Mic noise due to noisy phatom power?

"Chris Whealy" <[email protected]> wrote in message
news:g2orff$q1s$2@news.sap-ag.de...
> Been doing a recorded talk show where the guests wear Sony ECM-77 lavs
> plugged directly into the back of a Yamaha 02R desk. We've recently
> purchased a Focusrite ISA 828 8 channel pre and are now running the
> ECM-77s into this.
>
> There is now a significant improvement in general mic hiss - the
> background noise is almost inaudible whereas previously with the 02R it
> was audible.
>
> I'm taking a guess here, but all I can put this down to is that the
> Focusrite produces a much cleaner phantom power voltage than the 02R does,
> and consequently the mic signal sounds less noisy. Is this a correct a
> deduction or is there some other cause for the signal improvement between
> the 02R and the ISA 828?
>
> Chris W
>
> --
> The voice of ignorance speaks loud and long,
> But the words of the wise are quiet and few.
>

Not the phantom. The phantom appears equally across both legs, which nulls
the effect. The common method of delivery is through a pair of 5.6K
resistors, one on each leg.

I'm wondering if it may be a gain-structure issue. The Focusrite has less
internal noise, but the Yamaha certainly should not cause audible hiss,
unless there's something in the setup that's knocking down input to the 02R
A/D...? Could those ECM-77's be clogged?

Bob Morein
(310) 237-6511

"Chris Whealy" <[email protected]> wrote in message
news:g2orff$q1s$2@news.sap-ag.de

> Been doing a recorded talk show where the guests wear
> Sony ECM-77 lavs plugged directly into the back of a
> Yamaha 02R desk. We've recently purchased a Focusrite
> ISA 828 8 channel pre and are now running the ECM-77s
> into this.

Seems unecessary, as IME the mic preamps on Yammy digital desks are pretty
good.

> There is now a significant improvement in general mic
> hiss - the background noise is almost inaudible whereas
> previously with the 02R it was audible.

I'm surprised that the 02R mic preamps were noisy with electret mics, which
usually have much more output than say, dynamics.

> I'm taking a guess here, but all I can put this down to
> is that the Focusrite produces a much cleaner phantom
> power voltage than the 02R does, and consequently the mic
> signal sounds less noisy.

Unlikely.

> Is this a correct a deduction
> or is there some other cause for the signal improvement
> between the 02R and the ISA 828?

The 02R is a pretty old mixer, even the sequel 02R96 is out of production.
Maybe it is broken?

Also, the patching on a digital mixer can be non-obvious. Maybe you need to
re-initialize the 02R, and set it up clean?

Arny Krueger wrote:
> Seems unecessary, as IME the mic preamps on Yammy digital desks are pretty
> good.
>
>
That's what I thought until I plugged the Focusrite ...
> The 02R is a pretty old mixer, even the sequel 02R96 is out of production.
> Maybe it is broken?
>
> Also, the patching on a digital mixer can be non-obvious. Maybe you need to
> re-initialize the 02R, and set it up clean?
>
The input routing for the particular channel takes the mic input (AD21
in this case) and routes it straight to the Stereo out.
How would a factory reset would help here?

Chris W

--
The voice of ignorance speaks loud and long,
But the words of the wise are quiet and few.
---

"Chris Whealy" <[email protected]> wrote in message
news:g2p3bb$kva$1@news.sap-ag.de
> Arny Krueger wrote:
>> Seems unecessary, as IME the mic preamps on Yammy
>> digital desks are pretty good.
>>
>>
> That's what I thought until I plugged the Focusrite ...

>> The 02R is a pretty old mixer, even the sequel 02R96 is
>> out of production. Maybe it is broken?

A completely different tack - I've had mics that were noisy when the phantom
voltage was not in a certain range. It was a fault of the mic and eventually
corrected by the manufacturer. But, this fault would make a lot of noise on
consoles where the phantom power was near the top end of standard spec. On
consoles with a lower phantom voltage or one that was even substandard, the
mics were very quiet.

It is very likely that the phantom voltage from different consoles will be
slightly or significantly different.

Chris Whealy <[email protected]> wrote:
>Been doing a recorded talk show where the guests wear Sony ECM-77 lavs
>plugged directly into the back of a Yamaha 02R desk. We've recently
>purchased a Focusrite ISA 828 8 channel pre and are now running the
>ECM-77s into this.
>
>There is now a significant improvement in general mic hiss - the
>background noise is almost inaudible whereas previously with the 02R it
>was audible.
>
>I'm taking a guess here, but all I can put this down to is that the
>Focusrite produces a much cleaner phantom power voltage than the 02R
>does, and consequently the mic signal sounds less noisy. Is this a
>correct a deduction or is there some other cause for the signal
>improvement between the 02R and the ISA 828?

Nope, it's just a better mike preamp. Might have something to do with
the load impedance on the mike too.

The thing about phantom power is that it's applied to both leads in
parallel, and the signal is applied to both leads differentially. So
if the mike is perfectly balanced and the preamp is perfectly balanced,
you can have a huge amount of noise on the phantom supply but because it
is common mode it all cancels out.

Back in the days of transformer-coupling, it was reasonable to assume
everything was perfectly balanced, too. It isn't really any longer.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Scott Dorsey wrote:
> Nope, it's just a better mike preamp. Might have something to do with
> the load impedance on the mike too.
>
The impedance of the 02R input is 3K ohms and the ISA 828 has four
variable settings (600, 1k4, 2k4 and 6k8). When the ECN-77s are plugged
into the Focusrite, I set the impedance to 2k4.

So what actually makes a good pre-amp?

A friend of mine who is alot better at electronics than me thinks that
most of the hype surround top end pre-amp is just marketing BS. His
explanation for this opinion is that at the end of the day, there are
only a very small number of decent op amp chips for manufacturers to
choose from, so he reasons that the amp stage of all these pre-amps are
all much of a muchness.

I can't say I agree with this opinion, but then I don't know enough
about circuit design to say if its right or wrong.

I dare say that Neve and SSL charge a premium for their products because
they know people will pay not just for the quality, but also the
reputation that goes with the name. So there is probably some hype in
there, but I just don't know how much. 80:20 hype to technical quality?
60:40? 50:50?

I know what my ears tell me when I listen to a good quality pre and a
cheap one. But are the components in the top end pre's circuitry that
much better to warrant the much higher price tags? I know you could use
only the best components with the lowest tolerances, but is that enough
to warrant a price jump from £50 a channel to £500?

(BTW, in the UK the Focusrite ISA 828 works out at just over £200 a
channel, which given its quality, I would say is pretty good value for
money)

Could someone enlighten me here...
> The thing about phantom power is that it's applied to both leads in
> parallel, and the signal is applied to both leads differentially. So
> if the mike is perfectly balanced and the preamp is perfectly balanced,
> you can have a huge amount of noise on the phantom supply but because it
> is common mode it all cancels out.
>
OK, that makes sense.
> Back in the days of transformer-coupling, it was reasonable to assume
> everything was perfectly balanced, too. It isn't really any longer.
>
>
Do you have an example of where such an assumption would turn out to be
false? Budget equipment?

Chris W

--
The voice of ignorance speaks loud and long,
But the words of the wise are quiet and few.
---