Friday, 5 February 2016

Montarbo Sinfhoton

Here's a pedal that caught my eye while looking for a new fuzz. It was made by an Italian company in 68.

It's very Big Muff like in tone, but seems to have a bit more clarity in the notes, and no Fuzz control, just Bass, Treble, and Volume. In my search for the schematic I found that over time there's been 3 different versions, so I made a layout for each, and then found that a few years ago by Miro actually made a layout for the later version. My layouts are a bit more compact, has less cuts & jumpers, and I put the CLR for the LED on the board, so I thought I would still post all of them, since while they have the same feel, each sounds a bit different.

Also, through my searching I found some mods people have done, and made a massively modded layout as well that I'll post later with a description of it all.

The video compares the stock vintage unit to a clone made by Brad at Creepy Fingers. Not sure what the extra pot does, but since it's full CW I can only assume it's a Fuzz Pot.

Earlier Version

Later Version

2 Knob Version

  1. You may need to adjust the 2 10k resistors to the Collectors of Q1 & Q2 to bias the transistors  right and have it sound good.
  2. Since BC149's are hard to come by, although I've seen them on eBay, you may want to try the usual suspects with higher gain. So transistors like 2N5088/2N5089, MPSA18, BC109, BC108, etc.
  3. For the 2 knob version you may want to change the 2uF caps to 10uF caps like the later version.
  4. For more fuzz remove the 100k or 1M resistor parallel to D1 & D2. 
  5. I left out the usual 1M pulldown resistor as in all the videos I heard I didn't hear a pop from the vintage unit, so I felt it's not needed, you can always add it on the switch or just add an extra column.


  1. is there a decent way to add a gain control?

  2. Maybe like a pot between the two stages?

    1. not sure if that will work, but it's a possibility. i did find that johnk added a gain pot by connecting lug 2 to the junction of D1, D2, 100k, 47k or 22k, 4.7k, 100k, and 10uF, which is Row I on the all of the veros, and ran lug 1 to ground, which you can put on Row J. He also upped the 47k or 22k that runs to ground to 180K. from what i've read this also cleans up really nicely with your guitars volume, so another way to to add a gain pot could probably be running a voltage divider in front of it to lower how much the first transistor is getting hit by your guitars signal.

    2. Sounds like my kind of nasty, and it looks ripe for experimenting/modifying. I will be building one of these. Thanks Zach.

    3. my pleasure beaker. i'm planning on building one too fairly soon, just have to figure out my enclosure design first so i can pop it in as soon as it's done. but, since you and i think alike, are you thinking what i'm thing?

    4. Let me guess.. MP38A version with Ge diodes :P

  3. I had the chance to try out an original one, couple of years ago.
    Montarbo is planning to reissue this model, along with the other pedals they used to make in the 60s-70s.
    They announced it last summer, and it can be found on their website:

  4. Was gonna take a crack at the later version today,but appears I only have 1 10k pot on hand :/

  5. You can tag the two knob version as verified Zach. Built it as stock layout values.
    I've been playing with it for hours now, and can't seem to find a better combo than 2N5088 for both transistors, and some Russian 2D503b diodes (1N417 equivalents, so nothing special really).
    What is the huge 330uF cap for - I socketed this as I don't have any at the moment, so tried 220uF and 470uF. I can't notice any difference with either value, and it sounds exactly the same with it removed altogether!

    It's not very loud though and quite polite really,not hugely impressed, UNTIL I removed D3 (keep the north facing diode in place). Now it's plenty loud, and rip your face off aggressive. Aphex Twin "Come To Daddy", paint blistering aggressive. I like!

    1. And yes I did try an all Germanium version - sounded like poo!

    2. It's just for supply filtering so you would probably be fine with the usual 100u. If you're in a experimental mood...try removing the 1M parallel to D1 and 2. Should give much more out of Q1. Also play with the 10K collector resistors. Combined with the 5.6K give you a 15.6K Rc. In the big muff these ranged from 10K total to 33K.

    3. agreed about the parallel 1M. it seems to adjust the amount of gain, and i would think the amount of output as well. kinda disappointing that a Ge version doesn't sound that good. i wonder if changing it to PNP and using some of the higher gain transistors available would be better to get a Ge one up.

      even though the layouts are different, i'm pretty sure if the 2 knobbie one is good that the others are good. i'm thinking that using the typical 100uF electro for the 330uF power filtering should be fine, i just made the layouts exactly as the schematic shows. btw, did you see the massively modded layout i posted? would be interesting to see what the changes to.

    4. It's not "bad" sounding as stock, just sitting under a duvet with a nice mug of cocoa, which might just be your cup of tea, but it does need more volume as it barely reaches unity. The tone control woofs out pretty quick, so I don't think swapping the caps will be a good move, but I'll try it anyway.

      I will reduce or remove the 1M resistor, which as you say, should give more volume. I will go with 100uF for the filter cap, so thanks for the info guys.

      I'm really liking the sound of this without D3 though, it's like a cross between a Muff and a Meathead.

    5. oh man that sounds fantastic. i'm off to design the enclosure so i can get to work on one when i'm done with my modified brassmaster, which i just finished designing the enclosure. it's going to be great, i think you'll really appreciate it beaker.

    6. i forgot to say. if changing the 1M resistor increases volume and removing D3 still sounds really good and different enough it might be worth putting a switch to either remove or add D3 to the effect.

    7. I have been playing with this all weekend, and come to the conclusion that it is pretty much perfect - meaning the designers did their jobs properly.

      After tralling all sorts of variations and value changes, the only mods I have made are as follows:

      Transistors are now 2N5089
      Tone pot changed to 10k lin
      Volume pot changed to 50k log
      330uF cap changed to 220uF
      D1,D2,D4 are 2D503b
      D3 changed to an orange LED.

      Reducing/removing the 1M parallel to D1 and D2 effectively does nothing, altering the transistor bias resistors does not improve things, IMO the bias is just about perfect with the stock values.
      Changing the 47k resistor to ground has minimal effect as well.

      The magic bullet seems to be the orange LED for D3 (it has to be north facing), that removes the sonic duvet, and lets it breathe.

      Your guitar volume control then works almost like a blend control, letting clean notes ring out over a sea of fuzz.
      I'm highly impressed with this simple little circuit.