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From: Rune Allnor on 30 Jul 2010 08:25
On 29 Jul, 19:20, robert bristow-johnson <r...(a)audioimagination.com>
wrote:

> why can't a low-impedance source simply connected to a high-impedance
> load suffice?  why are matching transformers needed (unless level is
> important)?

Level *is* important, as the impedance matching takes place
before the ADC. I bought a Sony ECM-MS907 mic specifically to
use with my PC. The impedance of that mic is tuned to the
impedance of the PC mic line.

The mic does *not* work with my Edirol R-09 recorder. When I
connect it, the recorder level display only shows the strongest
peaks. I haven't tested this fully, so it might be that I
can get the sony to work with the edirol, but if so, the
useful dynamics window will be a lot narrower than it would
be with a well-matched set-up.

Rune
From: Mark on 30 Jul 2010 09:23
On Jul 30, 8:25 am, Rune Allnor <all...(a)tele.ntnu.no> wrote:
> On 29 Jul, 19:20, robert bristow-johnson <r...(a)audioimagination.com>
> wrote:
>
> > why can't a low-impedance source simply connected to a high-impedance
> > load suffice?  why are matching transformers needed (unless level is
> > important)?
>
> Level *is* important, as the impedance matching takes place
> before the ADC. I bought a Sony ECM-MS907 mic specifically to
> use with my PC. The impedance of that mic is tuned to the
> impedance of the PC mic line.
>
> The mic does *not* work with my Edirol R-09 recorder. When I
> connect it, the recorder level display only shows the strongest
> peaks. I haven't tested this fully, so it might be that I
> can get the sony to work with the edirol, but if so, the
> useful dynamics window will be a lot narrower than it would
> be with a well-matched set-up.
>
> Rune

to RBJ,

the usual reason to load an audio device with it's "rated load" (which
is not necessarily the same as "matched" is so that it works the way
it was designed to work most importantly delivering the right
frequency response and level....

This is usually more important with hi z devices like guitar pickups
and less important with electronic outputs..

Level is important for low level transducers like guitar pickups,
phono pickups and mics.

Audio transformers are used in professional equipment to provide
galvanic isolation and common mode rej where you NEED it to work the
first time without any fuss despite the fact that the guy next to you
is controlling 10,000 Watt worth of lighting equipment and his cables
run next to yours, or your audio signal runs to the next building
whose "ground" is 10V different from your ground.

The folks at rec.audio.pro may have adidtional comments..

Mark




From: Clay on 30 Jul 2010 10:57
On Jul 29, 1:20 pm, robert bristow-johnson <r...(a)audioimagination.com>
wrote:
> i know it's not DSP, but i thought people on the list would have some
> experience, opinions, and knowledge.
>
> whether it's a microphone going into a preamp or mix board, or an
> electric guitar going into a guitar amp, how does impedance matching
> (or mismatching) affect *tone*?
>
> the way i see it (as an EE) is:
>
> 1. that if one of either the source or load impedance has reactive
> components and the other is resistive, changing the resistance will
> change the simple RC filter and the corner frequency of it, thus
> changing the tone.
>
> 2. if both source and load impedance are mostly resistive, changing
> one or the other will only change the level, not the waveshape.  if
> the input to the pre-amp or amp has some nonlinearity, a louder level
> will push that stage into the nonlinear region which will change the
> waveshape and affect the timbre.
>
> other than that, i do not understand the why "impedance matching" is
> considered so important in audio.  it's not like we need to maximize
> power transfer (in micro watts) from the transducer or pickup into the
> input of the amplifier.  and, at audio frequencies (and the length of
> cables), reflections along the transmission lines (due to impedance
> mismatch) should not be an issue either.
>
> why can't a low-impedance source simply connected to a high-impedance
> load suffice?  why are matching transformers needed (unless level is
> important)?
>
> --
>
> r b-j                  r...(a)audioimagination.com
>
> "Imagination is more important than knowledge."

