DIYstompboxes.com

Recently I had to repair the secondary coil of an Accutronics tank because it's a real PITA to import even small packages where I live.
I decided to kill two birds with one shot, a plan B in case the tank fails again, and a reverb unit for my Epi Valve Jr.

I started with ferrite magnets inside coils, and springs and rubber bands.
As audio source, guitar to npn preamp to LM386 to first coil. Second coil to npn to amp.

Some drawbacks, the magnets move perpendicular to the coil axis, not a big deal.
And the coils were big, so the first one induced dry audio into the second coil.
Besides, the magnets I have are not strong, and to hold them in place make weak vibrations.

I decided to use a cheap speaker, a piezo and a copper spring, one side taped to the cone, the other side taped to the piezo.

The spring in the sound demo is about one meter of solid core, alarm systems wire.
I used a pencil to form the spring.
In the pictures you can see bigger one too.

(https://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=53452&g2_serialNumber=1)

(https://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=53455&g2_serialNumber=1)

Schematic is almost included in the first picture. Please do not make me draw one :)

The demo starts with clean sound, then a bit of reverb, a bit more, and finally Marty McFly settings,

https://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/reverb1.mp3.html (https://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/reverb1.mp3.html)

Mobile recording.
The speaker distorts, and the coil should be glued, not taped.
It's getting feedback from the Epi speaker too.

But it's good sounding, nice balance of lows, mids and highs. And when I tried the bigger spring lows were deeper, at the expense of highs, of course.
Two springs would be killer, one inside the other.

I'm having trouble trying to play the sound demo from the Gallery, maybe a Flash thing.

Enjoy,

mac

Pretty cool idea mac.   

Actually works better than I expected from the pics.  There's a bit of "single delay"  reverb sound like you get from a Flanger.   Normally the spring is in tension and they tune the wire-size, winding pitch, spring length and tension to fine tune the sound.  Many even have damping at the mounts.

Check out this one with tension tuning  (note more turns of thin wire on the spring.)
http://www.instructables.com/id/DIY-Spring-Reverb/

The spring is too loose on the video.  Maybe need even thinner wires and more turns.

There are lots of thing to tweak, wire gauge, lenght, materials, compression or expansion, number of piezos, feedback, etc, etc.

I have to re-think the driving and mixing circuit.
Opamps might be a better choice.



:)

mac

This one uses a transverse drive and undamped pivot points for the spring.  IMHO this set up is more like the reverb tanks.  It should have a lot less damping at the ends but there's still some as the spring is connected to the speaker.  Probably would sound better.
https://c1.staticflickr.com/3/2613/3948219750_e1abb6ab48.jpg

QuoteWorld's biggest spring reverb

Yeah there's some insane reverb constructions.

The flickr pict is similar to what I was trying to do with coils, but in axial mode.
I was getting transverse vibrations.
If I place the coils facing upward, the magnets or ferrites vibrations would be along the spring.
I can compare transverse and axial waves.
Thanks.

QuoteYeah there's some insane reverb constructions.

Speaker -> non-newtonian fluid -> speaker, just another idea :)

mac

QuoteSpeaker -> non-newtonian fluid -> speaker, just another idea :)

Good idea.  Maybe a piezo driver and piezo receiver.  They would be easy to adapt to a liquid.
I'm pretty sure blood is non-newtonian.   Mac's Blood-Bath reverb sounds cool.  Ideal for scary movies.  You can even plop things in the liquid for sound effects.

[PS: If I get a chance I might try to extract the spring parameters off some reverbs I have.   I don't want to stuff them.]

QuoteI'm pretty sure blood is non-newtonian.   Mac's Blood-Bath reverb sounds cool.  Ideal for scary movies.  You can even plop things in the liquid for sound effects.

Unfortunately it's illegal to sacrifice animals, or humans, in the name of science.
I'll have to resign myself to use Ketchup  >:(

Or corn and red water, which behaves inversely.

mac

QuoteI'll have to resign myself to use Ketchup

That's the movies for ya.

QuoteUnfortunately it's illegal to sacrifice animals, or humans, in the name of science.

There's actually blood mimicking fluid you can buy for that.   For example when you test heart pumps and valves. When we did ultrasound testing for blood flow we put cornstarch in the water to mimic the reflections off the red blood cells.  For the magnetic flow meter we used salt.  We didn't need to mimic the fluid dynamics.

Quote from: mac on May 03, 2018, 06:24:28 PM

QuoteI'm pretty sure blood is non-newtonian.   Mac's Blood-Bath reverb sounds cool.  Ideal for scary movies.  You can even plop things in the liquid for sound effects.

Unfortunately it's illegal to sacrifice animals, or humans, in the name of science.

both are allowed, however, in the name of art.

Another demo, but with a longer and thicker copper wire.
4" speaker.
Spring about 40cm long, diameter 1cm.

I'm getting a bit of short delay, and lots of mids and lows.

https://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/reverb2.mp3.html (https://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/reverb2.mp3.html)

mac

Don't know why, but I can't listen to the demos :icon_frown:

QuoteDon't know why, but I can't listen to the demos :icon_frown:

I had that problem the other day.  On the left bar there's a duplicate name.  Just click the other
link with the same name and it has the sound file (.m4a).   The ones that come-up blank for me are the .mp3 links, there's actually no link.

QuoteAnother demo, but with a longer and thicker copper wire.
4" speaker.
Spring about 40cm long, diameter 1cm.

I'm getting a bit of short delay, and lots of mids and lows.

That one sounds pretty good.

Thanks for the tip, Rob. Now I got it working. This one really sounds pretty good! Just to clarify, you've ended up with just the coil glued to the speaker and the piezo, no magnets?

It´s taped in the demo.
The speaker side must be glued.
The piezo side could be soldered, or glued as long as the coil is in contact with the piezo.

QuoteDon't know why, but I can't listen to the demos :icon_frown:

I had that problem the other day.  On the left bar there's a duplicate name.  Just click the other
link with the same name and it has the sound file (.m4a).   The ones that come-up blank for me are the .mp3 links, there's actually no link.

I can't play the mp3 either, that's why I left the .m4a  ???

New host issue?

.m4a links,

https://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/reverb1_m4a.html (https://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/reverb1_m4a.html)

https://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/reverb2_m4a.html (https://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/reverb2_m4a.html)

mac

I can't play them in the browser.  I could download the .m4a, had to change the end of the file name from "_m4a" to ".m4a" to play it.  Don't see a download option for the .mp3.

What happened with the mp3 files is what Rob mentioned. No download link, then click on the duplicate name on the left side. Then the file is downloaded as "_m4a", but it plays correctly after renaming it to ".m4a".

Question on the schematic. Could you draw one? :icon_lol: just kidding.
"As audio source, guitar to npn preamp to LM386 to first coil. Second coil to npn to amp."
The npn preamp is something like the LPB? Then you split the signal into an 386 (pins 1 and 8 opened?), which feeds the speaker. The piezo is connected to an npn preamp (LPB?), then the wet signal is mixed with the dry signal that cames from the first LPB. Is that correct?

In the first demo I used a mpsa42, 10k to vcc, 1M C to B, 2k2@emitter to reduce gain,  then a 50kA pot to lm386 to speaker.
The mixer amp is a BMP 1st stage, 1k@emitter.
Guitar to input cap to 47k to Npn.
Piezo from B to Gnd, not a good idea because it cuts highs, piezo capacitance?

Second demo,
Signal to Npn buffer to 50kA pot to lm386 to speaker.
Mixer unchanged.
One side of the piezo to base, the other to 47k to gnd.
Guitar to cap to 250kA to mixer base.

mac

QuoteWhat happened with the mp3 files is what Rob mentioned. No download link

The audio player is missing, new host issue?

