technology of the AKG BX spring reverbs, mainly the BX-20

[haima]

i'm wondering if anyone has any knowledge pertaining to the design of these beautiful reverb boxes?

specifically what sets them apart from the common tanks made by accutronics etc

i know the springs are obviously longer, and hanging vertically - but is there any tricks? i.e. phase cancellation / summing of multiple springs, how the springs are driven, dampening etc. that we could learn from?

anyone know of any patents filed by AKG relating to this design?

the AKG website has service manuals for most of the BX units - but there's not a lot of information about how they specifically work... how the springs are wired, joined etc.

i'd love to own one of these units, but they are pretty hard to find - i'm wondering if there's a way we can make a "poorman BX-20"  

perhaps using parts from a accutronics tank, but with extended springs etc...

thanks for any info or ideas anyone has!
haima

SOME COOL INTERNAL PHOTOS HERE:
http://www.villaraspafactory.org/mediagallery/thumbnails.php?album=44

http://www.villaraspafactory.org/mediagallery/thumbnails.php?album=27

anyone know who "villa raspa factory" is?

SERVICE MANUAL:
http://www.akg.com/mediendatenbank2/psfile/datei/23/BX204055d1e1a05c1.pdf

 

[haima]

from the villa raspa factory site:

this image is quite intriguing! are they big C magnets? almost looks like a ribbon mic... what's that gold legged thingamejig? and IC?


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it seems as though there is two of these driver/pickups at one end.


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this photo shows the other end where you can see the spring do a 180 degree turn. there also appears to be another set of C magnets at this end? i guess that makes sense - 2 drivers and 2 pickups...

this photo shows the bend in the spring.

 

[mad.ax]

One of the difference is that BX20 use moving coils transducers which allows for big external magnets, thus a powerful magnetic field.
Conversely, Accutronics tanks use moving magnets. Those magnets have to be small enough to be fited on the spring...

Think about the old vinyl P.U. Moving magnet vs moving coils...

Axel

 

[abbey road d enfer]

Probably the biggest difference is in the dual servo-drive. Each spring has two coil assemblys, one at each end. Each coil assembly has a drive coil and a feedback coil. The procedure states positively that proper tuning of the electronic damping gets rid of flutter echoes (unproperly translated as "shutter echoes"). This is also, unfortunately, one of the most frequent causes of failure of the AKG spring reverbs. With time, the delicate balance stability margin gets impaired by softening of materials, suspension sag, aging electrolytics or deterioration of the trimmers, and the unit starts to oscillate and the drive coil gets burnt. When buying a BX series reverb, it is paramount to check the integrity of coil assemblys and springs, since they are extremely difficult and costly to repair. These parts can be found NOS some times, but $$$...
I'm a fan of spring reverbs; I think they offer unmatched density (except real echo chambers). I must admit I don't use them anyway, because of the impracticalities (my failure of finding a reasonably priced BX20 or MIC-MIX), risk of terminal failure, bulk, acoustic feedback...). AKG went to great lengths designing THE ultimate spring reverb, but the approach taken by MIC-MIX, although being much simpler, gave also excellent results. Their approach was a purely mechanical one; they relied on logarithmically scaled spring length. Check this thread
http://www.picocompressorforum.com/forum/php/viewtopic.php?f=6&t=213
Fairchild had a tube spring reverb; we had one at Barclay studios, but I never had the opportunity to listen to it; it had been decommissioning years before my time there (mid '70's)

 

[speakercoil]

You mean like this one....

http://cgi.ebay.com/Fairchild-Reverbetron-Model-659-Vintage-Spring-Reverb_W0QQitemZ300380686569QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item45f01588e9#ht_8719wt_1167

I myself have been a fan of Spring Reverb for a while, and always wanted a BX20... However I have recently been toying around with just designing my own... It seems like it could be a pretty simple project... Wouldn't be a BX20 by any means... But it would be something unique.

-Grant

 

[abbey road d enfer]

Thanks for the link. The one I remember was bigger than that, like a 4u main rack; the spring rack was probably the same size as this one.
Have you checked the pico link? The moderator had done some work on the springs and had some kind of agreement with Accutronics that they would make the custom springs as soon as they would be approved. But he said he had been sidetracked.

