From: merchant@cbnews.cb.att.com (shahrukh.s.merchant)
Newsgroups: rec.audio,rec.audio.pro
Subject: Questions on Microphone/Line/Headphone voltage levels/impedances
Date: 8 Apr 92 19:32:17 GMT
Reply-To: shahrukh_merchant@att.com
Organization: AT&T

I have several questions on voltage levels and impedances for various
audio inputs and outputs.  In some of these, I use the term "normal
listening level."  I don't know if there is such a term defined (as a
reference point); for my purposes, let us say it is the level (zero-to-
peak) of a 1 kHz sine wave at a level equivalent to "quiet conversation."

Basically, the reason for these questions is that I need to make up
a number of attenuator networks, so that I can connect headphone outputs
to line inputs, line outputs to microphone inputs, even speaker outputs
to line inputs, etc., all without stressing either the output or input.
In some cases I need to mix stereo outputs into a mono input (without
losing too much L-R channel separation.  In all cases, I wish to use
only passive resistive networks.

I'm listing the specific questions below--if you can offer any other
related information or suggestions in addition, I'd appreciate those
too.  In all cases, I'm talking about middle-of-the-road consumer
equipment.

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

2. LINE INPUTS AND OUTPUTS
   a) What is the typical input impedance?  10 K-ohms?
   b) What is the typical range of output impedances for line outputs?
      Can these be connected directly to high-impedance terminations?
      What is the lowest impedance termination that should be connected
      to a line output?
   c) What is the loaded output voltage at "normal listening level"?
   d) Is there any DC bias to a line output, or is it zero volts for
      "no audio input"?

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

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

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

						Shahrukh Merchant
						merchant@mvuxd.att.com
						shahrukh_merchant@att.com