Micky's Big Blog

BOSS OS-2 Modifications

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Gonna mod this pedal, it is kinda noisy and harsh




First a little extraneous info. My little pedalboard is a BOSS BCB-30 (I think...) and it has room for only 3 boss pedals (or pedals that size/form like Digi-Tech or possibly a TS or similar) and maybe a power supply. I use this small, portable unit in the living room upstairs with my smaller tube amps. I have a tweed Champ stacked on top of a Marshall DSL5c, and I slave the hi input of the Champ into the front of the DSL. (a trick I learned from Alvis, I think...) I only turn on the DSL when I need reverb or a lot of volume. The combination of these two amps is outstanding (to me anyway) and the 10" WGS in the Champ and 12" WGS Green Beret in the DSL seem to be very well matched.

So anyway, on this little board I have a BOSS RV-5 reverb (last in line) as well as a GE-7 equalizer. These 2 pedals are connected to the FX loop in the DSL, and the only time they are active is when that amp is on. This leaves only 1 slot left. I like to use 2 other pedals, a BOSS SD-1 (slightly modded) as well as a BOSS DS-1 (heavily modded) in front of the Champ. The SD-1 is great for a clean boost, but it is an either/or situation where the boost is great and the overdrive is only so-so. The DS-1 however is the closest I can come to a Big Muff type of distortion I have found, especially since I have the Frommel mods in it. There just isn't any more room for my 2 other favorite pedals.

In walks the BOSS OS-2. As it says, it is an Overdrive / Distortion pedal, basically an SD-1 and a DS-1 built into 1 enclosure. I think as with anything, when you try to combine 2 things in 1 box, you lose a little of each when you do it. I have always felt that seperate components are always best, and combining them is almost always a compromise. This is the case with this pedal. It is mediocre at best, trying to make an SD-1 into a combination pedal kinda fails miserably.

For example, an SD-1 is an excellent overdrive pedal, doing exactly that. Taking a signal, and increasing the level of it into an overdrive situation. The ability of an SD-1 to boost a clean signal is outstanding, (with a few modifications) as it can have an extremely low noise floor and a very high signal-to-noise ratio. In layman's terms, it is really quiet (no hiss) and can get very loud with no increase in noise. You do this by using a few high-quality component replacements such as precision metal-film resistors and a low-noise op-amp IC.

On the flip side, a DS-1 is an excellent platform to mod into a versatile distortion pedal in much the same way. In this case, BOSS had it right the first time around, when their pedals were still made in Japan before the move to Taiwan. Back then, the op-amp IC's they used were very much in demand, and their mojo lives on today. Fortunately, the OS-2 uses that same old op-amp in the older DS-1's so I lucked out in that respect. Not a lot has to change in the distortion side of the pedal except the frequencies of some of the filters used to cut high or low signals as they travel thru the pedal.

I hope I haven't lost you, this crap kinda bores me, but I have had a lot of real-world experience with it as a Ham Radio operator with an Extra Class license since 1990. Filters and signals rule on that side of the fence, as well as oscillators and amplifiers. Figuring out how all of this works in the REAL world is the interesting part. And I would imagine that this is the reason you are still reading this.





This pedal was obtained from eBay, I tried to find one that was clean and not too beat up. Also one that didn't have extrobitant shipping charges. Let's just say I got lucky and got it at auction for around $50. Just about the right price for any BOSS pedal. The serial number indicates this particular pedal was manufactured in July 1996. You can find the serial decoder here: https://serial-number-decoder.com/boss/boss.htm

The pedal arrived from a US shipper, and apart from being old, was in near mint condition. There were a couple of dings (you can see them on the very bottom of the pedal) but no scratches. I connected it to my amp and guitar by itself, and it worked pretty much as expected. A couple of the pots were relatively scratchy, but I expected that from an almost 30 year old pedal. It was also incredibly noisy with the level maxed out. The Color control (to adjust between SD and DS) was very subtle, with not much difference at all. I needed the Tone control all the way up or else the pedal seemed muffled.

