From: hanke@nessie.cs.id.ethz.ch (Norbert Hanke)
Subject: Re: Questions on Microphone/Line/Headphone voltage levels/impedances
Organization: Swiss Federal Institute of Technology (ETH), Zurich, CH
Date: Thu, 9 Apr 1992 08:16:11 GMT

In article <1992Apr8.193217.15730@cbnews.cb.att.com> shahrukh_merchant@att.com writes:
(some stuff deleted)
>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?

Normal "big" headphones usually have an impedance of about 600 Ohms.
"Walkman" (TM of Sony) type headphones have a lower impedance (to get more
SPL out of the low supply voltage of those devices) which usually is about
30 Ohms, as you stated.

>   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?

On Hifi equipment the output impedance normally is about 100 Ohms, on
Walkman equipment it's less, I guess a couple of Ohms.

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

Looking at the specifications of headphones they are normally rated to handle
a maximum power of about 100 milliwatts. For a 600 Ohm headphone this means
7 Volts, for a 33 Ohm headphone 1.8 Volts.

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

On studio equipment usually 10 kOhms, on hifi equipment for home use usually
100 kOhms.

>   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?

It's normally very low (50..100 Ohms), sometimes 600 Ohms an can be connected
to high-impedance terminations. Studio equipment is normally capable of hand-
ling a load of 600 Ohms or less, home equipment usually more, althoug seldomly
stated. I guess 10 kOhms or more load should work on every output, be it pro
or consumer equipment.

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

Studio Equipment most often has a maximum output level of +24 dBm which 
is roughly 12 Volts. Consumer Equipment (e.g. CD players) have output levels
of 0.5 to 2 Volts maximum.

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

There should not be a DC bias, however if there are decoupling capacitors
both at the line output and the (loading) line input, there might be DC bias.

>
>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?

It's normally "much lower" indeed. The data of power amplifiers normally
states the "damping factor" which is the ratio between rated load impedance
(4 or 8 Ohms) and the output impedance. A damping factor of 100 means 4/100
= 0.04 Ohms. A well built power amplifier doesn't require a minimum load.
The more "resistive" the load, the better for the amplifier.

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

Depends on loudspeaker and "listening level". A loudspeaker with rather high
efficiency produces a SPL of 90 dB out of 1 Watt electrical power. If you
consider that a "normal listening level", the voltage across 8 Ohms is
about 2.8 Volts. At 110 dB SPL (which is very load) this meand 100 Watts
or 28 Volts.

>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?

Normally all microphones used nowadays are "low impedance". Dynamic micro-
phones usually have an output impedance of typically 200 Ohms, Condenser
microphones may be lower, in the range 30..50 Ohms.

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

Most microphones are specified to work with a load impedance of 600 Ohms
or more. Typical imput impedance of studio equipment mic inputs is in the
range 2 kOhms to 5 kOhms. Note that a too-low input impedance alters the
frequency response of dynamic microphones. 

>   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)?

First, it's a very bad idea to attenuate the output of a high impedance
microphone with a low load impedance, in the case it's a dynamic microphone.
Output levels vary depending on the type of microphone: dynamic ones have
a lower output voltage (roughly 5 times) than phantom-powered studio condenser
microphones.
If I remember correctly a studio condenser microphone has a output voltage
of usually 10 mV at 1 Pascal SPL, which corresponds to about 90 dB SPL. I may
be wrong here.

>
>Thanks for any answers.  You may reply by email to:
>
>						Shahrukh Merchant
>						merchant@mvuxd.att.com
>						shahrukh_merchant@att.com


Norbert Hanke
ETH Zurich, Switzerland

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