Summary of design choices:

• Use an active drive stage, since line outputs don't have enough current to move the springs
• Leave the makeup gain out and use external microphone preamps to keep cost down
• Leave EQ mostly to other devices
• Select the tanks by ear after testing what's currently available

Input Stage

The outputs on audio interfaces, converters, and consoles are line outputs. They send a voltage (around 1 or 2 volts) that represents the waveform, and the device receiving it reads that voltage.

It's helpful to think of the input side of a spring reverb tank as being similar to a speaker. A coil of wire sits in a magnetic field, but instead of moving a cone, it's attached to one end of the springs.¹

Speakers and headphones need power to be driven, which is why they have amplifiers. A line output sends voltage but does not have enough current to make anything move.

stereospring includes a drive stage that turns the line signal into enough current to drive the springs.²

Note: the tank connections are unbalanced RCA, but the inputs to stereospring are balanced TRS, which are better at rejecting hum and interference picked up along the cable.

The Springs

I tested many of the currently available options for springs, which you can hear on the Samples page. The main differences are decay time, number of springs, and input and output impedances.³

Output Stage

At the other end of the tank is a second coil that works in reverse. The springs move it, and that motion becomes voltage. A dynamic microphone works on the same principle, except the coil is moved by a diaphragm in air rather than by springs.

Like a microphone, the level that comes out is very small and needs a lot of gain to be useful.

External Preamps

Most spring reverbs have a preamp inside the unit for makeup gain. But the stereospring tanks outputs go straight to the XLR connectors at microphone level and require you to use your own microphone preamps.

This decision came down mainly to cost. High quality preamps are expensive. Rather than paying for preamps built into the unit, you can use what you already have.

Note: the tank's output coil is inductive, so the microphone preamp's input impedance changes the sound, with higher impedance usually keeping more of the top end.

EQ and Predelay

I chose to leave EQ mostly out of stereospring. There is a gentle high-pass filter around 65 Hz in the drive stage, but I recommend experimenting with your own filters both before and after stereospring.

Predelay is worth experimenting with too. The tank's decay is fixed, but moving where the reverb starts is a good way to fit it to different tempos.

Components

The components come from reputable manufacturers with published specifications and consistent tolerances, chosen for reliability and availability:

• THAT Corporation 1246 balanced line receivers
• Texas Instruments NE5532 op-amps
• STMicroelectronics BD139 / BD140 output transistors
• WIMA MKS2 polyester film capacitors
• Traco Power TEC 2-1223 isolated DC-DC converter
• Nichicon Muse ES bipolar electrolytics
• Neutrik XLR outputs
• Switchcraft TRS inputs and DC power inlet
• Cicor T6639ST 12V regulated power supply
• Accutronics / MOD / Belton reverb tanks

The circuit boards are manufactured by JLCPCB and populated with the surface-mount parts (resistors, ceramic capacitors, diodes, op-amps and output transistors), which helps reduce cost and build time significantly. Everything else is soldered and built by hand in Minnesota.

Enclosure

I chose to use wood partly to keep costs down and partly because I like the look and feel of it. Since the tank's own shielding varies with orientation, I added a steel sheet to the bottom of the enclosure for additional shielding against magnetic interference.