DIY Spring Reverb Rack?

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dissonantdissident
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DIY Spring Reverb Rack?

Post by dissonantdissident » Thu Mar 13, 2014 10:27 am

So, my next little project will be building a stereo spring reverb rack unit, and it doesn't seem that there are any affordable units, currently in production. I don't know if any folks would be interested, but, if I get a working version going, I'll share the info. My goal is to get the unit together for $300-$400, tanks, rack case, and all. Also, just wondering if folks would be interested in a kit.
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Post by timh » Thu Mar 13, 2014 12:18 pm

Absolutely. That'd be very cool.
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Post by JohnRichard » Thu Mar 13, 2014 12:46 pm

I would be interested in a standalone version. But I assume one could change the case.
I don't rack a lot of stuff myself.

Once I was on a recording project and asked the engineer if he had an old school spring reverb box to send the guitar through. The next day, he showed up with a hand built custom spring box that sounded great!

Ever since I have wanted to build my own.

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Post by Matt C. » Thu Mar 13, 2014 12:59 pm

I'm interested, not necessarily in a kit, but just sharing some information about how it was built, circuit design, etc. I've been wanting to build one for a while, although I might just do mono (for the time being at least). I've been reading the Rod Elliot page about the driver and recovery circuits, doesn't seem too tough to put together but I'm curious to hear people's experiences with different circuits.

I'm thinking about doing the thing where each channel has two tanks wired out of phase, which i think cancels some hum and some of the sproing-y sound.

I'm also wondering whether I should put the whole thing in a regular rack case, or have part of it in a box elsewhere so that it doesn't pick up hum from other gear in the rack.

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cool

Post by dissonantdissident » Thu Mar 13, 2014 6:51 pm

Sounds like some other folks are kicking this idea around, as well! If I get one together(and working), I'll share what I find out.
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Post by Jim Williams » Sat Mar 15, 2014 8:41 pm

I built custom spring reverbs in the early 1980's. I used Accutronics new 3 spring tanks. I had them custom made with reversed polarity magnets on one of the return coils. That way they were wired in series creating a hum cancelling pickup.

The driver circuit was an opamp with a couple of hefty TIP power transistors in the discrete output drive section. The reverb tank driver coils were also wired one tank out of phase. That cancelled the sproing effect on transients by reducing the level of the fundamental signals. 3 springs added depth 2 spring tanks don't have.

A low noise opamp was used as the reverb recovery preamp. I made a one knob compressor/expander circuit that had no effect 1/2 way. Turned left it kicked in the 1/2 expander shortening the reverb time. Turn it right and the 2/1 compressor kicked in lengthening the reverb time. It did about 1/2 second to 3 seconds reverb time with the medium time reverb tanks.

I use Lexicon and Bricasti now days.
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Post by The Scum » Mon Mar 17, 2014 4:28 pm

With the phase/antiphase drive trick, and using the recovery coils in series, how would this compare to the ol' Paia Hot Springs?

For circuits, there used to be some Accutronics app notes with a current-mode driver that i had pretty OK success with.
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Post by Jim Williams » Tue Mar 18, 2014 8:11 am

I don't know PAIA. They probably used the short springs, Accutronics make a 3 spring version, not so good.

I've also used a current drive design, that seems to smooth out the impedance drive. You place the driver spring coils into the feedback loop of the system instead of driving the outputs through the coils to ground.

There are many variations to this theme, all include one variable that I can't overcome, the non-linear and scattered response of the springs themselves.

Another reason I use Bricasti and Lexicon these days. 35 years ago I had more smarts than money. Now I have more money than smarts.
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Post by Matt C. » Wed Mar 19, 2014 6:33 am

here's the Paia schematic. seems to be similar to what Jim is talking about.

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Post by Jim Williams » Wed Mar 19, 2014 8:40 am

That is a typical current feedback drive design, probably best with a high current opamp like a LM6172. Those will dump over 100 ma allowing a lower impedance drive coil to be used. They come from 8 to 600 ohms from the factory. 600 ohm versions were popular when designers used the 5532 opamp as a driver as those will run a 600 ohm load, but not below that. The LM6172 will drive 150 ohm loads easily. Use a pair of current driving discrete power transistors and that 8 ohm coil can be used.

The recovery amp design is poor, an inverting opamp design with a 4.7k input resistor and a huge 2.2 meg feedback resistor. Better to use a non-inverting design with a low noise opamp. Something like an Analog Devices AD8599 would be good. Load the input at 50k ohms and set gain through the gain shunt resistor from pin 2 to ground. An el cap will filter out subharmonic noise. The driver amp design is a good example of how the recovery amp should look.
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Post by germaniac » Wed Mar 19, 2014 12:28 pm

I'd be interested in following the general progress of the project. I have a Sound Workshop 242 that I've been meaning to try a dual-tank noise-cancelling mod on. Or depending on the results here, maybe I'll just build up a new unit from scratch. . . .

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Post by L?Andratt » Sat Apr 26, 2014 4:19 am

Jim Williams wrote: A low noise opamp was used as the reverb recovery preamp. I made a one knob compressor/expander circuit that had no effect 1/2 way. Turned left it kicked in the 1/2 expander shortening the reverb time. Turn it right and the 2/1 compressor kicked in lengthening the reverb time. It did about 1/2 second to 3 seconds reverb time with the medium time reverb tanks.
Very cool, I was thinking about using an adsr envelope generator + vca.
But your approach sounds better. Did you use a 4301? I could imagine the rms+rectifier signal going to the wiper of a pot panning between +cv and -cv for expand/compress, or how did you do it?
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Post by Jim Williams » Sat Apr 26, 2014 8:20 am

This was 1982 folks. The only part I found back then that worked as a compander chip was the NE570.

I also used that part in 1988 when I designed a PWM subcode audio noise reduction scheme for Hasbro video games. I got about a 90 db s/n from using the 'black bars' between video frames to bury the subcode in.
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Post by L?Andratt » Sat Apr 26, 2014 11:43 am

Jim Williams wrote:This was 1982 folks. The only part I found back then that worked as a compander chip was the NE570.
I was 9 then. :P
Thanks for the answer though.
I also used that part in 1988 when I designed a PWM subcode audio noise reduction scheme for Hasbro video games. I got about a 90 db s/n from using the 'black bars' between video frames to bury the subcode in.
I don?t even get what you are talking about, but sounds like I would love the idea!

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Post by Drone » Tue Apr 29, 2014 7:46 am

What are the likely max specs on reverb tanks? I see there's a bunch of springs with an 8 ohm input impedance, which you'd think would work great on the output of a tiny tube amp, but I'm imagining even a single watt might be more than the input section can handle.

Can't see any dissipation / handling specs for the Accutronics :cry:
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