Last Breadboard of 2022: The Timbre Man


Well-known member
Why is that 78L09 there? you can't regulate 9V down to 9V.

I don't know, that's an exact copy of PedalPCB's Basic Relay Bypass. I posed the exact same question a while ago, but got no replies.

Anyway, I just hacked in the missing R10/27k I mentioned in my previous post, now it sounds legit! Definitely a low gainer, but a serviceable amount of gain. Frankly, it sounded pretty bad without the 27k! My kids just went to bed so I could only do a quick and quiet test, but I can already tell it's right in line with what I was expecting.

I'll spend some more time with it when I can open the amp up a bit. But I can say with certainty I just flipped from discouraged to encouraged!


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Well-known member
So I've spent a decent amount of time playing on this. I've played at home a couple times, both through a real amp and my headphone amp, and I used it a bit at band practice a couple nights ago. It sounds great through my headphone amp (DSM Simplifier), some pedals that sound great through a real amp sound like poo through the Simplifier.

Assuming the R10 error mentioned above is the only "real" mistake, and I didn't make any build errors (bold assumptions, admittedly!), my personal preference would be to see two tweaks:
  • I find only the last 1/3 or so of the treble pot sweep to be useful, it's much too dark otherwise. I'm playing through a G&L ASAT with the jumbo MFDs into a Hiwatt DR504 clone, I think most would consider this a naturally fairly bright rig.
  • I would like a little more dirt capability. The clipping diodes are a 3+2 config, what I made it a 2+2 config, with each side being a SI and GE? Or is there a "better" way to get a little more drive out of it?

I re-did the PCB layout. I'm not sure if it's really any better than before (except that the known bugs have been fixed of course). I went "premium" and did a four-layer board: signal traces on the top and bottom layers, power traces on one internal layer, and a solid ground pour on the other internal layer. Probably falls into the deliciously overkill category, but if nothing else, it sure makes routing easier!

I haven't submitted this updated revision for fabrication yet, but here's KiCad's raytraced render of the PCB:

Chuck D. Bones

Circuit Wizard
Definitely overkill. You'll pay a premium for the extra layers will little or no performance benefit.

You can get 6dB more gain by removing R8. Not a huge increase, but maybe enough.

Chuck D. Bones

Circuit Wizard
More likely it's to limit the relay's coil current. LM555 (NE555, SE555, etc) max rating is 16V. When a 78L09 is powered by 9V, the output is around 7.5V. The 555 drops about 1.5V from Vcc to OUT and we lose another 0.2V in D2. That gives us ~5.8V on the coil when powered by 9V and ~7.3V on the coil when powered by 18V.
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Well-known member
More likely it's to limit the relay's coil current. LM555 (NE555, SE555, etc) max rating is 18V. When a 78L09 is powered by 9V, the output is around 7.5V. The 555 drops about 1.5V from Vcc to OUT and we lose another 0.2V in D2. That gives us ~5.8V on the coil when powered by 9V and ~7.3V on the coil when powered by 18V.
Thanks for clarifying @Chuck D. Bones! I didn't catch the absolute maximum rating when I checked the NE555 datasheet.


Well-known member
I finally finished the buildup of my fixed/revised PCB, build report here.

I have a few extra PCBs I'm willing to give away, please PM me if interested.

Here are various project files if you want to have your own fabricated and/or further tweak the design. There are two PDFs, the BOM and schematic. There are two files that end in ".zip.txt", they are actually .zip files, but the forum won't let me attach zip files, so I simply added ".txt". They are the Gerbers I submitted to JLCPCB, and the KiCad project files.

Thanks again @HamishR and @Chuck D. Bones!

Edit - Note that the PCB uses a 4.5v non-latching relay, tested only with the Fujitsu A-4.5W-K. Latching relays (such as the Panasonic TQ2-L-5V) definitely will not work. Furthermore, even among latching relays, other manufacturers' relays may use a different pinout, so it's impossible to say if they'll work without checking on a case-by-case basis.

That said, it won't be pretty, but you can omit the integrated bypass circuit entirely and wire to a mechanical 3PDT using these instructions:
  1. Omit the following components: K1 (the relay), IC101 (78L09) IC102 (NE555), Q101, D101, D102, R101, R102, R103, R104, C103, C104, C105
  2. The FOOTSWITCH solder holes (lower left) will be unused
  3. C10 can be reduced to 100uF (optional, left at 220uF is ok)
  4. For the input/output jacks: wire the grounds to the PCB, and the signal wires to the 3PDT (like you would with a typical PedalPCB board); the IN and OUT solder holes at the top of the board will be unused
  5. Wire the 3PDT switch as you would for a typical PedalPCB board. My PCB doesn't have the familiar IN, GND, SW, OUT solder holes, but the equivalent holes are (mostly) in the relay solder pads. In particular:
  6. Relay pin3 = IN
  7. Relay pin7 = OUT
  8. Relay pin10 = GND
  9. For SW: your LED anode ("positive" side, longer lead) should be wired to the board's LED+ solder pad; the LED cathode ("negative" or "ground" side, shorter lead) will be wired directly to the 3PDT solder lug that would typically go to a PedalPCB SW pad (top middle if you follow PPCB's build docs). My board's LED GND pad will be left unused.

Relay pin numbering: if you are looking at the board as in the rendered picture above: pin1 is top left, pin5 is lower left, pin6 is lower right, pin10 is top right. Like this:
1 -------------- 10 (GND)
2                9
3 (IN)           8
4                7 (OUT)
5                6

I haven't tested this, but assuming I thought this through correctly, it should work ;)

Edit2: I just added another attachment, the offsets of the pots for enclosure drilling. This assumes a 125B enclosure. I used a random PedalPCB build doc for the top I/O, and the footswitch and LED go "at the bottom" (see my build report for an example), I don't have specific measurements, for those, I basically made enough room for the momentary switch, and put the LED on the same center.


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