I nicknamed this build the BSMSB – Buffer-Splitter-Mixer-Switcher-Boost. It does pretty much what the name suggests, although the splitter-mixer part doesn't really work particularly well. More on that later.
A simple non-inverting op amp circuit provides adjustable gain from unity to +20 dB via the boost pot. The op amp is an OPA1641, chosen mainly because it's similarly spec'd to the OPA134 (single channel version of the OPA2134) at less than half the price. The entire active circuitry can be bypassed with the buffer/boost switch on the side.
The idea for the switchable input impedance was taken from @JTEX's "An Idea for a Universal Input Buffer/Booster (onboard)" thread (https://forum.pedalpcb.com/threads/an-idea-for-a-universal-input-buffer-booster-onboard.21235/). This was also the initial inspiration for the buffer/boost circuit. If I were to build another one of these, I'd probably stick with a fixed 2.2M or so resistor instead of the rotary switch, as I didn't find its effect on tone that useful for my purposes.
I'm using a voltage boost module with an XL6007 DC-DC converter chip to get +/-15V from 9V. This improves headroom of the op amp boost stage and removes the need for a VREF voltage divider, as the op amp is simply biased to 0V (ground). This particular module (DD1912PA) was only a couple of bucks on AliExpress, but it's filtered quite well and I'm having no issues with power supply noise. I'm wondering why these types of chips are generally not used in guitar pedals. It seems like a much better alternative to charge pumps, which in my experience always inject low levels of switching noise into the audio signal.
The two loops are true-bypass and can be toggled via the footswitches. These can be operated either in parallel or in series with switchable order, selectable via a single 4P3T rotary switch. Unfortunately, the parallel routing doesn't really work. Or rather, it kind of works with some pedals and not others. Well, mostly the latter. There are no mixing resistors on the returns, maybe that's the issue. The whole build was kind of a pain in the ass to be honest and I certainly don't feel like undoing any of the wiring, so I guess I'm just going to have to live with it.
If I'm ever building a version 2.0, I'll include a summing amp and balance control. An option to switch the phase of one of the return signals would be a good idea, too.
My grounding scheme might have been a little overkill. The piece of stripboard by the output jack gathers all ground connections in a single point. The 1/4" jacks are isolated from the enclosure, which is instead grounded via a wire to the housing of the series/parallel switch. I suspect the strict star-grounding was largely unnecessary, but it was one of those "it probably won't hurt, so why not?" type of things. Overall, the wiring for this build is kind of a rat's nest, but it does the job.
My measurements for how it should all fit together were pretty off, I honestly don't know what happened. In the end, I was able to make it fit by clipping off and bending some of the rotary switch lugs, moving jack positions slightly, the sorts.
All in all, the planning stage for this build was much more fun than actually putting it together and I'm glad it's done now. It sucks that the parallel mode doesn't really work, but it is what it is. Maybe I'll do a version 2.0 at some point with the above mentioned improvements, maybe not. We'll see!
A simple non-inverting op amp circuit provides adjustable gain from unity to +20 dB via the boost pot. The op amp is an OPA1641, chosen mainly because it's similarly spec'd to the OPA134 (single channel version of the OPA2134) at less than half the price. The entire active circuitry can be bypassed with the buffer/boost switch on the side.
The idea for the switchable input impedance was taken from @JTEX's "An Idea for a Universal Input Buffer/Booster (onboard)" thread (https://forum.pedalpcb.com/threads/an-idea-for-a-universal-input-buffer-booster-onboard.21235/). This was also the initial inspiration for the buffer/boost circuit. If I were to build another one of these, I'd probably stick with a fixed 2.2M or so resistor instead of the rotary switch, as I didn't find its effect on tone that useful for my purposes.
I'm using a voltage boost module with an XL6007 DC-DC converter chip to get +/-15V from 9V. This improves headroom of the op amp boost stage and removes the need for a VREF voltage divider, as the op amp is simply biased to 0V (ground). This particular module (DD1912PA) was only a couple of bucks on AliExpress, but it's filtered quite well and I'm having no issues with power supply noise. I'm wondering why these types of chips are generally not used in guitar pedals. It seems like a much better alternative to charge pumps, which in my experience always inject low levels of switching noise into the audio signal.
The two loops are true-bypass and can be toggled via the footswitches. These can be operated either in parallel or in series with switchable order, selectable via a single 4P3T rotary switch. Unfortunately, the parallel routing doesn't really work. Or rather, it kind of works with some pedals and not others. Well, mostly the latter. There are no mixing resistors on the returns, maybe that's the issue. The whole build was kind of a pain in the ass to be honest and I certainly don't feel like undoing any of the wiring, so I guess I'm just going to have to live with it.
If I'm ever building a version 2.0, I'll include a summing amp and balance control. An option to switch the phase of one of the return signals would be a good idea, too.
My grounding scheme might have been a little overkill. The piece of stripboard by the output jack gathers all ground connections in a single point. The 1/4" jacks are isolated from the enclosure, which is instead grounded via a wire to the housing of the series/parallel switch. I suspect the strict star-grounding was largely unnecessary, but it was one of those "it probably won't hurt, so why not?" type of things. Overall, the wiring for this build is kind of a rat's nest, but it does the job.
My measurements for how it should all fit together were pretty off, I honestly don't know what happened. In the end, I was able to make it fit by clipping off and bending some of the rotary switch lugs, moving jack positions slightly, the sorts.
All in all, the planning stage for this build was much more fun than actually putting it together and I'm glad it's done now. It sucks that the parallel mode doesn't really work, but it is what it is. Maybe I'll do a version 2.0 at some point with the above mentioned improvements, maybe not. We'll see!
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