TELOS TWOX12 USER’S MANUAL
INSTALLATION
CHAPTER 2 32
Why do I need a mix-minus?
Adaptive hybrids, commonly used in broadcast applications since Telos pioneered the
technology, work by comparing the audio sent to the audio received. The system then
adjusts itself to minimize send audio in the audio coming back (i.e. minimizes leakage).
Thus the audio fed to the hybrid is, in effect, a reference signal. If the audio sent into the
hybrid contains the caller audio (the audio we want) then there is no reference signal and
the system cannot do its job.
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CURIOSITY NOTE!
This is why we at Telos do not claim our hybrids will work without a mix minus.
Sure, they might work without a mix- minus (it has been done), giving borderline
performance, but that’s not good enough. Do the job right and use a mix- minus!
An additional problem is the potential for feedback through the hybrid. In any real world
situation there will be some leakage of the send audio into the received audio. With gain
through the AGC and console, this path creates a loop and feedback can occur. The Telos
TWOx12’s extremely low leakage makes this scenario unlikely, however a clean mix-
minus is still essential for optimal performance.
More on Mix-Minus
Simple Mix-Minus
The simplest way to create a mix-minus is to use a distribution amp to feed the mic preamp
output directly to the hybrid, as well to the board (mixer, console, mixing desk). This
approach lacks flexibility, and requires additional equipment if more than one mic is used
and/or more than one mix-minus is required.
Sophisticated Mix-Minus
Most modern broadcast consoles have some provision for mix-minus. The best allow
selective feeds to the phone system. This is useful since sometimes you want only one mic.
feeding the phone, sometimes you want to three or four mics (during the morning show, for
instance), and sometimes you want to play a cart machine, CD, or other device when callers
need to hear and react to contest sound effects, etc. Some even provide for separate “on
line” and “off line feeds”. And when two hybrids are used, each caller will normally need a
separate mix-minus since each hybrid will need to have the output of the other, if callers are
to hear one another (actually, the TWOx12 allows for a single Mix-Minus, though it is less
flexible, see section 7.3.1 for details). A simplified figure of a two-buss two-hybrid mix-
minus configuration follows:
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