Spare Room Reverb

Phenomenal Reverb Powers, Itty Bitty Pedalboard Space

Overview

OK, so maybe attributing phenomenal reverb powers to it is overselling it a bit much, but there is no denying that you don't need a pedalboard mansion to have some reasonable reverb. Just a little spare room (get it!?) and you can eliminate the dryness from your pedal chain. While the Spare Room won't get you the same lush, dense reverb that the T60 or Riptide will, it is a fantastic effect for the space. Again, no brick here, so you know it will be economical, as well.

In the DIY world, reverb projects seem to be few and far between. Most that are out there rely on the Belton brick or the FV-1 DSP chip. Both are great options, but both are also approximately $20 each, which makes for a pricey component. One of the more popular DIY reverb projects that requires neither of these is the Equinox II by Merlin Blencowe (AKA ValveWizard). When researching for the T60 Reverberator, I built the Equinox II and didn't really like the sort of metallic ringing sound it had to it. I moved on from it without any real experimentation.

After completing the T60, I decided to try my hand a simplified reverb. In doing so, I came back to the Equinox II and found that a few moderate changes made for a much more pleasing effect, thus the Spare Room.

The Spare Room PCB is just a little larger than the potentiometer footprint, making it ideally suited as a standalone reverb module for small practice amps, multi- effects pedals, or just in a small enclosure to use the spare room on your pedalboard.

How It Works

As mentioned, the Spare Room started life as the Equinox II, but there are some notable changes which make it it's own thing. The primary changes from the Equinox II are:

Remove JFET bypass for tails
Send the output of stage one through a filter and feed it into the input of stage two
Modulate the delay time for second delay stage using a simple LFO

The removal of the JFET and associated components for tails bypassing was a more logistical thing, as I wanted to put this into a 1590a enclosure. Could I have squeezed it in there? Maybe, but I didn't feel comfortable doing it with the board space I had left due to ground plane considerations. If you are building it in a larger footprint and want tails, go for it.

Taking the output of stage one and feeding it into stage two allows for a perceived third delay line in the signal. We all know that the Belton brick has 3 PT2399's in it, so I figured if I could get a third delayed signal, it would help provide a sense of additional space. However, as sound propagates, some frequencies get lost. In a room, the high frequencies get dampened due to absorption of both surfaces and air more readily than lower frequencies, so I passed the signal through a single order low pass filter to roll off a touch of the highs to give a more realistic effect.

The addition of an LFO providing subtle modulation of the delay time of the second chip makes a world of difference. The source of the metallic ringing in the Equinox II is having static delay times that build on top of each other. With just a little modulation to the delay time, this effect is drastically reduced and the resulting reverb effect is far more pleasing. The LFO used in this case is a single transistor design from the modified EA tremolo. This is a great LFO for space-conscious designs, which was a requirement for fitting all of this into a 1590a, even going full surface mount.

The Spare Room uses the same input buffer as the Equinox II, as it works well enough and I already had it on the breadboard.

Spare Room Input Buffer

Next up is the first delay stage. The input filtering to the PT2399 uses a slightly different topology from my other designs. Instead of being a multifeedback design, this is a more straightforward low pass filter design. This is a very basic stage, though notice how pin 12 has two outputs. The TO2 marker indicates that it will go to pin 12 of the second delay stage, just like the original Equinox II. However, R7 and C11 provide low pass filtering and R8 connects the filtered output to the input of the second delay stage.

Spare Room First Delay Stage

The second delay stage is another fairly straighforward delay stage, except for the addition of the LFO signal between R32 and R10 off of pin 6. The delay time is set as two similar resistors so that the LFO does not (a) go to ground and (b) go directly to pin 6. If the LFO goes to ground, then there is no modulation. If it goes directly to pin 6, it can actually cause the chip to latch up under certain circumstances. Notice the filtering networks starting with C20 that filters the outputs of both delay stages before being sent to the mix control and output buffer.

Spare Room Second Delay Stage

Once both delay lines are filtered, the signal gets sent back to the feedback input for both chips as well as the mix control. The mix control and output buffer are set up the same as the Equinox II, because if it ain't broke, don't fix it. The output buffer adds some additional brightness compared to the true bypassed signal, but overall volume remains comparable.

Spare Room Output Buffer

In order to minimize size, a single transistor LFO was chosen. Opamp-based LFO's are great, but take up much more space, which is at a premium with a 1590a project. This LFO comes from the modified EA tremolo that has been a mainstay in the DIY community for some 20 years now. The depth and rate were carefully tuned to provide a subtle modulation that results in delay times that shift just enough to eliminate the metallic ringing effect of the static delay times. It is a slow and subtle LFO, but extremely effective. I actually quite like this LFO and will likely use it whenever a static rate and depth LFO is needed.

Spare Room LFO

The power section is pretty standard. It provides +9V, +4.5V for VREF, and +5V for the PT2399 chips with appropriate filtering.

Spare Room Power Section

Conclusion

Now that you know what it's doing and why, how about building one yourself? You can grab the Gerber PCB files, build documents, and schematic here. Please remember that this design is provided to the DIY community free for personal, non-commercial use. If you want to use it commercially, please contact me. Now get building!