Other web Sites
Harmonica Blues  Harmonica Amps
Harmonica Links Harmonica Pages
• Archives Home
• Years
 · 1992
 · 1993
 · 1994
 · 1995
 · 1996
 · 1997 		 · 1998
 · 1999
 · 2000
 · 2001
 · 2002
 · 2003
 
Web HarpL
Ebay Searches:
Amps:
Microphones:
Effects:
Harmonicas and Gear:
Harmonica Music and Instruction:

 

 

Harp-L Archives

[Previous Message] [Next Message]
[Previous in Thread]
[Start of Thread] [End of Thread]

From: Pete Brunelli
Date: Thu, 12 Sep 1996 12:51:33 -0400
Subject: Re: loZ/hiZ debate. techies jump in

My reference to my experience was not meant as a snub, just a reference. I
am often confused, and see others confused between the impedance of the
device, and the impedance of the load. I also think that my use of
generalitites caused this whole mess.

No arguments from me this time, just a snip from the pro-audio faq located at:

http://www.cis.ohio-state.edu/hypertext/faq/usenet-faqs/bygroup/rec/audio/pr
o/_AudioFAQ%3apro-audio-faq.html

Here, guitar pickups are characterized as high-Z. I have always treated
them as low-Z, so i guess that i have done more learning.

Q3.2 - What is meant by "impedance matching"? How is it done? Why is it
necessary?

We can talk about the characteristic impedance of an input, which is to
say the ratio of voltage to current that it likes to see, or how much
it loads down a source. (You can think of this as being an "AC resistance"
and you would be mostly right, although it's actually the absolute
magnitude of the vector drawn by the resistive and reactive load
components. Dealing with line level signals, reactive components
are going to be negligible, though).

In general, in this modern world, most equipment has a low impedance
output, going into relatively high impedance input. This wastes some
amount of power, but because electricity is cheap and it's possible to
build low-Z outputs easily today, this is not a big deal.

With microphones, it _is_ a big deal, because the signal levels are
very low, and the drive ability poor. As a result, we try and get the
best efficiency possible from microphones to get the lowest noise
floor. This is often done by using transformers to step up the voltage
or step it down, to go into a higher or lower Z load. Transformers
have some major disadvantages in that they can be significant sources
of nonlinearity, but back in the days of tubes they were the only
solution. Tubes have a very high-Z input, and building balanced inputs
with tubes requires three devices instead of one. As a result, all
mike preamps would have a 600 ohm balanced input, with a transformer,
driving a preamp tube. Today, transistor circuits can be used for
impedance matching, although they are often more costly and can be noisier
in cases.

As a result of the expense, consumer equipment was built with high-Z
microphone inputs, and high-Z microphones. This resulted in more noise
pickup problems, but was cheaper to make. Unfortunately this still
held on into the modern day of the transistor, and a lot of high-Z
consumer gear exists. Guitar pickups are generally high-Z devices,
and require a direct box to reduce the impedance so that they can go into
a standard 600 ohm mike preamp directly.

Many years ago, the techniques that were used in audio came originally
from telephone company practice. Phone systems operate with 150 or 600
ohm balanced lines, and adoption of this practice into the audio industry
caused those standards to be used. In the modern age where lines are
relatively short and transformers considered problematic, the tendency
has been to have low-Z outputs for all line level devices, driving
high-Z inputs. While this is not the most efficient system, it is relatively
foolproof, and appears on most consumer equipment. A substantial amount of
professional gear, however, still uses internal balancing transformers or
resistor networks to match to a perfect 600 ohm impedance. [Scott]

[Ed. note: Modern equipment works on principles of voltage transfer
rather than power transfer. Thus a standard audio circuit today is
essentially a glorified voltage divider. You have a very low output
impedance and a very high input impedance such that the most voltage
is dropped across the load. This is not an impedance-matched circuit
in the classic sense of the word. Rather, it is a "bridged" or
"constant voltage" impedance match, and is the paradigm on which
nearly all audio circuits operate nowadays. -Gabe]

and FYI

Q3.4 - Which is it for XLRs? Pin 2 hot? Or pin 3 hot?

Depends on whom you ask! Over the years, different manufacturers have
adopted varying standards of pin 2 hot and pin 3 hot (and once in a
while, pin *1* hot!). But nowadays most manufacturers have adopted
pin 2 hot. Still, it is worth taking the extra minute or two to check
the manual. The current AES standard is pin 2 hot. [Gabe]

/-----------------------------------\
|--/=====================================\--|
||| a | h | a | r | m | o | n | i | c | a |||
||| i | n | m | y | p | o | c | k | e | t |||
|--\=====================================/--|
\-----------------------------------/
Pete Brunelli p~onnix.com http://www.connix.com/~pcb