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From: glen herrmannsfeldt on 29 Jul 2010 14:31
robert bristow-johnson <rbj(a)audioimagination.com> wrote:
> On Jul 29, 1:30�pm, spop...(a)speedymail.org (Steve Pope) wrote:
>> 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)?

>> Many audio connections are in fact a low-impedance source connected
>> to a high impedance load.

> i know. i'm having a discussion with someone who designs custom
> pickups and he has a version that transformer couples the output to
> the guitar amp. it's a step-up which increases the voltage (and
> decreases current) by the turns ratio factor and also the apparent
> source impedance (by the square of the turns ratio). now my thinking
> is that this is a levels issue (the louder level in the guitar amp
> makes for a hotter sounding pickup), but he insists that the reason is
> impedance matching and that results in a "better tone".

As mentioned, you normally don't want to impedance match, but just
not be so far off. If you connect a low impedance (50 ohm)
microphone to a high impedance (100K ohm) input you lose a lot
in amplitude.

I just looked up the AT-822, which is the microphone that I have.
It looks like they just took the balanced AT-825 and put a three
conductor (stereo) instead of five conductor (stereo differential)
plug on it. The AT-825 is expected to be used with transformers,
while the 822 is not, but both have amplifiers built in so the
output voltage isn't so low. (One AA cell.)

So, with the AT-822 I can still use long cables without worrying
about high frequency loss, but still don't need a transformer.

>> Using transformers on some connections is thought to prevent ground loops.
>> Matched connections (e.g. a 600 ohm load), when they are used, are
>> thought to prevent hum on long lines. �I'm not sure how scientific
>> either of these beliefs are. �

> i dunno if the matched connection is responsible for suppressing hum,
> or if it's the natural differential input that a transformer primary
> winding is. you get a damn good common-mode rejection ratio (CMRR)
> with a transformer floating around in all this low-frequency slew of
> E&M radiation.

Yes. Especially if you have long microphone leads (studio or
concert hall) possibly running along side power cables.

-- glen
From: glen herrmannsfeldt on 29 Jul 2010 14:42
steveu <steveu(a)n_o_s_p_a_m.coppice.org> wrote:
>>Steve Pope <spope33(a)speedymail.org> wrote:
>>Note that transformers are also used on both ends of UTP
>>ethernet to remove the common mode signals.

> They do that, but their primary purpose is electrical
> isolation for safety. They aren't tested at 4kV for their
> common mode rejection qualities. :-)

Well, it is both. Remember, it is running 30MHz or so signals
through an unshielded cable. Any common mode in the source will
radiate very well. (The coding used for 100baseTX is carefully
designed to keep most of the power below about 30MHz.)

It would be difficult to make an appropriately balanced source
without a transformer. A differential receiver might not be
so hard to build without a transformer.

But yes, even without the RFI need, they would still want the
transformers for isolation.

But people still worry about running ethernet cables near
power lines. I have heard that UTP ethernet has been tested
with 120VAC common-mode directly onto the cable and it still
worked fine.

For coaxial ethernet, the transceiver was directly connected
to the coax cable, with shielded twisted pair cable in between,
and transformers on both ends. (And a DC-DC converter to get
isolated power to the transceiver.)

-- glen
From: Jerry Avins on 29 Jul 2010 17:13
On 7/29/2010 1:20 PM, robert bristow-johnson 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)?

It's not always important, but it often is. For example, a typical
magnetic phono cartridge should be loaded with about 50 Kohms; that is
the usual input impedance of the pre-amp. A departure affects the
frequency response by altering the L/R filter. (Interestingly, a crystal
cartridge that should ordinarily be feed into a high-impedance (half a
meg ohm or more) input without a pre-amp's gain or frequency
compensation, will have its highs killed by the pre-amp's low input
resistance, and sound pretty good coming out of the pre-amp..) What is
true for magnetic cartridges is also true for dynamic mics. You could
maintain the response by loading one with a resister, but giving up the
gain and other benefits of a transformer invites noise. For a resistive
source with low enough impedance, the load matters very little.

Jerry
--
Engineering is the art of making what you want from things you can get.
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From: glen herrmannsfeldt on 29 Jul 2010 18:35
Jerry Avins <jya(a)ieee.org> wrote:

(snip of questions on microphone impedance)

> It's not always important, but it often is. For example, a typical
> magnetic phono cartridge should be loaded with about 50 Kohms; that is
> the usual input impedance of the pre-amp. A departure affects the
> frequency response by altering the L/R filter.

In other words, the design of the cartridge frequency response
includes the load.

> (Interestingly, a crystal
> cartridge that should ordinarily be feed into a high-impedance (half a
> meg ohm or more) input without a pre-amp's gain or frequency
> compensation, will have its highs killed by the pre-amp's low input
> resistance, and sound pretty good coming out of the pre-amp..)

For many years (high school and college) I had a magnetic cartridge
connected to an amplifier without the appropriate amplifier.
Apropriate adjustment of the volume and tone controls usually
worked fine. Finally I built a preamp from a dual op-amp data
sheet, and not so much later bought a nice integrated amplifier
with magnetic phono inputs. I still have the amplifier, and
haven't used those inputs for years.

> What is
> true for magnetic cartridges is also true for dynamic mics. You could
> maintain the response by loading one with a resister, but giving up the
> gain and other benefits of a transformer invites noise. For a resistive
> source with low enough impedance, the load matters very little.

-- glen
From: Andreas Huennebeck on 30 Jul 2010 03:18
glen herrmannsfeldt wrote:

> Jerry Avins <jya(a)ieee.org> wrote:
>
> (snip of questions on microphone impedance)
>
>> It's not always important, but it often is. For example, a typical
>> magnetic phono cartridge should be loaded with about 50 Kohms; that is
>> the usual input impedance of the pre-amp. A departure affects the
>> frequency response by altering the L/R filter.
>
> In other words, the design of the cartridge frequency response
> includes the load.

Yes, and this load includes the cable capacitance. This is true for MM cartridges (MC
cartridges have such a low output impedance that cable and input capacitance
do not matter). Since the recommended load capacitance varies from cartridge to
cartridge and the cable capacitance from cable to cable good phono preamps
have a switchable input capacitance.

MM cartridges have a L-C resonance whose resonance frequency and
Q depend on the load impedance. A lower resistance dampens the
Q and a higher capacitance lowers the resonance frequency. The
recommended load impedance shifts this resonance down into a region
above 10-15 khz. The increased output in this region (due to the
resonance) compensates for the loss of output due to inherent characteristics
of the cartridge such that a linear frequency response is reached up to
20 kHz.

If the load impedance is not as recommended then you can eather get
a frequency response with sligthly reduced output above 15 khz or a hefty
peak below 15 kHz and a large loss above.

bye
Andreas
--
Andreas H�nnebeck | email: acmh(a)gmx.de
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