it directly, maybe it will sound good like that? Filter capacitor could be solution, just be careful if standard (unipolar) elyt is OK or if you need bipolar type (see what's inside).

Cheers, T.

[kmos]

Thank you for sharing the project.
Being a beginner, I would really appreciate if you could answer some questions before I try and make the mod myself:

• I have a Ditto X2 looper by tc electronic that I want to use with my Micro Cube. It has "Loop level" turn knob built in. Does this mean that I can do without Master pot at the fx loop schematic? If I do without the master volume pot, but still use your basic circuitry, will it still take care of the correct impedances of the whole circuit?
• As far as I understand, you are using a jack socket with additional contacts that simply short the circuit when the jack isn't plugged in?
• You are using a dual opamp, one channel for attenuation and one for gain, right?
• What's with the strange voltage of 11V that you use at VCC? Is it the standard voltage inside Micro Cube that you are getting directly from power circuit?
• See picture attached. What does this part of the circuit do? (sorry, I am very inexperienced)

[Thomeeque]

 Hi there, pleasure

• I think you still need to use Master pot, Loop Level knob probably influences only level of the loop playback, not the direct ("dry") signal coming thru, right? About impedances the answer is yes
• Yes, if male jack is out, signal comes thru. It is simple and easy solution (most of the female jack connectors have these additional contacts). Small drawback is that circuit is not bypassed this way, it may influence passing signal (add some noise, distortion and tonal changes). Puristic solution would be to add true-bypass toggle (or maybe relay, controlled by jack additional contacts), which would allow to switch MC to the original state. I did not find it necessary.
• Yes. Hence IC1A and IC1B. You could use single opamps, half of the quad (but read this) etc., this circuit is not very picky. But dual opamp is the most convenient here, also most common setup, all common types should work here.
• Exactly. It is voltage taken from MC power rail labelled "A+8", it is used for powering surrounding opamps in original circuit. 11V is value I measured when powering MC by original wall power supply (0.3mA was quiescent current taken by fx loop circuit). I never tried with batteries (maybe it is 8 volts then, loop should work).
• It is sort-of classic audio input circuitry you will see a lot in other designs (high frequency noise filter, voltage decoupling, impedance matching, high voltage protection, voltage bias). But actually I made a mistake here, one essential part is missing (resistor /cca 10k/ in series with signal at the input). I will fix it and explain deeper then 😇

Cheers, T.

[kmos]

Thank you for the answers, Thomeeque. I'm afraid, I need some more clarification:

• As far as I understand, the resulting sound that returns to the power amp is a sum of 2 components: the processed guitar sound out of DSP (going through the whole loop) and the added looper playback. So I can change the volume of the first with the Micro Cube's "Volume" knob, and the second one with the "Loop level" knob on the Ditto. Why the need for the Master pot then?
  Is it because if I add some other (especially analog) device into the FX loop, the parameters of the circuit will change and the volume might drop?
  Thing is, I am not too happy about drilling a hole in the top panel of the MC unless it is really necessary. On the other hand, it seems like a simple way of fixing the pcb inside.
  
• Since you didn't find true bypass necessary, I assume there is no audible hum/noise or tonal changes, that you noticed after the mod? I would really like the true bypass relay solution, but only if it solves a real problem. Me playing on a Micro Cube kinda suggests that I am not a purist in terms of sound. 

[Thomeeque]

 No problem

About Master pot - there are two issues:

1st is inconvenience - if you will want to change overal volume (of both direct guitar and looper playback), you will need to dial two potentiometers, one on Micro Cube and one on the floor.

2nd I already touched few posts above: Micro Cube's "Volume" knob can be used this way but with some limitations, based on fact that regulation would be happening at the input of connected circuit (=effect chain in loop) - signal level may be too small and lead to bad SNR or too big and overload the circuit or even affect it's function (if it would be e.g. some envelope follower based FX).

But none of these issues is fatal, impact depends on your setup and style of use, there is no reason why not to try it without Master volume pot first and add it later if necessary. Actually you can go even further, do the same thing as junor suggested - start with just passive FX loop. Remove C32, solder two wires to it's pads and one wire to AGND (C107- pad), install SEND/RETURN jack and connect it (both signals thru 22uF capacitors with + at MC side):



It is the least invasive most puristic solution and it may work fine for you. If not, you can upgrade to active version later

Cheers, T.