As I also anticipated (mentioned above) this pedal is a compromise in a few ways, but it may be a worthwhile sacrifice in order to save a space on my board. After opening it up, it looked incredibly clean, with no mods whatsoever. A perfect platform to work from. No need to click on the photo below, you can see the underside of the board in high resolution. (not really hi res, but big enough...)





If you are viewing this blog on a desktop system, these photos are probably 4 times as large as the actual pedal. Each solder pad on the board is about 1.5-2.0MM and difficult to identify. Below is a photo of the topside of the board with all the original components. You can easily identify 2 op-amps, one is a Single Inline Package (SIP) and the other is a Double Inline Package (DIP). IC1 is a Mitsubishi M5218AL, a run-of-the mill general purpose op-amp with similar characteristics of IC2, a JRC 1458D. The 1458D is actually a desirable op-amp in many pedals, as it is no longer produced and very difficult to find. The 1548D was used in many older Taiwanese SD-1's. It is functionally equivalent to the JRC 4580D although many builders prefer the 4580DD version.

A lot of people won't really care about these chips, they might think they are unnecessary in a REAL audio design. I used to think that way as well, I didn't even really like transistors, even they when they were used for functional things like switching and weren't even used in the audio signal path. Then after an Engineering degree, I finally understood transistors and realized that op-amps (especially dual op-amps like are used here) are really just a couple of transistor circuits built onto one package. While not particularly complex, things like slew rate (including rise time) and input and output resistance play very important roles in a pedal's design. I can link a couple op-amp comparisons and descriptions later in this blog if you are really interested.




While I feel the op-amp selection is a VERY important aspect of the BOSS pedal design, even more important is the circuitry in and out of these chips. Op-amps play a big role in defining the 'sound' of a pedal, especially how 'harsh' or 'smooth' it can sound. The frequency response (a large part of how we 'hear' tones) is dictated by the filters in the signal path before and after these op-amps. At the very bottom you can identify C31, a mylar film capacitor that is very inexpensive, and very typical of many of the replacements I made. These type of very inexpensive (I am trying hard to avoid using the word CHEAP here...) components are inherently unstable, varying widely straight from the factory and even more so with variations in temperature and humidity. Eventually I will replace all of them, basically for tolerance and stability in the long run.

A lot of people will say 'it's only a pedal, and component values don't matter too much as long as they are close...'. Many times I will tend to agree with them, as in most audio circuits and especially in older tube amplifiers this is very much the case. Close enough is generally good enough, especially in hand grenades and tube amplifiers. But what is actually 'close'? Frequency response, or sometimes referred to by many as 'bandwidth' is dictated by the R/C (resistance & capacitance) filters that allow or restrict certain frequencies. For example, something that restricts high frequencies can or might be heard as 'muffled' or 'boomy'. When there is an emphasis on mid frequencies, it can eliminate what is called 'mid scooped' by restoring or enhancing certain frequencies above and below a certain point.

Anyway, back to the task at hand. As you can see from above, a 2mm solder pad is really not a lot to work with. This BOSS pedal is a single-layer board, with the traces etched on only one side. This is inherently less sturdy, as plated-through holes/traces can withstand a LOT more abuse, such as soldering and de-soldering. To say I needed to be careful with this is an understatement. Throughout the whole process I only ruined half of one trace, and that was only due to my impatience while pulling a capacitor. Below is a photo with all the capacitors replaced.




Next is a photo with the IC2 op-amp removed and a machined 8-pin socket installed. I prefer this type of socket, it works well and stands up to repeated insertions and removals.




Here is a close up of the socket:





Thanks to T-Rex for the iPhone 6+ that took this macro photo, it does a great job!

Updated 01-31-2020 at 07:14 AM by Micky

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  1. Micky's Avatar
    You can see by comparing the last 2 photos above, many of the mylar caps have been replaced such as C27, C20, C17 and others. Some of the electrolytics have been replaced as well, with ceramic/mylar/poly (or whatever) dipped caps. As long as the value is under 1u you can generally get away with this, especially in lo-voltage circuits such as this.

