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 Here’s the post, but you need to register to see it. Here’s the drawing I studied:


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


Tagged , , , , , , , , , ,

26 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.


  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.

    • 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?


      • 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.

      • 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.

  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?

  5. honi says:

    Hello! First of all, great build, looks awesome.

    I’m having some trouble with my build and was wondering if you could give me a hand.

    I’m confused about all the grounds in the schematic. It looks like there are three grounds: GND(T), GND and PGND.

    PGND is the regulator’s (LM78L05) ground pin, connected only to PGND pin on the BTDR-2H.

    What about GND and GND(T). Are they ever connected together? I think not, GND(T) is the ground from the 9V power source, only used in the “power supply” section on the upper left of the schematic, while GND is obtained on the negative pin of C11 cap.

    The input/output jacks connect to this GND, which I believe is the signal ground?

    So where do all these grounds interconnect? I believe this configuration is called “star grounding”? If so, would this happen inside the BTDR-2H? It has PGND and GND, so it could connect them together inside. But GND(T)…?

    I get very strange readings on the regulator. If I manually connect the grounds, then I can successfully read 5V in the output of the regulator. Could this be a faulty BTDR-2H? Is there anyway to test this?

    Sorry for the long post! I’m reading the link you posted about virtual grounds. Any answer or link you can provide would be helpful. Thanks!

    • simonallaway says:

      I rechecked the schematic and the layout that I used and there’s definitely only two grounds to worry about. PGND is, as you say, the connection between pin 2 of the voltage regulator and pin 2 of the BTDR-2 (not the BTDR-1).

      The common GND is the same as GND(T) on mine, so I’m not sure what the significance of the ‘T’ is. On the layout I used that’s treated a a ground rail for the entire circuit, and connects to pin 4 of the BTDR-2. The input/output jack rings will connect to this too.

      The layout I used is here:

      Look for picture 3 of 3.

      • honi08 says:

        Hi Simon, thanks for the reply!
        I’ve managed to get it running! I was missing wire between pins 6 and 7 of the TL074.

        Now the only problem I’m having is that there is no mix with the clean signal. If I turn the 25K reverb pot all the way counter clockwise, there is no sound at all. The other way around, only the reverb. Do you have any ideas?


      • simonallaway says:

        That implies there’s something wrong with the last op-amp (IC1D on the schematic), as that’s where the dry signal should appear. The reverb pot is just a mixer at that point. I’d check the connections on pins 12, 13 and 14.

        You’re so close!😉

  6. honi says:

    It works!! I have the effect built in the breadboard, and there was a wire too short and it wasn’t making any contact.

    Thanks a lot for all your help!

  7. Dio666 says:

    I would like a version that is reverb only with no dry signal, (I’ll be running it parallel to the dry signal into a different amp channel). Can I just eliminate the second op-amp? And maybe the first op-amp too? I basically want it to operate like honi’s did before he “fixed” it.

    • simonallaway says:

      You might try simply leaving out the connection between the two op-amps. I.e. take out the dry signal. The knob will still give you a level, if you still need it. Otherwise replace that B25k pot with a wire.

  8. Dre Rosa says:

    Nice sound! Did you use the long, medium or short decay belton module?

  9. honi08 says:

    Hello! So I’ve finally managed to build the circuit. I’ve designed a PCB with Eagle, etched it and soldered everything in.

    The effect is working as expected / But I’m having a grounding issue… as there is a constant humming whenever I plug in the effect. I’ve checked all connections and tracks and can’t seem to find anything wrong. I’ve also tried with 2 different 9V power supplies.

    Any ideas on how to track down this kind of problem?

    • simonallaway says:

      You can try the “poke at it” approach. Take a length of wire connected to the circuit ground and poke it at the places where you would expect to have a ground connection….and see if the humming goes away. You can confirm the validity of a power supply buy trying it with a different pedal.

      If you suspect it is power supply, then I would check all voltages around the circuit. And check for continuity from one cord sleeve to the other, and obviously with the ground points on your board.

      If that doesn’t reveal anything then I’d try an approach with a signal probe. Take a piece of wire connected to your amps input, and poke that around the signal path of the circuit, beginning at the input, and make sure you have a clean signal as you work your way back to the output. In essence you’ll be bypassing most of the circuit at first, and then including more as you follow it back. This should help spot where the noise is being introduced. This is easier with a signal generator of some kind as the input into your circuit, rather than a guitar; I have a cheap one I made from a kit. I use a sine wave at 1,000Hz at 0.125V as the input.

  10. Devon Arreola says:

    Hey I was trying to build your circuit but for some reason every time I connect the ground of the voltage regulator to the power ground of the btdr-2h it starts to get really hot. I have the ground connected to the signal gnd of the btdr-2h. I was wondering if you could maybe give me some advice on why this is happening. Thanks.

    • simonallaway says:

      I have no idea why that is happening, to be honest. Voltage regulators nromally get hot when they’re being forced to drop a load of voltage, in the form of heat. Are you definitely supplying just 9 volts to it?

      Is it still functional?

      • Devon Arreola says:

        I checked the voltage, and I am supplying 9 volts to the LM7805. The problem seems to be that the power ground of the btdr-2h is not seeing the ground connected to the signal ground, so the voltage regulator is not properly grounded.

        I can get the circuit to work with a small amount of noise if I connect the ground of the voltage regulator straight to ground instead of connecting it to the power ground of the btdr-2h.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s


Get every new post delivered to your Inbox.

Join 30 other followers

%d bloggers like this: