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TatJSJ Offline OP
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Hi
We just replaced a very old amp with a Bogen TPU100B. The amp is used for paging, night ring and alarm system. Moved over the paging and night ring with no issues. The alarms system is connecting to a Link Passive Direct Box where it connected to the microphone ports on the old amp using what looked like an RCA jack. I cut off the RCA jack and connected the wires to the microphone connection, tested the alarms and worked. We found soon after that there is a constant low static coming thru the speakers. When I remove the alarm connection, it goes away. Not entirely sure how to resolve so looking at the experts for advice.

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Maybe an Impedance mismatch? Are you sure it was wired to a Mic input previously? RCA is usually an auxiliary input.

Double-check the output from the alarm. If all you have on the Bogen is a Mic input, there are impedance matching devices that will let you correct the problem.

Sam


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You either have a problem with the alarm tone generator (let's hear more about that thing and what it is)or there is a level mismatch. Kind of unusual to need to connect the alarm tone gen to a microphone input. A microphone input is a low level input whereas a tone generator should be able to provide a line level or greater. So I'm thinking that some hack connected to the mic input because that's all that there was. Does that DI box have a level control? Is it passive or active? Maybe its turned way down and the mic input up?

-Hal


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TatJSJ Offline OP
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Hi Sam,
I took a pic of the old am when everything was connected. It was connected to the the Microphone INput where there is a label reading "Low Impedance". I also moved the connections from the alarm system from the Microphone input to the music with no change. Are you saying I need another device between the Link Passive direct box and the Bogen?

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TatJSJ Offline OP
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The DI box does not appear to have any level control. It appears very old. there are two outputs coming from the alarm system, each has a different tone. They connect to the DI box where a single output from the DI box using a RCA jack went into the Mic input of the old amp. The bogen has no RCA jack input that I saw for this so I snipped it and spliced the two wires (the hack). I was wondering if one of the controls on the Bogen could be turned down to help with this but unsure which would be directly related.

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The Bogen is looking for a Balanced Low Impedence microphone. Is the cable running to the Bogen shielded? If it is, do you have the shield terminated to a ground or common screw on the amp?

Sam


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The bogen has no RCA jack input that I saw for this so I snipped it and spliced the two wires (the hack).

I'm not sure what two wires you spliced together.

Quote
I was wondering if one of the controls on the Bogen could be turned down to help with this but unsure which would be directly related.

I would think that would be the mic level control.

I believe what's happening is that the alarm system was never intended to connect with an amplifier. The internal tone generator is designed to drive an 8 ohm speaker which is normally a horn located someplace in or outside the building. As such any noise on the output(static or hiss)is of little consequence because it is so low in relation to the high volume of the tones. So a certain amount of noise on a speaker output is normal. But it could also be that the noise is abnormally high because of ageing components in the alarm tone generator.

But as I said, this is not really the way to do this. The right way would be to use a separate tone generator designed for the purpose- assuming the alarm panel has dry contact outputs for each of the alarm conditions.

-Hal





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Thinking this over some more- a DI box is a high impedance in to a low impedance balanced out with probably a -20db attenuation to connect to a mic input. Most common use is to connect a guitar to a mic input. There are some that are designed to connect to a guitar amp speaker out for those musicians who don't like to have their amps mic'd (I can't see why but that's topic for another discussion). So that said, I'm thinking about the original RCA plug on the DI output that you cut off and what you spliced together. A balanced line has two "hots" and a ground, usually the shield. The Bogen TPU microphone input is balanced. The DI box output is balanced. Now, I can't tell from what you are describing what the output connector on the DI box is (XLR, Phoenix terminal plug)or what kind of wire was used. I kind of think from what you are saying that there were two wires and the ground shield connected to the old RCA plug. If that were the case, one of the "hots" went to the center pin and the other "hot" was tied to the ground then both to the shell of the RCA.

Did you splice the two "hots" together and connect them and the ground shield to the + and - on the Bogen mic input? Then you cranked the mic volume up to hear something?

-Hal

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First things first:

1. Get the specifications on the alarm system alarm outputs. At this point, everyone here is spitting in the wind.

2. Mic inputs are looking for, as Hal wrote, a balanced input. It is imperative to find out what is coming out of the alarm panel to determine if the mic input of the Bogen will work properly. There are matching transformers available that can match a balanced to unbalanced as well as high/low impedance.

3. Most modern alarm systems are dirty, meaning there's a lot of digital noise on all the terminals. It could be the Bogen is picking up some residual data noise. In that case, either a shielded cable or a choke may be needed.

Rcaman


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There are matching transformers available that can match a balanced to unbalanced as well as high/low impedance.

Well, that's exactly what that passive DI box is- a matching transformer. Apparently it worked before this guy touched it or at least nobody noticed the noise. I agree though that without more information we are shooting blanks here. How about some pictures that show how the alarm is connected to the DI box as well as some showing the DI box output, the cable and how you connected it to the Bogen?

-Hal


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