Outragous Idea....DIY EMT 240

[Consul]

Has anyone considered taking the "eBow" approach to this problem?

If you use steel for the base metal, due to it being ferromagnetic at room temperature, you can both drive it and pick up signals off of it without anything needing to touch it. Furthermore, reverb times and characteristics could be set by setting up a feedback loop between the pickups and the driver coils and controlling the feedback amount, phase, envelope, etc. Then you could do wacky things like modulate the phase, or run the feedback loop through a tape loop, to get some really interesting stuff happening.

As far as I know, pure gold is not ferromagnetic at room temperature, so a magnetic pickup could never work on it, as a magnetic pickup depends on the pickup magnet turning a steel string (or in this case, a foil) into a magnet so the vibrating string (or foil) will induce a signal in the coil. I agree with Gyraf, then, that if the 240 is using a magnetic pickup to obtain the signal, then the foil can't be pure gold, and the base metal must be ferromagnetic at room temperature. Nickel would work.

EDIT: Oh, hey... It would appear that nickel can be gold-plated just fine, as nickel is the most common substrate metal used when gold-plating other metals. New theory: the EMT240 foil is nickel, with a gold-plating to... I was about to say "resist corrosion", but that wouldn't be necessary with nickel, would it? Perhaps if they were thinking about long-term reliability, they would. Clearly, there are a few factors to think about, still.

 

[abechap024]

Excellent Ideas, I think having an airtight contraption would be beneficial in 2 ways, you could control the decay times and how tight you want the sheet. I know a trick when people can food is they heat up the mason jars and then stick on the lids and when the cans cool of the lids are sucked onto the jars...I'm wondering if the same thing couldnt be done with aluminum foil and a cake pan. but then again it wouldn't let one change the amount of air inside the pan. otherwise a simple pump type apparatus?

I believe the original 240s' pick up design used a moving coil pick up design there is a picture of it in the manual at the beginning of this thread.

OK so I got 2 peizos from rat shack. They are absolute crap with a resonant frequency around 3k (Ouch!!!) but despite all that it gave for some decent test. I made a very simple jig with an empty picture frame and a cake pan. I taped the foil to the frame and laid it on top of the cake pan. Super glued the 2 peizos on and hooked up the system. initial results were crap. It sounded just like you would imagine something would sound going through a piece of aluminum foil. I then applied some pressure to the frame by pushing on it. Soon I got a very lofi "room" sounding echo. then I applied a ridiculous amount of weight (My knees on the lower 2 frame corners and my hands on the top 2, I'm glad no one walked in on me) and it began to sound like a re verb. Now not an AMAZING sounding reverb but something that could maybe be worked with....maybe. Keep it mind this is the first try. I'm sure its going to take much much more. I then got excited and hooked the foil up to record the beast, and when i went down to give the plate all my loving i didn't have it lined up exactly square and ripped the poor thing.

I think Gyraf might be onto something about moving the air out if the device....Or mybe an electrostatic pic up, It sounds like the foil is really bogged down. Is the foil so thin we have to take into account air resistance?

I'm going to get around to testing another jig today, using clamps to apply constant pressure. then i want to try sucking the air out of at least one side of it. I imagine the foil like a guitar string or speaker. We want to make it tight so it will sing. But how do we avoid making it sound like the inside of a 12 x 12 piece of foil? gold or not. I think there is a vital piece of engineering we are missing... Anyone ever dissect a 240?

 

[haima]

where's CJ?

 

[Kit]

Nice work, refreshing to see someone doing something out of the ordinary.
That being said, I think you will have a hard time getting any usable sounds out of that kitchen foil. Im thinking we need a foil made out of a material with high density and more important high tensile strength. Alu has none of these qualitys. Steel does.
But as you said, for initial experiments Alu foil may answer some basic questions.
Steel foil as thin as your kitchen foil is sold as sheets for making your own gaskets.

