Holy crap Batman! I built an EBow!!!

[Pedro Freitas]

Hi guys!

Finnaly I got something that worked!
First I took a ferrite rod 2cm long and 7mm diameter.
Then I got this copper wire coil from an old telephone buzzer and took
8ohms of wire out of it.
Now, this wire is way thicker than PU wire, I got about 16m of wire
totalizing 8ohm. From a AWG Wire chart this corresponds to AWG22 wire,
right :?:
After hand winding the darned long wire to the tinny ferrite I melted
candlewax and imersed the bugger into it.
Wile it dried, I built the 386 driver circuit someone sugested in the other
thread:

http://logosfoundation.org/kursus/4047.html

Then just taped both the coil assembly and circuit to a piece of plastic
laying around, one after the other like a pen, with enough space between
them.
Now, in this space goes a permanent magnet that just touches the ferrite
rod to magnetize it. The magnet is just a plain magnet... er... I guess its
ceramic tipe... it's small and it ain't a rare earth tipe, there  
Plugged the guitar right into the opamp's input (pin3) and opamp's output to coil.
Power up,...

And we have lift off!!    :twisted:

Worked right away! The strings vibrated strongly.
3 minutes later the 386 died. It got a little warm and died.
What did I do wrong :?:  :?:
I could have done many things diferently, like winding directly on
a magnet rod, but I have none. I did forget to add an input cap to
the 386. The potting wasn't something to brag about because I could
feel the wire in the bobbin vibrate in sympathy with the stronger signals.
I think I just fed too much signal, hey :?:

Anyway, I'm just so happy I could do this ebow thing without using
black magic and chicken teeth  :lol:
I don't have a digital camera so pictures will only come in a few days.
Maybe I can take some crappy ones with a cell phone...

Any suggestions to avoid killing some more opamps???
Pedro

[Alex C]

That's awesome, great experimenting!  I have no helpful comments or suggestions though, sorry.


Alex

[Transmogrifox]

The 386 chip absolute maximum power dissipation rating is 700 mW.

If you DC couple the inductor to ground (as in lm386 output pin to coil, to ground, no cap), you have the 386 maintaining the output at 4.5VDC.

I = V/R

I = 4.5 VDC/8ohms
= 562.5 mA

So your 386 is sourcing 563 mA at a 4.5 V drop across the output current source, and

Power = V*I = 4.5V*563 mA = 2.5 Watts!!!

That's 3 and a half times the absolute maximum rating for a 386.  No wonder it burned up quick!

Here's the solution:  Use 2 386 chips.  One of them you just connect the inputs to ground and the output is your "virtual ground".   The other is connected as you had it before.  Instead of connecting the coil to ground, connect it to your virtual ground.

That way there is no DC current in the coil, therefore no power dissipation through the 386 chips unless there's signal.  Since the output of the 386 is a current source, the AC power on 8 ohms will remain within specifications for this condition.  The 386 is design to drive a 8 ohm load, but not to maintain 4.5 VDC across 8 ohms to ground.  Most app circuits in the data sheet are capacitively coupled to the load for this reason.

take care.  I'm excited for you that it worked.  I might have to make one now that you've demonstrated that it's this easy.

[toneman]

Trans,
isn't your idea called a "bridge amp"
think it's in the National Audio Handbook.
Also,
U could run a current limiter to the power pin
of the 386.  Something like your garden variety
LM317 current limiter.  see datasheet.
The MemoryMoog used a current limiting
scheme on their headphone out.

[lovekraft0]

...If you DC couple the inductor to ground (as in lm386 output pin to coil, to ground, no cap), you have the 386 maintaining the output at 4.5VDC...

The circuit mentioned has an output cap before the load, so it's not DC coupled.  :?
Ebow Amp

[bioroids]

Hi! great man! congratulations!

Luck

Miguel

[gak]

wow, i can't wait to try it by myself
did you use just one coil instead of two like described in the circuit? the one you described is the output one and instead of the input coil you plugged the guitar?
i wish i could get a ferrite sooner, i really want to try this!

congrats!

[Pedro Freitas]

Hello!
Just woke up!    Thanks for the replies.
I used a 100uF cap on the output like the schematic said. Didn't
have a 220uF around thoug...

Yes, I described the output coil and yes, I plugged the guitar directly.
And beleive me, the output coil is not a mechanical marvel  :lol:
The wire is thick (think AWG22) so I could have many turns of it into
a bobbin, it's insulation varnish is thick so I think the commonest wire
can be used, there is no said bobbin - the wire is wound directly over
the ferrite rod with some tape separating wire and core.
And its scatter wound with wire a bit loose in some places (I did it
by hand...).
Everything is pretty low tech  :shock:

Now I'll try adding the input cap and do some more experiments.
Lukly I have an E-store nearby with a big supply of 386s  

Pedro

[Ge_Whiz]

Try using the 386 to drive a power MOSFET, and have the MOSFET drive the coil. You may need to re-jig the input to keep it in phase.

Sounds like a darned good experiment to try!

