Making the SpikerBox louder for Classrooms
Our engineering department is working hard on the next generation of the SpikerBox. The chief user request we’ve received is to make the SpikerBox louder; in fact, loud enough for large classrooms. Below is the audio component of our circuit, using the infamous LM386. Making this audio component both loud AND stable has been a major challenge, but we like challenges at Backyard Brains.
We know we can put a 10 uF capacity across pins 1 and 8 to increase to 200 gain, but stability goes out the window. If any analog electronics engineers in the world know an alternative low-cost, low-power audio driver, or a stable LM386 arrangement, please let us know; the solution is near.
Another common user request is to have the ability to both use the iPhone/iPad and hear the spikes at the same time. At the moment, the iPhone OS doesn’t allow playthrough (please Apple, do this for us?), but below is a cheap hardware solution that also solves the loudness issue. Go to your friendly neighborhood RadioShack, and purchase:
1) A mini Audio Amplifier for $14.99: This portable audio speaker amp will allow your spikes to be loud enough for a large classroom or lecture hall. It needs a 9V battery.
2) A Y-adapter for $3.59 to split the output of the SpikerBox to two outputs, so that you can plug in both the iPhone/iPad and the loud audio speaker
3) a 1/8 audio cablefor $3.99 to go from the SpikerBox (or Y-splitter plugged into the SpikerBox) to the audio speaker.
Note: the Y-adapter and audio cables listed are the monaural versions. The Stereo Versions (cable and Y-splitter links here) will also work fine but are $1-3 more. We usually buy the stereo versions anyway, as they are useful for other audio projects. Here is the standard lab class set-up. The iPad works even better for lecture halls.
Note: our kind friends at RadioShack customer service actually sent us the circuit diagram for the audio amplifier! This scan is how we actually received it. Notice they have put a transistor in front of the LM386. We are experimenting with a similar arrangement. Stay tuned!
Is there any reason why you can’t provide a lower-current line-level output for a pair of computer speakers, or alternately why you can’t slap on an additonal low current voltage-gain stage prior to your power amplifier?
And there are indeed dozens of low power audio amplifiers out there, particularly if you’re willing to go SOIC or mSOP (both hand-solderable with a little practice.) TI alone has about 40, including this little number:
90 cents apiece at mouser, though I don’t know what the ceiling on voltage gain is. Again, a small voltage gain stage can be put ahead of it to get you up to where you need to be.
Comment by Craig Smuda — 2011-Jan-05 @ 17:06
is available in SOIC and offers up to 40dB gain.
Comment by Craig Smuda — 2011-Jan-07 @ 16:47
I have an nice alternative to using a LM386 on one of my websites:
Maybe the gain would be too low for what you require, though you can increase it by changing one resistor value. Why fight a stability battle with a LM386 when a cheaper circuit is better.
Comment by Sean O'Connor — 2012-Jan-07 @ 19:39