 

[Consul]

I still like the idea of using magnets and coils to both feed and pickup the signal in a steel sheet and then using feedback between the two to control reverb time and character.

As for vibration isolation, take a tip from home holographers: build the reverb mechanism on a large paver and float that on a small inner tube at about 60% inflation. I suppose springs could be used instead of an inner tube, at a cost of complexity. Keep in mind, this isn't something being built for retail sale. Instead, it's a essentially a prototype being built for everyday use by the person who built it.

We are all eternal prototypers. (I think it's a very fitting term, myself.)

 

[abechap024]

hi all,
made another prototype last weekend, wanted to give an update in case it might help anyone. I put the foil in a picture frame and tightened it over a pan I got from out toaster oven (gosh I feel ghetto!) and super glued a stick to a speaker cone and the other end of the stick to the foil....
used a peizo for the pick up. The system transmitted signal from speaker to the pick up just fine, but with not much reverb sound to it. I think maybe the peizo is too heavy and damping out all the vibrations. When I just let the speaker play and got my ear close, I'm either high or I can hear some nice reverberation going on. I attempted twice to disassemble some headphones to make a lighter pickup using the method described in the 240 emt manual, but both times those hair thin wires broke.I felt like a thick handed mule....oh welllll no success so far, as I trashed it the next day...(that aluminum foil sure is delicate) but i'm sure I'll get around to building another prototype again sometime

AC

 

[vinyvamos]

I am assuming it ok to dig up this ancient thread, as it has much valuable discussion, rather than me start another thread on exactly the same topic and need to reference this one...

Yesterday I decided to fab up a very rough version of the EMT 240 concept. Attached are some photos of the setup. I am using an alu foil/sheet which is used in the offset printing process. Far thicker than 18 microns! More like 100 microns, coated on one side with a green photosensitive chemical, plus a base material, and seemingly raw or possibly treated alu on the other side. I washed away the green with acetone but the grey base coating remained unfortunately. I am using small piezo discs, one for drive in the middle and another two for pickups. The electronics are an in-progress spring reverb project. Driving with a solid-state line output amp through an edcor 600:10K trafo in attempt to get higher voltage drive. Recovery through a pair of triode stages on each channel. This is all working ok, but now for the issues...

Firstly the frequency response of the piezos is a bit of a pain. Any ideas there? Maybe I should try a conventional voice coil driver (I know the 240 uses the opposite; piezo for drive & tiny coils for pickups). I have added the counter-mass as the EMT manual mentions on the back-side of the driver, in the form of a danish coin and lots of solder ;-). This made a huge improvement to the sound.

The other issue is the sound of the reverb itself. It is working, which is cool, but it has a nasty kind of barrel or tunnel sound, with what sound like standing waves. The decay is about 3 seconds at best, and then nicely reduces when I bring a facecloth closer to the foil. One reason I might be getting standing waves is because I cut the foil as a perfect square (why oh why!) rather than a rectangle. Any thoughts folks? And one reason I might not be getting more decay could be due to the plate being too thick. I will try removing this base coating with more chemicals and test again...

My other aim now is to source thinner foil. I came to the conclusion through reading this thread that the base foil is probably nickel, with gold electroplated on. Sourcing nickel foil could be rather tricky and pricey so any ideas are welcome there! As for the gold, pffff, don't need that kinda bling in my life .

I think that Jakob's idea of creating a vacuum chamber is in fact awesome, and I am keen to try it. In fact, I do have a vacuum pump asking to be used in something (also from an offset print machine in fact!). Let's see about this!

 

[vinyvamos]

The other big question I have is how EMT were suspending/tensioning their foil. It looks like rather small wire hangers with two at each corner and one in the middle of each side. My approach is in comparison brutal and probably overbuilt, but I was considering that I would need to give more tension due to the thickness of my foil/sheet...

 

[PRR]

> Sourcing nickel foil could be rather tricky

You don't have Google where you are?