QuoteQuestion on the schematic. Could you draw one? :icon_lol: just kidding.

>:(

(https://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=53521&g2_serialNumber=2)

mac

I was kidding on drawing the schematic, but thanks a lot :icon_lol:. Nice and simple. Need to get me some solid wires.

QuoteNeed to get me some solid wires.

I like copper, it's softer than steel springs of the same gauge. But that's my taste.
I was about to try the wire around the core of wound strings too.

QuoteI was kidding on drawing the schematic, but thanks a lot :icon_lol:. Nice and simple.

I had to draw it by hand since I'm in bed right now :(
It needs to be polished, a better mixing, feedback, etc.

mac

FYI, I dismantled a spring reverb the other day and started taking measurements of the springs.   Haven't had a chance to get back to it.  I had some unexpected results.   The springs have a degree of pre-tensioning in them ie. you need to apply a minimum force before the turns separate.   I realized that when I got different spring constants at different extensions.   BTW the only reason I found that out is because I removed the springs.  Initially I calculated the spring constants from the dimensions which would have given me incorrect results.

http://mae3.eng.ucsd.edu/machine-design/force-torque-power-analysis/ftpe-springs

mac

Quotehttp://mae3.eng.ucsd.edu/machine-design/force-torque-power-analysis/ftpe-springs

Thanks, at least it confirms the spring model I'm using for pre-tensioning.

I'm using one of these to measure the force,

http://media.rs-online.com/t_large/F0432211-01.jpg

Measuring deflections of 1mm using adhoc methods is a bit dubious.  It's not that important anyway I only want a reasonable ball park.

Anyone know how pre-tensioned springs are manufactured?  I can't imaging them winding them hot and cooling them there has to be a trick.

QuoteMeasuring deflections of 1mm using adhoc methods is a bit dubious.  It's not that important anyway I only want a reasonable ball park.

If the spring follows Hooke's Law you could measure at say 1cm and the costant K should read the same.
Another silly way to get K is to hang a small weight, Mweight>>Mspring, and measure how many times the weight goes up and down in a given period of time, say 30 seconds. Then divide 30sec by N and that's your period. Roughly. Do it many times to reduce the counting error. Statistics :)
But you need to know Mw  ???
2.pi.f=sqrt(K/M)

QuoteAnyone know how pre-tensioned springs are manufactured?  I can't imaging them winding them hot and cooling them there has to be a trick.

That's what blacksmiths have been doing for centuries, fire, hammer and water... fire, hammer and water... :)

mac

QuoteIf the spring follows Hooke's Law you could measure at say 1cm and the costant K should read the same.

Initially I had a few points for large extensions but depending on what points I used I got different numbers.  That's when I knew the springs were pretensioned.   I then did a measurement at small extensions like 1mm to 5mm just to confirm the problem.    I've actually got a least squares line fit now, which essentially does the statistics for me.  It won't get rid of systematic error at small deflections so I'll chuck out those points.

The reason I'm not concerned about the accuracy is that I'm sure even good sounding springs vary a lot.  Moreover, it seems the spring constant isn't an important factor, at least not directly.

QuoteBut you need to know Mw  ???  2.pi.f=sqrt(K/M)

I remember doing that in physics labs.    The problem is the spring has mass distributed along the length and you need to take that into account somewhere/somehow.   My favorite standard masses are using coins.

Quotehat's what blacksmiths have been doing for centuries, fire, hammer and water... fire, hammer and water...

At 4am this morning I woke and it came to me that tensioning the wire while winding might also work.   That method is probably more controlled.   The only concern is will the wire break to get the pretension you see in the real springs.  And is that what manufacturer's do?  (How to do it is only for interest anyway.)

I'll try to post some more stuff today. 

BTW, what is the diameter and number of turns (or length of wire) of your good spring?  I'm assuming it's 22AWG wire.  I was going to try to compare it against the spring reverb springs.

QuoteThe reason I'm not concerned about the accuracy is that I'm sure even good sounding springs vary a lot.  Moreover, it seems the spring constant isn't an important factor, at least not directly.

Young's modulus and spring constant,
https://www.physicsforums.com/threads/difference-between-youngs-modulus-and-spring-constant.887403/ (https://www.physicsforums.com/threads/difference-between-youngs-modulus-and-spring-constant.887403/)

Lenght, tension, mass, cross sectional area shape, speaker in this case, etc, are likely to be more important than the spring constant.

mac

QuoteBTW, what is the diameter and number of turns (or length of wire) of your good spring?  I'm assuming it's 22AWG wire.  I was going to try to compare it against the spring reverb springs.

The thicker spring was used in the 2nd sound demo. It was from the secondary of a 220v-6v wall wart. About 240 turns.
Looks like half a mm, so it could be 22 awg or higher.

The shorter and thinner spring was used in the 1st demo.

(https://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=53532&g2_serialNumber=1)

(https://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=53535&g2_serialNumber=1)

mac

QuoteThe thicker spring was used in the 2nd sound demo. It was from the secondary of a 220v-6v wall wart. About 240 turns. Looks like half a mm, so it could be 22 awg or higher.
The shorter and thinner spring was used in the 1st demo.

Awesome mac thanks.  I'll plug in those parameters and see how it goes.

Finally got a chance to get back to this.  I found this paper on calculating reverb delay times,

http://research.spa.aalto.fi/publications/papers/dafx09-sr/
http://dafx09.como.polimi.it/proceedings/papers/paper_84.pdf

Then there's this paper which discusses loading (ie. stretching or compressing) the springs,
http://newt.phys.unsw.edu.au/music/people/publications/Fletcheretal2001.pdf

From what I can work out so far.    The behaviour of the coil doesn't depend so much on the tension in the coil at the operating position.  It depends on the length of the coil at the operating position (H)  (and not the free length L0).   These papers calculate Delay Time not Decay Time.  Decay time relates to damping.  Also note R is the average radius.  In the table below I used Dav, the average diameter.

The tension in the coil prevents it from sagging.

The first paper gives some example of springs.

I disassembled two reverb tanks and extracted the spring data.

Old reverb from an organ.  Probably early 70's.
No brand the only marking is "Mod 003.930.861" which I assume is the model
(https://s9.postimg.cc/j1iw21m6z/mod_003.930.861.jpg) (https://postimg.cc/image/j1iw21m6z/)

Accutronics 1AB_D1_  from an old Washburn amplifier (probably an SX60C).
I don't know the full number because the top of the reverb tank is corroded.  I found it on the side of the road.  It was only the box and the reverb.  The speaker and amp chassis were removed.
(https://s9.postimg.cc/qhi5nupbv/acc1_AB_D1.jpg) (https://postimg.cc/image/qhi5nupbv/)

BTW all the reverb springs have pretensioning.  I suspect this is done to keep the length of the spring down?

I plugged data for the various springs in the formulas from the first paper.   So there's:
- the examples in the paper
- the springs for the two tanks I measured.
- your two handmade reverb springs.
- an example where I tweaked the dimensions to increase the frequency response

(https://s9.postimg.cc/d1v4ygtpn/Reverb_Spring_Calc_2018_05_14.png) (https://postimg.cc/image/d1v4ygtpn/)

The key results are Td and Fc.  Td is the delay time and Fc is the upper cut-off frequency.
For the split spring I added the Td's for each spring calculated individually.  For Fc I'm not sure
how to combine it correctly.  It will be between the values in the table and the values I annotate at the bottom.