    The modification kit I purchased for this is the Fromel Electronics out in Seattle. This kit is a good start to make a mediocre pedal into a very good pedal. Beyond this kit I have chosen to make additional modifications and replacements, to get things into a bit of a customized for my situation type of sound. You can see the kits for pedal mods here: https://fromelelectronics.com/collec...pedal-mod-kits and they have many other products as well. Although the kit I purchased was complete, the instructions were geared towards advanced builders, you can do it yourself or have them mod your device. I felt the lack of a step-by-step listing should really discourage many pedal modders.

    The mod kit I purchased is very similar to the mods I found here:

    https://www.roboticbeast.com/modific...-la-boss-os-2/ These show some very detailed photos outlining the various component replacements he made, I needed to translate it from French.

    The schematic (basic one anyway is here: https://0x4c.files.wordpress.com/2018/10/os-2_orig.png

    Some op-amp information and links here:
    https://www.cycfi.com/projects/six-p...-amp-shootout/

    http://www.ti.com/lit/ds/symlink/opa2134.pdf

    https://www.rcscomponents.kiev.ua/da...4ufguygf43.pdf

    https://pdf1.alldatasheet.com/datash...I/M5218AL.html
  2. Micky's Avatar
    The above modifications are pretty much common knowledge, but one of the aspects still haunts me, day after day.

    You see, this type of pedal is inherently noisy, that is, there is a fair amount of hiss when you activate the circuit. One of the things I usually do with BOSS pedals that still allow for modifications is change the op-amp from the stock choice, to one more suited to the task.

    One of my go-to choices is the Texas Instruments NE5532. Here is a link to a Monte Allums site where he provides a very nice section and describes several op-amps in detail. http://monteallums.com/pedal_mod_parts.html#ops Here is what he said about the 5532:

    NE5532 - Made by Texas Instruments. Very similar tone-wise to the JRC4558D and RC4558P. This is a more Hi-Fi, lower noise chip than both the JRC4558D and the RC4558P. I really like the tone and gain of this chip. In some ways I like it better than a JRC4558D or RC4558P. Rich, smooth gain with nice overtones. This is a very popular chip with pedal modders.

    The reason this is my go-to chip is simply because of the low noise. I especially like this chip in the GE-7 equalizer pedal because if the low noise, (high signal to noise ratio as well as the incredibly low noise floor) and the way it corrects a deficiency in the OEM chip, where lowering one setting can actually INCREASE the noise in the pedal.

    In this particular application, it is the lowest noise op-amp that you can reasonably obtain that will still fit in the pc board as designed. The Dual Inline Package (DIP) is an old-school design that does not lend itself to close component placement such as some newer smd designs that have incredibly low noise, and when the design is right, can offer some unbelievably low noise floors. With the design of the OS-2 you really don't have a lot of choices, and you need to work with what will fit. Even the 1/2 watt resistors I used were actually too big, and I needed to install them vertically because I did not have 1% metal film resistors in 1/8 or 1/4 watt sizes.

    Another thing not many will notice (mainly because I did not go into detail about the replacement components I used...) is that I replaced a couple electrolytic caps with monolithic dipped caps. You can get away with this about up to 1uf or so. The smaller metal film caps used in BOSS pedals are incredibly inexpensive, and with time, their values tend to drift a bit.

    Anyway, if you were looking for sound clips you are gonna be disappointed. None will be found here. Sorry. Just take it for a fact the pedal sounds better and is much quieter. Actually the biggest change had nothing to do really with any of the components I changed. It had to do with cleaning the 30-some year-old pots that had become scratchy and irregular. I sprayed them (with waaaaay too much) DeOxIt cleaner and lube, and then had to wipe up the excess lube after the pots had dried a bit. The 12mm pots with knurled shafts used (4-in-a-row) are difficult to find, although I found a source for the smooth shaft ones. If I had some small knobs with set-screws I would have just changed them out rather than try to bring them back to life. Anyway, with the lube they should be good for another 30 years or so.

    Feel free to contact me if you have questions, I can be reached at mcorrow@gmail.com