[Paul Marossy]

Umm, couldn't you use something like an LM380 to increase the wattage rating? Also, is that ebow page linked above in dutch or something?!

[bwanasonic]

Sounds fun and useful, but is there a potential for pickup damage with this design? I know the Ebow design is safe, but I'm not sure about this one. I would also hesitate to call it an Ebow. For one thing, having to plug the guitar into it creates performance hassles that do not exist with an Ebow. And as I have mentioned elsewhere, the Ebow is not merely a sustain device. The ridges on the bottom of an Ebow are a vital part of Ebow technique. I guess "Guitar Sustaining Thingy" doesn't have quite the same ring to it though, eh?  

Kerry M

[moogatroid2000]

LOL. I think many peoples first impressions of the ebow is to call it a guitar sustaining thingy, so in that light I think there is much potential in this experimentation. Even if it never got past a guitar sustaining thingy, it is nothing to sneeze at.
Just a quick qeustion. Could a sound bug be useful to this in any way?
http://www.gadgets.co.uk/soundbug.html
Stuck tot he back of the headstock or some such?
I gotta get me a pair of these to play with.

[Paul Marossy]

Looking at the Ebow patent documents, it looks like the two ridges on the bottom are to align the string with the input and output coils of the circuit. My guess is that the purpose of that is to optimize the conditions for string vibration to occur. I wonder if it would even work at all if you rotated it 90 degees to the strings...

[Pedro Freitas]

Sounds fun and useful, but is there a potential for pickup damage with this design? I know the Ebow design is safe, but I'm not sure about this one. I would also hesitate to call it an Ebow. For one thing, having to plug the guitar into it creates performance hassles that do not exist with an Ebow. And as I have mentioned elsewhere, the Ebow is not merely a sustain device. The ridges on the bottom of an Ebow are a vital part of Ebow technique. I guess "Guitar Sustaining Thingy" doesn't have quite the same ring to it though, eh?  

Yeah, I see your point. Having it plugged to the guitar is somewhat
"uncool"  
There is no pickup damage with this design. Also, I didn't kill the 386!
The pickup selector on my Strat is getting finnicky and craped on me.
Soooo,... only mod I did was to add an input cap to the schematic.
This time I pluged the guitar to the amp wile playing with the Sustaining
Thingy and the bugger emites lots of uncontrolled feedback when near
a pickup. Also, the magnetic field is kinda weak due to the poor magnet
arrangement.
Hey, this Sustaining Thingy can be self suficient if an input pickup is made
just for it.


A few months ago I also did the little speaker srewed to the headstock
thing and it actually worked very good too.
Drove the speeker with a 386 too.
I'm planing to build 380 amp fed with 12VDC from a regulated PSU on
the floor and running the DC to the guitar via stereo cable.
With the 380 amp I can have enough power to vibrate any place on the
guitar (not just the headstock) or enough power to drive a replacement
pickup, like an ebow for all the strings.
I like having only one chip doing all the work because I can save alot
of space doing so. And if the EBow uses just one 386, so can I  :twisted:  :twisted:  

Pedro

[Transmogrifox]

sorry about the unhelpful information.   I should have looked at the schematic before I went running off on a tangent.

There's no good reason I can think of the 386 overheating in that circuit unless something was hooked up wrong or your capacitor is bad, or you have a hard-to-see short on your board somewhere.  If you have an 0-scope, you can check for electronic resonance to make sure you're not getting an excessively-high apmplitude at some frequencies.

take 'er easy.  That's a cool thing to accomplish.  My eBow that I bought stopped working as well.  Stupid thing.  You pay $70 for something you can make for less than $10 yourself.  They epoxy sealed everything in there so bad I don't know if I can even salvage the coils.

[bwanasonic]

Also, the magnetic field is kinda weak due to the poor magnet arrangement.

I think a weak magnetic field is what you want. The way the people at Heet explained it to me is that their device *puts out*/ (induces?) roughly the equivalent of a good whack on the strings.

BTW-the thing I keep talking about with the Ebow kind of comes down to the extent you use the Ebow purely mechanically as a bow or *pick like device*, in combination with the driver/string excitation circuitry. This and the degree you can precisely manipulate the relation of the field to the strings, is "where the magic is". BTW - I'm all for as many possible "sustainy thingys" as can be imagined and implemented! I guess I'm just picky about what you call them :lol: !

Kerry M

[Paul Perry (Frostwave)]

I'm completely impressed! As rthey say "I dips me lid".
After telling people not to bother trying, I'm pretty :oops: as well!
One problem though.. I guess it isn't really fair to call these string excitign dDIY devices 'Ebows" because, that's a registered trade name. Can anyone think of a catchy & amusing alternative?

[Jason Stout]

I guess it isn't really fair to call these string excitign dDIY devices 'Ebows" because, that's a registered trade name. Can anyone think of a catchy & amusing alternative?

Pedrobow?
GST? (Guitar Sustain Thingy)

[birt]

go for sustaining thingy  :lol:

[Pedro Freitas]

Eheheh, Pedrobow?  :lol:  No way :!:  
Sustainy thingy or other


Pedro