By playing with the coil diameter it is possible to tweak the frequency response (Fc) and then you tune the length to give you the desired delay.   I've added an example where I took your second coil and reduced the diameter then tweaked the delay to be in the order of the commercial units.

According to Accutronics they give this guide:
LONG   41 ms   (guitar)
MEDIUM   37 ms   (organ)
SHORT   33 ms   (voice)

Just an observation..
You are feeding the piezo transducer into a really low input impedance transistor stage.. i would use a jfet or an opamp with at least 10meg input impedance if you want decent frequency response from it..

QuoteJust an observation..

There's also the current drive vs voltage drive issue.


Interestingly I found a reference to a commercially available piezo based Reverb Tank from a 1976 Electronics magazine:

(https://s9.postimg.cc/fxriqmwpn/RE-4_Piezo_EA_May_1976.png) (https://postimg.cc/image/fxriqmwpn/)

QuoteJust an observation..
You are feeding the piezo transducer into a really low input impedance transistor stage.. i would use a jfet or an opamp with at least 10meg input impedance if you want decent frequency response from it..

Yeap.
A TL072 can do it, one ic as the input buffer, the other to mix the signal.

mac

Quote from: Rob Strand on May 14, 2018, 07:03:19 PM
Interestingly I found a reference to a commercially available piezo based Reverb Tank from a 1976 Electronics magazine

The 60's Danelectro/Silvertone amps (1484 etc) used a piezo tank, didn't they?  I think those were made in-house though.

QuoteThe 60's Danelectro/Silvertone amps (1484 etc) used a piezo tank, didn't they?  I think those were made in-house though.

I don't know.   Getting out of my knowledge zone.    Probably a good way to avoid stepping on Hammond's toes.

I remember some DIY spring reverbs in 60's electronics magazines that used a ceramic cartridge (from a stereo) for the sense end.
--------------
Edit
Apparently 1960's Danelectro 9100 tube spring reverb unit.
Also saw VOX used a commercial ceramic cartridge.

I've got a wacky one here.  I recently tore apart a dead '67 Silvertone solid state amp (scored several nice Si and Ge transistors, most are in pedals now).  The reverb tank is in a cylindrical cardboard tube.  It has a shielded wire (output?) and two unshielded red wires coming out.  I get about 60 ohms across the red wires, but the shielded wire reads open.  Maybe it was driven with a transducer, but has a piezo pickup?  Who knows!  I haven't tried to open it because I might list it on Ebay.

Thanks Rob, a good reading while I'm still in bed. (thanks Yellow Fever vaccine :) )

I'd like to take a look at the Love-Kirchhoff theory.

Parker & Bilbao say: "The magnetic bead is driven torsionally by passing a signal into
a nearby electromagnetic coil. Torsional vibration of the magnet/wire
system translates to vibration tangential to the path of the
wire at a point within the helix."

The ratio R/r on the Fc eq. looks like the rotation of one closed turn.

It makes me wonder,
-the speaker gives a nice punch to the spring along the axis. The movement of the turns is visible, so the spring must be expanded enough to avoid turns hitting each other.
-and the edges of the coil are fixed at both sides, no rotational force at all.

This gave the idea to use a 2D sawtooth like Fig 1 in the Parker & Bilbao paper.

mac

Quote from: mac on May 15, 2018, 09:33:13 PM
It makes me wonder,
-the speaker gives a nice punch to the spring along the axis. The movement of the turns is visible, so the spring must be expanded enough to avoid turns hitting each other.
-and the edges of the coil are fixed at both sides, no rotational force at all.

This gave the idea to use a 2D sawtooth like Fig 1 in the Parker & Bilbao paper.

mac

What if you fix the spring with one of those swivels, allowing it to rotate?
(https://s7.postimg.cc/kwwuz9prb/shop_items_catalog_image7109.jpg) (https://postimg.cc/image/kwwuz9prb/)

QuoteWhat if you fix the spring with one of those swivels, allowing it to rotate?

Good idea.

Those in particular can introduce noise with a loose spring like the ones I used.
With a tighter spring, a plastic/metal swivel can work without introducing noise or damping.

mac

QuoteThanks Rob, a good reading while I'm still in bed. (thanks Yellow Fever vaccine :) )

That paper is a good start.  The numbers seem to make sense.

QuoteParker & Bilbao say: "The magnetic bead is driven torsionally by passing a signal into
a nearby electromagnetic coil. Torsional vibration of the magnet/wire
system translates to vibration tangential to the path of the
wire at a point within the helix."

Yes, I've read that before in the National Semiconductor Audio Handbook.  It actually describes things is a bit more detail.   I'm fairly sure most information around came from Accutronics literature of the 1970's.
See part 2, section 5-7 to 5-10 from this document,
https://e2e.ti.com/support/amplifiers/audio_amplifiers/w/design_notes/3315.blast-from-the-past-1980-nsc-audioradio-handbook
https://e2e.ti.com/cfs-file/__key/communityserver-wikis-components-files/00-00-00-00-03/1980-NSC-Audio-_2600_-Radio-Handbook-_2D00_-Part-2.pdf

It also implied the torsional drive has a slow propagation which helps give the long delay.

In order to get a torque that rotates the end of the spring the magnet must magnetized across the cylinder.
What concerns me about having this type of system is the magnet also needs to be pointing in the right direction.  If the magnetic field points across the gap you don't get any torque.  The magnetic field needs to be in parallel to the faces of the gap and perpendicular the axis of the spring.  That's going to be a headache for manufacturing.  It also means you can stuff up the motor or sensor by rotating the magnets.   I used to think the magnets were polarized along the axis and the magnet applied a torque but it did so by rotating along the magnet diameter line .  That would pull the wire up and down.  That way you don't care about the magnet orientation.

Because of that I'm scared to pull one apart to the point that I can get to the magnets and measure the direction of the magnetic field!

QuoteThe ratio R/r on the Fc eq. looks like the rotation of one closed turn.

I've always approached this stuff as the cut-off is near a resonant frequency where the forces for against the spring and the forces against the inertia of the mass are about equal.  More or less like the sqrt(k/m) example you gave before except more general in that freq is proportional to sqrt(stiffness / mass).   If you apply this to the spring or a wire you get the r/R^2 term.    I did that a couple of ways (not expecting the exact answer) and what I noticed is the Fc in the paper doesn't depend on N which I suspect you can only get by solving the equations for the distributed system.

Other than that, I haven't tried to analyse this thing any further than the paper.

QuoteIt makes me wonder,
-the speaker gives a nice punch to the spring along the axis. The movement of the turns is visible, so the spring must be expanded enough to avoid turns hitting each other.
-and the edges of the coil are fixed at both sides, no rotational force at all.

The thing about these 2D/3D systems its when you excite one mode of oscillation it tends to oscillate the others.  So it turns out any drive system will work.  Maybe if you drive the intended mode directly it gives a more efficient drive and receive system.   There was one DIY reverb which used a speaker but instead of driving longitudinally, the speaker was mounted side-on and an arm from the face of the speaker connected to the end if the spring.  It was done in a way that the in-out motion of the speaker caused a rotation of the spring.   I also remember reading the magnetic field across the diameter can prevent mechanical noise and help with feedback immunity.  So maybe rotational sensing is also good.

QuoteWhat if you fix the spring with one of those swivels, allowing it to rotate?

Friction/stiction might cause creaks.   I suspect that why an elastic end piece is better.

QuoteThe reverb tank is in a cylindrical cardboard tube.  It has a shielded wire (output?) and two unshielded red wires coming out.  I get about 60 ohms across the red wires, but the shielded wire reads open.  Maybe it was driven with a transducer, but has a piezo pickup?  Who knows!  I haven't tried to open it because I might list it on Ebay.

