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Re: Spaced Omni's for Stereo + MKH110?

Subject: Re: Spaced Omni's for Stereo + MKH110?
From: Walter Knapp <>
Date: Sun, 12 Jan 2003 15:33:19 -0500
Tom Thorley wrote:
> Hi all,
>
> I made a post a few weeks ago about a pair of DPA omni's I have just boug=
ht,
> about whether they were well matched enough. Thank you for all your respo=
nses
> you have allayed all queries - time to get out there and put them into ac=
tion.
> This post is one asking for the experience of those who have employed omn=
is in
> a stereo rig before.
>
> 1. Spacing - how far apart (interested in whether you tend to used fixed
> spacing or need to change to suit the location + what results/artifacts a=
re
> obtained with various different spacings)
>
> 2. Angle - do you mount them forward firing or angled out (if so at what =
angle)
> and what are the different results obtained by using different angles.
>
> 3. Practical - what is the best way to mount them. Should it be a fixed
> mount/one with variable angles/lengths re. qq. 1+2. If it needs to be var=
iable
> does anyone have any suggestions for a setup based on a tripod and two ry=
cote
> windshields for a stereo bar that might be attatched to a tripod with it'=
s
> screw and then have slideable/angleable (easily used in the field and
> recallable if I like a particular setting for a particular location) moun=
ts
> with the appropriate thread for rycote's. (the DPA bar besides being very
> expensive does not have the right threads and is designed for indoor use)=
. I do
> not have access to a machine shop.
> Are there any other solutions that you might recommend?
>
> 4. Technique - My thoughts are to go for a spaced pair technique (I alred=
y have
> a really nice binaural setup) are there any other techniques that anyone =
feels
> I must try before settling on the spaced pair setup?
>
> I would be very grateful for any suggestions and eagerly await your respo=
nse's

I would suggest if you are going to get into this that "The New Stereo
Soundbook" might be a nice book to have. It goes into details about all
sorts of setups without being too technically dense. I'd have to
duplicate at least a couple chapters out of that to fully respond to
your questions. Here's a link:
http://www.stereosoundbook.com/

1) a number of spacings have been used. So this is not a fixed solution.
In general the greater the spacing the more problems with mono
compatability due to phase differences. And as you space the mics out
you will eventually develop a "hole in the middle" of the stereo field.
And if too close together you don't get much of a stereo field.

2) If it's a true omni, angle does not matter, by definition they pick
up the same from all directions. Should they have any deviation from
true omni, then point the axis of the mic at the main subject.

3) I would think for omni's without a central barrier you would need
them far enough apart as to require separate supports and so on. Even a
stereo bar would be a bit short for outdoor work. For spaced omni's the
spacing inside is dependent on the acoustics of the space and is
critical. Typical for a concert hall might be at least a couple meters.

Stereo bars are usually used with cardioid mics, which is a different thing=
.

4) I would suggest looking into Jeklin Disk or SASS as another approach.
The original modified SASS was a Crown model designed to take the DPA
mics. Lang Elliott did not like the quality of the noise floor of the
DPA and substituted the MKH-20's for his. Mine are based on the same
idea. Crown no longer makes the SASS model that took the DPA, so you
would have to modify a standard housing from their parts department,
just like Lang and I have done.

Jeklin Disks are available commercially, but are pretty simple to make
yourself.

> Lastly a brief one for Walter or anyone else with experience of the Sennh=
eiser
> MKH110. I do not have a background in electronics but I understand it is =
not
> phantom/T powered is there a readily available commercial solution to pow=
ering
> it? If not I would be incredibly grateful for an idiots guide to powering=
 this
> mic.

A MKH-110 needs 8 volts plus/minus 1 volt, which is not phantom
powering. It's also not T-power. It's a special setup. It's not even the
same as other MKH mics of the period.

On rare occasion you may see offered the Sennheiser power supply that
ran this. Usually the battery one that used a whole stack of mercury
batteries in a small inline holder. There also used to be a third party
power supply, I've never seen one of these.

The mic is easy to power, however. If you go to my SASS/MKH-110 page
there is a link to a scan of Sennheiser's powering diagram:
http://frogrecordist.home.mindspring.com/docs/sass_mkh110.html

For my SASS I duplicated the 8 volt supply using a 9.6 volt NIMH battery
for the source. (it can also use a regular 9 volt Alkaline throw away)
That puts out about 10 volts at full charge, so the dropping resistor
needs to take off 1+ volt to meet specs. The current draw is about 8ma
if the supply voltage is exactly 8 volts, I used a 1K trimpot and it's
adjusted somewhere around 180 ohms. Using a voltmeter, I set the
initial, fully charged voltage at 8.8 volts to give the longest margin
as the battery discharges. I'm using a 470 ufd cap in the power supply,
that makes powering up/down more gradual. I did not put a zenar diode in
as I could not find a 8 volt one locally.

The Sennheiser diagram shows a 2nd 5 volt battery, but I could not
figure out how that went and just left it out. It appears to be
optional. Note that pin 2 (output +) has 5 volts DC relative to pin 1
(ground/output -), so you have to put a cap in to isolate the DC and
just pass your signal. I ended up putting two .47 ufd mylar caps, one on
each line out.

The third issue I had is if using them as pairs, you cannot have the
housings of the two mics electrically connected. I end the shielding at
the connector and let the mic housings float. If you don't do this the
two mics interact and produce a nasty whine in the output. I also have
dual power supply circuits, only the battery is in common. A single mic
will run ok with the housing connected to the cable shield.

Walt




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