mic preamp modification

I read in sci.electronics.design that Erik Walthinsen wrote (in ) about 'mic preamp modification', on Sat, 9 Apr 2005:

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.

I read in sci.electronics.design that Joerg wrote (in ) about 'mic preamp modification', on Sun, 10 Apr 2005:

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

I read in sci.electronics.design that rex wrote (in ) about 'mic preamp modification', on Sat, 9 Apr 2005:

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.

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.

I read in sci.electronics.design that rex wrote (in ) about 'mic preamp modification', on Sun, 10 Apr 2005:

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.

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...

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...

Hello John,

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

I read in sci.electronics.design that Joerg wrote (in ) about 'mic preamp modification', on Sun, 10 Apr 2005:

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.