That's a funky set-up.  Probably not worth risking damaging the unit.

The magnetic drive and ceramic sense seems to be the way to go.   I don't know about the unit I posted from the magazine.   I seemed to recall the magazine implied it had a low impedance and they put a resistor in series with it.   Unfortunately that doesn't help identify the drive system.    The old Motorola piezo tweeters used to have a similar set-up, so maybe it was piezo drive.

Thanks again Rob for those TI pdfs.

QuoteIt also implied the torsional drive has a slow propagation which helps give the long delay.

In order to get a torque that rotates the end of the spring the magnet must magnetized across the cylinder.
What concerns me about having this type of system is the magnet also needs to be pointing in the right direction.  If the magnetic field points across the gap you don't get any torque.  The magnetic field needs to be in parallel to the faces of the gap and perpendicular the axis of the spring.  That's going to be a headache for manufacturing.  It also means you can stuff up the motor or sensor by rotating the magnets.   I used to think the magnets were polarized along the axis and the magnet applied a torque but it did so by rotating along the magnet diameter line .  That would pull the wire up and down.  That way you don't care about the magnet orientation.

Because of that I'm scared to pull one apart to the point that I can get to the magnets and measure the direction of the magnetic field!

In a standard reverb tank the magnet moves in circles, not along the spring axis, and that movement creates torsional waves along the spring.
It's like the Earth's orbital and axial (or spin) momentum.
I had noticed this when I tried magnets inside coils, as I wrote before in this post.

http://sound.whsites.net/articles/reverb.htm (http://sound.whsites.net/articles/reverb.htm)

QuoteI've always approached this stuff as the cut-off is near a resonant frequency where the forces for against the spring and the forces against the inertia of the mass are about equal.  More or less like the sqrt(k/m) example you gave before except more general in that freq is proportional to sqrt(stiffness / mass).   If you apply this to the spring or a wire you get the r/R^2 term.    I did that a couple of ways (not expecting the exact answer) and what I noticed is the Fc in the paper doesn't depend on N which I suspect you can only get by solving the equations for the distributed system.

Roughly speaking, Fc is calculated from the group velocity, which is an "average" speed, ie, the envelope of many waves. L or N are likely to get lost going in circles along the spring :)

QuoteThere was one DIY reverb which used a speaker but instead of driving longitudinally, the speaker was mounted side-on and an arm from the face of the speaker connected to the end if the spring.  It was done in a way that the in-out motion of the speaker caused a rotation of the spring.   I also remember reading the magnetic field across the diameter can prevent mechanical noise and help with feedback immunity.  So maybe rotational sensing is also good.

I'll try this. I hope the piezo can read it.

I also thought of two speakers face to face, and two thick wires glued to the cones. One wire soldered to the upper side of the spring, the other to the lower end.
When the speaker cones move they do what you do with your hands when your are turning the car's wheel.

mac

Main difference between this speaker-piezo reverb and a standard reverb:

- In a reverb tank the spring is under torsion forces, not along the axis.
The model in the PDFs Rob posted before describes a wave travelling along the spring.

- The speaker force on the spring is mostly axial. Radial and angular displacements are negligible as the frequency goes down.
The turns move along the axis, and nodes and peaks are visible, like a gas on a pipe.

Things to have in mind if you try this,

>The separation or tension of the spring is critical to avoid turns hitting each other at lower frequencies, or larger wavelenghts.
>Spring should be hold tightly at both ends.
>Thick wires roll off highs.
>A floating piezo absorbs part of the speaker energy.
>Piezos are sensible, any speaker distortion will get in the way.

I update the circuit, I'm using two opamps as buffers and I'm getting an overall tone improvement.
Uploading soon.

mac

QuoteIn a standard reverb tank the magnet moves in circles, not along the spring axis, and that movement creates torsional waves along the spring.
It's like the Earth's orbital and axial (or spin) momentum.
I had noticed this when I tried magnets inside coils, as I wrote before in this post.

Thanks for the link.   OK I get that part.  I still don't know which way the magnets are polarized, which determines how the forces/torque are applied.  When the magnet orbits I suspect this is a *result* of the spring.   It's like a guitar string if you pluck it any way, or bang the guitar, the strings eventually follow an elliptical motion.

I applied the maximum DC current to the reverb and tried to observe/feel force.   I could not feel or see the any deflection or rotation.  I could hear a small "tick".   I might repeat the test again with the spring removed.

QuoteRoughly speaking, Fc is calculated from the group velocity, which is an "average" speed, ie, the envelope of many waves. L or N are likely to get lost going in circles along the spring

That makes sense.   The results which have N are likely to be macro effects, not wave propagation, like the transverse motion when you hit the reverb.   The frequencies are much lower.

QuoteI also thought of two speakers face to face, and two thick wires glued to the cones. One wire soldered to the upper side of the spring, the other to the lower end.
When the speaker cones move they do what you do with your hands when your are turning the car's wheel.

On one hand I think it's the old problem that if the system isn't perfectly symmetrical all the propagation modes exist.   So it doesn't matter how you excite or detect you end up getting a bit of each.   On the other hand, when I calculated the longitudinal wave speed, I get delays about half that of what the Parker paper predicts.    That could mean the dominant propagation in your set-up is longitudinal not torsional (as assumed in Parker paper).

            mac1    mac2
mass [g] 0.74   13.61
Td [ms] 14.16   55.87

Perhaps that explains why your mac2 spring sounded about right (?).   The Parker equations predict a longer delay.

Here's a good summary of the simpler equations,
http://www.physics2000.com/PDF/Non-CalcText/Ch13WaveSpeedNonCalculus.pdf

Keep in mind that L in this paper, the length of the spring in the operating position, is actually H in the Parker  paper.

What you do think?   

Grrrr.  Forgot to post this. 

Here's the link for the DIY unit with the ceramic cartridge,

http://www.ozvalveamps.org/reverbs.htm

Scroll down to half way until you see.
   Radio Constructor
   Reverberation Unit by Hal Moorshead

From what I can work out the article is actually from Practical Wireless (UK) July 1970, not Radio Constructor.

-

I haven't had a chance to go over this one properly.   It's referred to in the Parker paper,

http://newt.phys.unsw.edu.au/music/people/publications/Fletcheretal2001.pdf

The paper starts with a beam.  In this context the orientations of transverse/longitudinal/torsional take on different meanings.  (Later on it translates to the axis of the helix.)

QuoteI still don't know which way the magnets are polarized, which determines how the forces/torque are applied.  When the magnet orbits I suspect this is a *result* of the spring.   It's like a guitar string if you pluck it any way, or bang the guitar, the strings eventually follow an elliptical motion.

And I still don't know IF the driving magnetic field is
- like in a standard transformer, the lines along the lamination, with the upper and lower lines running in opposite directions
- if there is some leaking field between laminations, forming a vertical field, a kind of N-S magnet.

I found this post at diyaudio,

http://www.diyaudio.com/forums/parts/289882-spring-reverb-transducers.html (http://www.diyaudio.com/forums/parts/289882-spring-reverb-transducers.html)

It seems it's magnetized radially.
Neodymium magnets of this type can be found for a few bucks.

QuoteOn the other hand, when I calculated the longitudinal wave speed, I get delays about half that of what the Parker paper predicts.    That could mean the dominant propagation in your set-up is longitudinal not torsional (as assumed in Parker paper).

            mac1    mac2
mass [g] 0.74   13.61
Td [ms] 14.16   55.87

Perhaps that explains why your mac2 spring sounded about right (?).   The Parker equations predict a longer delay.

