Impedance matching of mics with pres?
WideGlide
I just noticed that some of the newer mic pres on the market have adjustable
impedance knobs. What's the deal with this? Is it the type of thing that
might be more critical with only certain types of mics like ribbons, or is
it something that may be useful for almost any mic?
This whole idea of an adjustable impedance on a pre has me wondering now
what the stock impedance rating is for many common and vintage pres... I
once thought there might have been a "standard", but I guess not... or?
The GT Vipre can adjust for 300, 600, 1200 or 2400 ohms. The Avalon 2022
can adjust for 50, 150, 600 and 1500 ohms. The UA 610 tube pre can adjust
for 500 or 2000 ohms. Ok, they're all in a similar range, but now one would
have to question which impedance selections would be most useful. The
Avalon goes down to 50 ohms, considerably lower than the others.... but the
Vipre goes up to 2400 ohms, way higher than the highest on the Avalon, etc.
I understand that this may be relative to a specific user's needs, but
finally if someone is gonna plunk down a decent sum of dough for one of
these things, it should be very universal and cover almost all bases.
I realize that these are all just numbers, and numbers don't really mean
anything, only the sound does... but I'm just curious as to how these
numbers RELATE to the sound. After all, if a manufacturer goes through the
trouble of putting such a "feature" on a product, I'd like to think that it
does have SOME relevance, as opposed to being purely a marketing gimmick.
I seem to remember reading a review in some audio magazine where a reviewer
said something about how switching the impedance on one of these adjustable
pres made a big difference in the sound of a Shure 57... something like
that... if this is true, then perhaps this adjustable impedance stuff is
useful.
Thanks. -wg
ken
loading is changed. Experiment.
Ulysses
<so...@no-email.com> wrote:
> I just noticed that some of the newer mic pres on the market have adjustable
> impedance knobs. What's the deal with this? Is it the type of thing that
> might be more critical with only certain types of mics like ribbons, or is
> it something that may be useful for almost any mic?
>
> This whole idea of an adjustable impedance on a pre has me wondering now
> what the stock impedance rating is for many common and vintage pres... I
> once thought there might have been a "standard", but I guess not... or?
The "standard" impedance for most microphones is (was) 150 ohms. Ribbon
mics tended to have lower impedance, most often 30 or 50 ohms. I suspect
the reason for the lower impedance for ribbons may have been that the
ribbon element itself has a very very low impedance (on the order of about
an ohm, give or take) and transformers get worse performance the higher
the ratio. I could be wrong about this. At any rate, it used to be
common for all audio systems to try and match the load impedance to the
source impedance, which is why the transformers on old mic preamps often
have 150 ohm primaries. When people outside the phone company started
designing equipment, they started to realise impedance matching wasn't
necessarily the best way to go, and eventually we ended up where we are
today, with most interfaces having a low-impedance source feeding a
high-impedance load. The degree of this seems to have evolved somewhat
gradually, to the point where today equipment often has output impedances
of practically zero, often artificially raised to 50 or so ohms with
build-out resistors; while input impedances can be 100,000 or more.
The problem lies in the fact that a lot of microphones have transformers
in them. Transformers are very sensitive to loading, and they are
designed with a specific termination in mind. Or at least, their response
is flattest with a particular termination. The exact ideal termination
will depend on who made the transformer, and when. If there were a
"standard" loading for microphone inputs, I would guess it's about 1200
ohms. That's the impedance of most Neve preamps and it's in the general
ballpark of ten times the source impedance, which is a rule of thumb many
designers follow. But you can see why it might be a good idea to have
different impedances available.
Furthermore, since recording is an artform, you often times choose
colorations a shade to the side of neutral for aesthetic reasons and
microphones with frequency response that isn't flat are popular for that
reason. So it makes sense that an engineer might want to choose an
impedance other than what the mic designer had in mind. High-end ringing,
top or bottom roll-off, and increased distortion are some of the euphonic
defects that might arise from technically incorrect termination of the
microphone's output transformer. This can be a good thing.
However, it's my suspicion that the lower, and more "proper" input
impedance of high quality transformer-input mic preamps is what makes them
bring out the glory of an SM57 so much more than even a "good"
high-impedance transformerless preamp can.
To answer another way, I think a 50 ohm input impedance is too low. I
suspect that the Avalon is simply labelled differently than the others.
It may be that the numbers indicate the setting for a microphone's source
impedance. Lots of gear is specked this way, and it's kind of annoying
and leads to confusion.
Ulysses
>
> The GT Vipre can adjust for 300, 600, 1200 or 2400 ohms. The Avalon 2022
> can adjust for 50, 150, 600 and 1500 ohms. The UA 610 tube pre can adjust
> for 500 or 2000 ohms. Ok, they're all in a similar range, but now one would
> have to question which impedance selections would be most useful. The
> Avalon goes down to 50 ohms, considerably lower than the others.... but the
> Vipre goes up to 2400 ohms, way higher than the highest on the Avalon, etc.
> I understand that this may be relative to a specific user's needs, but
> finally if someone is gonna plunk down a decent sum of dough for one of
> these things, it should be very universal and cover almost all bases.
>
> I realize that these are all just numbers, and numbers don't really mean
> anything, only the sound does... but I'm just curious as to how these
> numbers RELATE to the sound. After all, if a manufacturer goes through the
> trouble of putting such a "feature" on a product, I'd like to think that it
> does have SOME relevance, as opposed to being purely a marketing gimmick.
