[BC] Xmitters at aol.com

Wed Apr 11 10:53:07 CDT 2007

I understand how transformers work. That said, with a 111C you have four
identical windings. These windings are optimized for use  (best frequency,
phase  or other response) when used with  300 ohm resistive sources and loads.
 You can put two windings in series (to make 600 ohms), leave them as they are
as 4 300 ohm ones, parallel two for a 150 ohm source or load, even probably
put 3 windings in series or parallel for even more permutations.

What many don't realize about transformers is that they have to be sourced
properly. For example, if you connect the average Distribution amp directly to
a transformer wired 1:1, you will be sourcing the unit with about 200 ohms
(the average DA uses 100 ohm build out reisitors on each output). This will
reflect through into the secondary, which will now also look like a 200 ohm
source (give or take, because the wire used to wind the transmformer has a bit
of resistance). If telco has set up an equalized circuit using an oscillator
with a 600 ohm output    
impedance, then sourcing this transformer with this DA (or many other
compressors which have low output impedances) will cause frequency response
errors in the program circuit. This is why I put a pair of 270 ohm resistors
in each output leg of a Compellor directly feeding a program line for example.
The compellor has a very low output impedance (about 75 ohms-read the book)
and you need to "build up" the source impedance to 600 ohms to properly match
the line.  Also, it's essential to put some kind of pad between two
transformers connected back-to-back lest they screw up each other's response.

-D

------ Original Message ------
Received: Wed, 11 Apr 2007 08:06:23 AM EDT
From: RichardBJohnson at comcast.net
To: Broadcasters' Mailing List <broadcast at radiolists.net>, "Broadcasters'
Mailing List" <broadcast at radiolists.net>Cc: "Dana  Puopolo"
<dpuopolo at usa.net>
Subject: Re: [BC] Xmitters at aol.com

No! No! No!

The transformer does __NOT__ represent an impedance in itself. The transformer
is specified to work in a circuit with certain impedances. To do this, it was
designed so that its own reactances and resistance (complex impedances) are
inconsequential for the range of frequencies and range of impedances in which
it was to operate. The transformer does __NOT__ have a bunch of "300 ohm"
windings that can be connected in series or parallel. It has four identical
windings that can be connected as desired. This gives one the following turns
ratios: 1:1, 2:1, and 3:1. The impedance ratios vary as the square of the
turns ratio. Therefore, if a transformer was fed from a 600 ohm source, the
possible output impedances are 600, 150, and 66.6 ohms. To best distribute the
internal reactances, when the 1:1 configuration is used, pairs of windings are
series-connected. When 2:1 is used, the input pair is series-connected, and
the output pair is connected in parallel. These are the connections that 
would be used in program channels. For mid frequencies, there would be no
difference in impedance transformation for the 1:1 configuration if the
windings were paralleled. However, the transformer would not have enough
inductance to properly pass frequencies below about 100 Hz without distortion
(the volts-per-turn would be too high). That's why the higher impedance side
has the windings in series. The lower impedance side ends up with the same
volts-per-turn because of the impedance transformation. To correctly
distribute the leakage reactance (for high frequency response) the low
impedance windings are paralleled.

To correct other misconceptions about wire transmission, one only  needs to
look at the limits. Since an audio transmission line is a distribution of
series inductance, series resistance, shunt capacitance, and some shunt
(dielectric loss) resistance, we have relatively low Q delay line. If a delay
line has identical elements, or is physically long enough so that its
distribution "seems like" a bunch of identical elements, then the following
occurs when you muck with its source impedance: If the source impedance is
equal to its load impedance, and the delay line is matched, the line acts like
a lowpass filter with a smooth (single pole) roll off at its high frequency
limit. If its input is mismatched by feeding if from a lower impedance, the
frequency response with contain a peak in amplitude just before its roll-off.
Also, the roll-off with be faster, containing an additional pole that the
mismatch creates. This characteristic can be used to "bring up" the high end
when attempting
 to equalize the line. If the line is fed from
an impedance that is higher than its designed impedance, there will be a
high-frequency roll-off that occurs  at a lower frequency than the natural
lowpass characteristics of the line. Playing with the load impedance does
nearly the opposite, a lower impedance starts an early roll-off, and a higher
impedance causes peaking.

In the limit of a zero source impedance, and an infinite terminating
impedance, we have a resonant
circuit as f = 1 /  2pi * sqrt(LC)  where the L is the series inductance and
the C is the shunt capacitance.
The wire-line has become a pi-network.

--
Cheers,
Richard B. Johnson
Read about my book
http://www.AbominableFirebug.com

 -------------- Original message ----------------------
From: "Dana  Puopolo" <dpuopolo at usa.net>
> You can use ONE winding to source 300 ohms. Put a 300 ohm resistor across
the
> ubused winding.
> 
> -D
> 
> 
> ------ Original Message ------
> Received: Tue, 10 Apr 2007 09:05:25 AM EDT
> From: RichardBJohnson at comcast.net
> To: "Broadcasters' Mailing List" <broadcast at radiolists.net>,
> broadcast at radiolists.netCc: K7qa at aol.com
> Subject: Re: [BC] Xmitters at aol.com
> 
> 
> The impedance of various telephone cables is well documented. 16-gauge
> spiral-four toll entrance cable is 540 ohms -j460, 22 AWG emergency cable
is
> 464 ohms -j449, paper insulated (inter-city toll cable) is 910 ohms for 26
> AWG, 22 AWG is 756 ohms, 19 AWG is 453 ohms, 16 AWG  is 320. All from Page
> 823, Wire Transmission, Reference Data for Radio Engineers, fourth edition.
> Nothing is close to 150 ohms. It is well known that the series 500
"standard
> black telephone" was called the "500" because its hybrid (anti sidetone
> network) was designed
> for 500 ohms. Historically, the telephone lines have been called "500 ohm"
> circuits.
> 
> Also, there are no 300-ohm windings on a 111-C. The transformer consists of
> four identical windings that are electrostatically shielded so they can be
> connected in any manner. The usual (read correct) connection is for two
> windings to be series-connected for the output, and two windings to be
> series-connected for the output.
> 
> See http://www.oldradio.com/current/111c.html , It shows the two usual
methods
> of connection. Note that both the line-side and the drop side can be
connected
> either in series or parallel. Don't be fooled by the "typical use" note.
It's
> for "short" i.e., unequalized circuits.
> 
> I would guess that your response was either a test to see if I was still
> awake, or not very well thought out!
> 
> --
> Cheers,
> Richard B. Johnson
> Read about my book
> http://www.AbominableFirebug.com
> 
> 
>  -------------- Original message ----------------------
> From: K7qa at aol.com
> >  
> > In a message dated 4/9/2007 6:41:47 AM Pacific Daylight Time,  
> > RichardBJohnson at comcast.net writes:
> > 
> > The  circuit feeding the telco pair should not be connected as 150 ohms  
> > unless
> > there has been a problem obtaining the required frequency response.  
> > 
> > 
> > Most standard balanced telco multiconductor trunk lines used for many
years 
> 
> > had an internal surge impedance of 135 to 150 ohms at audio frequencies.
The
>  
> > 111C had individual 300 ohm windings for a good reason.  Connect the  111C

> > transformer split windings in parallel to achieve 150 ohms on the line
side 
> and 
> > in series for the 600 ohm equipment side. 
> >  
> > tm
> > 
> > 
> > 
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