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Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Posted: Sun Feb 03, 2019 8:14 pm
by mediatechnology
I just finished cutting and assembling springs for a client.

Sample sound file: ... 5_Demo.mp3

As a kid doing sound in high school and college I always looked for a stairwell, hall or small room that sounded like this.

The equally-tempered tuning of the MicMix Master Room reverb.

Posted: Wed Mar 13, 2019 6:49 pm
by mediatechnology
I wanted to provide a mathematical and musical analog to explain the Master Room spring timings.

The equally-tempered musical scale has a ratio between neighboring notes that is equal to the 12th√2 or 1.059463. (plus change)
An octave spans a factor of two - there are 12 notes in each octave.
For example "A4" is 440 Hz; "A4#" is (440 * 1.059463) = 466.16 Hz.
B4 is 1.059463 * A4# or 493.88 Hz.

Open the lid of a piano, hold the damper open and sing into it.
It will be some of the smoothest reverb that you've ever heard.

For smooth reverberation, like that of a piano, the goal is to make the delay timings of the springs equally-tempered in the time domain.
If the springs are equally-tempered in the time domain, they will be equally-tempered in the frequency domain similar to the musical scale.

There's one simple difference: An octave is a ratio of "2."
Springs produce an initial delay we'll call T.
An impulse at the driver will produce an initial impulse in the pickup at T.
That first pulse is reflected off the pickup and back to the driver where it is reflected again back into the pickup.
The initial pulse appears at time T, but the reflected pulse of the second echo has to transverse the length of the spring twice.
The second-order echo appears at time 3T. (T+2T=3T).

An equally-tempered octave has to span a frequency ratio of 2 in 12 steps; an equally-tempered spring reverb has to span a time ratio of 3 with a number of steps equal to the number of springs.

The Master Room XL-305 had 12 springs.
For the twelve springs to span a ratio of 3 in equally tempered steps the ratio is the 12th√3 or 1.0959.

If you take the 13.5 ms delay of the first spring and multiply it by 1.0959 you come up with 14.8 ms.
Multiply the 14.8 by 1.0959 and you get the delay of the third spring.
Repeat the series and you wind up with the same numbers in the original table.

The point is that the Master Room reverb is tuned musically - to an equally-tempered scale.

I've been contacted by a manufacturer who may want to make these in a rack unit similar to the original XL-305. Stay "tuned."

Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Posted: Thu Mar 14, 2019 7:53 am
by emrr
well spotted!

Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Posted: Tue Jul 16, 2019 7:50 pm
by gzifcak
Finally sharing a photo of the unit I built based on this thread. Thanks so much to Wayne for all the help, tips, pcbs, and for providing this info in the first place! This has become my go-to reverb, I'm having fun putting it on all kinds of different stuff. I'm also really enjoying shortening up the decay by damping the springs with pieces of foam. From what I can tell, while the number of turns determines the delay time, the tension of the springs is what determines the decay. Two of the switches shift the HPF from 600 to 1200 Hz and the other two cut out the longer two tanks so I can easily just manipulate the shorter two for sound effects.

Anyway, if anyone is thinking about trying this project I wholeheartedly recommend it. Cutting the springs was intimidating at first but once I got in the groove it was not that difficult and testing the timings became pretty easy with an impulse and two DAW tracks. I found the Accutronics tanks much easier to work with than the MODs I originally started with.

In the pic you can see that I crammed it all into a cheap 1u enclosure by bolting the sleds together and suspending them from the case with spare bits of spring. I even repurposed one of the reverb trays to hide the electronics so I can safely run it with the springs exposed.

Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Posted: Wed Jul 17, 2019 7:47 pm
by mediatechnology
Great work! Glad you like it. :D

Thanks for the pics.

I remember MicMix installing foam plugs inside the springs.
I don't fully remember why some lines had them and others didn't but I think it was to tame a particular line that was longer than the others.
Are you putting the foam under the springs to damp them or inside the turns?

Accutronics recently explained that the transducers have damping material inside the brass tips to control decay.
Need to re-read the email to see if it was the driver or the pickup.
That's what determines the short, medium and long decays.
I'm still not sure how they visually identify which is which.
I used to recall color dots on the brass tube.

Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Posted: Thu Jul 18, 2019 1:16 pm
by gzifcak
Oh cool, thanks for that info. I've just been sticking little pieces between the springs, which shortens the decay a lot. I've also tried resting some bigger pieces across the top of the springs for less severe damping.

Now that you mention it I may try them underneath and see how it sounds. That might give me more flexibility to adjust the tension with slightly different sizes on the lines.

Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Posted: Thu Jul 18, 2019 6:57 pm
by mediatechnology
I found that when the springs are laid out horizontally - like yours - that you can almost play it like an instrument damping some of the springs with your fingers but not others to change the tone.

You might play with using a small block of foam and then moving it between the transducers along the length to see what effect it has.

Were you able to ever find anything that dissolved the glue on the Mods?
Acetone dissolved the lacquer on the Accutronics pretty quickly.
(Where they glue the spring to the hook on the transducers.)

Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Posted: Tue Jul 30, 2019 2:43 pm
by gzifcak
I wasn't able to dissolve the glue on the MODs, so what I did was just cut the coils from the middle and then just re-hook them there. I ended up abandoning them though, I think I broke one off the transducer while doing this and also realized the staggered spring heights wouldn't work as well for me.

Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Posted: Mon Aug 09, 2021 12:49 pm
by mediatechnology
Shortly after I revived this project back in 2018 I was contacted by Chris Yetter of
Chris was interested in a reverb product and had heard great things about the MicMix MasterRoom XL-305.

I agreed to help Chris with the re-issue doing both the circuit design and teaching him how to cut springs to the original MIcMix timings. Chris and his team did the circuit board layouts and mechanical design. Now that he's shown it at NAMM I'm able to show the prototype he sent me for evaluation.

This is a pre-production prototype. I'm blown away by the execution and the many sonic improvements over the original I designed over 40 years ago.

AudioScape XL305 Reverb Prototype

The design uses all new electronics and provides signal flow improvements over the original.

It's far quieter using a balanced instrumentation amp front-end for the pickup transducer and a lower distortion driver.

Metering is now at the output which provides a more meaningful indication of drive level.
The meter is based on this circuit: ... f=6&t=1162

A peak flasher indicates current overload in the driver using the TL431 circuit I described here: ... =30#p12657

Soon you'll be able to buy an XL305 without having to cut all those springs...

Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Posted: Fri Oct 08, 2021 10:38 am
by emrr
Cool to see this play out.