[sdiy] Opamp ICs (slightly OT)

[harrybissell]

Hello Robotboy8

If I understand you... you are going to try and induce a current in the
string itself...
by having it vibrate in a strong magnetic field.  This is not likely to
work... because
the signal would be so small.

Guitar pickups have thousands of turns to get a signal in the 10-100mV
range.

I'd try to use existing guitar pickups, or coils from relays with a
magnet beneath them...

You might want to try and cause the feedback electrically... as is done
with a commercial
product called the E-Bow.  The Fernandez "Sustaniac" series guitars use
a pickup and an
electomagnet (with feedback) to drive the strings.

Another possibility would be to use a "photonic" pickup consisting of an
LED and Phototransistor on either side of the string.  Shield it from
ambient light and it would work quite well. This was used as a hex
pickup on the (duh - guess what?) "Photon" Guitar / Midi interface.

I tried making my own and it worked well. The problem with THEIR version
was that fretting
the string caused the angle to change and it would not be in optical
alignment... YOUR version
has no frets and you don't press on the strings so you're golden !!!

The nice thing about the optic pickup is you could drive the strings
with an electromagnet and
not ever get direct feedback into a sensing coil.  You would not pick up
stray magnetic fields
either.

Driving an electromagnet is a little more than the typical opamp can
handle... but a trashed
stereo amp or similar from radio, cassette player etc... would do the
job.

My Favorite places for opamp applications info would be AN-31 Op Amp
Circuit Collection
from National Semiconductor and also AN-20 An Applications Guide for Op
Amps.

I'd start by trying the 741 opamp.... its cheap, rugged, and does not
have nasty tricks up its sleeve.  You'll need the data sheets, which are
all online as well.  Search Google for Texas
Instruments, National Semiconductor, etc and you'll find it all.

Others will likely give you more info. Good luck. Contact me if you get
stuck somewhere...

H^) harry

Robotboy8 at aol.com wrote:

> I'm tentatively working on a fretless 2-string drone bass guitar (not
> a synth... but i might add a lowpass filter whose cutoff is controlled
> by a ribbon controller under the fretboard).  I'm planning on not
> using a standard pickup but instead on using the strings as pickup
> with a strong magnet (from a car speaker) beneath them.  I do not
> know, however, whether the signal attained this way will be strong
> enough without using a magnet so strong that the strings tend to stop
> vibrating rather quickly (undesirable, as I intend this for drone
> use).  I'm thinking that I'll put an opamp IC right before the output
> (ratshack seems to have some for relatively cheap, if i recall
> correctly).  I've not used many ICs before, though, so can anyone
> recommend one for this use and explain where to connect which pins
> (okay, my rather incredible newbiness shows itself...)  Am I correct
> in assuming that I need basically just the + and - connections from
> the string and the + and - connections from the powersource (in this
> case batteries)?  Or are there many other connections to make
> (possibly a ground... any others?)
>
> Also, does this make any sense?  Will the output voltage just be too
> low for suitable audio use?  Or will I not need an opamp anyway?
>
> Thanks in advance