mic preamp modification

[John Woodgate]

I read in sci.electronics.design that Erik Walthinsen
<omega@pdxcolo.net> wrote (in <42585EDE.3050109@pdxcolo.net&gt about 'mic
preamp modification', on Sat, 9 Apr 2005:

Problem is, the current audio runs are decent wire but are exposed runs
in the attic of the building which is the same space we'd probably be
putting the dimmer packs.

If the cables are screened-balanced, and your mixer has balanced inputs,
AND pin 1 of the XLR mic input connectors goes to the case and not the
printed circuit board, you should be OK.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[John Woodgate]

I read in sci.electronics.design that Joerg
<notthisjoergsch@removethispacbell.net> wrote (in
<ag_5e.2767$RQ7.1377@newssvr14.news.prodigy.com&gt about 'mic preamp
modification', on Sun, 10 Apr 2005:

The trick will be to figure out if there are any current products out
there with prices we can handle, that use sine-wave dimming. ...

That is tough. If you walk through a hardware store they have a wide
spread of dimming products in terms of pricing. But I never saw any
hints as to technology, except for "this one buzzes less" or so.

Dimmers for household applications still use 'phase-control turn-on',
because the other techniques cost more.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[John Woodgate]

I read in sci.electronics.design that rex <notat@hotmail.invalid> wrote
(in <v6og51l9il3o7nm2fdjoipb9rtbdie9gvq@4ax.com&gt about 'mic preamp
modification', on Sat, 9 Apr 2005:

On Sat, 09 Apr 2005 23:01:37 GMT, Joerg
notthisjoergsch@removethispacbell.net> wrote:

Hello Rex,

....and have posted it as 'mic preamp' in ABSE.
5heg51df30ubgvcvdjb4u6pfjv969b79sp@4ax.com

Maybe it's just me but I cannot see the attached file in this ABSE post.

Regards, Joerg

Sorry, when you post text and graphics there it splits it into two
messages with the same subject. I linked the text part, here is the
image part...
infg511uht42r0d8cn2hjh3pbsof9h0d1i@4ax.com

ABSE doesn't split files with attachments into two bits. Either your

news client is doing that, or something en route.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[rex]

On Sun, 10 Apr 2005 07:50:37 +0100, John Woodgate
<jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that rex <notat@hotmail.invalid> wrote
(in <v6og51l9il3o7nm2fdjoipb9rtbdie9gvq@4ax.com&gt about 'mic preamp
modification', on Sat, 9 Apr 2005:
On Sat, 09 Apr 2005 23:01:37 GMT, Joerg
notthisjoergsch@removethispacbell.net> wrote:

Hello Rex,

....and have posted it as 'mic preamp' in ABSE.
5heg51df30ubgvcvdjb4u6pfjv969b79sp@4ax.com

Maybe it's just me but I cannot see the attached file in this ABSE post.

Regards, Joerg

Sorry, when you post text and graphics there it splits it into two
messages with the same subject. I linked the text part, here is the
image part...
infg511uht42r0d8cn2hjh3pbsof9h0d1i@4ax.com

ABSE doesn't split files with attachments into two bits. Either your
news client is doing that, or something en route.

I never posted with an attachment in a newsgroup before. I notice
Phantom's postings there look like mine, so maybe it has something to do
with Agent.

 

[John Woodgate]

I read in sci.electronics.design that rex <notat@hotmail.invalid> wrote
(in <ofrh51hebv8q7runtaoaih262u8dptkjlt@4ax.com&gt about 'mic preamp
modification', on Sun, 10 Apr 2005:

On Sun, 10 Apr 2005 07:50:37 +0100, John Woodgate
jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that rex <notat@hotmail.invalid> wrote
(in <v6og51l9il3o7nm2fdjoipb9rtbdie9gvq@4ax.com&gt about 'mic preamp
modification', on Sat, 9 Apr 2005:
On Sat, 09 Apr 2005 23:01:37 GMT, Joerg
notthisjoergsch@removethispacbell.net> wrote:

Hello Rex,

....and have posted it as 'mic preamp' in ABSE.
5heg51df30ubgvcvdjb4u6pfjv969b79sp@4ax.com

Maybe it's just me but I cannot see the attached file in this ABSE post.