While not great audio quality, telephone circuits use transformers to
achieve common mode rejection. Imagine your signal cable running in
parallel with power lines for miles. I use to often see ground
referenced 60 Hz in the hundreds of volts often on phone lines where
the audio was routinely at -10 or -20 dBm. I recall the old stereo
amps ( I still have one) where the user selects one of three impedence
and tone matching functions for the turntable. I.e., moving coil,
moving magnet, and piezo. I think about microphones and phon
cartraiges having small signals, so optimizing power transfer seems
like it would be important. My 2 cents worth.
Clay
From: Jerry Avins on 30 Jul 2010 11:12
On 7/30/2010 10:57 AM, Clay wrote:


> ... I think about microphones and phon
> cartraiges having small signals, so optimizing power transfer seems
> like it would be important. My 2 cents worth.

Matching impedance and maximizing SNR often work out to nearly the same
constraint. For maximum power transfer, one wants to match the load to
the source, not (if it's different) match the source to the load. When
efficiency matters, the source impedance should be much lower than the
load's. Power lines and signal lines follow different rules.

Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
From: robert bristow-johnson on 30 Jul 2010 11:48
On Jul 30, 9:23 am, Mark <makol...(a)yahoo.com> wrote:
> On Jul 30, 8:25 am, Rune Allnor <all...(a)tele.ntnu.no> wrote:
>
>
>
> > On 29 Jul, 19:20, robert bristow-johnson <r...(a)audioimagination.com>
> > wrote:
>
> > > why can't a low-impedance source simply connected to a high-impedance
> > > load suffice?  why are matching transformers needed (unless level is
> > > important)?
>
> > Level *is* important, as the impedance matching takes place
> > before the ADC. I bought a Sony ECM-MS907 mic specifically to
> > use with my PC. The impedance of that mic is tuned to the
> > impedance of the PC mic line.
>
> > The mic does *not* work with my Edirol R-09 recorder. When I
> > connect it, the recorder level display only shows the strongest
> > peaks. I haven't tested this fully, so it might be that I
> > can get the sony to work with the edirol, but if so, the
> > useful dynamics window will be a lot narrower than it would
> > be with a well-matched set-up.
>
>
....
>
> the usual reason to load an audio device with it's "rated load" (which
> is not necessarily the same as "matched" is so that it works the way
> it was designed to work most importantly delivering the right
> frequency response and level....
>
> This is usually more important with hi z devices like guitar pickups
> and less important with electronic outputs..
>
> Level is important for low level transducers like guitar pickups,
> phono pickups and mics.

i understand how level is changed when a transformer is tossed in. my
primary question has been to how is *tone* changed? is it because
higher levels push the guitar amp into a little bit of distortion and
that makes the tone "better"? the issue with some of these custom
pickups is that their initial level is lower and, when going into a
guitar amp, the pickups "just ain't hot enough" and *tone* is part of
the subjective experience (because cranking up the volume knob doesn't
get us to the same sound). putting a transformer in between and
"matching impedance" reportedly fixes this.

now, if, instead we connect that custom pickup to a transformerless
booster amp that is perfectly linear, high input impedance, low output
impedance, and bring the level up to whatever the transformer did,
then will *tone* suffer? if so, then since the levels are the same,
if must be something with the source impedance of the pickup and/or
the input impedance of the guitar amp. no?

> Audio transformers are used in professional equipment to provide
> galvanic isolation and common mode rej where you NEED it to work the
> first time without any fuss despite the fact that the guy next to you
> is controlling 10,000 Watt worth of lighting equipment and his cables
> run next to yours, or your audio signal runs to the next building
> whose "ground" is 10V different from your ground.

i understand how transformers are useful for isolating grounds and for
providing a natural differential input (and high CMRR) besides the
function of changing voltage level and reflecting impedance.

> The folks at rec.audio.pro may have additional comments..

i was thinking of posting there, but i hadn't really been hanging
there since the Gabe Weiner days and it seemed to have gotten really
noisy. maybe i'll post this question there. i *did* post it to (the
newly created) sci.physics.acoustics (the alt newsgroup is moving to
there).

L8r,

r b-j



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