[BC] Re: Copper program Loops

[Dana Puopolo]

My standard line driver was a pair of Jensen 990 opamps differentially driving
a 111C coil connected 150:150 with 75 ohm 1/2 watt resistors between the
opamps and the coil. The 990's were run at +/- 24 volts. This setup could
easily put +30 dBm across a phone line. Worked great with long, noisy lines. 

-D

------ Original Message ------
Received: Fri, 06 Apr 2007 02:50:35 PM EDT
From: Bob Tarsio <Bob at Broadcast-Devices.com>
To: "'Broadcasters' Mailing List'" <broadcast at radiolists.net>
Subject: RE: [BC] Re: Copper program Loops

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