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Custom Unit: Spring - feedback loop based on Tilde Elektriske FJÆRLETT

Heres a custom unit i made called Spring.
It’s based on Tilde Elektriske FJÆRLETT.

It’s 10 band graphic eq in a feedback loop with a spring reverb - no input. You shape the feedback with the eq. Each fader is assigned to each eq band. I also added a Grain Delay / Reverb / Limiter. For this patch to work you need a Faderbank / Sweet Sixteen to control the EQ’s and an external Spring Reverb. This unit is already premapped to a faderbank using Teletype SC.CV channels 1 to 16.

I’m using firmware 48k v0.4.27.
You also need Joes “Accents” installed as i am using the bandpass unit.
Note : The CPU runs at 70% at 48k because it contains 10 bandpass units also the internal reverb and grain delay. You may want to bypass the grain delay / reverb and run an external reverb instead to save CPU.

Channel 1 to 10 = different EQ bands volume gain from low (bass) left side to treble / high right side.
Channel 11 = Reverb Size
Channel 12 = Reverb Wet / Dry
Channel 13 = Grain Delay Time
Channel 14 = V/Oct of the Grain Delay
Channel 15 = Grain Delay Wet / Dry
Channel 16 = Feedback Master Gain to boost the feedback if too low.
All sliders down = 0

The top half of each EQ slider is set self oscillate the higher up you go. Be very careful here as its very easy to lose control of the feedback.

Tip : You balance everything by keeping 1 hand on the EQ sliders and the other hand on slider 16 - Master Gain and very gently boost and cut eq channels. You play this by manipulating the springs on the spring reverb whilst adjusting the EQ bands on the faderbank.

Tip : When touching the springs, open up the low and mid EQ’s and that is also the best time to play the 3 faders of grain delay. There is an envelope follower that modulates the Grain Delay using the OUT volume. Bass especially triggers that envelope generator.

Tip : Try using an external reverb with this patch - something like the Eventide Blackhole works well. A long cavernous reverbs are great for this.

Warning - It can be fiddly and can take some time to get used to as you are dealing with feedback on a knife-edge so be very careful with your speakers - there is a limiter in the patch. Don’t open up all the eq’s at the same time to the max.

Also feel free to change the bands i used by looking at all the Bandpass Filter frequencies.

Loading Instructions :
Unzip Spring to ER-301/v0.4/presets/units/custom-effect/xxxx
Unzip Accents to ER-301/libs/accents/xxxx if you don’t already have them installed.

Click on “Custom Effect” in the menu.
Click on the custom effect self generated name (left side - not where it says “open”)
Load Preset -> goto ER-301/v0.4/presets/units/custom-effect/xxxx
Another alternative way to load custom units is to press “insert” in the bottom smaller window. Then move the cursor to “Presets” and load that way.

Connections : This is a mono unit for Channel 1 with the speaker output taken from Channel 2. With the custom unit installed connect Spring reverb return/or OUT to IN jack on the ER301. Connect the OUT on Channel 1 on the er301 to the IN on the spring reverb. *On Channel 2 route the internal Channel 1 OUT to IN. Take the Channel 2 out on ER301 to your speakers.
Alternatively you could just multi the same Channel 1 OUT 1 and take that to the speakers instead of using a separate ER301 channel.

spring.zip (5.7 KB)


Fascinating! Have you tried this with something like a guitar as the input? Just curious if you think some other sources would be interesting since I don’t have a spring reverb lying around.

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I haven’t tried a guitar and you don’t necessarily need a spring reverb (but are a lot of fun) The Doepfer A199 whilst not a particular great reverb is great for feedback loops especially with a fixed filter bank.

You could replace with anything really that has an in / out - oscillators, wavefolders, filters, phaser, freqshifter, BBDs and Delays, resonators, dividers. Processor modules like bipolar VCA’s / inverters for negative feedback. Having control over the gain staging is important, so are mixers, and especially matrix mixers and attenuators. So having control points within the loop is what you need to aim for. In this case the control points are all the VCA’s and EQ and master gain slider.

There is loads of info on feedback loops on muffwiggler forum going back years and years. If you google this “muff wiggler feedback loops site:www.muffwiggler.com” - you’ll find lots of suggestions…

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Reminds me of the hours and hours of fun I use to have with my pair of Serge Resonant Equalizers. I should dig them up and put them back in my case. I remember feedback patching the COMB 1 and 2 outputs was an endless source of fun, particularly when you get that overblown wind instrument warble.


I just received one today! Was this patch you outlined a no-input one? Any other patching tips? :slight_smile:

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I haven’t tried it yet @josker but I believe it needs an input of some sort. I’m sure you could patch something internally too but may not be as malleable as the spring reverb unit!

Keen to try it out! Thanks @mudlogger :slightly_smiling_face:


There’s no other inputs, so you have a feedback loop with in/outs connected between the er301 Eq chain, and the Reverb tank only, the same as the Tilde. But it’s very easy just to add another input by adding another mixer unit (with vca inside) in the eq chain and routing it in from any of the other external jacks.

The other option would be make a stereo unit so you have 2 inputs / 2 outputs but pretty sure the cpu would max out since it’s already running at 70%.

You can always use a no input external mixer


I’ve had alot of fun with erbeverb in place of the spring reverb and using pressure points to modulate the size parameter! Thanks Mudlogger

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