Mot. amplified speaker / use with scanner

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RRR

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I have an amplified Motorola 2 way speaker I want to use in conjunction with my vehicle mounted SDS200. Audio isn't up to par sometimes.

I saw somewhere that it needed to be modified? How so, and why?

Thanks in advance
 

kruser

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The 536HP and SDS200 use a BTL audio amp. Neither lead is tied to ground. When you use most amplified speakers, the ground wire or sleeve on the speaker plug will become grounded through the speakers power connection. This basically shorts out one half of the SDS200's audio amp chip.

You basically need to build a 1:1 transformer circuit that will isolate the ground lead. Some users have tied the ground lead at the speaker plug to the radio chassis instead of letting it connect to the amplifier in the radio. This method does reduce the amount of audio power by one half but most amplified speakers will overcome that power loss.

Some people have also used the simple ground loop isolators that lift the ground. Those usually don't provide the correct impedance to the radio though plus many can't really handle speaker level audio power.
Somewhere here is a thread that has drawings showing how to build a proper interface for these scanners with BTL amplifiers in them.
I don't have the link to that thread as it was a while back like when the 536HP came out.

Using an amplified speaker that does short half of the BTL output can potentially damage the audio amp in the radio. I don't think I've seen any posts claiming the audio output component was fried by putting half of it into a short but I'd not take the chance when you take into account the cost of the SDS200.

Non amplified speakers can do the same thing if one side of them are tied to ground although that is rare as most non-amplified speakers have both speaker leads going directly to the speaker.

To build a isolator using a transformer would be best built with a 1:1 transformer that lets the radio see an 8 ohm impedance or whatever impedance the factory speaker is inside the SDS200. I've built a few but they had an input impedance from the BTL amp more around 20 to 25 ohms. They worked great and even though they are not 8 ohms, they provide a non-shorted input from the BTL amp. I never built one for the SDS200. All mine were made for the 536HP which probably has an identical audio stage as the SDS200 for the speaker level output.
Radioshack used to sell small 1:1 transformers for this purpose but of course they are gone today.

Here's three threads discussing the problem: BCD536HP: - Audio Out



One of them should have schematics of what is needed or at least a link to the info.
 
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107pilot

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I use a twelve volt powered Motorola speaker (Black Speaker) without any modifications. It works perfect for my scanners. This is an older picture and I use the amplified speaker on my SDS200 with no issues. Loud, Clear and deep rich tone with my Moto Speaker.

83626
 

kruser

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I use a twelve volt powered Motorola speaker (Black Speaker) without any modifications. It works perfect for my scanners. This is an older picture and I use the amplified speaker on my SDS200 with no issues. Loud, Clear and deep rich tone with my Moto Speaker.
Some amplified speakers do have floating inputs where neither lead is tied to ground.
I'm not familiar with the design of the black Motorola in your picture but it could be built with floating inputs.
All of my Motorola amplified speakers are from a newer design judging by the style of the speakers housing compared to yours.

An amplified speaker with floating inputs are fine to use as is.

I have an old amplified speaker with Nokia's name on it. Not sure if they actually made the thing but possibly they did. It came with a hands free car kit that I had from back in the days of the old AMPS cell band and bag phones. It has floating inputs on both leads and worked as is.
I still like the sound of the Motorola models though.
 
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RFI-EMI-GUY

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A lot of the amplified speakers were used with vehicular adaptors to boost audio from a hand held Motorola radio. The ones I am familiar with do have balanced inputs to match the MOTOROLA radios. However I would still recommend an isolation transformer and an attenuation pad (Some resistors) so you don't overdrive the amplifier. In short, read the schematics, tread lightly.
 

gmclam

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The Motorola speaker I purchased has a VOX circuit that "powers on" the unit when it detects audio. I was missing the first portion of most transmissions so I bypassed the VOX so the speaker is always on. It has a volume control & power switch so it can still be turned off.
 

kruser

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"This Isolation Transformer works great!

~Doc"

I might be wrong, but I don't think it is a transformer at all, rather two capacitors (non polarized?) on each leg. I would be wary.
I'd opened one of these up after buying a couple. I also thought they may just be a pair of caps but I found a pair of transformers instead.
No other components at all other than the two jacks. At first glance, the transformers appeared to be center tapped on the output side but that turned out to be false. The center lead is probably just used for correct orientation.

The transformers are close to a 1:1 ratio. The side marked input was 47 ohms between sleeve and tip or ring while the output side was 38 ohms between tip or ring to sleeve.
 

N0GTG

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Or, if your vehicle has a radio with an auxiliary (AUX) input, you could feed the scanner audio output with a simple 3.5mm cable. I've done that with a handheld, and it works great.
 
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