 

[haima]

thanks abbey, mad.ax, speakercoil...

i agree that the micmix seems a much more doable project! i wonder if wayne (mediatechnology) is still planning on finishing his? maybe we can encourage him.

a diy BX-20 would be a very ambitious project indeed - perhaps one day someone will do it, but i don't know if i'm the guy, at least not at the moment ;D. in the mean time let's collect as much info as we can, maybe one day it'll assist the one crazy enough to attempt it.

 

[speakercoil]

We could use this forum to come up with a group DIY design for one. Source the parts... etc.

 

[abbey road d enfer]

That's more or less what Wayne intended to do, but as I said, he has been sidetracked. I think we should talk to him about it.
Anyway, the most important thing to do is experiment with the spring length. Wayne has had a bunch od springs from Accutronics and he started to play with them, counting the turns, etc, and someone has to take over. It'll not be me, my eyes aren't good enough. Maybe we should revive the already existing thread...

 

[speakercoil]

I second that notion... I am young, ambitious, and have good eyes... The only thing that would slow me down is money... But I think this would be a relatively cheap project. Especially purchasing old parts from e-bay. This isn't the type of thing I am going to fill with audionote capacitors and what not... I just want it to function properly.

I have a few ideas too...

OK... For example... Why not have different lengths in the springs... Think of it as looking at a reverb tail. Start with the longest spring and maybe do tank of 6 springs each getting shorter. It would provide a different kind of decay, and different resonant frequencies among the springs I would imagine... Providing a unique kind of sound.

-Grant

 

[abbey road d enfer]

OK... For example... Why not have different lengths in the springs... Think of it as looking at a reverb tail. Start with the longest spring and maybe do tank of 6 springs each getting shorter. It would provide a different kind of decay, and different resonant frequencies among the springs I would imagine... Providing a unique kind of sound.

-Grant

that's really what the MicMix patent is about. You really should read the thread http://www.picocompressorforum.com/forum/php/viewtopic.php?f=6&t=213

 

[speakercoil]

lol yeah sorry... I only briefly glanced at it. I've been sooooo wrapped up with studying the U47, that it was hard to switch modes. But I will check it out here in a little bit. I need to give the U47 a little break in my mind anyway.

-Grant

 

[Crusty2]

My uncle had a couple of the Fairchilds at his studio, They were probably 9u high in thier own open frame racks. I'm sure they were solid state. They were tall because the spring units (3 of them) were mounted vertically. I think I remember the Reverb'Rocket had the same type springs and were vertically mounted as well.
I used them a few times. They were cool, pretty dark and dense. Good fit for the psychobilly guitar stuff I was working on at the time but too much character for a general reverb I thought.

 

[abbey road d enfer]

Yes, it seems to be more what I had in memory; so there were different models then. The one on Ebay is likely to have only one or two spring tanks. Interesting is the 3 position Decay switch (S, M and L). I had a brief thought that they would switch tanks (Accutronics offer three srandard decays), but in fact there's only one set of connections to the spring box. So I suppose they do it electronically (probably some kind of compansion).

 

[Dan Kennedy]

Bucket brigade delay?

I vaguely remember a couple of units done that way.

 

[Crusty2]

A little positive feedback maybe?

 

[abbey road d enfer]

BBD's were not currently available at the time.
Positive feedback is so uncontrollable with spring reverbs that it's not likely.
Compression/expansion seems to be a more valid candidate.

 

[PRR]

The oldest BellLabs and Hammond spring reverbs used a huge motor and hung long springs vertically.

Hammond and/or Accutronics later developed the compact low-cost tank we all know from Fender Reverb.

The recent eBay listing is clearly the "new" tank type. It is in two boxes so you can put the knobs in the control room and put the springs elsewhere so they don't get bumped.


> Interesting is the 3 position Decay

What are the "remote control" screws? The knob-unit appears to be two halves, and these screws are in the same half with the SML knob. Could they have put a relay in the spring-box, to select different tanks?

 

[Crusty2]

There's a "REM" position on the mode selector switch, so the terminals are probably for a console remote. The springs only have high/low/gnd in and out terminals, but I guess you could switch a relay with 'phantom' DC...

 

[speakercoil]

There's a "REM" position on the mode selector switch,

SICK!!!! So if I use that unit and run my mix through it I will sound like REM!!! YES!!!!!!!

lol... sorry couldn't help myself.

In reality... I think designing and building our own is very doable. Especially after poking around that link a little more. I would want to use longer strings... possibly of different widths and lengths. Kind of like a mini-piano in a way. I think vertical makes much more sense when using long springs. The BX20 actually looks quite large, which I never realized before.