Build Report: Torchy's BassThru Layout (Now With PICS!)

[tommy.genes]

I've recently finished building a BassThru (a/k/a Bass Paralooper) using Torchy's layout, so here is my report. It would have been nice to post this while Torchy was still here but, after fixing a few errors, it works! Interestingly enough, I found someone at the eden-electronics.com bass amp forum (screen name: Warwick) who wanted to build the circuit at the same time, so we worked out the bugs together.

First, the errors. The original schematic can be found HERE, but you have to PM me with your email to get Torchy's layout.

1. The layout shows R9 connecting to pin 7 of U2, but it should go to pin 7 of U1. This is a critical fix. Without it, your pedal really won't work, and can squeal with oscillation if you turn the Blend knob all the way to "dry".

2. The layout incorrectly lists R11 as 1M when it should be 10k. This is not so critical. Because of the gain structure of a non-inverting opamp (1+R2/R1), you can still get close to unity gain out of the pedal even with the 1M resistor installed for R11. That's the way mine is now. To be the most flexible, however, it would be nice to be able to add a little  boost at the final stage of the pedal, so I plan to replace the 1M with the 10k in the near future.

3. At the bottom right of the layout, a label which reads VR2-2 should actually read VR3-2. It's right below the label for VR3-3.

This was my first-ever pedal build, although I have done some minor soldering and repair before on various household electronics. I might have liked to start with an even simpler circuit, like the Bazz Fuss, which I plan to build next, but I really wanted this one right away. It's still pretty simple, there are no high-speed clock signals for example, but there are more components than other simpler circuits, and there is more involved with stuffing it all in a box. I'm glad I used an Eddystone C box, which is similar in size to a 1590BB but deeper, to help me get it all in. I'll try to plan out my wiring looms a little better in advance next time.

Now for circuit impressions. As a basic bass blender, it works great; clean and clear. If you want to use effects on bass, you really need one of these or a similar unit like the B Blender. Unlike the original schem, I wired mine with a 3PDT true-bypass, but it probably doesn't matter much because the first stage of the circuit is a unity-gain buffer. My only complaint is that the LPF filter used in the "dry" path of the circuit is a bit, well, whimpy. At the very least, I will try a bigger pot in the RC circuit to get even lower cutoff frequencies. I suspect, however, that a higher-order filter or even active boost/cut bass and/or treble controls would be more useful here; perhaps even a more bass-tuned BMP tone control. If you you don't feel a tone control is necessary, omit it, in which case you are essentially building a B Blender. If you really want the options of a tone control here, I'd investigate something more sophisticated.

I also tried to be a bit fancy and added a feedback loop as shown below. Problem is, it doesn't work. There is no noticeable difference in sound at most settings of the pot, except when the pot is all the way up. Then, instead of getting maximum feedback, the effects loop is completely bypassed and a clean sound (no effect at all) is heard. Any ideas as to why this is happening? This seems to be how all the other feedback loops I've seen work.



Finally, my biggest "lessons learned" from this, my first build:

1. Verify any layout described as "unverified" before you build it.

2. Breadboarding any circuit, even one with a finished layout, is probably a good idea before building. (I actually already knew this, but I wanted this pedal ASAP.)

3. Just go ahead and buy a Unibit. I know they are pricey, but the alternative is crappy-looking holes and ruined conventional bits.

4. Don't use too heavy of a wire gauge inside the box just because you have it. It is harder to get into terminals and board holes, and the lack of flexibility hurts during final assembly.

I will post pictures either later today or after the weekend. Maybe I'll get ambitious enough to post sound clips, but I don't think that would be terribly illustrative. It's possible to get effects to sound good on bass at home, but on stage is where you really notice that the bass just disappears when the effect is turned on. So it's in a live situation where this pedal (or others like it) really shine, and I don't think I can capture that with my home computer.

-- T. G. --

[Disco_Gee]

Thanks for the report! I'm about half way through making the Paralooper with Moosapotamus' layout. I haven't seen Torchy's layout, is it drastically different? What caused you to choose that layout over Moosapotomus' one?

I can't help you with the feedback loop problem. I'm not too cluey with this electronics stuff. I can read a schem, I can solder, and that's about it! I only really understand the very basics.

[tommy.genes]

I haven't seen Torchy's layout, is it drastically different? What caused you to choose that layout over Moosapotomus' one?

Mooseapotomus' layout is for a PCB, and Torchy's layout was for stripboard a/k/a Veroboard. I went with the stripboard layout because I was trying to limit the number of new skills I would have to learn on my first build. I'll look into etching PCBs in a little while.

As for circuit differences, Torchy based his layout directly on the original BassThru schematic. As far as I can recall (don't have it in front of me), Moosapotomus had made some changes to the filter section -  toggle switch between two cap sizes versus a pot for cutoff freq. - and substituted some values for other parts, I'm guessing based upon what he had in stock at the time.

-- T. G. --

[tommy.genes]

Closeup of circuit board showing jumper to correct R9 error on layout


The circuit as tested, before boxing


The inside, after boxing


The outside, showing the "temporary" execution of the graphics which is gloss photo paper held on for now by epoxy. I plan to do a more permanent version with paint and water-slide decals or iron-on transfer once I'm fully satisfied with the circuit.


Networking geeks should recognize the icon...

-- T. G. --

[nex_06]

could someone explain what it is that this does? :oops:

[niftydog]

in very basic terms, it passes the low frequency component of the signal straight through, whilst sending the high frequency component via an effects loop then mixes the two back together.

It's intention is to allow bass players to retain the low frequency in their signal whilst using tone sucking effects pedals. Especially useful if you have guitar effects in your signal chain, as they are not designed to pass the low frequencys.