>
> I seem to remember reading a review in some audio magazine where a reviewer
> said something about how switching the impedance on one of these adjustable
> pres made a big difference in the sound of a Shure 57... something like
> that... if this is true, then perhaps this adjustable impedance stuff is
> useful.
>
> Thanks. -wg
--
Justin Ulysses Morse
Roll Music Studios
Minneapolis, MN
www.rollmusic.com War is terror.
Mike Rivers
> I just noticed that some of the newer mic pres on the market have adjustable
> impedance knobs. What's the deal with this? Is it the type of thing that
> might be more critical with only certain types of mics like ribbons, or is
> it something that may be useful for almost any mic?
Well, Recording said they'd eventually publish the article I wrote
about that several months ago. The short version is that
transformerless condenser mics like moderately high impedances but
aren't fussy, and anything inductive like a mic with an output
transformer (which means dynamics including ribbons) are affected
significantly by preamp input impedance. This is one reason why mic
preamps sound different with the same mic, and why people trying
different mics with the same preamp don't always report consistent
impressions.
--
I'm really Mike Rivers (mri...@d-and-d.com)
David Satz
> The short version is that
> transformerless condenser mics like moderately high impedances but
> aren't fussy, and anything inductive like a mic with an output
> transformer (which means dynamics including ribbons) are affected
> significantly by preamp input impedance. This is one reason why mic
> preamps sound different with the same mic, and why people trying
> different mics with the same preamp don't always report consistent
> impressions.
It's also one reason why some engineers experience sonic
differences among mike preamps more vividly than other
engineers do--if you always use condenser microphones that
have very low output impedance, the interactions with the
particular impedance characteristic of most high-quality
preamps will not be as diverse and complicated as they
will be if you sometimes use higher driving impedances.
It's a little like the situation with tube power amps and
loudspeakers, in which audiophiles love to boast that their
tube amps (which generally have higher, and more complex,
output impedance than most solid-state units) can "resolve"
the differences among loudspeakers better. Of course there
is a certain willful ignorance in that attitude, but if you have
as much money as some of these people throw into their home
playback systems, you may not feel that you need knowledge.
Francis Fisher
> It's a little like the situation with tube power amps and
> loudspeakers, in which audiophiles love to boast that their
> tube amps (which generally have higher, and more complex,
> output impedance than most solid-state units) can "resolve"
> the differences among loudspeakers better. Of course there
> is a certain willful ignorance in that attitude, but if you have
> as much money as some of these people throw into their home
> playback systems, you may not feel that you need knowledge.
Hi, guys, here I am again, writing against the mic pre industry, an
industry that can only exist when the pre significantly loads the mic.
Otherwise, when a pre bridges a mic, which is the ordinary case then a
57 always sounds like a 57, a 77 is always a 77, a 44 is always a 44,
etc., etc.
Does anyone here recall the earlier times phrase for spec for mic
input: "150 ohms, unloaded"? This means what it says, zero loading.
Historically, the strategy was not the transfer of power to the pre,
rather, designers wanted the greatest possible voltage presented to
the class A tube-based voltage amplifier which drew no grid current
and couldn't begin to use of any microscopic power that the mic could
create.
Francis
____________________
Francis Fisher
Mountain Stage
WV Public Radio
mailto:f...@verizon.net
Mike Rivers
> Hi, guys, here I am again, writing against the mic pre industry, an
> industry that can only exist when the pre significantly loads the mic.
> Otherwise, when a pre bridges a mic, which is the ordinary case then a
> 57 always sounds like a 57, a 77 is always a 77, a 44 is always a 44,
> etc., etc.
Right - but properly loaded, a 57 can sound better than a 57
improperly loaded.
> Does anyone here recall the earlier times phrase for spec for mic
> input: "150 ohms, unloaded"?
No, I don't think so. For many years though, preamps typically have
had an input impedance in the range of 1000-1500 ohms, and most mics
worked pretty well into that load. The issue was never one of maximum
power transfer, just reasonably light loading of a source impedance
that was typically less than 100 ohms. That was before we had
inexpensive but reasonably good sounding dynamic mics, transformerless
condenser mics, gourmet ribbon mics, and a signal path with good
enough resolution so that we could hear the effect of loading on
microphone output transformers.
Presto! A new industry, and a new place to spend money. Ain't life
grand?
Francis Fisher
> Presto! A new industry, and a new place to spend money. Ain't life
> grand?
You put your finger on it! As such, when you need to color the sound
of a mic then a new pre could be what a person needs. And, as such,
such a purchase would be taken in the same vein as when an electric
violin player goes to the music store to buy a distortion pedal for
his rig. Fine.
Sorry, I get set off when an impression is left that my basic Neve or
Yamaha board is deficient because of it's preamp design. Baloney!
People should feel free to purchase any effect box.
Scott Dorsey
Francis Fisher <f...@example.com> wrote:
>On 9 Dec 2001 08:04:38 -0500, mri...@d-and-d.com (Mike Rivers) wrote:
>
>> Presto! A new industry, and a new place to spend money. Ain't life
>> grand?
>
>Yamaha board is deficient because of it's preamp design. Baloney!
>People should feel free to purchase any effect box.
The problem is amplified by the huge numbers of cheap mass-produced
consoles out there that _do_ have deficient preamps.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."