Distortion pedal build - diyAudio
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Old 8th November 2011, 01:21 PM   #1
shanx is offline shanx  Canada
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Default Distortion pedal build

Many years ago I had built up a perfboard based on an MXR Distortion Plus circuit and was pretty satisfied with the sound. But the enclosure was not too practical to bring along to practices or shows. I was recently on a thread here on DIY Audio that was discussing different distortion circuits, using transistors in the op-amp feed-back loop, LED clipping, hard /soft clipping etc. Based on that, I decided to use this circuit as a base and build up a pedal that I could more easily modify the circuitry and explore some different techniques and hopefully interesting sounds.

FYI Here is a link back to the original topic thread.
Modifying in-built distortion: transistors in an op-amp feedback loop?

As I go along I will post some pictures and schematics, feel free to send comments and/or suggestions, as it will be a work in progress.
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Old 8th November 2011, 01:48 PM   #2
shanx is offline shanx  Canada
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Here are some pics of the perfboard circuit. I decided to use a larger enclosure than the more standard Hammond cast aluminum type. I had an unused case in sheet steel that was bigger so I could easily make mods as I go..one thing though: the sheet metal is not as rigid so I installed an aluminum plate over where the foot-switch goes to spread out the force when pressing it. Looks a bit different, but works OK
Attached Images
File Type: jpg Picture 006.jpg (16.2 KB, 420 views)
File Type: jpg MXR DIST Build Mod 1.jpg (15.8 KB, 413 views)
File Type: jpg MXR DIST Build Mod Front.jpg (9.0 KB, 411 views)
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Old 8th November 2011, 02:57 PM   #3
shanx is offline shanx  Canada
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First modifications from original circuit:
1-Installing a DPDT footswitch in place of original SPDT for true bypass of the effect board.If all goes well on the build I plan to change it to a 3PDT to put an LED for active/bypass indication.
2-Original MXR circuit uses a pair of Germanium diodes (IN270) for symmetrical clip located at the output of the op amp circuit. That type of post op-amp circuit is considered ''hard clip'' type at least for purposes of discussion. The forward drop on this pair is 0.310 and 0.312 volts and has a more rounded switching curve than silicon, but clipping at lower voltage so not much headroom.
I jumpered some wires in there so I can switch select from the stock pair to a non-symmetrical type post op amp clipping.
3-Another switch will be added to select from hard clipping (post op amp) to a soft clipping circuit within the op-amp NFB loop.
4-Future addition of a tone control, as the original MXR Dist + only had gain and level pots.
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Old 8th November 2011, 03:17 PM   #4
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cant you use a DPDT and just switch the source for the output? isn't it still true bypass, but both inputs are connected at the same time, used it alot with success. i also found that some serious bass rolloff gave quite good results (6db cut at 2k) but needs a fair boost at the end. any schematics for what you've done?
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Old 8th November 2011, 04:00 PM   #5
shanx is offline shanx  Canada
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Hi Razorrick,
Sure, agree it does work to leave both inputs connected, just as with original MXR circuit that I had, but in that bypass arrangement you have the source signal going to the input impedance of this circuit always in parallel with the impedance of next element or elements(amp, another pedal effect). If you have a few effects all bypassed that way it could seriously degrade signal. I say ''could'' because it depends on all the individual effects, their input impedance and bypass modes.
When manufacturer's specs say ''true bypass'' they refer to the bypass as completely switching out the effect from the signal chain at both input and output. Most of the early pedals did not have true bypass, it cost a little bit more money to put in there. My DOD 250 had a SPDT with no LED indicator either. IMO if you find that the straight guitar signal sounds weak when a lot of pedals are plugged in and bypassed, you could look at adding true bypass to them to restore the signal. When I get the mods installed and checked I will post a schematic.
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Old 8th November 2011, 04:07 PM   #6
shanx is offline shanx  Canada
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FYI Here is a diagram (linked to muzique.com) on ''true bypass''
Click the image to open in full size.

Click the image to open in full size.
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Old 8th November 2011, 06:16 PM   #7
shanx is offline shanx  Canada
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The SPDT footswitch I am using is Miyama type. A lot of people use the Carling type, I find the Miyama to be really solid and reasonable price (about $9 CDN)
Click the image to open in full size.
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Old 8th November 2011, 07:13 PM   #8
shanx is offline shanx  Canada
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Sorry meant to say DPDT footswitch
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Old 9th November 2011, 02:08 PM   #9
shanx is offline shanx  Canada
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Cool Locking toggle switches

Thanks to Arick for those really cool, locking toggle switches. Merci!
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Old 9th November 2011, 06:09 PM   #10
shanx is offline shanx  Canada
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Default front panel wiring

DPDT footswitch is installed and wired per previous diagram with in/out 1/4'' jacks. 9VDC adaptor power jack mounted and wired, not using battery connector on this build. FYI on most commercial 2.1 mm jacks for powering pedals, the polarity is negative on pin and positive on barrel. Some advice: use the insulated type jack that won't short to the case, as the case is referenced to negative. Double check to make sure your adaptor is polarized correctly before power up.
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File Type: jpg DIST Build Wiring 1.jpg (13.9 KB, 48 views)
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