[kmos]

Thomeeque, what about this question:

2. Since you didn't find true bypass necessary, I assume there is no audible hum/noise or tonal changes, that you noticed after the mod? I would really like the true bypass relay solution, but only if it solves a real problem.

Also, how are you dealing with Volume and add-on Master knobs when the loop is not used? I assume, they are not interchangeable in terms of sound, as the original Volume knob determines the loudness of the digital signal inside the DSP, and is restricted by the DSP's bit resolution. And then you have the Master pot, that is more "analog" in a sense that you actually can overdrive the poweramp further down the line, and get undesirable sound characteristics.
How do you coordinate the values at those two?

[Thomeeque]

2. Since you didn't find true bypass necessary, I assume there is no audible hum/noise or tonal changes, that you noticed after the mod? I would really like the true bypass relay solution, but only if it solves a real problem.

I would put it the way, that I did not notice any changes which would bother me. I did not do thorough A/B testing, I liked the sound and that was it. I am sure there are some audible changes, it is unavoidable, but IMO nothing calling for true-bypass.

But thanks tou your questions I bumped into quite serious issue today (happening at high volumes at some circumstances), see the next post.

Also, how are you dealing with Volume and add-on Master knobs when the loop is not used? I assume, they are not interchangeable in terms of sound, as the original Volume knob determines the loudness of the digital signal inside the DSP, and is restricted by the DSP's bit resolution. And then you have the Master pot, that is more "analog" in a sense that you actually can overdrive the poweramp further down the line, and get undesirable sound characteristics.
How do you coordinate the values at those two?

If set correctly (by ATT/GAIN trimmers), FX loop circuit with SEND and RETURN pins connected and Master knob at maximum should have unity gain (=1; 0dB). That means, that you will not be overdriving PA any further than with original Volume knob. It also means, that you can achieve original Volume knob range by putting Master pot to the maximum. But otherwise it is definitely valid question, I was quite worried at the beginning also, Volume pot could do much more in DSP then just adjusting signal level (equalisation, compression, some "simulation", whatever). But it actually seems like it does not, using any of these two pots feels pretty much same. My usual setup is Volume pot somewhere at 50% and Master at third, sound is really sweet

Cheers, T.

[Thomeeque]

Yesterday I have revised original circuit - it had problem at higher volumes, where voltage ripple at supply line mirrored into IC2b input thru R2/R3 bias divider and whole system got into low frequency oscillation (it was "pumping" or how to describe it). I had rearranged it to get filtered VCC/2 bias voltage (R2, R3, C10, R6 - quite a classical arrangement)*, it solved the problem completely. If somebody already built the original circuit, I apologize for inconvenience and I advise you to do this rearrangement (value of R2 and R3 can stay at 220k, 100k is value I used from beginning as I was short of parts here, this time I have just rather wrote exact values I used to avoid more surprises).

Another change I made was adding resistor R5 at the input, to make AM/high frequency noise filter (R5+C2) and IC2b input overvoltage protection (R5+D1/D2) more effective.

And I have also renamed FX in/out jack pads to SEND/RETURN, as it is maybe more intuitive.



Cheers, T.

* Other option would be to take VCC/2 voltage from Micro Cube board, but I prefer solder as little wires there as possible, this is IMO cleaner solution.

[kmos]

"SEND/RETURN" is definitely more comprehensible, thank you.
Is there a chance that you update the PCB layout as well? I can try tracing this myself, but this being my firest ever project will probably be a bit tricky 

[kmos]

No problem

About Master pot - there are two issues:

1st is inconvenience - if you will want to change overal volume (of both direct guitar and looper playback), you will need to dial two potentiometers, one on Micro Cube and one on the floor.

2nd I already touched few posts above: Micro Cube's "Volume" knob can be used this way but with some limitations, based on fact that regulation would be happening at the input of connected circuit (=effect chain in loop) - signal level may be too small and lead to bad SNR or too big and overload the circuit or even affect it's function (if it would be e.g. some envelope follower based FX).

But none of these issues is fatal, impact depends on your setup and style of use, there is no reason why not to try it without Master volume pot first and add it later if necessary. Actually you can go even further, do the same thing as junor suggested - start with just passive FX loop. Remove C32, solder two wires to it's pads and one wire to AGND (C107- pad), install SEND/RETURN jack and connect it (both signals thru 22uF capacitors with + at MC side):



It is the least invasive most puristic solution and it may work fine for you. If not, you can upgrade to active version later

Cheers, T.

