[GreenKeys] Far OT: audio inputs

[gil at baudot.net]

Hey Eugene:

I am curious to see posts about this as well.  I don't have your
answers, but I can add a couple of things.  I always wondered where the
600-ohm standard evolved from -- anybody?  It seems too high to be the
characteristic impedance of a cable;  I expect that it is a practical
value for transformer design, but undoubtably there will be some other
reason.  Bell telephone perhaps?

As for the inputs on your gear:  a transformer input allows you to
connect an audio signal and keep both sides electrically isolated from
your chassis -- a balanced signal input with neither side referenced to
ground.  This is used to prevent a ground loop between systems, which
can result in a hum superimposed on the audio.  I'm not sure what was
generally done with the center tap in those applications, but I presume
you could leave it open, or tie it to ground (via a short, resistor, or
capacitor; whichever helped).  I'd like to hear from folks who used this
type of gear, with their experiences.  A 600-ohm transformer input is
driven from a 600-ohm source (active, or transformer output), not unlike
50-ohm antenna lines or 75-ohm video signals (except that those coax
lines are unbalanced).  Your differential-amplifier inputs sound like
they are also balanced inputs, but I can't suggest why one would be
better than the transformer input on that gear of yours.

I can add a bit of information related to unbalanced and balanced audio
equipment, and used in consumer and pro-audio applications:

A ground-loop hum may be noticed when an unbalanced-audio output from a
consumer audio source (eg: cd player)  is connected to
amplifiers/speakers in a distant room. A line-level coaxial cable is
connecting the source and the amps, which are on different circuit
breakers. These ground-loop problems are caused by different ground
potentials (perhaps only
millivolts rms) of the two AC power circuits feeding the two systems
(source/amplifier). When you connect these systems
together with an unbalanced line-level coaxial cable, the outer shield
of the cable connects to each chassis, and hence to the two AC grounds,
resulting in a small AC current flow in the ground shield. In an
unbalanced signal connection, this AC
current flow in the shield becomes an audible signal: the 60-cycle hum
you hear (and harmonics: 120-Hz...) In Europe and other places that use
50-Hz power you are, of course, hearing 50-Hz, 100-Hz...).

You can convert an unbalanced signal to balanced, and this is just what
is done with pro-audio gear, where you see the three-pin XLR connectors
on everything.  Transformers can be used in this application, but
generally don't have enough bandwidth or linearity for a high-fidelity
application, so active systems are generally used (transformers are just
fine for voice use).  An active system can also use a specific
source/termination impedance (like 600-ohms), or can be a
low-Z-source/hi-Z-termination design (which has less loss).

Balanced audio cables are usually twisted pair, either unshielded (like
100-ohm CAT-5 cable), or shielded twisted pair, in which case the shield
is usually connected only to the ground at the receiving end of the
cable.

Unbalanced and balanced audio systems use different references for their
nominal volume level:
The nominal level for unbalanced signals (eg: consumer cd-player) is -10
dBV. The unit dBV, is a voltage level with respect to 1 Vrms. A nominal
-10 dBV signal is 0.316 Vrms from center pin to ground (-10 =
20log(0.316/1)).  The nominal level for balanced (pro-audio) signals is
+4 dBu. The unit dBu, is a voltage level with respect to 0.775 Vrms. A
nominal +4 dBu signal is 1.23 Vrms from + to - terminals (+4 =
20log(1.23/0.775)).  Why 0.775V ref?  I have no idea.

An older reference is dBm, which is 1 mW into 600-ohms -- this came from
days when balanced signals used 600-ohm transformer coupling, a lossy
conversion (which also limits bass response). Sometimes dBm is used in
specs of balanced systems
that are not 600-ohm impedance -- this is incorrect. Transformer-coupled
systems use 600-ohm output and input impedances, for maximum power
transfer over these balanced links.  Most electronic balanced audio
systems today use a low output impedance and a high input impedance, for
minimum voltage loss, and amplify/attenuate as needed for the
conversion, that is, -10 dBV is amplified to +4 dBu, and vice-versa.
This is not a 14-dB difference -- since there are different reference
levels, there is electrically just an 11.8 dB difference.  

It is easy to use opamps to build balanced drivers and receivers for
audio;  one I did drives well over 1000-feet of CAT-5 with virtually no
distortion or noise.  

Buy you may not need to convert signals.  You PC output is unbalanced
(the computer ground is the shield of the coax) and typically sees a
load of perhaps 5K or greater -- if you connect this to a 600-ohm
transformer input, you will get the benefit of the balanced input since
the transformer's input side is floating.  The PC needs to be able to
drive the 600-ohm impedance, which it likely can, or you could insert
some resistance in the line to lighten the load on the PC's driver. 
Whether there will be a compatible signal level is another story, but I
would guess that you would be able to adjust the PC output to a decent
volume to drive your gear.  If the PC cannot drive a 600-ohm
transformer, you will need to insert a small preamp, but again, it does
not necessarily need to have a balanced output if you are connecting to
a floating transformer. 

Your microphones are likely a different matter, and one that I can't
comment on -- but I presume that some sort of preamp will be needed to
drive a 600-ohm input.

FWIW,

gil


greenkeys moderator
gil at baudot.net
www.baudot.net
Vaux Electronics:  480-354-5556



> -------- Original Message --------
> Subject: [GreenKeys] Far OT: audio inputs
> From: <eugene at hertzmail.com>
> Date: Sun, February 15, 2009 5:41 pm
> To: <greenkeys at mailman.qth.net>
> 
> I have an exciter (rf-1310) that has several audio inputs. Any audio
> input can be selected for any type of mode (two different inputs are
> selected for ISB mode).
> 
> Two of the inputs are transformer input with 3 connections. 2 outside
> and one center tap connection.
> 
> Three of the inputs have differential amplifier inputs (op amps?) with
> only two connections (basically inputs to the inverted and non-inverted
> inputs to the op amp)
> 
> All inputs seem identical in terms of expected input levels (0dbm
> nominal into 600 ohms). So why would I choose one type of input over the
> other? Whats the benefit/difference/proper application?
> 
> The inputs I am interested in using are 
> 
> a)       PC audio out 
> 
> b)       A Sure 55S dynamic mic with High (100K ohms) Medium (150-250K
> ohms) and Low (30-50) impedence selection 
> 
> c)       A D104 mic non-amplified
> 
> Do I need an amplifier beteween the mics and the 600 ohm 0dbm inputs of
> this exciter? 
> 
> So what do I need to know about audio inputs? 
> 
> Thanks
> 
> Eugene