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"Pop" in amplified speaker

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kf4sqb
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"Pop" in amplified speaker

Postby kf4sqb » Mon Mar 16, 2015 9:44 am

Ok, so it's not Motorola, but I know someone here can help me. I've got a Kenwood amplified speaker hooked to a Cobra 19 Ultra CB. The speaker wasn't designed for an isolated ground installation (input negative and power negative connected), so I used a 220nf polyester film cap to isolated the input negative. Audio sounds great, but there is a loud, annoying POP! every time the radio squelches/unsquelches. Any ideas what I can do to eliminate, or at least reduce, the pop? Maybe bypassing the cap I installed with a fairly high ohm resistor? Thanks!
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Wowbagger
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Re: "Pop" in amplified speaker

Postby Wowbagger » Mon Mar 16, 2015 11:28 am

Could you use an audio transformer to isolate the radio side of things?
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Bill_G
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Re: "Pop" in amplified speaker

Postby Bill_G » Mon Mar 16, 2015 2:29 pm

+1 on the transformer for isolation.

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kf4sqb
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Re: "Pop" in amplified speaker

Postby kf4sqb » Mon Mar 16, 2015 2:33 pm

I suppose so. I'm out on the road with work, and the best option the local Rat Shack, which is (surprise!) in the process of closing, had was a capacitor. I may try running back by there today and see if they have a resistor assortment pack left (it'll be cheap if they do!), and try bypassing the cap with around a 100k resistor and see what that does for it.
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kf4sqb
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Re: "Pop" in amplified speaker

Postby kf4sqb » Mon Mar 16, 2015 6:17 pm

No dice with the bypass resistor. Tried a few values from 100k all the way down to 1k, which will make the CB squeal on transmit, with no noticeable change in the pop. I guess I'll have to live with it until I get home. I have a 600/600 audio isolation transformer in my junk box that I'll try on it. I'm beginning to worry that the pop is actually in the radio's audio output, and the amplified speaker is simply doing its job and amplifying it.
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Wowbagger
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Re: "Pop" in amplified speaker

Postby Wowbagger » Tue Mar 17, 2015 3:18 am

The pop is because the system is powering up the amp when it unsquelches, and powering it down when squelched - so you are seeing the DC component of the audio line go from 0V to 6V. There's no way you can prevent the pop with just a capacitor.

See if you can get a audio PA transformer - I know DratShack usually carried those.
This is my opinion, not Aeroflex's.

I WILL NOT give you proprietary information. I make too much money to jeopardize my job.

I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.

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jackhackett
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Re: "Pop" in amplified speaker

Postby jackhackett » Tue Mar 17, 2015 5:18 am

These things are quite different than the Motorola bridged amplifier audio PAs.

On CBs the audio PA chips are usually single ended and feed a transformer through a large value electrolytic cap (this is because they use the same audio PA to AM modulate the transmit PA). In the one in question (unless they've radically changed the design recently) this is an autotransformer, and the speaker audio is taken off one of the transformer taps through a second large cap. The transformer also has one tap connected to +12v and one feeding the transmit PA through a diode.

The speaker is grounded on one side through the PTT switch so that pressing the PTT opens the circuit and un-grounds the speaker, thereby cutting it off during transmit.

In the end, an isolation transformer would be your best bet.

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kf4sqb
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Re: "Pop" in amplified speaker

Postby kf4sqb » Sat Mar 28, 2015 11:34 am

Now this gets interesting. I installed an isolation transformer (1:1, 600 Ohm, 51 Ohm DC resistance primary, 70 Ohm DC resistance secondary, its what I had) with the radio's audio output to the primary, and the speaker's audio input to the secondary. Now the pop is WORSE!!! I'm beginning to think it is actually in the radio's audio output, and there is no way to get rid of it. I guess that's what I get for buying a cheap CB.
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