Analog Spring Reverb : How it works

Reverberation, you said?

No need to be an insider to understand the effect of reverb, come into an empty room, clap your hands and the sound produced by your hands will reach your ears in different ways. The delay is very short because the sound spreads relatively quickly so the delay will not be noticeable (<20ms) but enough to give a lot of information to the listener.

This type of reverb comes from an acoustic phenomenon, the sound emitted by a speaker will be broadcast according to a cone of propagation. Basically if you are in front of the speaker, you receive everything right in the teeth but at the same time, the sound spreads around the speaker so you will receive this data with a really short delay following a few rebounds on the walls. The bounces that can be seen on this drawing.

This acoustic phenomenon depends on a LOT of parameters, this is a science and it requires a lot of knowledges. This is a simple reminder to understand the rest of the article.

Finally, as you will understand, a hall digital reverb will try to find these very characteristic signal rebounds, which depend on the room where the samples are recorded. To create this effect, some engineers go into physical and specifics rooms to “record” the acoustics and retranscribe it in an algorithm. Some just tweak some algorithm to create a funny response.

Why springs

Now comes the problematic of most of the studios, it is fine to want to capture very atypical rooms but it is not really convenient in your daily purpose. Then, some brillant genius have racked their brains and started to find mechanical systems that could simulate this acoustic phenomenon without using any algorithm (we were in a vintage world folks).
This is how the first plate, corridors and crazy system appeared!

This was still too big for a studio!
Then an other idea emerged, a really simple one, but yet really efficient.

Let’s start by the basic, the audio sound is a wave, instead of spreading the sound in the air and encumbering itself with a lot of architectural constraints, we will mechanically propagate the signal in springs that will also make plenty of bounces through their vibrations and thus create a beautiful artificial reverb!

A simple link between electronic and mechanics

We finally onset the heart of the subject! In this chapter, I will explain how to transfert your guitar signal into the springs and then, receive a delicious reverb effect in your ears.

The transducers

Theoretical approach

Let’s start by a small schematic :

The tank usually has 2 RCA connectors, the white one is for the input and the red one for the output. Input and output have different role that we will describe thereafter.

At the input

At the input of the system, we want to transform an electric guitar signal to a mechanical disturbance propagated in the spring by creating a mechanic oscillation.
In order to achieve this, we will use what is called a transducer. It is on the electronic side a coil that captures and stores the electric guitar signal and on the mechanical side, a system that will disrupt the magnetic field so that the springs will begin to move.

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The electric guitar signal causes an electric field at the entrance of the coil, this signal is dynamic (its level changes permanently depending on what you play, you are a disrupter of our system!). So we know thanks to Maxwell’s law that, this disturbance of the electric field will, after passing in the transducer create a disturbance in the magnetic field lamination.

And this lamination goes around the magnets to which the springs are attached. This disturbance will create a magnetic field, oscillating with a waveform really close our electric field. This oscillating magnetic field will then provoque a movement of the magnet of the springs and then this is how your springs finally move according to your guitar playing.

At the output

This is the same phenomena but on the opposite way! The springs oscillate to your guitar waveform and also has bounces thanks to rebounds on each side of the tank. Then the magnetic field of the lamination is disturbed because of the oscillation of the magnetic and this will create an electric signal on the output coil.

How to create the reverb effect

As the spring will diffuse the different waves, it will at the same time receive input disturbances from our transducer, while the output signal will bounce on its fasteners. The different signals will mix and create a very atypical sound. Each system therefore has its own characteristics that depend on the materials used, the stiffness of the springs and their size.

Guitar signal shaping

So the first element of our transducer is a coil and this is where we need to send our signal. To send a signal in a coil with 100% of the initial power, in theory, it’s better to adapt the impedances! It’s like withh your amp, you are asked to connect an amp in 8Ohms on a cabinet in 8Ohms, otherwise we loose some signal. It’s the same thing !

As you can see, the quality of the audio system that will drive your spring tank depends first of all on this impedance adaptation. To do this we have several solutions, and here is one with valves:

For most of the Accutronics tank, the input impedance works with 8Ohms and the output with 2250Ohms.

Your guitar will first enter the tube to get some gain and probably correct some frequencies. Taking in consideration that the 12AT7 has an output impedance of about 20kOhms when its triodes are in parallel, as on this schematic. (to be confirmed by those who work with tubes more than me)
Therefore, the output signal of the tube will not be adapted to enter the spring tank, it will need to use a transformer to go from 20 kOhms to 8 Ohms.

Recover an imperfect signal

The signal is injected into the transformer, it disturbs the magnetic field and finally it spreads in the spring! Now that it makes a lot of rebounds in the system (that’s what gives the beauty of this reverb) we are facing a problem!

All these electrical / magnetic / mechanical conversions … it creates loss! On our first tests we lost almost 20dB of signal! In addition, we do not even talk about the bandwidth … There are only mids! Forget about your bass and treble.

That’s why on the output of the reverb tank, we must provide a preamp capable of recovering what has been lost without creating noise. It also needs to reshape the signal for the rest of the circuit.
On the Element we have used very low noise OPA, this had permit us to offer you a reverb that manages full wet signal with the same level of output as at the input!

Then we developed a 2-band EQ that could sufficiently reshape the spectrum so that it is balanced and not only dependent on the bandwidth of the spring tank.

Precautions

You get it, the signal that enters into the tank is over-vitaminized! Due to its low impedance, the current is very important. He got every good characteristics to be able to go through the transducers in the perfect way and drives perfectly your springs.
On the other side, the output side of the spring tank is not that lucky, we have to recover a weak signal because of the mechanical and transducers lost. So we apply our beautiful layers of make up to give it a second youth.

So if you like your signal, leave the output connector as for as possible from a 60 cycle socket or source!!! The current is so weak at the output that it will be sensitive to strong disturbances. Your 60 cycle will go into the preamp, it will be re-amplified and maybe even with a little luck it will saturate and create harmonics (100Hz, 150Hz etc …) Then good luck to remove them!

But do not worry, the solution is very simple! Leave the input close from the 60 cycle and put the output at the opposite side. Or, stick your spring tank into your speaker cabinet and run a cable from your amp to your cable. You can use at maximum a 20m mini-jack to RCA cable. In addition, we are working on a video or an article filled with tips and tutorials to show you how to connect your tank safely 🙂

I hope with this article that the spring reverb will never have any secrets to you! If there are some points you want me to develop, go for it! Also if you want to discover our Element, here we go :