The BX series by Boss has advanced from “do the job, I guess“ mini mixers to some sort of “magic techno distortion“ units over the past few years. Many swear by them, even more swear about them (their high noise floor), and others go to the church of Mackie instead. With the hoarding impulses we witnessed in 2020/21, these little boxes also became more expensive on the second hand market, so mind you, first and foremost: that BX may not be your preferred main mixer as it imparts a sonic character that might not be your kinda thing. (We keep saying analogue character… you know, a sandwich with anchovies and limburger cheese also has character…) Secondly, do check for brittle plastic and crackling pots. Anyhow, that’s babble, so best let’s listen to some distortion to wipe that off our minds…
This would be the sound your little Boss mixer is known for, and mind you, we just listened to input overdrive and that you can still smash that mix buss for more!. Yet, even more subtly used, this mixer can be handy for giving your audio some character (I happen to like).
Before turning to modifications and why I think they are cool to have, here’s some service help. The BX series mixers are technically very close to each other, so you can get by without a schematic of the BX800. Yet since I could not find any service manual freely available, I traced one channel (using the BX8/16 notes as a template). Here you go:
(schematics of modifications are further down the page…)
Now, why would one want to modify something like this? Well, the desk itself is sweet and neat and I see why folks tend to like the distortion. Yet, sometimes too much is too much, I feel, and after the whole signal is distorted some more distortion does not always help. I guess what I noticed in toying around with some different setups around this thing is that you want to hit red and then take the gain a bit back again. Especially with more than two or three signals your mix gets muddy very quickly otherwise.
GAIN REDUCTION: If it is sludge-fest you want, different story, of course, yet I reduced the gain by half on all channels of my unit and still get enough crunch. What you do is to replace R106-806 by 560k resistors.
I assume the op-amp chosen in this design plays more of a role in the character of the overdrive than the diode clipping in the gain circuit. First I thought this would be the other way round but listen to me putting diode clipping in and out on the fly:
The one well known Eurorack adaptation of the BX series uses the same op-amps, as far as I know, that’s M5218L, which are still available here and there in SIP8 package, but I guess NJM4558s (available in Soic8 too!) should do the trick as well as these two amplifier ICs seems to be pretty similar. I have not tested one against the other yet though!
By the way, one hack to increase distortion even more is to put some diode clamping (as you see in the gain amp input) between the gain pot PIN1 and ground. This f…s up your sound quite a bit, and you might want to have this mod, if at all, on a switch.
Channel Mute Switches: One big performance mod I implemented are mute switches per channel. For this I sacrificed the gain LEDs as that location seemed perfect, and although visual feedback for clipping helps in a busy mix, I’m willing to gamble a bit in this regard. Rather than disconnecting the signal via this switch, I simply pull the signal at the wiper of the treble potentiometer to ground by means of a switch. This works as if you rapidly pulled down the volume slider. On my unit I placed the switches where the overload LEDs. used to be. Yep, you read right, I’m flying that channel gain by ear! 🙂
EQ mods: the EQ section of the BX800 is a simple passive treble/bass setup consisting of two potentiometers and some capacitors. While I like the treble and how bass is reduced, I do not like at all the boxy boomy smudge introduced by boosting bass. Hence, also of course for the heck of doing it, I rewired the EQ a bit, so that treble takes off more high frequencies when fully anticlockwise and that bass is a simple bass cut when anticlockwise. I find this helps me sculpting sound more efficiently than the existing setup. But taste are different, innit…
If that mod is interesting to you, it’s super easy to do: remove C110 (to C810) and replace C109 (to C809) with a 10nf capacitor. For increasing the high frequency dampening on the treble knob, simply wire a 100nf capacitor across C106 (to C806).
More Sends: The BX800 has just one send and return. Good enough, but hmmmm…. I couldn’t help but fiddle with this too, so on my unit I replaced the panning knobs on channels 1-4 with a second FX send that is post-fader like send 1.
The idea here is that I usually use sounds like kick, bass and snare on the first channels, all sounds which I usually keep in the middle of the stereo field. Having a second send here is more useful to me so I can pull in some reverb while sounds on later channels go through delay. Removing the panning knobs is easy. These are centre dented 10k pots and when you take them out, you get straight centre panning by simply putting in a 5.1k resistor between PIN1 and PIN2 and another 5.1k resistor between PIN2 and PIN3 on the PCB. That’s like having your pot in the middle position. You can then put in A50K pots in the place of the former panning pots, yet leave the pins out, i.e. just put them in their place by soldering them in by their brace. That’s a wee bit dangly but good enough if you do it right. You then connect PIn1 of the existing effect send pot to Pin1 of your Send 2 pot (that’s ground); you also connect Pin3 of Effect send pot with Pin3 of your new Send 2 pot (that max audio signal). Pins 2 of your send 2 pots you wire via 100k resistors to a summing node. That summing node I send on mine via the (former) FX return opamp, which is IC9-B. (Here you need to remove the clipping diodes and replace teh 220k resistor in the feedback loop with a 27k resistor – then wire op-amp out to the former effect return socket, which is now your “send 2” socket.
Stereo Inputs: As you see, I ditched the FX return, partly because I can “afford to” do so. if I want a mono return, then I’d just use a mono input, and if I want a stereo return, I can use one of two extra stereo inputs I put in. For doing so, I chucked out the headphones amp and rewired it, using the stereo pot of former phones and a second dual gang A50K pot I put n place of the former FX return pot. I guess stereo ins are always convenient but here especially so, since it saves you fiddling about with panning and level faders to get the stereo image right. By this logic I think there may be merits in changing the main volume controls too. Then and again, using them as two independent mono outs (with panning being your A/B) selector, is a cool trick to Keep up your sleeve…
Direct outputs: not done/tested yet, but should be super easy. Lazy way post-fader: tap volume fader wiper and wire in 10uf cap and 1k resistor in series, which you connect to your output socket. Probably better would be to use a opa as a voltage repeater/buffer.
Modded sections are marked blue and different values are in bold. Some refinements might come along later…
And here’s some “jam”
Eurorack Adaptation Schematic
I don’t plan on stepping on existing turf but might suggest someting further down the line that is sufficiently different from existing setups. How teasy…. (and no promise though!)