cell phone interference

[gearhead]

I made a tiny amp for my electric guitar with the LM386 chip and a
small speaker
http://www.national.com/ds/LM/LM386.pdf
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.

 

[Bob Masta]

On Mon, 17 Dec 2007 17:43:29 -0800 (PST), gearhead
<nospam@billburg.com> wrote:

I made a tiny amp for my electric guitar with the LM386 chip and a
small speaker
http://www.national.com/ds/LM/LM386.pdf
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.

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
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!

 

On Dec 17, 8:43 pm, gearhead <nos...@billburg.com> wrote:

I made a tiny amp for my electric guitar with the LM386 chip and a
small speakerhttp://www.national.com/ds/LM/LM386.pdf
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.

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