When a spring is compressed or expanded by a force, a speaker, you have to use the shear modulus, not Young's modulus.
The Young modulus is used because a torsion force can be thought as a hammer hitting the spring in the "s" direction, as defined in the Parker Bilbao paper. You are compressing the wire a bit along that direction, ie, along the wire.
That is, a "longitudinal" wave running along the wire. From the distance, we called this wave torsional.
In my case, the speaker deforms the spring in a different way, almost perpendicular to the wire. This looks like a "transverse" wave along the wire. But again, from the distance we say it's a longitudinal wave along the spring axis.
Messy, isn't it? :)

IIRC, and this is stuff I haven't seen in 30 years, my delay time should be,
Tm=2*Lt/sqrt(G/d)
where
Lt: the uncurled lenght of the wire
G: shear modulus, 44,7*e(9) N/m2 [kg/m/seg2] for copper
d: density, 8960 kg/m3 for copper

I am hearing a short echo, similar to the audio latency of my old Power Mac 7300/180 of about 13ms.
The formula above gives me near 6ms for mac2 spring.

Anyway, the axial spring model is similar to a gas on a pipe. Have to dig deeper for better aproximations.

http://home.uni-leipzig.de/prakphys/pdf/VersucheIPSP/Mechanics/M-16E-AUF.pdf (http://home.uni-leipzig.de/prakphys/pdf/VersucheIPSP/Mechanics/M-16E-AUF.pdf)

mac

I wonder if an unused guitar mic can be used to drive a magnet attached to a spring, or a magnetic spring made of a guitar string.

And I was thinking of a different kind of spring, a flat balance spring.
The external side of the wire perpendicular to the speaker to have torsion motion, and the center attached to the piezo.

(http://www.damaskowatchtechnology.com/images/tech_epsspring-small.jpg)

mac

QuoteAnd I still don't know IF the driving magnetic field is
http://www.diyaudio.com/forums/parts/289882-spring-reverb-transducers.html

It seems it's magnetized radially.

I can't see how a radially polarized magnet can work.    They only work when
you poke magnetic material inside the hole; that conducts the field out of the hole.
(The other use is putting things inside the hole.)

I'm pretty sure I've got a handle of how the motor system works.  There is just a vertical field
inside the gap between the core faces.   The magnet tries to align with the field and that produces
a torque on the magnet.   For case C it's already aligned so to the torque is low.  There can be a weak vertical force due to a different mechanism; basically the field is slightly stronger near the core and that causes a force due to the field gradient.

Here's the cases for the different magnets.   The arrows represent the field direction.  The circular paths in B and D represent the rotation due to the torque.

(https://s31.postimg.cc/43t7p1fk7/reverb_motor_2018_05.png) (https://postimg.cc/image/43t7p1fk7/)

What isn't clear is if is case B or D.    Case B requires alignment of the magnet during manufacturer which borders on impractical to me.   Misalignment of the magnet could result in case C which is fairly useless.

QuoteWhen a spring is compressed or expanded by a force, a speaker, you have to use the shear modulus, not Young's modulus.

I understand what you are saying here.   

The longitudinal speed in the simple paper is in terms of k and M.   k is the compression/expansion spring constant.     k actually contains G.   (When you design springs for a certain k you use G plus the geometry and turns.)

If you look at equation 2.4 and figure 1 of the Fletcher 2001 paper he formulates the longitudinal speed in terms of G (inside of c_psi_0).   The interesting thing is in equation 2.4 the longitudinal speed, along the helix axis, is slower by a factor of 1/sqrt(2).  That and the fact G is less than E explains why the longitudinal speed is slower by a factor of 2.

QuoteThat is, a "longitudinal" wave running along the wire. From the distance, we called this wave torsional.
In my case, the speaker deforms the spring in a different way, almost perpendicular to the wire. This looks like a "transverse" wave along the wire. But again, from the distance we say it's a longitudinal wave along the spring axis.
Messy, isn't it?

(Yes) The geometry of the beam and the geometry of the helix flip some of the co-ordinates.   I guess the significant contribution from Wittrick 1966 was to unravel the calculations.

QuoteI am hearing a short echo, similar to the audio latency of my old Power Mac 7300/180 of about 13ms.
The formula above gives me near 6ms for mac2 spring.

That seems quite short.

QuoteIIRC, and this is stuff I haven't seen in 30 years

Same here.   I've only use small amounts of this stuff over the years. 
My deeper intuition seems to have gone out the window.

QuoteAnd I was thinking of a different kind of spring, a flat balance spring.
The external side of the wire perpendicular to the speaker to have torsion motion, and the center attached to the piezo.

I don't see why it won't work. I guess the issue is to get enough delay.
If you tried to connect more than one spring (by connecting the inside to the inside then the outside to the outside) it's probably easier to just use the cylindrical spring.

I really love this thread the longer it goes on.  I think spring reverb is "just one of those things" that some clever guy invented because nothing else was possible at the time.  It didn't really do what it was supposed to do perfectly, but it took on a life of its own and is now highly sought after (at least in some musical situations).  I'm really interested to hear what anyone comes up with using no traditional springs, especially if it can do long (4 to 6 second perhaps) decay times.

Even though I have digital reverbs galore I put a real spring in when I built up my modular synth system.

As a kid we used to "zing" on the garage door springs with a screwdriver.  Hours of cheap entertainment!

QuoteI think spring reverb is "just one of those things" that some clever guy invented because nothing else was possible at the time.  It didn't really do what it was supposed to do perfectly, but it took on a life of its own and is now highly sought after (at least in some musical situations).

So very true.  Like a lot of stuff from the early days.  The Fender Rhodes comes to mind.

The thing that amazes me most about guitar stuff is the sound of guitar is defined by a combination of Tube amps, totally flawed speakers from a hi-fi perspective, and the inherent limitations of pickups.   A good deal of technical improvements only seems to subtract from that winning combination.   You wonder how it came to be.

Quote
I'm really interested to hear what anyone comes up with using no traditional springs, especially if it can do long (4 to 6 second perhaps) decay times.

Even though I have digital reverbs galore I put a real spring in when I built up my modular synth system.

You might be interested in this stuff.   Stefan Bilbao, the co-author of the Parker paper, seems like he's got this stuff down at all levels,

http://www.acoustics.ed.ac.uk/group-members/dr-stefan-bilbao/
http://www.ness-music.eu/wp-content/uploads/2013/06/dafx13.pdf
https://www.researchgate.net/publication/224586163_A_Virtual_Model_of_Spring_Reverberation

After lot of pain I've managed to work out the way the reverb springs are driven.

The reverb motor system causes the magnets to rotate about their axis.  That implies the magnets are polarized diametrically and the magnets field is orientated horizontally.

A positive current into the center pin causes the magnets to rotate in a clockwise direction when looking at the magnets from the spring side.    From what I can make out a positive on the center pin gives a B-field vertically upwards across the gap (opposite directions to my diagram posted previously).   That means the magnets are polarized diametrically and orientated horizontally from right to left.

(https://s31.postimg.cc/c2dv2x97r/reverb_motor_drive_direction_BM_2018_05_22.png) (https://postimg.cc/image/c2dv2x97r/)

Comments:
- The torques on the left hand magnet seem stronger than the right hand magnet.  This might be due to the fringing around the end of the core.  It would be better to extend the core further to right.


