Wednesday, August 14, 2013

Avoiding "blowed up" speakers!

Several years ago I finally got around to installing my dual-band FM transceiver in my car permanently and in so-doing, I put a speaker in a location under the dash where one might have been had it not already been factory-equipped with in-door speakers.

The radio, a Kenwood TM-733, resides under the back seat and is "remoted" to its control head, microphone and speaker with several cables.  Under the dash, the speaker is about 3" diameter and rated for 3 watts RMS, 4 watts maximum at 4 ohms (Jameco P/N:  99996) and I figured that I would be OK as that was more-or-less the radio's audio amplifier rating - plus I had the resistance of the fairly long run of wire to the speaker, as well.

After about a year, the speaker quit.

"Bad luck", I assumed as I replaced it with another, identical model, but just a few months later that speaker quit, too and it, like its predecessor, suffered an open voice coil.

With speaker #3 in hand I was determined that I wasn't going to let this happen again.  Since I couldn't really fit a larger, higher-power speaker at that same location I needed to protect it somehow.

The most obvious answer to this would be the use of a series resistor.  I figured that something in the 3-8 ohm range (at 3-5 watts) would probably do, so I temporarily tried several values in line to judge the effects. While still fairly loud, this extra resistor did make a pretty good dent in the volume and I was worried that I'd have trouble hearing a "quiet talker" on the radio while driving down the road with the windows open.

Another option:  A light bulb in series!

Rummaging around in my box of light bulbs I tried almost everything in there.  When I would test a light bulb I would first try it at a low volume, listening for the difference when temporarily shorting out the bulb and I would then turn up the volume all of the way and then temporarily short out the bulb again, noting the difference in volume.

In my tests I concentrated on 12 volt lamps as this approximately matched the amount of voltage swing for which the radio's audio amplifier was capable since it, too, was running from 12 volts.  In briefly experimenting with a small 6 volt lamp (such as a #47) it would not only glow too brightly on audio peaks - making me suspect of its potential longevity - but its small filament and relatively low current ratings meant that it had a lot more resistance and that made the audio far too quiet for my needs.  "Smaller" 12 volt automotive light bulbs (such as the "wedge" types often used for instrument panel lighting in cars) would glow more brightly at full volume, but either had what I considered too much effect at low volume, or seemed too "aggressive" in reducing the volume for loud audio.

Finally, I settled on a common 1157-type tail light bulb - the type with two filaments.  Trying each filament in turn (and in series) I determined that the "taller" filament by itself worked best for my purposes so I soldered short wires to it and glued it to the backside of the plastic trim that held the speaker using RTV (silicone) adhesive.  In my case the 1157 bulb had only a very slight effect on "quiet" audio, but made a rather marked difference on very loud, ear-splitting audio.

The speaker and its light bulb protector - an 1157 bulb - wired in series with its voice coil and secured in place with
RTV ("Silicone") adhesive.  Note that in this picture, the filament (the taller of the two contained within the bulb)
is actually glowing slightly - this, from the radio running open squelch at full volume and blasting noise.
Click on the image for a larger version.

After thinking about it for a while one of the problems that likely led to the speakers' failures was that it's common to run the volume up all of the way - either to overcome road noise or, more likely, because someone on the radio isn't talking very loud.  When this person un-keys there is often a very load burst of noise (the squelch tail, a.k.a. "kerchunk") and/or another person will talk in a more normal voice - either one being much louder!  Rather than "ride" the volume control all of the time I would just put up with these bursts of extra loudness, and there was no doubt that this was likely how the speaker got "stressed" and damaged.

The light bulb has the advantage that at low volumes, the filament's resistance is quite low and will have relatively little effect.  If the audio gets very loud, however, the filament will start to light up and its resistance will go up, reducing the amount of power reaching the speaker and in this manner it not only offers a degree of protection to the speaker, but it also reduces the volume of very loud audio (sort of like an "AGC" - Automatic Gain Control circuit in a radio) without affecting "quiet" audio to a significant degree!

While the first two speakers lasted about a year and a half total, the newer speaker (which is the same model as the first two) has lasted 3-4 5-6 years and counting!

Note: I have since replaced the old, aging TM-733 with a newer TM-V71a with about the same audio output power:  The same, old speaker with the light bulb in series is still working fine!


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