I found many offers for small sheets.
99.96% Pure Nickel Ni Metal Foil Thin Sheet 0.05mm x 200mm x 1000mm  $17.60
99.96% Pure Nickel Ni Metal Thin Sheet Plate 0.3mm x 100mm x 200mm  $17.54

 

[mig27]

Instead of working with a vacuum to prevent oxidation, one might just as well use a noble gas like helium.
Readily available and chemically uncritical.

 

[Jarno]

But, there is a shortage of helium, and it is expensive! (we use it in superconducting coils to make giant electromagnets)

 

[Whoops]

I am assuming it ok to dig up this ancient thread, as it has much valuable discussion, rather than me start another thread on exactly the same topic and need to reference this one...

It's great that you decided to do this and revive this thread

 

[gyraf]

Instead of working with a vacuum to prevent oxidation, one might just as well use a noble gas like helium.

Yes, but we're not trying to prevent oxidation, but rather to alter damping characteristics of the plate. Thinning out air gives longer decays. Variably so.

Still, there are some of the original questions unanswered so far (We don't have a 240 at our studios)..

Jakob E.

 

[mrclunk]

That's some awesome diy!

We've a 240 here that i'm happy to open up again to solve mysteries.
But...  it's heavy and awkward to get out the machine room so i'd only want to do it the once!
If we come up with a list of things for me to check and measure i'll pull it out and crack her open.
I have some picture from last time i opened it up. I'll try and dig them out.

edit:
https://photos.app.goo.gl/0YpJoux2ynlFecQz2

 

[Whoops]

I might have one in for repair in the next months.

I can give more details then

I have the full instructions and technical docs for the EMT240,
I would upload it to the tech docs section but the pdf is 8.6MB so it's over the attachment limit

what other solutions to host the file?

 

[mrclunk]

Well if your going to open one up anyway...
If you have a google account you can use google drive as i did above?
I can share it from my account if u like?

 

[Tubetec]

I was dozing in the twilight zone the other morning and an idea for a foil reverb came to mind .

First take a used or broken bass drum head ,preferably not a pinstripe.

Now cut a circle out of the middle of the drum head leaving say 5cm all the way around the edge ,now as flat as possible glue your foil around the edge of the drum skin ,maybe just for a proof of the principle use thin aluminium as its cheap and if it breaks you can just start over

Next mount up the head on a bass drum and gradually and evenly tension it up , once upto tension attach what ever transducers ,piezo might be worth a try ,also maybe a small headphone drive unit might work ,but you'll need to mount it seperate from the skin and link the drive coil/cone solidly to the aluminium foil ,not sure the best way to do this ,but you could always experiment .

 

[shabtek]

Forgot about this thread--

What about mylar?

Would a ferromagnetic membrane not be able to resonate freely with electrmagnetic drive and or pickup-- be damped  by one or the other.

a diy 'vacuum' be subject to swings in temp/pressure ; may only change tuning, but cycling could weaken adhesion if mechanical means were not used to hold membrane:
Drum head idea allows for variation in tuning.

Would an asymmetric frame make a more interesting  reflection pattern?

 

[Tubetec]

I think anything will beat a square panel , offsetting the transducers from dead centre  might be worthwhile in that respect too ,It  means your reflected paths will have different delays and hopefully a more rich complex pattern to the verb.

There is the principle used in electrostatic speakers where a mylar sheet is rubbed with graphite powder ,it makes it hold charge and when placed between two plates with a high voltage and driven through a high stepup ratio transformer . There is a possabillity it could work ,but you'll most likely need kilovolts to energise the plates and very large ac swings applied to the membrane ,would be quite an engineering job to do it that way ,but still worth investagating .
Maybe a laser interferometer could be used to pick up the signal from the plate ,that would be great as you wouldnt have the mass of the drive coil to dampen the good vibes .

 

[gyraf]

I think anything will beat a square panel

I'm not sure - we are after surface (bending) waves in this, and these may reflect best off straight edges. But I agree that we should most definitely experiment with this..!

Jakob E.