The problems & solutions:
-  I've never spent so much time deciding a motion was a rotation or a vertical vertical movement.
- The forces and torques are tiny.
- With the springs loaded you cannot see any spring motion.  The tension is too high.
- The stiffness/friction in the pivot sometimes makes a rotation look like a vertical movement.
   This was particularly a problem when the springs were removed.
- I used about 800mA DC to give the largest force/torque possible.
- I checked the movement two ways: 
   The first way was to removed the springs and attach wires with just enough mass and moment that they were the dominant load.  That got round the pivot friction issue.
   The second  was to insert the spring but to add a long wire link at one end so I could tune the spring tension just enough to see the movements.

Anyway,  the good thing is the rotary drive was mentioned in the NSC Audio Handbook, the Parker Paper, and the Rod Elliots ESP website.   So I can only conclude it is that way.    That brings up the question on how to align the magnets in production.   It also means the magnet must be mechanically attached to the end-pivots, otherwise the magnets will spin without transferring torque to the springs.

To maybe state an obvious:

You can find the poles of a round magnet with a very fine pin. If a pin sticks top/bottom, or left/right, it is magged across a diameter.

If it IS poled that way, then in production you lay the magnet on an iron surface. Presto, one pole goes to the iron and now you know which way to go. Logically this could be an iron jig which then slides into the pickup assembly.

QuoteYou can find the poles of a round magnet with a very fine pin. If a pin sticks top/bottom, or left/right, it is magged across a diameter.

I'm working on it  :icon_wink:.  I've got to find something suitable.   I tried sticking stuff to the magnets but I couldn't feel anything.  The magnets are very weak and/or the things I've tried are actually stainless.

QuoteIf it IS poled that way, then in production you lay the magnet on an iron surface. Presto, one pole goes to the iron and now you know which way to go.

Good idea!  I noticed the magnets are also phased correctly, so maybe take that one step further and use a magnet so it points in the right direction.   It didn't occur to me before but they might first make the little wire pivot assemblies with the magnets attached then magnetize them later.  There's little hooks for the springs which are mounted in a consistent way so that ensures the magnets would point in the right direction.

QuoteIf it IS poled that way

I'm very certain of it.  I played around with it for ages until I was sure.

> I played around with it for ages until I was sure.

Your interpretation agrees with all I have read, but I am very glad you are doing the confirmation.

(Yes, random assembly and post-magnetizing makes sense too.)

A better circuit for the speaker-piezo,

(https://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=53541&g2_serialNumber=1)

mac

Rob,
your last picture will go viral :)

As soon as I finish playing with the speaker-piezo combo, I'll try a coil-magnet-spring-piezo with torsion force to get more delay with a shorter spring.

mac

I replaced the speaker with a coil and a flat cylinder neodymium magnet.
I placed the magnet perpendicular to the coil, as seen in the picture below.
Again, the coil is driven with a LM386.
A speaker cone can also be used, and the impedance matching will be better.
But I used the coil to get more torque.

I am having a visible torque. I can feel it in my fingers too.
And when the magnet flat side is horizontal, the torque is gone.

I need to modify the circuit a bit, but the good thing is... it works :)

(https://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=53544&g2_serialNumber=1)

mac

I uploaded a video showing standing waves in the speaker-piezo reverb. Just for fun :)



The spring in this case acts like a 1D gas in a tube, which is closed in one side and has a moving piston in the other side. In equilibrium, gas and air pressure are equal.
But if the gas inside is compressed a bit by the piston, and then the piston is released, there will be a net force acting on the piston, because the gas pressure in now bigger than the air pressure. The piston will move back to the equilibrium position.
The force exerted by the gas on the piston surface is PA or using ideal gas equation, nRTA/V=nRT/x
Assuming small displacements from equilibrium 1/x can be aproximated to A+Bx... and boila! harmonic oscillator!
(the demonstration is a bit more complicated, I tried to make it more DIY friendly -> https://en.wikipedia.org/wiki/R%C3%BCchardt_experiment )

This reverb is less complicated for a DIYer. A decent speaker, a LM386 or any small power amp, and a piezo.
But you a pay a price.
Delay time is shorter. You need a larger spring to get the same delay of a standard reverb can.
I'm getting short delay with long decay and lots of lows and mids.
Is it bad? Well, it's a matter of taste.


Rob attached some papers from Parker & Bilbao.
The delay time of a spring under torsion is R/r longer.
How is that?
Think the compression spring as kids in a row playing a silly game. When a kid is touched by the kid before, she/he has to touch the next kid. Kid 1 touches kid 2, immediately kid 2 touches kid 3 and so on.
In the torsion case, kids must describe a full circle with their arms before touching the next kid in the row. Kid 1 makes a circle then touches kid 2, and so on.
In the last case the speed is slower.

This is the best analogy I can think of to remark the differences without the use of Lagrangian or Hamiltonian Eqs :) :)

I'm still experimenting.

mac

Looks cool.  I like your coin-flipper motor.  You will get a lot more force for a given drive current with a closed iron path similar to the reverb tanks.  With the coil you have it will get quite large, however, with the iron in place you can shrink down the scale of things somewhat.

I did not put an iron because the magnet is so strong that it sticks to the iron.
Maybe a non magnetic material?

I also thought of a divergent field outside the coil, but it can generate transversal waves adding a more complex pattern.

I will continue experimenting when I return home. Too cold in my town, in NE Brazil right now  :icon_lol:

mac

QuoteI did not put an iron because the magnet is so strong that it sticks to the iron.
Maybe a non magnetic material?

Hmmm, that *is* a problem with those magnets.
It's probably a delicate balance of choosing the right size magnet, gap height and spring tension.
Keep in mind with the close magnetic path you will probably get away with a much smaller/weaker magnet.
You could spend days tuning that in!

The magnets on the reverb tanks are tiny and very weak.  That gives you an idea of the lower limit of a working system.

Some types of sticking, like sticking when you hit the assembly, could be solved with plastic sheet on the face.  This limits the minimum gap and the maximum force when the magnet is displaced.
If the magnet is so strong it sticks through with plastic present then the it's obviously too strong for the gap or spring.

Quote
I also thought of a divergent field outside the coil, but it can generate transversal waves adding a more complex pattern.

Maybe that would sound better?

Without wishing to derail this thread,  I have a query about the springs in use in a spring reverb (something I am sort of working on at the moment, so this thread is very interesting). I recently bought a set of extension and compression springs, but in the great tradition of buying online bought some rather short springs instead of long ones. Can I connect a few springs together to make a longer spring, or would doing this destroy the reverb effect? If I can connect them together would using a hot glue gun be a good idea to reduce any damping effect on the sound to a minimum? Thanks.

While I think of it, I presume I could make my own spring by just coiling a length of bare copper wire around a cylinder of some sort - would you know what thickness of wire would be best? I assume thin wire for more 'springiness', I can get hold of 0.25mm thick copper wire easily. The springs I bought are quite stiff and are 0.5mm thick.

Also I will be attaching the spring to a speaker at one end and a piezo disc at the other, would you recommend using hot glue for this or something else? Thanks for any advice you can give.

Martin,
I am using solid Cu core wires, those used in alarm systems.
5m to 10m and a pencil will do it.

Is Cu better than steel? Cant say.

Hot glue something with a hole and a screw to the speaker. This way you can try different springs.
I am using an alligator on the piezo side.
Leave the piezo floating.
See my pics.

mac

Rob,
If the magnet surface vector, blue line in the picture, is now 45° respect to the spring axis, transversal waves  be generated.

mac

Quote from: mac on August 24, 2018, 08:57:04 AM
Martin,
I am using solid Cu core wires, those used in alarm systems.
5m to 10m and a pencil will do it.

Is Cu better than steel? Cant say.

