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Spring Reverb Driver with Synthacon VCF Filter | Electronic Musing
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Spring Reverb with Synthacon Filter

Wednesday, May 14, 2014 at 1:46 am

Spring Reverb with Synthacon Filter

Spring Reverb with Synthacon Filter, in its natural habitat.

Saying reverb is important to music production is like saying that Jesus is “kind of popular”: the level of passion can’t be stated plainly; it demands some sort of humorous understatement. So, as I’ve just finished a tune-up on my custom spring reverb, I figured I’d write about it!

I previously covered Impulse Reverb (a.k.a. “convolution reverb”), which represents the bleeding edge of reverb technology. Spring reverb is completely the opposite of that: Before DSPs, engineers had to rely on physical or analog processes to get the effect, and the most popular techniques break down into three major categories:

Hall / Chamber Reverb – The simplest way to apply reverb to a recording is to skip the simulation and use the real thing: Speaker at one end of a room, microphone at the other. Alternatively, if it’s acoustic music, just make the recording there in the first place! Indeed, this is where the effect started: You’re not always able to record in a fantastic-sounding cathedral, but you can build a giant, echoing room anywhere. Studios like Abbey Road had specially designed rooms in the basement specifically for this purpose. While this lets you get a good reverb sound without going on-location, it’s still not very portable — and it takes a lot of time and money to even pull off in the first place, unless you happen to live in a building with an echoing stairwell. However, it still marks the line where we crossed over from a recording location to a repeatable FX unit: bottled lightning. Having captured the effect, engineers then sought to scale it down, and make it more controllable, via crafting analog models of the physical process.

Plate Reverb – A large slab of metal is buzzed and vibrated with the source signal. Vibrations reflect and gradually taper off; just add a pickup and you have what sounds like natural reverb. If this sounds confusing, just think of a gong. Unfortunately, to get a good sound, you still need a large chunk of metal set up in a precision manner: Beatles-era plate reverb units easily clock in at around 600lbs. The only control was a dampener.

Spring Reverb – Same idea as plate reverb, except smaller and cheaper: flappy springs, instead of a huge slab of metal, allow the effect unit to be scaled down to the size of a pencil box. While not considered adequate for studio usage (They don’t sound nearly as nice as plate reverbs!), spring reverbs were well-suited to being built into guitar amps and organs. You’ll still see spring reverb tanks in guitar amps today, even though many have gone digital — along with all the organs. Spring reverb has the sonic richness of a physical, analog process… along with the corresponding headaches and hassles. It is very subject to noise and interference. There are no controls on the effect unit itself: Fixed decay, fixed everything.  The spring responds as the manufacturer designed it.

Since you have no control over the response of the spring reverb tank, you are forced to rely on the supporting circuity: EQ can alter the sound before it goes in, and again after it comes out. Feedback loops can be used to artificially enhance the decay. Compression/expansion can be used to warp the spring’s response curve. Starting out, however, I didn’t know any of this!

My trip began in 2006 or so, when I purchased a Belton reverb tank off eBay for $10, planning to simply plug it into my mixer. Very quickly, I discovered that this didn’t really work. It was hard to get much sound out of it, and what sound I did get wasn’t very good: It was totally swamped in buzz, noise, and AC hum. Besides that, it sounded incredibly muddy. Turns out, you need a dedicated circuit to drive the reverb — you can’t just use a bog-standard preamp. So, it sat on the shelf for a year or three, until I learned more about electronics.

Eventually, I found Rod Elliott’s writings on spring reverb, which I highly recommended. With an LM386 and a few components, I immediately got better results (fig 2 on the linked page). After spending some time with this setup, I began to discover previously-hidden sweet spots: It responded differently to some frequencies than others, and by boosting or cutting those frequencies (along with the overall gain) I actually began to get significant variety out of what had hitherto been a one-trick pony.

After EQ and a proper driver got me using the thing on a regular basis, I discovered something else: It’s even more fun when you throw in a proper filter — a modular-grade, multi-mode VCF: in particular, bandpass and highpass allow you to bring out some of the more subtle characteristics. Lowpass is merely useful to cut off excess frequencies. In all cases, resonance gives some tone control, and being able to voltage control the cutoff means being able to automate the wackiness. During this time I used the VCF on my PAiA Fatman, which I’d previously modified for audio input and additional filter modes. I typically used it as an aux send, and brought it back in on a dedicated channel rather than the aux return: This allowed me to send it back out to the aux send again, feeding the effect back on itself. You can hear the end result in my track Electric Taste. It reminds me of a red-hot filament…

Eventually, I got tired of having my Fatman’s filter monopolized as a personal assistant to the spring reverb, and decided to rebuild the driver with a dedicated filter. I wound up settling on Ken Stone’s Synthacon VCF due to its weird an unique nature. Most filters have a single audio input, and separate outputs for the various modes: HP, LP, BP, whatever. However, he Synthacon VCF has inputs for each mode instead, and a single unified output. Feeding sounds into multiple jacks at once results in some weird goulash. To me, this sounded like the perfect thing to go along with all the feedback: have this quirky filter in the loop, and allow me to further sculpt the tone.

Unfortunately, I had problems getting the filter to work right: Fresh off the success of a few projects, I got overly-ambitious. Since the driver still worked fine, I decided to put it back in the studio and just leave the filter set on “bypass” until I cleared out my project backlog (Note: If you do this sort of thing to yourself as well, the best advice I could ever give is: don’t start new projects until you finish existing ones! ). Here’s a video I shot while working on it. As the video date indicates, it’s been almost two years since I last had it out of the case. Last weekend, however, I finally got around to fixing the filter:

Driver and filter, original circuit.

Driver and filter, original circuit, with newly-added power connector.

Driver/Filter bottom view

Bottom of the board. I took this photo for my own reference — digital cameras are a lifesaver for this stuff!

Reverb driver and Synthacon VCF, all boxed up.

Reverb driver and Synthacon VCF, all boxed up. I used the case from a busted Delta 1010 audio interface. 🙂

Reverb and filter, back in the studio, with tank

Reverb and filter, back in the studio, with tank, hooked up and ready to go!

Here’s a video of me testing it out:

The three switches enable/disable HP/BP/LP, and the fourth switch optionally bypasses the filter entirely. Then there’s input gain and tone control (for the driver), and cutoff/res (for the filter). It’s a very simple configuration, and I have a lot more planned for it. There are a few things I want to do as soon as possible:

– Needs an output preamp — The input preamp (driver) is really what matters, but the output still requires me to ramp the gain on the mixer’s preamp, which can be a bit noisy.
– Internal feedback loop — I want to be able to feed the thing back on itself without relying on a mixer.
– Need level controls for the three filter modes in addition to the on/off switches.
– CV input jack for filter cutoff
– Power LED

And then there are some blue-sky ideas, which I’m not sure when I’ll get to:

– Dedicated EQ
– LFO for filter cutoff
– Envelope follower circuit to control filter cutoff
– Alternate driver circuit (the LM386 is a bit rough)

For the moment, though, I’m stoked to simply have it working as I intended it to back in 2011, and I’m going to allow myself the luxury of enjoying it a bit before I tear it up again. Stay tuned for Pt. II! 🙂

Meanwile, have a few useful links…

Categories: circuit bending / SDIY, FX, gear, studio, Uncategorized

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