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#607016 01/14/17 12:22 PM
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davetel Offline OP
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my customer bought an xblue from amazon. they didnt know it didnt have a paging port. xblue tells me i can use a co line port with our existing bogen amp if i give it 24vdc in parallel. does the transformer go in ahead of the amp or behind the amp. thanks, dave.

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I highly recommend that you obtain a Viking Electronics FXI-1 Paging port to CO Line interface.

From the way you asked the question, I would hesitate to try and describe how to PROPERLY design a simple circuit to provide the DC talk voltage while maintaining the proper impedance of 600 ohms for the CO trunk port.

Please don't think I am talking down to you, but you should not try and just "parallel" on DC talk voltage and expect things to work.

The FXI-1 will resolve all your problems in a nice, neat unit priced at under $100.

Rcaman

Last edited by Rcaman; 01/15/17 12:54 PM. Reason: Impedance Mistake

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yes save yourself a lot of grief, this requires a filtered battery not just 24vdc


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Isn't the c.o. port 600 ohms?


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Yep, John, you are correct. CO trunk ports in the United States and North America are 600 Ohms real. In the rest of the world, they are 900 Ohms complex.

The point is the Viking Electronics unit will match and provide the filtered talk voltage needed for this situation.

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I've never used that unit so I guess it's switchable 600 or 900 to match?
Thanks


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I guess it's switchable 600 or 900 to match?

Doesn't have to match. If you want to know more look HERE and click on the manual link to read the manual.

-Hal


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Looks to me that it is a 600 ohm matching device. Says so twice in the spec.'s my guess is it is for US and North American applications.


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Impedance matching really is an issue when your source is 600 or 900 ohms and your amplifier is 10K Ohms. The matching is done to reduce hum and get the signal as close to intelligible as possible.

Since the OP is using a Bogen amp, it is likely there is a Telephone paging port that would match.

Rcaman


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Impedance matching really is an issue when your source is 600 or 900 ohms and your amplifier is 10K Ohms. The matching is done to reduce hum and get the signal as close to intelligible as possible.

I disagree. Gone are the days when we matched the source impedance to the load. Today it's a low impedance into a high impedance and that's both for an unbalanced and balanced line. It matters little what the source impedance is as long as it's lower than the load, all that matters is that it is at a suitable level that the particular amp input wants to see.

BUT for long balanced lines it is best to use transformers at each end that will provide good balance and an impedance match to each other. Then, yes, you do that to minimize or eliminate hum pickup and other common mode noise.

The 600 ohm impedance is for audio out of the FXI that gets connected to the paging amp input which is generally something like 10k. Nothing to do with the phone line.

-Hal



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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....

Sam


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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


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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.

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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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Originally Posted by hbiss
Ask an IT wienie how to connect to a paging system and see what answers you get.

-Hal

Well, I'm kinda of an IT weenie and I can do most paging hooks ups.

Last edited by Mercenary Roadie; 01/22/17 06:56 PM.

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Hal,

I respect your comments and opinions, however, I stand by my original comments.

To prove the point, I sent a 1 KHz sine wave audio signal through a telephone and connected an oscilloscope to the phone system paging port. The level was -10 db. The trace was clean of distortion.

I connected the telephone system paging port to a Bogen TPU-100 telephone input port. The scope view showed about a .05% distortion that was added. I looked at the output of the Bogen and, as expected, the amplitude was clean except for the small distortion noted at the input.

I took the same telephone system output and connected it to the Bogen AUX input. There was about a .1% distortion added by that input. The amplified output had a 10% distortion in it. It was audibly noticeable. I inserted a WMT-1 between the telephone system input and the AUX on the amplifier. There was no distortion at the input. The amplifier output was clean and only registered a .05% distortion. This was not audibly noticeable.

I took that same telephone system output and connected it to a mic input. With no transformer, the sine wave was greatly distorted at the input and the output did not resemble a sine wave. Even limiting the input level to -60 db, the sine wave was noticeably distorted. Using a mic to line level transformer, which matched the 600 ohm telephone system output to the balanced 150 ohm mic input produced a clean sine wave with about .01% distortion. The distortion increased as the level increased, however, within the normal paging range, the distortion was not noticeable.