Hot glue something with a hole and a screw to the speaker. This way you can try different springs.
I am using an alligator on the piezo side.
Leave the piezo floating.
See my pics.

mac

Thanks Mac

I thought I could get solid copper wire but it turned out to be braided :( I assume that would be worthless so did not buy any. However while doing some DIY for my mother this weekend I found a shop near her that sold piano wire at a very low price (bare steel wire, very springy, and a website told me that it is used to make piano strings AND musical springs!). So I bought 30 meters of it... and have now wasted an entire afternoon trying to make a spring from it :( . It will coil into a loose spring shape but it is so resistant to being bent into shape that it took me the whole afternoon to wind 2 meters worth of it round a pencil, and it refuses to be any less than 1m long now! So it won't fit in any enclosure and is not long enough for a reverb sound. I thought I'd let you know that whatever metal spring reverbs are made from; it isn't piano wire, so don't ever buy any. I have ordered some solid copper wire (0.6mm thick) online, hopefully it will arrive in a couple of days and I can continue with the project.

It IS piano wire, but their winder is much stronger than you can hack-up in a woodshop.

QuoteCan I connect a few springs together to make a longer spring, or would doing this destroy the reverb effect? If I can connect them together would using a hot glue gun be a good idea to reduce any damping effect on the sound to a minimum? Thanks.

Many reverb units joined springs.  I noticed the join points on the spring are rigid and lossless they use small metal crimps.  You don't want any movement in the join as it could cause "creaks".
Maybe the best home friendly method is to epoxy or super-glue the joint but let it dry with the two springs under tension.

mac

Quote from: PRR on August 27, 2018, 10:33:47 PM
It IS piano wire, but their winder is much stronger than you can hack-up in a woodshop.

Or even standing in my garden using a G clamp and brute force :) I did plug the spring in eventually and discovered it does not need to be as long as I thought - the 2 meter's worth of spring I have made does indeed create laser sounds and some hint of signal (mostly fuzzy noise, but it is there) however placing a piezo disc about 50 cm along the spring (from the speaker) does indeed give a reverby sound, although one that is far too loud on the bass strings of a guitar and too quiet on the high strings. I will work on making a high pass filter to calm the bass response down... although maybe that is the material? Mac, have you noticed if your copper springs make certain frequencies louder than others?

Quote...placing a piezo disc about 50 cm along the spring (from the speaker) does indeed give a reverby sound, although one that is far too loud on the bass strings of a guitar and too quiet on the high strings. I will work on making a high pass filter to calm the bass response down... although maybe that is the material?

If you use a LM386 to drive the speaker, lower the output cap at pin 5, that's your high pass filter.

QuoteMac, have you noticed if your copper springs make certain frequencies louder than others?

I get a similar freq response.

mac

All the Fender reverb paths have heavy bass-cut.

There are MANY things going on. But the primary thought is that mechanical things we can pick-up with fingers do NOT like to shake 20,000/second. They are too heavy (massy).

Cone loudspeakers all roll-off above 200Hz. They are flat to >2KHz only because typical cone-size will not "grab air" below 1KHz. So a high-cut due to mass, a low-cut due to too-small cone, works out pretty flat over half the audio band. You are indeed likely to want a rise from bass to treble.

Quote from: Rob Strand on August 27, 2018, 11:27:12 PM

QuoteCan I connect a few springs together to make a longer spring, or would doing this destroy the reverb effect? If I can connect them together would using a hot glue gun be a good idea to reduce any damping effect on the sound to a minimum? Thanks.

Many reverb units joined springs.  I noticed the join points on the spring are rigid and lossless they use small metal crimps.  You don't want any movement in the join as it could cause "creaks".
Maybe the best home friendly method is to epoxy or super-glue the joint but let it dry with the two springs under tension.

Thanks Rob, I solved the issue using the video Mac posted - using a drill to wind the spring creates a (with copper anyway) tighter coil that contains a lot of metal in terms of length, so I don't think I need to join multiple springs now (I hope).

Quote from: mac on August 28, 2018, 05:34:01 PM

Quote...placing a piezo disc about 50 cm along the spring (from the speaker) does indeed give a reverby sound, although one that is far too loud on the bass strings of a guitar and too quiet on the high strings. I will work on making a high pass filter to calm the bass response down... although maybe that is the material?

If you use a LM386 to drive the speaker, lower the output cap at pin 5, that's your high pass filter.

QuoteMac, have you noticed if your copper springs make certain frequencies louder than others?

I get a similar freq response.
mac

Thanks for those videos Mac :) They enabled me to try the below:
(https://i.imgur.com/J79Wtkv.jpg)

These are two steel springs from piano wire and two copper springs. The shorter ones have 1.5m in length of wire, the longer ones contain 3m of wire. I haven't tested the longer wires yet (everyone here has gone to bed, so it will have to wait until tomorrow). The shorter wires do have a sonic difference to them. Admittedly it's not the fairest of tests; the steel spring is not a great spring, but it was made in exactly the same way as the copper spring - it demonstrates that doing this in steel without a special machine for it as PRR noted is a semi waste of time - the spring just uncoils all over the place. The copper holds its form well and I would recommend to anyone else making their own springs to use copper in future. The two metals sound different. The steel spring sounds more like spring reverb sounds - more metallic and hollow, although the 1.5m length isn't long enough to create the drippy spring reverb sound (which I don't mind). The copper spring has a warmer, softer reverb sound. I prefer the copper sound, but the steel is interesting;  might put both in the tin box I have ready for the springs and use a pot as a mixer to blend between the two sounds as they are reasonably distinct. Both require low frequencies to be removed, but the copper is not effected as badly by low frequencies as the steel spring is.

I decided not to change the cap on the amplifier (thanks for suggesting it though), instead I will add two high pass RC filters after the audio has left the amplifier circuit. The reason for this is I want to blend the dry signal from the amplifier back into the audio after the springs so I can have full undistorted clean sound (all frequencies) plus the reverb with reduced bass frequencies.

Quote from: PRR on August 28, 2018, 09:07:45 PM
All the Fender reverb paths have heavy bass-cut.

There are MANY things going on. But the primary thought is that mechanical things we can pick-up with fingers do NOT like to shake 20,000/second. They are too heavy (massy).

Cone loudspeakers all roll-off above 200Hz. They are flat to >2KHz only because typical cone-size will not "grab air" below 1KHz. So a high-cut due to mass, a low-cut due to too-small cone, works out pretty flat over half the audio band. You are indeed likely to want a rise from bass to treble.

Thanks PRR - I didn't know that about Fender reverbs, but it's nice to hear that it's not something I've done wrong but an actual physical issue that professionals have had to deal with. I suspect the low cut is required severely in my circuit because the speaker cone I am using is only about an inch across. It sounds OK when I listen to sound coming through it (it's obviously not bass heavy or particularly loud) but it reproduces higher pitches best. A larger speaker would be better for bass, but I don't have any of those spare at the moment and there isn't enough space in the enclosure I'm using to put a larger speaker in it.

QuoteAdmittedly it's not the fairest of tests; the steel spring is not a great spring, but it was made in exactly the same way as the copper spring - it demonstrates that doing this in steel without a special machine for it as PRR noted is a semi waste of time - the spring just uncoils all over the place

Looks like a used guitar string :)
That's why I did not consider steel to make the spring.

mac

QuoteThat's why I did not consider steel to make the spring.

Most springs in the world are steel.  They use special tools and jigs to wind them.
Copper is more DIY friendly.


[How to make springs:
http://educypedia.karadimov.info/library/springs.pdf]

Thanks Rob - the world of spring construction is a lot more complex than I had previously considered.

QuoteThanks Rob - the world of spring construction is a lot more complex than I had previously considered.

