Newsgroups: rec.audio

In rec.audio, merchant@cbnews.cb.att.com (shahrukh.s.merchant) writes:

    I have several questions on voltage levels and impedances for various
    audio inputs and outputs.  

I am doing this from memory, and corrections/clarifications are welcome.

    1. HEADPHONES
       a) What is the typical input impedance of headphones.  I read
          33 ohms somewhere.  Can this be modelled as a resistive load for my
          purposes?

Some consumer headphones may have impedences as high as 2000 ohms.
I think many "pro" headphones are in the range you mention.

       b) What is the typical range of output impedance of headphone outputs.
          Can these be connected to high-impendance terminations or do
          they require a certain minimum load to function?i

If we are speaking of a dedicated headphone amp, the impedance may be
a few ohms at most.  These things act essentially as voltage sources.
Many headphone outputs on the front of power amplifiers are just a 
large-value dropping resistor in series with the main output.  Obviously,
this is a much different situation.

       c) What is the loaded output voltage at "normal listening level"?

I can't give you a very precise number here, as it depends on the phones.
A typical _power_ rating for a headphone amp is 300 mW.  They often have
+/- 12V supply rails.  With most phones, such amps can play "very loudly."

    2. LINE INPUTS AND OUTPUTS
       a) What is the typical input impedance?  10 K-ohms?

10k is typical.

       b) What is the typical range of output impedances for line outputs?

Typically 100 ohms.

          Can these be connected directly to high-impedance terminations?

That is the expectation.

          What is the lowest impedance termination that should be connected
          to a line output?

This depends on the rail voltage and the power rating of the output
"protection" resistor.  I personally do not attempt less than 1 k
without examining the schematic of the equipment in question.  This
may be over-cautious.

       c) What is the loaded output voltage at "normal listening level"?

There are two defacto standards for "0 dB" (peaks will be a few dB above
this).  Pro studio equipment is: 0 dB = +4 dBm (i.e. re: one milliwatt
into 600 ohms -- this despite the fact that noone uses 600 ohm matched
equipment anymore!).  "Semi-pro" equipment is 0 dB = -10 dBV (i.e.,
10 dB below one volt).  Converting these both to voltages, they are:
1.23 V and .707 V, respectively.  Which "standard" do consumer products
follow?  I have been surprised enough times to conclude "neither."  
But one can safely assume "around a volt."  For those of us who listen
quietly, the number is correspondingly lower.

       d) Is there any DC bias to a line output, or is it zero volts for
          "no audio input"?

It is "supposed to be" zero.  Equipment with bipolar supplies often does
not use a blocking capacitor on the output.  Thus there may be some
slight offset voltage.  Line inputs nearly always have DC blocking
capacitors.

    3. SPEAKER INPUTS AND OUTPUTS
       a) What is the typical range of output impedances of speaker outputs?
          I would assume much lower than 4-8 ohms, but how much lower?
          Do they require a certain minimum load to function properly?
          How much?  Can this load be strictly resistive?

Power amplifiers have DC impedences below one ohm.  Due to feedback,
the impedence (within the bandwidth and complience of the amp) appears
to be essentially zero.  All amplifiers specify a minimum load for
full power output.  This is governed by the current capability of the
output devices.  At lower levels, there is still a minimum load, 
dictated by what it takes to keep the amp's protection circuitry happy.
Some amplifiers do not like 2 ohm loads.  There is no problem with the
load being purely resistive for modern solid-state amplifiers.  I 
will not speak to tube-with-transformer designs, due to lack of knowledge.

       b) What is the loaded output voltage at "normal listening level"?

This depends on the impedance and efficiency of your speakers.  A 
common supply voltage for a power amp output stage is +/- 40 V.  Such
an amplifier might be rated about 100 W rms into 8 ohms.

    4. MICROPHONE INPUTS AND OUTPUTS
       a) What is the typical output impedance of a microphone (non-
          balanced output)?  I understand there are low-impedance and
          high-impedance microphones, so what is it for each?

Low impedance professional mics are generally 150 - 250 ohms.  I
do not know about high impedance mics.  (I think they are a bad idea.)

       b) What is the typical input impedance at a microphone input
          jack (non-balanced).  I've seen 600 ohms somewhere--is this
          standard?

600 ohms is often used as a design target by makers of matching transformers.
For non-tranformer "low impedance" balanced mic inputs, one typically
sees 1k - 3k from each leg to ground.  "High impedance" mic inputs
may be 10k - 20k (single-ended).

       c) What is the loaded output voltage at "normal listening level"
          for each type (low- and high-impedance)?  Is it true that it is
          the same for both types (the higher output level of the
          high-impedance mike being attenuated proportionately more owing
          to its higher output impedance into 600 ohms)?

I can only speak of professional microphones, which are low impedance.
There are several popular methods of rating microphone sensitivity,
and it can be difficult to compare specs for two microphones for 
this reason.  Some manufacturers specify open circuit voltage, some
specify loaded voltage.  There are two different sound pressure levels
used for reference.  With a fairly loud sound source miked closely,
one should be prepared for output levels anywhere from 2 mV to about
30 mV, depending upon the microphone, and what "loud" means to you.
Good microphone preamps always have adjustable gain.  The gain range
might be 15 to 60 dB, with a 10 dB input attenuator "pad" available
for truly extreme conditions.

    Thanks for any answers.  You may reply by email to:

Such a carefully constructed set of questions -- I think I'll waste
the net bandwidth with the answers!

David L. Rick
(associated through a circuitous set of contracts with:)
Hewlett-Packard Company
"Versatile Bus Modular European Card Extensions for Instrumentation
Systems Division"
Loveland, Colorado
drick@hplvdo.lvld.hp.com


Disclaimer:  I _told_ you I was doing this from memory!