The original C32 cap on Roland Micro Cube is marked 10uF 50V.
The Cube 30 schematic that you found says that it is 10uf 16V.
The simplest cirquit you proposed says to put two 22uF 16V caps into the send/return lines. Why those values? 

[Thomeeque]

Is there a chance that you update the PCB layout as well? I can try tracing this myself, but this being my firest ever project will probably be a bit tricky 

I plan to do that, but I don't know when. Will you start with the passive variant first?

The original C32 cap on Roland Micro Cube is marked 10uF 50V.
The Cube 30 schematic that you found says that it is 10uf 16V.
The simplest cirquit you proposed says to put two 22uF 16V caps into the send/return lines. Why those values? 

Simple rule is that voltage limit of capacitors should be above the voltage the circuit works on, here we have 11V, so the closest standard value above is 16V. We could elaborate if there was some smart audiophylic decision behind 50V capacitors in MC (some people say that higher voltage capacitors sounds better and it may be true), but as the Cube 30 schematics shows, it had probably some more prosaic reason (price, availability, dimensions, stuff like that). Feel free to use higher voltage capacitors, take proposed voltage values as the minimum you should not go below.

22uF I have chosen to achieve (approximately) same capacity as original C32 has, when loop is not used and signal goes just thru these two capacitors.

T.

[kmos]

Thomeeque, thank you! You've been a great help.

I decided to go with the final version of your solution. I currently formed a BOM from your EAGLE project and am going to make a shop order for the components. I am gonna use a breadboard at first, but later I will definitely need an updated tracing layout to order the PCB.

I need one clarification for the BOM: do trimmer and potentiometer values (10k, 22k, 50k) represent the total resistor range? Or are those the values to which the pots should be set, and i need to double the values so that pots are set roughly to the middle of their working range?

[Thomeeque]

 Hi kmos,

cool, I hope you won't be disappointed.

Trim/pot values represent whole range, B in pot value says it is linear potentiometer (A would be logarithmic, C exponential).

I will try to update PCB layout with revision changes soon.

Try to buy plastic SEND/RETURN jack (e.g. like this), so you don't have to worry about insulation (I just hope it will fit, it was quite tight there).

Good luck, T.

[Thomeeque]

 I have updated PCB with Revision 1 changes, find at: http://thmq.mysteria.cz/ucfx/#build

Good luck and share your results, please.

Cheers, T.

[kmos]

Good news, I got the readymade boards of the currently latest (v1.1) revision. I will solder the board and run the first tests soon.

To whom it might concern: since I ordered 3 boards, I will end up having spares, which I am happy to send away for free, shipping price not included.

Bad news is that the vertical board-mounted jack socket that Thomeeque has used seems to be the only model that can fit nicely inside the Micro Cube casing. Mine doesn't fit inside, unfortunately (notice the long scratch parallel to the edge of the chassis - it represents the casing edge):

I have checked other available socket models as well, and their width is still over 16mm, which is the physical limitation of the casing.

So, I will have to probably cut away a small region of the casing edge (marked on the photo). It is made of MDF, so shouldn't cause much trouble:


While reflecting on this, I have realized several things:

• Master Pot at max setting is supposed to give unity gain (0dB), so it gives no real gain (pun intended) in terms of driving additional effect loop cirquit.
• My looper pedal has its own "Loop Volume" knob and I am quite happy to operate two knobs, MC volume and Looper volume to mix the output sound.
• I really do not want to drill more holes than absolutely necessary in my Micro Cube.
• I like the way Thomeeque's board is mounted to the chassis using Master Pot's threaded nut, and my model of jack socket allows me to do the same.

So I came up with a solution of my own: I will solder the jack socket directly onto the board and will use the socket nut it as a mounting point. Here is a rough mockup:


While this solution will require me to trace a new board, it will at the same time lower the number of loose wires from seven to four (C32+, C32-, AGND, 11V) as well as keep the original neat outlook of my MC. And since I am already planning to cut the chassis edge, the board will fit right into the cutaway. And of course, I will be able to add a Master pot in the future, if I ever need one.

So I am looking for ways to trace and produce a new board. Meanwhile, to make sure that I really can do without a Master pot, I am going to solder a Thomeeque v1.1 board and try it out.