Plate reverb next  :icon_lol:

mac

That's how this started! I was trying to build a small plate reverb in a metal box without realising how large they had to be to work when someone here (Duck_arse I think) told me to put some springs in the box instead. I originally tried to persuade my wife we really needed a 2m x1m sheet of steel in our house but she disagreed with that idea (the only place I could see it fitting was hanging from the ceiling in the dining room, but that wasn't her idea of good interior decoration). Anyway I finally assembled my spring reverb today and lots of it doesn't work... I'll put the info in my thread to stop me from cluttering your thread up. Go here if you want to look:

https://www.diystompboxes.com/smfforum/index.php?topic=120755.40 (https://www.diystompboxes.com/smfforum/index.php?topic=120755.40)

QuoteAnyway I finally assembled my spring reverb today and lots of it doesn't work... I'll put the info in my thread to stop me from cluttering your thread up.

You can post your results here.

QuoteI originally tried to persuade my wife...

"Don't make me choose between you and the plate!"

mac

Just throwing out a crazy idea here, but is there any way you could anneal the spring wire?  This would take some of the stiffness and "springback" out of it.  One way to do this would be to wind it on a form (like a steel rod or tube), and heat it in the oven.  About 45 mins. to an hour at 450 F should do it.  This will give it a very mild annealing under stress.  If you want to go full on, you can heat the wire with a torch until red, then allow to cool in the air (don't quench).  This will significantly affect the properties of the wire.

One other thing, and please forgive me if I'm overexplaining things; there are three modes of propagation in a spring. 

The first is what I would call a "mass effect" - the wave or impulse travels down the spring, compressing and expanding it, much the way a sound wave moves through air.  I think this is the way most spring reverbs work.

The second method is through the metal (or other material) of the spring at the velocity of sound through that material.  It doesn't matter whether the material is coiled into a spring or stretched out straight.  I don't think this is very usefull to us.

The third mode of propagation is torsional; through the twisting of the coiled spring.  If you had a transmitter and receiver (for lack of better terms) that worked *only* in this torsional mode, it could solve a lot of problems.  It would reduces the effects of outside stimulation (being knocked around) to almost nothing.

One other thing I've noticed.  A stretched out spring that has a compression/expansion wave traveling down it, will have the wave reflected back from the opposite end.  This might be (just guessing) why a lot of the reverbs were made from two springs connected together.

I recorded a quick demo of the new version, a speaker coil driven by a LM386, and a neo magnet at one end of the copper spring.
You can see in the video that the magnet is rotated to get torsional and transversal waves. Compression waves, if any, can only stretch the spring because the sewing thread restricts the magnet movement.

IMHO sounds better than the speaker version.

Again, thanks Rob for pointing me in the right direction.

Guitar w/old strings to reverb circuit to small SS amp.



mac

QuoteI recorded a quick demo of the new version, a speaker coil driven by a LM386, and a neo magnet at one end of the copper spring.
You can see in the video that the magnet is rotated to get torsional and transversal waves. Compression waves, if any, can only stretch the spring because the sewing thread restricts the magnet movement.

IMHO sounds better than the speaker version.

You have done well persevering and refining that project.
Sounds pretty good.
The only objection is a kind of fast "patoinggg" at about 1:50 in the clip.  Not sure if it's propagation speed or the need for some damping at the ends.

BTW, Nice chords.  The first part reminds me of the things Greg Lake used to play (from Emerson Lake and Palmer).

Thanks Rob

QuoteThe only objection is a kind of fast "patoinggg" at about 1:50 in the clip.  Not sure if it's propagation speed or the need for some damping at the ends.

I played single notes ("Set the controls for the heart of the sun", PF) and reduced the dry signal to emphazise a short delay.
The LM386 output cap is 100uf to filter lows. Lower values can be used to decrease the resonance at low and mid freqs.

About the coil, the LM386 has enough power to drive the small neo magnet (BTW, I did not try a ferrite magnet). I can see it moving.
A second coil can be placed above the magnet to avoid divergence of the field, ie, coil 4ohm - magnet - coil 4ohm. A 1cm diameter cardboard cylinder with a hole in the middle to host the magnet can do it.

As you can see the spring is held tightly with an alligator clip on the piezo side. It's better to solder the spring and the wire together to avoid overheating the piezo.

QuoteBTW, Nice chords.  The first part reminds me of the things Greg Lake used to play (from Emerson Lake and Palmer)

"The Sage"  :icon_lol:
After the intro I thought to play the arpeggio from film "Crossroads", but instead of sounding like Steve Vai I was going to be more like Ralph Macchio without Willie Brown mojo  :icon_mrgreen:

mac

QuoteAbout the coil, the LM386 has enough power to drive the small neo magnet (BTW, I did not try a ferrite magnet). I can see it moving.

Probably best with the neo.  It's actually cool you got the motor efficient enough to be driven by a LM386 - Like the commercial 8ohm tanks need that.

QuoteA second coil can be placed above the magnet to avoid divergence of the field, ie, coil 4ohm - magnet - coil 4ohm.

Yep, that would work.

Quote"The Sage"  :icon_lol: 

So it is!  I haven't listened to that for while.  He used to chew gum and sing while playing that stuff.   Made it look easy.

Quote
After the intro I thought to play the arpeggio from film "Crossroads", but instead of sounding like Steve Vai I was going to be more like Ralph Macchio without Willie Brown mojo  :icon_mrgreen:

It sounds good anyway.

I changed the coil.
As you can see below, I winded the coil around a cardboard former with two holes. One half of the coil is just above the holes, the other half just below.
Total DC resistance is 8 ohms.
The magnet is rotated 45ª to get transverse waves.
The flux is now almost parallel to the former axis.
I am getting a bit more decay.

(https://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=53605&g2_serialNumber=3)

(https://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=53608&g2_serialNumber=3)

(https://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=53611&g2_serialNumber=3)

mac

Looks cool.

I tried with two coils again.
I put the coils as shown below to avoid the field from driver to go into the other coil.

The one in the picture has more turns than a speaker and almost 3 ohms. I am driving it with a LM386. The driver could be a speaker coil too.

OTOT, the sensor must have lots of turns. It is connected to the (+) input and to vref.
Much like my Laney LC30 reverb return opamp :)

Same copper spring.

The sensor magnet is a bit heavy, and the coil is something I had at hand. However, the reverb is there. I can hear a short delay. Highs are filtered because of the magnet though.

(https://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=53625&g2_serialNumber=2)

mac

QuoteI tried with two coils again.
I put the coils as shown below to avoid the field from driver to go into the other coil.

Looks like a good set-up.   The copper spring still looking the way to go for DIY.

Quote
OTOT, the sensor must have lots of turns. It is connected to the (+) input and to vref.
Much like my Laney LC30 reverb return opamp

Do you get any difference in hum when you connect the outside or inside of the coil to the opamp input?   (Obviously there's signal phase shift.)

QuoteDo you get any difference in hum when you connect the outside or inside of the coil to the opamp input?   (Obviously there's signal phase shift.)

Hard to tell if there is a difference,
I live in a main avenue, there are all kind of EM around me.

QuoteThe copper spring still looking the way to go for DIY.

What about the winding around 6th string?

mac

QuoteWhat about the winding around 6th string?

I know it can be done but I suspect you need a proper spring winder.
What I've seen in the past (an also one of the posters a few pages back) is
everything looks OK when you are winding it.  Then you remove the spring from the
mandrel and it twists up and loses the cylindrical shape.  Some days come out better
than others.

This guy does it without any trouble whatsoever!
https://www.youtube.com/watch?v=XQBbGF4-t4s

The trick seems to be selecting a tube diameter with the right clearance for the
wire and mandrel.