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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11 thoughts on “Prototype reverb pedal using the Accutronics/Belton BTDR-2H module

  1. SLW says:

    That looks great. I am looking forward to the sound clips.

    SLW

  2. Jorge Lopes says:

    Hi Simon, how you’re doing?

    What are your toughts about this reverb compared with the expensive brand ones, like cathedral and etc?

    Tks for share your experiences.
    Jorge

    • simonallaway says:

      Hi Jorge,
      This reverb circuit definitely does only one thing, and that is a simulation of a spring reverb. So it’s not very flexible at all in the way that the EHX Cathedral is. But what it does do, it does it very well.

      Are you thinking of building one for yourself?

      Simon

      • Jorge Lopes says:

        Hi Simon,

        Yep, I was searching for a simple one spring like reverb and it seems that it does the thing quite like, right?

        I’ve been looking around these accutronics units form some time, researching and I’ve already drawn a schematic similiar with the one you’ve used. Based on the datasheet. Did you compared with other reverb units? What are your opinions?

        Tks for the blog. Nice see you sharing your experiences! Hope I can share mine sometime :)

        See you.
        Jorge

      • simonallaway says:

        Yes, it absolutely does do a great impression of a spring reverb unit. The only thing it won’t do is the sound you get when you kick it ;)

        I didn’t necessarily compare it with any modern units, but I’ve used so many different reverbs over the years I knew what the sound would be. I admit that I wasn’t expecting anything quite so high fidelity as this turned out to be. So I’m very pleased…especially for the price.

        I’m glad my blog has been useful to you, and I definitely encourage you to do the same :)

  3. Jorge Lopes says:

    Hi Simon. Really nice to hear your nice comments about this reverb. Now, I will certanly build a couple of those to test and tweak! :) I don’t think I will miss the sound of a reverb beign kicked xD.

    Yep, about the blog I’ve doing it in a little bit less technical way yet and in portuguese. But I’m thinking in doing something in both languages. Many toughts. =]

    tks again.
    Jorge

  4. dtaboso says:

    Today I built this layout. And it have low volumen and signal with many dustorsion.
    Voltage in 8.78v but voltage bias 8.72
    And it must be 4.5v more or less.
    I had cheked all and I dont see the problem
    Any idea plesse?
    Thanks
    David
    Spain

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