[BC] Re: Copper program Loops

Fri Apr 6 13:50:26 CDT 2007

To All:

A word about driving those long loops. Whatever you use to drive long, un
amplified telephone circuits care must be taken in choice of driver
amplifier. I would use a good quality low impedance driver for these
circuits. I would not try to drive them with a typical analog console with
mere 5532s in the output stage. They will run out of current in a hurry and
will have slew limitations too. You are driving a big long inductance with
distributed capacitance and older consoles had good totem pole push pull
discrete transistor drivers that could do the job nicely. PR&E BMX,
Auditronics 200 series, and our retrofit electronics for the Gates Executive
were all designed to drive phone lines directly. All three designs used high
quality Jensen output transformers which by the way make excellent repeat
coils if you can't find Western Electric 111C types. We still build a
headphone amplifier with this architecture and if you added a Jensen
123-DLCF you would essentially have a great telephone line driver circuit.
You can visit Jensen Transformers web site too for more
http://www.jensen-transformers.com/as/as005.pdf information about their
transformers and driving long inductive loops. This link shows an
application for a line receiver but it can also be repurposed as a line
driver. You still need a good amplifier to drive this. Check our website for
our HSA-100 Headphone Amplifier with discrete transistor outputs which
provides +20 dBm unbalanced or +26 dBm maximum output with a Jensen
transformer attached in a 1:1 configuration into a 600 ohm load. For a
nominal +4 dBm output from a console this provides 22 dB of headroom into
the phone line. You can even boost the level up to +10 dBm if these lines
are not amplified and you will still have 16 dB of headroom. The advantage
of doing this is that you can overcome noise on the line more easily with 6
dB more signal from the driver. The Jensen application note cited above
could be used with our driver amplifier. Simply locate the amplifier near
the transformers and drive them directly. The HSA-100 can accept a balanced
or unbalanced input and has -10, 0, +4 or +8 dBm input sensitivity. 

Jeff, there's nothing "old fart" about this approach either. No need to be
paranoid. Just because it isn't a digital solution doesn't mean it won't
work! When I left WLTW in 1997 they were still using equalized loops to the
Empire State Building and were driven by BDI line drivers putting +10 dBm on
the lines. These amps were capable of +30 dBm so there was still 20 dB of
headroom. We were able to achieve 90 dB of dynamic range from the console
right to the input of the processor at the transmitter site. Come to think
of it, on my last visit down there about six months ago I think I saw that
chassis in the rack! 

Sorry for the long winded post but this is a topic that I spent a lot of
time on when I was an "old fart" station engineer! Happy Holiday! 

Regards, 

Bob Tarsio
President

www.Broadcast-Devices.com

-----Original Message-----
From: broadcast-bounces at radiolists.net
[mailto:broadcast-bounces at radiolists.net] On Behalf Of Xmitters at aol.com
Sent: Friday, April 06, 2007 14:04
To: broadcast at radiolists.net
Subject: [BC] Re: Copper program Loops

In a message dated 4/6/07 11:39:29 AM Central Daylight Time, 
broadcast-request at radiolists.net writes:

<< From: Mike McCarthy 
 Subject: Re: [BC] Copper program loops
 To: "Broadcasters' Mailing List" 
 Message-ID: <5.0.0.25.2.20070406090718.02d0fc08 at pop.ais.net>
 Content-Type: text/plain; charset="us-ascii"; format=flowed

 Jeff,
 How long is the circuit?  If its less than 5000 ft. of wire feet, then a 
 pair of 111C's back to back in 150 ohm configuration should work 
 fine.  Otherwise, the associated equalizers will be needed.  Both can be 
 had surplus.  E-bay regularly has them for $50 ea.

 I have in the past used a graphic equalizer to smooth out the curve if it 
 was possible.  But depending on the amount of correction needed, that would

 be a worse off selection.

 Also, you could also run a DC closure on the pair by inserting a good sized

 cap between the coils at the center and injecting 12-48VDC on the pair.

 I have an equalizer and 111C if you want to try that route out before 
 committing.

 MM
  >>

Mike:

The split coil and big honking capacitor is "out" for the contact closures. 
That works fine for Two Way but that big cap and the inductance of the split

repeat coil winding produces a nice notch filter at some audio frequency.
Hum 
can also be an issue. No thank you. We will use a separate loop for
signaling.

It is a little over 5000 feet of wire. NIU does not have a purchasing 
protocol for buying things on Ebay. yes, the 111C is a great solution, even
tho
ugh it 
goes back to the days when dirt was new. NIU frowns on buying used things 
because there is no warranty recourse, among many other objections. 

I was really hoping for, and prefer, an inexpensive 21st century solution
for 
this project. I don't want any unlicensed radio solutions. I would prefer an

economy T1 encoder-decoder for copper or fiber. There is another dish on 
campus that I can have, and there's fiber from there to here. 

I cannot afford 25 grand for a high end T1 for a backup. It would therefore 
appear from your posts and those of others, that the 111C or Jensen 
transformers are the best choices. Anyone disagree? At my age, I'm a bit
parano
id about 
resorting to "old fart engineering" by using things that were popular 20
years 
ago. But if that's the best approach, then why not? I just want to be sure 
that it is :-)

Thanks,

Jeff Glass, BSEE CSRE
Chief Engineer
WNIU WNIJ
Northern Illinois University

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