Monthly Archives: June 2012

Prototype reverb pedal using the Accutronics/Belton BTDR-2H module

As a follow-on to my previous post about building a breadboard prototyping rig, here’s some information about my first attempt at a circuit.

The schematic

I found a schematic within the application note for the module itself, on this page but it did not include schematics for the power supply circuitry. I’m still so new to op-amps that I wasn’t about to make things up on the fly. I then stumbled upon this schematic on freestompboxes.org. Here’s the post, but you need to register to see it. Here’s the drawing I studied:

Box_of_Hall_Schematic_V1-1

So if you break it down into discreet sections, it’s actually rather simple:

Power supply

In many respects the power supply section was the eye-opener for me on this project. I’ve worked with tube amp supplies before, but never one designed for not only op-amps, but with provision for a ‘module’. So what you see at the top of the drawing is exactly that; 9V coming in from the wart/battery, a little bit of filtering, R1 and R2 dividing the voltage, to get 4.5V for op-amp bias, and the 7805 regulator to give the reverb module 5V. The voltage from the divider goes into one of the opamps to be regulated.

Shawn over at DIY Effects very patiently helped me understand the ins and outs of op-amp power, which was very useful. I had read a lab paper that has some great exercises for exploring op-amp behavior: here, which insisted that op-amps must be powered appropriately, regardless of whether the schematic indicates this. I’m used to tube amp schematics omitting the heater wires (they’re always there, so why bother?) so it became clear with Shawns help that this was true, and that I had simply missed the part of the schematic where pins 4 and 11 are connected to the TL074 quad-opamp. Duh!

Also, most articles focussed on theory talk about bipolar power supplies providing + and – voltage for the opamp, and then a virtual ground in between for bias. This didn’t make much sense until I realized the schematic here provides +9V from the main source, 4.5V for bias, and zero V ground. In relative terms this is identical. Here’s a good article on ‘virtual ground circuits‘.

Anyway, that top section of the schematic was my starting point, so I made reasonably fast work of getting it all working on the breadboard. I was able to measure 9V on the main rails, 4.5V on VB (meaning Voltage for Bias, we supposed) and 5V at VA (for the module. All cool so far.

The rest

The next day I just continued by working my way from left to right on the schematic, wiring up components and crossing them off the printed schematic. Again, through building tube amps I learned to mark progress as I went along; it really helps me as I have a terrible short-term memory.

The challenge at this point was to translate the schematic to a breadboard layout, which I had not done before (other than the 555 LED blinker project from ‘Electronics for Dummies’). I have no magical insight here, I was just lucky I think as this circuit doesn’t sprawl across the board; it’s very simple.

Mistakes were made, absolutely. When I first fired it up I got no sound, but this just meant I got to debug it. I went back with my multimeter and checked power. It was clear that something was wrong as I was getting about 7V for VB, which should’ve been half of the main 9V rail (due to those two 10k resistors between the rails acting as a divider). This led me to check all the connections surrounding the opamp, and I soon found that I had screwed up just two wires, and forgotten to ground the opamp at all. By this point I had a guitar and amp connected, so when I powered it up for the 3rd time, I knew I had a signal. And sure enough it works!!!

Bread-board chaos

Bread-board chaos

UPDATE! I managed to get this circuit onto veroboard, using the ‘Box of Hall’ layout that’s floating around online.:

The circuit on veroboard.

The circuit on veroboard.

Here’s a recording of the unit, with a clean sound to start of course:

UPDATE! July 29th The reverb pedal now has an enclosure…

The guts

The guts

The cheesy graphics

The cheesy graphics

 

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Breadboards and prototyping pedals

As time goes on I become more and more addicted to futzing with pedals, which is a good thing I suppose. But it is tempting to just keep building kits without really knowing whats going on in there. I’ve made attempts to tweak the pedals I’ve built from DIY Effects but only to the extent of messing with clipping diodes. And to be honest they sound great already, so there isn’t much to be done. What I really want is to go back to basics and learn how different parts of the circuit work and also how they interact with others.

So I needed a way to, for example, build a simple transistor gain stage (like the SHO I did before) and perhaps chain it with another one, to see what happens. Or build a generic fuzz and then play with bias values or different transistors. You get the idea. All this is harder to do if you are constantly soldering/desoldering components, so it’s time for breadboards 🙂

This project is nothing magical, just some parts assembly, but it was a lot of fun. I based my version on the Beavis Audio board that you could once buy in kit form. Here’s the link to the Beavis site, and a photo of mine:

The prototyping board

The prototyping board in its first incarnation.

What’s going on?

Power comes in from a small bench supply I have (not shown) via a standard 2.1mm socket on the back of the breakout box. That goes into the 10k pot, which lets you simulate crappy batteries. ALso not visible are the two 1/4′ jacks that serve as guitar in and effect out signals. You can clearly see the 3PDT stomp switch and the smaller toggle. All these connections are present on the connection block which I bolted to the side.

I also added a board on the back with some handy holes drilled in it. These will hold whatever pots and/or switches the circuit might need. Shawn from DIY Effects had done this, and it looked like a great idea, so I stole it 🙂 Here’s his blog entry showing a massively complex circuit on his protoboard. One day I’ll get to this stage!

What’s next?

  • Build a reverb circuit, probably the ‘Box of Hall’ circuit featuring the BTDR-2H reverb module. It should be very simple.
  • Build a generic fuzz-face and then progressively mod the crap out of it to see how it works.
  • Try some cascading gain stages (based on the Z.Vex SHO) to see what following the tube amp topology achieves.
  • Arduino!!! (this is a potential can of worms that will end with a switchable rack system…mark my words)

UPDATE! Here’s the Box of Hall circuit on veroboard. I still used the breadboard rig to hook up to power and I/O. And with great joy I realized that my little Marshall practice amp (if you can even call it that) clips onto the the back quite nicely 🙂

The circuit on overboard, connected to my huge Marshall stack.

The circuit on overboard, connected to my huge Marshall stack.

UPDATE 2: I did a prototype of the classic Fuzz Face. Not much to see, but it’s proof that the breadboard has made it very very easy for me to experiment with different circuits.

Silicon Fuzz-face!

Silicon Fuzz-face!

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