Doepfer A-199 Spring Reverb Schematic

• Oct 27, 2023

I recently found a second-hand Doepfer A-199 Spring Reverb module on sale near me for a good price. Nobody has posted its schematic online yet and I was curious so I decided to pick it up and trace out the schematic.

And here it is! Enjoy.

The circuit is mostly quite boring really. The amplifier driving the spring tank, which is the part I was most curious about, is just a LM386 set up pretty much exactly like the Typical Application circuit from its datasheet. And all the mixing and blending stuff is very standard (it all works well enough though so I can’t really complain). The interesting part of the circuit is the emphasis control. I don’t know exactly what that filter topology is called or how it works, but I did simulate it and found it boosts the frequencies around 3-5kHz by roughly 12dB when turned all the way up.

I might update this page with frequency response graphs later if I can figure out simulation loops in ngspice.

Power input rant

There are a couple issues with the power input of the module.

1. The module doesn’t have a shrouded power header, so there is nothing preventing you from pluggin in the power backwards.
2. There is no polarity indicator of any kind anywhere so you can’t know which way the power is supposed to be plugged in unless you read the manual or trace out the schematic.
3. There are no protection diodes on the power input so if you do plug the power in backwards (which is a lot more likely than it should be thanks to issues 1 and 2) you will fry the ICs.

This is awful design. Shrouded power headers cost a couple cents each, diodes are even cheaper, and putting a little stripe on the silk-screen is literally free. There is no excuse for this. Especially not in a module that’s otherwise really well made and that costs 120€ new.

At least the ICs are socketed so you don’t need to dig out your desoldering pump if you plug the power backwards and kill them…

The fix

To prevent accidents in the future I replaced the power header in this one with a shrouded one.

Tank switch mod

The person who owned this module before me (or the person who owned it before them) modded it to add a pair of RCAs and a DPDT switch on the front, so you can connect it to a bigger reverb tank outside of your rack if you want to. It’s a pretty neat mod. Below are some pictures.