cell phone interference

I made a tiny amp for my electric guitar with the LM386 chip and a small speaker

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I hooked it up just like the basic circuit in the datasheet, grounding the inverting input and leaving the gain and "bypass" pins open. I put a 0.1 uF cap across the power pins, and now it works great, with one little problem -- cell phone interference. Every so often my cell phone apparently gives a little shout-out to the system, whereupon the amp goes chirp chirp. When the cell phone rings, the amp goes nuts. I'd like to know where the interference gets in and what you do to put a damper on it.

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First thing to try is putting the amp in a metal box. Connect the shield from the input connector to the box. Ground the metal box to the powerline ground. (Assuming you used a decent transformer-isolated power supply.) Then put a small filter on the input, something like 1k series and 0.02 uF to ground. If this doesn't solve the problem, try removing the guitar connector from the input and seeing if the amp still picks up the interference. If not, you have to look at the guitar and amp as a system. Make sure the cable shields are intact and connect ultimately to the metal box.

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

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Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!

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Bob Masta

You are most likely using a GSM cell phone which are notorious for this sort of thing. Shielding may work, but the wires connecting to the amp will still act as antennas. CDMA cell phones (e.g. Verizon network in the US) do not suffer from the same problem. GSM phones use TDMA, a technique that allows multiple users to share the airspace by having the users take turns transmitting. That trasmit pulse rate, coupled with the appropriate information being transmitted, corresponds to an audible freqency.

CDMA always transmits, allowing multiple users through sophisticated mathematical codes. These codes are are pseudo-random, meaning they look like noise. So regardless of the information being transmitted, no portion of the signal is in the audible frequency.

Cheers, Chris

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