MicMix MasterRoom Spring Reverb Project

[mediatechnology]

How about a MasterRoom spring reverb clone? Accutronics graciously sent me springs and transducer assemblies to modify for the MicMix timings some time ago. After I dropped development on this in 2007 I received a number of e-mails asking "what happened?" I think I got bored counting spring turns and went crazy. The following is from my scrapbook capture of work done elsewhere.

Lets begin with this paper from Bill Hall of MicMix Audio who stumbled upon the invention while developing a pulsed acoustic intrusion alarm:

http://www.ka-electronics.com/Images/Hall_Reverb.pdf

And the patent:

http://www.ka-electronics.com/Images/Mi ... 350845.pdf

The original brochure:

http://www.ka-electronics.com/images/ha ... ochure.pdf

The MasterRoom spring mechanics were pretty simple - aluminum L channels screwed together forming a square cross section ran the length of the springs and held 4 Accutronics transducers at each end. The springs ran the length and were connected mechanically in series forming two pairs (x4) on each side. I think the drive transducers were paralleled and the receivers series-connected. The whole gizmo had spacers with bumpers on them to keep the spring assembly from hitting the sewer pipe. It was suspended by springs.

What might come in handy is, if I don't have the math right, a pulse response of the tanks. A soundfile of TDR pulse response might prove useful. What you do is pulse, say 1 KHz single-cycle click the tank. That's T0. You will get (in a stereo MR) first-order echos at T1, T2, T3, T4...T8. Those are the ones that really matter. The second-order echos are at 3T1, 3T2, 3T3, 3T4...3T8.

Why 3T? Because the second order echos have to reflect off of the receiver transducer at T1, go the length of the spring and reflect off the driving transducer (at T2 which is silent), and then return again through the spring a third time before being "heard" by the receiver at T3. Recursive delay lines are 2T systems and we had to re-write the patent to cover that.

Here are the schematics for the XL-305. These have also been posted over in Document. Nothing too special except for constant-current drive:

http://www.ka-electronics.com/MasterRoo ... _tile1.jpg
http://www.ka-electronics.com/MasterRoo ... _tile2.jpg
http://www.ka-electronics.com/MasterRoo ... _tile3.jpg
http://www.ka-electronics.com/MasterRoo ... _tile4.jpg

For the delay line timings (spring cuttings) I dug up the old log time-domain reverb formulas from mine and Bill Hall's original notes. These calculations are for a 6 or 12 spring system with an initial delay of 13.5 mS. The timings are equally spaced in the log domain. These calculations are in base e. For a six line/channel stereo reverb, the odd timings are assigned to the left channel, the even to the right. When mono'd they interleave.



I took the longest spring which equals the last line in the spreadsheet and worked backwards cutting the first spring at ~13.5 ms. Notice the difference in tension between the longest and shortest springs.

[raf]

Wayne

Glad to see that this project back again. I always wanted a Master Room reverb but couldn't justify it back when it was current. My 4 track R to R ate my budget! I even built my own NR from JRs 570 compander article in '81 IIRC. Homebrew mixer too.

Later this month after things aren't so crazy I will look at a layout for this. As I understand it, the xducers and springs are all that are being supplied by Accutronics? I was also wondering about the tank suspension springs. The metalwork part doesn't look too bad to DIY.

Thanks for putting this together!

rf

[mediatechnology]

Accutronics supplied the whole sled for this project. They can be bolted together and suspended by springs or O rings. They sell them online - the last time I checked they had an eBay storefront. For twelve lines (springs) there would be four sleds since they have a three line unit.

I was thinking the spring assembly could be separate (remote) with the power amp and preamp mounted with it. Then it would just be line level to and from.

[emrr]

Cool.

Off topic a bit:

I've had an ancient piece in mind to couple with a spring reverb, which I spotted in an early '50's magazine. Circuit is a proper variable vibrato for organ or instrument use, and it cries out to be done in stereo, with stereo spring reverb path. Done with octal tubes, of course.

[mediatechnology]

I've had an ancient piece in mind to couple with a spring reverb, which I spotted in an early '50's magazine. Circuit is a proper variable vibrato for organ or instrument use, and it cries out to be done in stereo, with stereo spring reverb path. Done with octal tubes, of course.

Was that the work of someone at Hammond? It's a cool concept and would aid diffusion in reverbs with a small number of lines. I think I may have seen a patent similar to that once.

[emrr]

Have to dig it out. It wasn't associated with any manufacturer I don't think. Very nice article I need to scan and post.

[raf]

Doug

I want to check that out... As long as it uses octal tubes! Sounds really interesting!

rf

[mediatechnology]

I originally wrote:

These calculations are for a 6 or 12 spring system with an initial delay of 13.5 mS.

I remember now that when I first checked the springs Accutronics sent me the "longest" spring (Accutronics longest) was ~37 ms. That's the last entry in the table. The 13.5 ms figure, dating back to Hall's work, was devised by working backwards from Hammond's 37 ms. In the MasterRoom MR-II, MR-III and MR-IV the springs were series-connected and much longer.

Here's the text in the original patent:



I should probably go back and check the math in my tables. I started at 13.5 and worked forward but they should be close.

[emrr]

At Roger's request, the vibrato article. Radio & Television News, April 1954. Reads to be a Wurlitzer circuit, or variation.

page 1
page 2
page 3

[raf]

Thanks Doug!

rf