Regards, Joerg

Sorry, when you post text and graphics there it splits it into two
messages with the same subject. I linked the text part, here is the
image part...
infg511uht42r0d8cn2hjh3pbsof9h0d1i@4ax.com

ABSE doesn't split files with attachments into two bits. Either your
news client is doing that, or something en route.

I never posted with an attachment in a newsgroup before. I notice
Phantom's postings there look like mine, so maybe it has something to do
with Agent.

I looked at your drawings and I think the design would be improved in

terms of signal-to-noise ratio with a long-tailed pair of very low-noise
discrete transistors before the op-amp. But it's not trivial to design
this. Unless the noise causes a real problem **when the mixer is
actually in use**, I would leave well alone. Or sell it on Ebay and get
a better one.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[rex]

On Sun, 10 Apr 2005 10:36:31 +0100, John Woodgate
<jmw@jmwa.demon.contraspam.yuk> wrote:

I looked at your drawings and I think the design would be improved in
terms of signal-to-noise ratio with a long-tailed pair of very low-noise
discrete transistors before the op-amp. But it's not trivial to design
this. Unless the noise causes a real problem **when the mixer is
actually in use**, I would leave well alone. Or sell it on Ebay and get
a better one.

Probably true. Current discussion in abse seems to be that the best
compromise improvement would be to replace the first op amp with one
with slightly better noise specs.

I settled on this device because the next step up in quality that seemed
comparable was almost twice the money. As it is now, I think the only
time I would notice the noise is recording something that is essentially
quiet or using a dynamic mic with its lower signal. It just bugs me a
bit that they built in the XLR inputs but did a bad job on the preamp.

 

[Joerg]

Hello John,

Dimmers for household applications still use 'phase-control turn-on',
because the other techniques cost more.

Leviton and others also make special ones for ceiling fans because that
noise can be really nasty. These often cost >$20 versus the usual $10 or
so but I don't know if they just post-filter or whether they are really
different architectures.

Regards, Joerg

http://www.analogconsultants.com

 

[John Woodgate]

I read in sci.electronics.design that Joerg
<notthisjoergsch@removethispacbell.net> wrote (in
<3Bg6e.706$t85.485@newssvr21.news.prodigy.com&gt about 'mic preamp
modification', on Sun, 10 Apr 2005:

Hello John,

Dimmers for household applications still use 'phase-control turn-on',
because the other techniques cost more.

Leviton and others also make special ones for ceiling fans because that
noise can be really nasty. These often cost >$20 versus the usual $10
or so but I don't know if they just post-filter or whether they are
really different architectures.

Not basically different, but ceiling fans present an inductive load.
This requires the triac to be triggered differently from the way it is
in a normal lamp dimmer, and that means that a few more small parts are
necessary. Advantage is taken of this to secure a better gross margin.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Erik Walthinsen]

Joerg wrote:

That is tough. If you walk through a hardware store they have a wide
spread of dimming products in terms of pricing. But I never saw any
hints as to technology, except for "this one buzzes less" or so.
All the stuff we'd be installing are full commercial DMX-512 dimmer

packs. Some bright person at some point decided that installing a bank
of 600W max dimmers (the biggest you'll find in any hardware store for
consumer use afaict) for the 750W spotlights. In a wall box just 2 feet
from the audio conduit. A few productions with that precipitated the
purchase of real dimmer packs for those lights...

My main concern is that we get no buzz in either the acoustic or RF
domains, or we can at least isolate RF noise. Acoustic noise is an
immediate killer, can't have nearly 20kW of lights buzzing all the time
with 250-500 people in the room. Listening to a hundred filaments
literally shake themselves slowly to death is not high on my list of fun
things to do...

Did you run differentially? If not, Radio Shack has "ground loop
isolators" which seem to be little audio xfmrs at around $17 for a
two-channel. Not too bad. Muxlabs has high quality ones but they may
also cost more.
It's all differential afaik, unless some of the aux outputs to the

floor-monitor amps are single-ended. I'll have to check the mixer
(Soundcraft Spirit LX7) docs for those outputs, and the cables. All the
mic channels are balanced XLR, though some of the instruments are on
single-ended line cables. But all that really didn't help at all with
those wall dimmers... <g>