Audio amplifier

[hiegtx]

Erik,
The speaker ground of the 396 is in electrical common with the antenna and power ground. If the amplifier input is also grounded, then the possible voltage differential between the scanner and the amplifier (or amplified speaker) could cause damage to the scanner.

The isolation transformer merely separates the two electricallty so that this can not happen. It's got two pair of lead wires. See Al42's earlier post for a simplified look. One pair of wires gets a 1/8" male phone plug that would plug into the scanner. The other pair of wires should get a 1/8" female jack or whatever matches the wires from the speaker you wish to use.

Steve

 

[MarkEagleUSA]

Will the RS amplified speaker work out of the box on my 396?

Yes... with one warning: make sure the audio cable is connected to the speaker BEFORE connecting it to the radio.

The isolation transformer is not required, though I think it's a good insurance policy. I would recommend rewiring the plug for the radio as described here.

 

[epbernstein]

Yes... with one warning: make sure the audio cable is connected to the speaker BEFORE connecting it to the radio.

The isolation transformer is not required, though I think it's a good insurance policy. I would recommend rewiring the plug for the radio as described here.

Those diagrams are so far above my head I can't even begin to describe it. How about I leave the cable connected to the speaker (even with tape or a red sticker reminder), and plug the other end into the scanner. I imagine that's what I'd do anyway...

Thanks for the patience...

 

[markab]

Clarification

Although capacitors can be used to effectively block DC they also act as frequency filters. IE: 1-5 mfd caps are used in series with tweeters to protect them from mid to low frequency signals that can damage them. I suspect it will make their use impractical for this application.

Regards,
Mark

Mark,

It's been a while since I've built kits. But you should be able to sovle this by inserting a capacitor in line (series) on the ground side of the patch cable from the 396 pr 246 to the amp. The capacitor would be transparent to the audio, which is an alternating current anyway. The dc-current of the ground circuit would see it as an open circuit, preventing a short and any damage. As far as capacitor value, I'd probably experiment with a couple of disc capacitors to see if one sounded any better than another.

Steve

 

[epbernstein]

Yes... with one warning: make sure the audio cable is connected to the speaker BEFORE connecting it to the radio.

The isolation transformer is not required, though I think it's a good insurance policy. I would recommend rewiring the plug for the radio as described here.

I picked up the speaker this evening, and I like it a lot. Another safety precaution (I think) is that my lighter plug is switched with the car's ignition, so I will generally only plug/unplug the radio when the speaker is powered off.

Thanks to all again for the assistance!

 

[mgosdin]

Wiring Harness

I followed this thread closely because I had just gotten one of the infamous 12 Watt Motorola speakers off of eBay.

I checked my RS PRO-83 to see if it cautioned about the use of grounded speakers and yes it has the same warning as it's Uniden cousins.

So, I used a spare DB25 female computer plug, one of the RS Isolated Transformers, a fused cigarette lighter adapter and various pieces of wire - solder - electrical tape to make a wiring harness for the Motorola.

Incredibly it works! I now have crisp clear LOUD sound from my PRO-83 in my '97 S-10 pickup. Yippee!

Mark Gosdin

 

[ILikeHatchChili]

If you want big time sound in your car from your police scanner, try using an FM modulator such as the Scosche FMTRNS01 which I use (bought from WalMart auto dept for about $19 (with the NM tax)) for my BC245XLT and my PRO95. The Scosche audio transmitter is plugged into your 12v "cigar lighter" and the other end is put into the scanner's earphone jack. Pick a frequency (88.1, 88.3, 88.5, or 88.7 MHz on the transmitter, and tune your car FM radio or your kid's walkman radio to that frequency and whammo - big time sound! -- turn your car into a wannabe police cruiser!. The airband transmissions sound great! I find my PRO95 can overdrive the transmitter into audio distortion; all I need to do is back off the volume control on the PRO95.

 

[Al42]

The Scosche audio transmitter is plugged into your 12v "cigar lighter" and the other end is put into the scanner's earphone jack.

All's well - as long as you don't connect a grounded antenna to the scanner.

 

[MarkEagleUSA]

All's well - as long as you don't connect a grounded antenna to the scanner.

Not exactly... if the audio ground is tied to electrical ground (in the modulator) then the warning Uniden has in the manual still applies.

 

[Al42]

Not exactly... if the audio ground is tied to electrical ground (in the modulator) then the warning Uniden has in the manual still applies.

Make that "unless the audio ground is completely isolated from the 12 volts ..." If it's connected to any voltage at all, 0v, 12v, something in between, it can still damage the scanner.

When it comes to playing a scanner over the car radio, those old cassette adapters are still the safest way, if you have a cassette player in the radio.

 

[GTO_04]

Amplified speakers

It would be great if an automatic volume control could be added to one of these amplified speakers so that the volume variation between analog and digital systems could be eliminated on my 796. Is this feasible or even possible?

GTO_04

 

[Al42]

It would be great if an automatic volume control could be added to one of these amplified speakers so that the volume variation between analog and digital systems could be eliminated on my 796. Is this feasible or even possible?

It's simple, actually - if you can handle a schematic and a soldering iron. There are a few AGC chips - pick one and use it between the amplified speaker input jack and the rest of the circuit. Or you can use an op-amp (even a 741 would work) - just rectify and filter the output and feed it back as negative feeback to reduce the gain. A pot would give you variable AGC.