When I install a telephone system to a paging amplifier, unless the amplifier has a "telephone input" of 600 ohms, I ALWAYS use a matching transformer. My hearing isn't what it used to be, the "Scope don't lie."

Rcaman


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Facts don't lie, and my only argument is as I stated above: phone systems aren't audio gear. Phone lines should be balanced 600 ohms and if the designer made the page port the same as a station port then you may or may not need a balun, depending on how he did it. I think your tests prove that with that particular system the WMT1 is needed, but there are others that won't. Read the manual and see what it says. In this case there is definitely something about the connection to the amp that the page port doesn't like. Should you always use a WMT1? If you know what you are doing and can understand what's going on like Ralph does I certainly would carry one in my truck. Try it without first and if that's no good see if the WMT1 makes it better. But either way make sure you know how to connect to the amplifier first so that an improper connection or input isn't really to blame.

-Hal


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Thinking some more about this, my experience is mainly with Partner and any paging I provided with it was almost always through a Bogen TPU amp. I always used the 600 ohm paging input for no other reason than it was there and provided ducking of the music on page. I do however remember using a PA amp in one installation and with that I went directly into an aux input from the Partner page port with absolutely no problem. The page output on the ACS is 600 ohms according to the manual.

I also did many Panasonic KX-T61610 systems before Partner. As I remember they had an RCA jack for the page output that I connected directly to a PA aux input with a shielded audio cable. It too was 600 ohms according to the manual.

So this kind of problem seems to be very much dependent on the system manufacture and from my experience of having no problems is surprising to me.

-Hal

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I go back a little. Back when I was installing crossbar PBX, the page port was always a station port of 900 ohms. The amplifiers were all tube type and, fresh out of the box, they had 5-10% distortion. They usually had one "input" and it was between 10K and 15K ohms. A matching transformer had to be used, no exceptions.

With the advent of electronic systems, Key and PBX, the output is usually provided to match a 600 ohm amplifier input. Some dinky systems have an 8 ohm output and they, usually, require a matching transformer.

We have paging systems in chemical plants and coal mine offices. The voice quality has to be absolutely clear as specific emergency instructions are announced via PA system when an emergency occurs. They will not tolerate ANY distortion, so it has been a pet project of mine to make sure everything matches and the distortion is kept as low as possible. That is why we have Tektronix scopes, distortion meters and spectrum analyzers. That is why I am obsessive about matching impedance parameters. Hal is correct in stating the situation is system specific. As he suggested, hook it up and if it sounds good, walk away. If you have hum and/or distortion, find the correct matching transformer that will resolve the issue and ALWAYS keep the level below overload which is the primary cause of distortion in most PA systems.

Rcaman



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Ralph, I assume you did this test on an ESI KSU. I find it interesting that, with the scope still connected to the page out, as soon as you connect it to the amp you see waveform distortion. I would think that even .05% occurring when connected to a 600 ohm input is indicative of poor design. Just for giggles did you ever try putting various resistive loads on the page output while looking at the waveform? Starting with maybe less than 600 and working up to 10k to see what happens?

-Hal


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Hal,

I expect to always see some distortion added unless the input pre-amp circuits of the amplifier are exactly matched. It does occur, but not as often as I would like. This circuit unbalance is the cause of the distortion. I have run these tests on Mitel, ESI, Comdial and X-Blue systems and the results are close.

As an exercise in futility, I took the pre-amp ICs of an out of the box Bogen that was adding a bit of distortion. I matched two input pre-amp ICs and changed one feedback loop resistor and I was able to achieve less than .001% added distortion. That's about the limit of my equipment, and, at that percentage, it is not audible.

I did play around with matching transformers and impedance. Adding resistors and matching the balance of baluns will, occasionally, improve performance. However, you made the statement: "poor design" is the normal with telephone system paging circuits which, more often than not, nothing more than a cheap transformer and, occasionally, in a real stroke of genius, a real matching circuit that delivers an actual 600 ohm impedance.

I count it a win when the output is clean and without noticeable distortion.

Rcaman


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Yes, you’ll are right. In America and North America we use 600 ohms.

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