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We can agree to disagree. Not all amplifiers are as forgiving as the Bogen. Hum may or may not be an issue, but intelligibility of the voice page can be seriously affected by impedance mismatch. Sure, a lot of systems are mismatched and they work fine, but there will come a time when a good quality amplifier will be looking for a properly matched input.

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Well, let's see if we can change your mind. cool

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Impedance Matching

Impedance matching went out with vacuum tubes, Edsels and beehive hairdos. Modern transistor and op-amp stages do not require impedance matching. If done, impedance matching degrades audio performance.

Read through this RaneNote Unity Gain and Impedance Matching: Strange Bedfellows

-Hal


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Hal -

I finally got a chance to look at that document and I'll admit I'm a little confused. Back in the day, I always used WMT-1 matching transformers to interface a telephone system (600 ohms) with an amplifier - either to a Balanced mic input of (approximately) 150 ohms or to an unbalanced auxiliary input of 15,000 ohms.

Are you saying this is not necessary?!?

I seem to recall being called out to "fix" a problem that was a tape deck (auxiliary output) plugged into a mic input and the sound was horrific (till I "fixed" it by changing the phone plug to an RCA and plugging it into the aux input).

Perhaps with advanced age I'm getting a little slow....

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Sam I too am confused after reading it. I connected telephone paging to an amp wth out a WMT-1A in the office just to see, quality was terrible. I put the WMT-1A in and it was fine. Also Bogen amps have a switch on an input specifically for Telco paging, if it is no longer necessary why put it in?


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You're overloading the Mic input with Line Level signal.

Microphone level is in the region of -60 dBV (0.001 volt) to -40 dBV (0.010 volt).

Line level is in the region of 0 dBV (1.000 volt). About 1,000 times greater than a mic-level signal

Aux-level is in the region of -10 dBV (0.300 volt).

Her is a link from Shure about it

https://tinyurl.com/hk3t3fx


Patrick T. Caezza
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I seem to recall being called out to "fix" a problem that was a tape deck (auxiliary output) plugged into a mic input and the sound was horrific (till I "fixed" it by changing the phone plug to an RCA and plugging it into the aux input).

You guys say that quality was terrible but you need to define terrible. Was it distorted? Only thing it could be is distortion caused by overdriving the input. Many amplifiers have gain stages directly after the input and before the volume or level control. So applying too a high level to the input causes the input stage to clip and the volume control would have no effect except for turning the whole mess up and down. This would be the case here. All MIC inputs are low level inputs because microphones (except crystal) provide a very small signal that needs extra amplification to bring it up to line level. Your tape deck AUX output is already line level so connecting that to the MIC input caused all kinds of clipping- and if you were using an amp or mixer that had clip lights you would see that right away. You fixed it by changing to the AUX input which is a line level input. Line level into line level= good.

So you have to know what your inputs and outputs are. Generally anything that is not a MIC input is line level and most outputs are also line level.

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Back in the day, I always used WMT-1 matching transformers to interface a telephone system (600 ohms) with an amplifier - either to a Balanced mic input of (approximately) 150 ohms or to an unbalanced auxiliary input of 15,000 ohms.

Depends. The transformer does two, actually three things: 1) it converts balanced to unbalanced and 2) changes the levels by virtue of the 600 to 15k ratio and 3)provides AC and DC isolation from one side to the other.

A good way to change from balanced to unbalanced and vice versa is with a transformer (balun) such as the WMT1 so if you have a balanced line and want to connect it to an unbalanced input it's one way to go.

It will also increase the level and note that the WMT1a has a changeable tap on the unbalanced (RCA) side that changes the output to 150 ohms from 15k (600:150) to go into a low level MIC input.

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I connected telephone paging to an amp with out a WMT-1A in the office just to see, quality was terrible. I put the WMT-1A in and it was fine.

Generally a phone system page output is line level unbalanced and going directly into a line level unbalanced input works fine. But without knowing or seeing the system and how you connected it I can't tell. Again, define terrible. Distortion or hum? Might be that it was a balanced output and connecting it directly to the unbalanced input caused hum. Maybe you had a ground loop. Maybe the level was wrong.

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Also Bogen amps have a switch on an input specifically for Telco paging, if it is no longer necessary why put it in?

Or they have a 600 ohm T&R input. It's a quasi-balanced (no transformer) input and the switch on those other amps I believe just puts a 600 ohm resistor across the input. There are some systems out there that probably do like to see a 600 ohm load, maybe some that use a station port for paging. Don't know. Just use whatever works best in those cases.

-Hal



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Well, I'm no less confused the I was before. Define "terrible" .....

Ok. Not so much hum as distortion (as I recall).

Sam


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It's like a woman bring her car into the dealer for service. The service writer asks her what the problem is and she says it sounds terrible. Not a lot to go on. If she said it squeaks or rattles that would give them a hint as to where to look.

If you are going to work with audio you need to know at the least distortion and hum and the common causes. Distortion is nearly always caused by something being over driven into clipping. That should tell you where to look. Check to see that you are using the correct input for the signal level you are supplying.

Hum can be caused by many things and can sometimes be a real PITA to solve. It's caused by 60Hz AC getting into your input somehow. Check to see that your input is wired properly. A balanced output connected to an unbalanced input will likely cause hum for instance.

Lastly, phone systems are not audio gear so don't expect page outputs to conform to any audio standard. Levels differ from manufacturer to manufacturer as does it being balanced or unbalanced. If possible read the manual to see if there are instructions for connecting to the amp.

The strangest one I ever heard of is the IPO. You need a WMT-1 on the station port that is programmed for the paging output during boot so the system thinks there is an analog phone connected to the port. If you connect to the paging port during normal operation you get nothing. You have to reboot to make it work.

-Hal


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Well Hal it's been 20 or 30 years since that last service call so if I'm a little fuzzy on the details, forgive me. As I recall the voice was distorted and probably overdriven - but it's been a very long time.

Sam


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I have a hunch phone system manufacturers in recent years have been all consumed with IP to the point where such unnecessary things like paging aren't given much thought. Ask an IT wienie how to connect to a paging system and see what answers you get.

-Hal


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