Hacking the iKEY-plus, part two.
I just picked up this wonderful new toy: the iKEY, that is a
little computer you hook between a turntable and a USB “key” (flash
drive) and then the iKEY will record what comes out of the turntable
on the flash drive as either a wav file or an mp3 file.
The iKEY-plus also has a microphone input and built in mic preamp.
The included stereo electret microphone works fine for recording
a rock concert, but there isn’t just enough gain/sensitivity for
recording a lecture or a string quartet.
I’ve built an outboard preamp that has 20dB gain. Now the noise in
the built-in electret microphone is the limiting factor. I found that
the microphone that comes with a Sony tape recorder is noticably lower
noise, so we’ll be using that for recording lectures.
The amplifier circuit is just a straight non-inverting op-amp circuit,
like this one or this one. I used 100K for the feedback resistor, 10K
for the gain-setting resistor, and 20pF across the 100K to limit the
high frequency gain.
I used a 5532 op-amp because it will run down to 3V, so it’ll be quite
happy on the approx 5v that the batteries in the iKEY-plus can supply.
One problem is that the iKEY-plus generates some noise when its
computer is running, which manifests as a hum (not 60Hz) audible in
headphones. I filtered this by putting a small 200mH choke in series
and a 500uF cap across the Vcc line from the batteries.
To avoid a power switch, I only run the battery plus wire to the circuitry.
When you plug the cable from the preamp output into the iKEY-plus input,
the ground is completed by the cable ground, powering up the preamp.
I plan on building the circuit into a box made out of copper clad board
material, built to replace the iKEY-plus’ aluminum lid.
Pictures to follow.
Hacking the iKEY-plus, part one.
I just picked up this wonderful new toy: the iKEY, that is a
little computer you hook between a turntable and a USB “key” (flash
drive) and then the iKEY will record what comes out of the turntable
on the flash drive as either a wav file or an mp3 file.
One problem is that all the LED indicators on the iKEY-plus are red,
so in a dark room, you cannot tell the record indicator from the -12dB
level LED, so you tell with a glance that the unit is in record mode.
Also, it would be nice to see if the 0dB LED is flashing, warning of
impending clipping.
To remedy this, I bought some high-efficiency green and yellow LEDs
and replaced the VU meter LEDS with those: -12dB through -3dB are
now green, and the 0dB is yellow. Unfortunately, the green and yellow
LEDs are not as efficient as the original red LEDs, so they are hard
to see in bright light. Good thing I don’t record outdoors much.
I don’t recommend this change unless you can either find brigher LEDs
or are willing to live with the decreased visibility in bright light.
The change itself is pretty simple, assuming you’ve got a decent soldering
iron, a solder sucker, and know what you are doing.
1. unscrew the 4 philips screws on the back of the unit.
2. carefully separate the case halves, and remove the bottom and put it
aside.
3. the electronics module is attached to the top case/battery box by four
or five small connectors, which can be carefully unplugged by using a small
screwdriver and needle-nose pliers to separate the connectors.
4. unsolder the wire that attaches the copper shield to the bottom of
the circuit board.
5. identify the pads that the 5 LEDs are soldered to. One or more of
the pads may be obscured by black goop that has been poured over one
of the circuits on the board, either to hide it to prevent reverse
engineering, or to protect a chip-on-board (COB) device. You can remove
this stuff from the LED pads pretty easily, however.
6. Carefully desolder the LEDs, taking to care not to pull the plated
through hole out of the board along with the LED lead. Mind the direction
the LEDs are soldered — you can see the cathode of the LED inside the
clear plastic envelope if you look carefully. All the LEDs are oriented
the same way.
7. Replace the LEDs with your new ones, careful to match the direction
(polarity) against the way the old ones were installed.
8. Re-assembly of the box, is, as they say, the reverse procedure.
Mind you don’t strip out the screws that hold the plastic bottom of
the unit on. They’re only threaded into plastic. Also, don’t forget
to replace the little felt masks that fit over the slide switch
handles, as these tend to fall off as you’re flipping the circuit board
over again and again.
9. pictures to follow if I take mine apart again.
Parts list:
Here’s what I bought, from www.superbrightleds.com: