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Attenuate or run hot mic directly into A/D convertor?



 
 
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  #1  
Old September 16th 06, 12:03 PM posted to rec.audio.pro
[email protected]
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Default Attenuate or run hot mic directly into A/D convertor?

We've got some nice pres here (Millenia, Symetrix, Great River, Manley)
but consistently run into problems of too much gain when tracking kit.
E.g. with the Millenia on its lowest settings a 451 on the bottom of
the snare clips our A/D convertor.

I'm considering two options, neither of which I know much about and
would appreciate any advice as to the merits of either:

1. Using an inline pad on the mic cable - Shure A15AS or Audio Technica
8202. Do these degrade the signal? Do they pass phantom power ok?

2. Skipping the preamps altogether and running the mics directly into
the A/D convertor. Would this work? My concerns are (i) would it be
loud enough? (ii) impedance issues? (iii) How to handle phantom power?
If we buy an inline phantom power box is this going to blow up the A/D
convertor? And does the quality of the phantom power unit affect the
sound quality? (e.g. Behringer do cheap little ones).

Any advice appreciated.

Thanks - Jon

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  #2  
Old September 16th 06, 01:14 PM posted to rec.audio.pro
Scott Dorsey
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Default Attenuate or run hot mic directly into A/D convertor?

> wrote:
>We've got some nice pres here (Millenia, Symetrix, Great River, Manley)
>but consistently run into problems of too much gain when tracking kit.
>E.g. with the Millenia on its lowest settings a 451 on the bottom of
>the snare clips our A/D convertor.


On the lowest setting, the HV-3 has about 18 dB of gain. I think the Great
River can be cranked down almost to unity.

>I'm considering two options, neither of which I know much about and
>would appreciate any advice as to the merits of either:
>
>1. Using an inline pad on the mic cable - Shure A15AS or Audio Technica
>8202. Do these degrade the signal? Do they pass phantom power ok?


They don't degrade the signal and they pass phantom power fine. Every
studio should have a drawer full of these. They are absolute and total
lifesavers for field recording work.

>2. Skipping the preamps altogether and running the mics directly into
>the A/D convertor. Would this work? My concerns are (i) would it be
>loud enough? (ii) impedance issues? (iii) How to handle phantom power?
>If we buy an inline phantom power box is this going to blow up the A/D
>convertor? And does the quality of the phantom power unit affect the
>sound quality? (e.g. Behringer do cheap little ones).


The phantom power unit has at least blocking capacitors and maybe a
transformer (unless it's the AKG unit, which allows phantom to appear
on the _output_ as well as the input). The quality of those capacitors
or transformer is still important.

The impedance is still an issue, but if you're going into an old-style
600 ohm input, most mikes will be happy. I used to run the RE-20 as a
kickdrum mike directly into the inputs of the Ampex with no problem.
There's no reason you can't do that also.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #3  
Old September 16th 06, 01:48 PM posted to rec.audio.pro
Sunflowermanuk
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Default Attenuate or run hot mic directly into A/D convertor?

Thanks Scott, sounds like a no-brainer solution then - I'm going
shopping for a clutch of inline attenuators to put between mic and pre.
Any preference make-wise or are they all much of a muchness?

Jon

Scott Dorsey wrote:
> They don't degrade the signal and they pass phantom power fine. Every
> studio should have a drawer full of these.


  #4  
Old September 16th 06, 02:05 PM posted to rec.audio.pro
Scott Dorsey
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Posts: 16,701
Default Attenuate or run hot mic directly into A/D convertor?

Sunflowermanuk > wrote:
>Thanks Scott, sounds like a no-brainer solution then - I'm going
>shopping for a clutch of inline attenuators to put between mic and pre.
> Any preference make-wise or are they all much of a muchness?


I like the Shures, because you can run a truck over them without breaking
them and my Shure dealer will give me a discount on them. They're all
really just a couple resistors in a barrel, so there isn't a substantial
difference other than ruggedness.

The A-Ts are fine. I also have a bag of old GenRad ones around here somewhere
that are 30 dB fixed and also perfectly fine.

Normally for field recording I'll split the whole stage snake and pull it
into my multitrack rig, which has Millennia front ends. Occasionally folks
will be running a line level signal from something on stage down the snake.
I'll just stick a pad in there and keep going. The S/N isn't as good as
it would be if I bypassed the preamp, but it's so good anyway that it's not
an issue.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #5  
Old September 16th 06, 02:13 PM posted to rec.audio.pro
Mike Rivers
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Posts: 8,744
Default Attenuate or run hot mic directly into A/D convertor?


Sunflowermanuk wrote:
> Thanks Scott, sounds like a no-brainer solution then - I'm going
> shopping for a clutch of inline attenuators to put between mic and pre.


For what it's worth, AKG used to make an attenuator for the C451 that
screwed between the capsule and body. Nowadays if you want one, you
have to pay about as much for it as a halfway decent preamp to some
greedy eBay seller. An outboard attenuator is fine, and there's no
appreciable difference between them. At least there shouldn't be - but
they used fixed resistors, and I doubt that any of them use better than
1% tolerance resistors. This could unbalance the input to the preamp
and reduce the common mode noise rejection, but if you're feeding a
huge signal voltage to the preamp, this isn't likely to be a concern.
But isn't it nice to have one more thing to worry about?

I believe Rich Chinn's web page http://www.uneeda-audio.com has a
section about building your own pads. If you care, you could build your
own out of 0.1% tolerance resistors. We got into this discussion not
too long ago and one of the Google wizards found at least one
attenuator's spec sheet that actually mentioned the tolerance or
matching of the resistors. It's most likely Shure since they're the
kind of company that would care to put that sort of data on their
sheets.

As long as the preamp isn't clipping, you might want to use it to get
whatever sound it imparts to the mic (putting a pad between the mic and
the preamp will change the loading of the preamp on the mic, which may
change the sound), and put the attenuator between the preamp output and
A/D converter input. Note that "change" does not necessarily mean
"degrade" unless you consider that any change, good or bad, is
degration.

But most A/D converters have input level controls unless yours is one
of those boxes that hooks to a computer and doesn't have any controls.
If you have knobs, use them before putting anything else in the signal
path.

  #6  
Old September 16th 06, 02:25 PM posted to rec.audio.pro
Sunflowermanuk
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Default Attenuate or run hot mic directly into A/D convertor?

> As long as the preamp isn't clipping, you might want to use it to get
> whatever sound it imparts to the mic (putting a pad between the mic and
> the preamp will change the loading of the preamp on the mic, which may
> change the sound), and put the attenuator between the preamp output and
> A/D converter input.


I considered that putting a pad after the mic pre - would these little
mic attenuators also work in that conif (on line-level signals)? If so
I guess I could make some custom leads up and run them off the
patchbay.

FWIW I'm using an RME ADI8-Pro (into a Digi ADAT Bridge/Mix system).
It has switchable +4/-10/low gain inputs, but even on the latter it's
still too hot..

Thanks guys, invaluable stuff. Jon

  #7  
Old September 16th 06, 02:25 PM posted to rec.audio.pro
Sunflowermanuk
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Posts: 3
Default Attenuate or run hot mic directly into A/D convertor?

> As long as the preamp isn't clipping, you might want to use it to get
> whatever sound it imparts to the mic (putting a pad between the mic and
> the preamp will change the loading of the preamp on the mic, which may
> change the sound), and put the attenuator between the preamp output and
> A/D converter input.


I considered that putting a pad after the mic pre - would these little
mic attenuators also work in that config (on line-level signals)? If
so I guess I could make some custom leads up and run them off the
patchbay.

FWIW I'm using an RME ADI8-Pro (into a Digi ADAT Bridge/Mix system).
It has switchable +4/-10/low gain inputs, but even on the latter it's
still too hot..

Thanks guys, invaluable stuff. Jon

  #8  
Old September 16th 06, 02:29 PM posted to rec.audio.pro
anahata
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Default Attenuate or run hot mic directly into A/D convertor?

Mike Rivers wrote:
> I doubt that any of them use better than
> 1% tolerance resistors. This could unbalance the input to the preamp
> and reduce the common mode noise rejection, but if you're feeding a
> huge signal voltage to the preamp, this isn't likely to be a concern.


Doesn't the usual configuration for a balanced pad involve no ground
connection? In that case any common mode signal will hardly be
attenuated at all by the pad, and hence there will be negligible
unbalancing of it either.

--
Anahata
-+- http://www.treewind.co.uk
Home: 01638 720444 Mob: 07976 263827
  #9  
Old September 16th 06, 02:33 PM posted to rec.audio.pro
anahata
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Posts: 163
Default Attenuate or run hot mic directly into A/D convertor?

Sunflowermanuk wrote:

> I considered that putting a pad after the mic pre


No use if the mic pre itself is clipping. Then you'll just feed an
attenuated clipped signal to the converter..

--
Anahata
-+- http://www.treewind.co.uk
Home: 01638 720444 Mob: 07976 263827
  #10  
Old September 16th 06, 02:36 PM posted to rec.audio.pro
Mike Rivers
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Posts: 8,744
Default Attenuate or run hot mic directly into A/D convertor?


Sunflowermanuk wrote:

> I considered that putting a pad after the mic pre - would these little
> mic attenuators also work in that config (on line-level signals)?


No problem.

 




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