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Old 10th January 2012, 05:07 AM   #1
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Default Active Mic splitter

I would like to build an Active Mic splitter. 1input/ 2out. All connections should be balanced. Does anyone have a schematic??
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Old 13th January 2012, 03:46 PM   #2
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Why active? Are you intending to have gain in there? And should the outputs be totally isolated (for example, splitting to an external recording console and a PA system)?

I had a twenty four channel rack of one input four out splitters (FoH PA, monitors, recording truck and TV truck) which had transformers everywhere, and still had to have pad switch, polarity invert and phantom power for each input, ground lift and pad for each output. The flight case weighed as much as an amp rack. For a mere two way I'd suggest a splitter transformer; a few dBs of loss on each output, but a lot less to go wrong and no power supplies to get kicked out on stage, lighter and a lot cheaper…

The trouble is that with active electronics your best noise figure is with a certain amount of gain, so the circuit which is best for low-level signals distorts on really loud ones, while the optimum for screaming rock hisses on solo acoustic sets.

If the following stages are guaranteed balanced it's fairly easy to do a balanced driver for each line on a twin op amp; if it's a bit of an unknown, you need a servo balanced stage, taking up potential imperfections on the destination lines, which is quite a bit more difficult to build (particularly at minimum noise levels). Either way, the outputs must be protected against 48 volt phantom.

The input stage is a mic preamp. Perhaps a bit less gain range than most console preamps, but any noise or distortion here, or lack of slew rate, or compromised frequency response, will be reflected to both of the outputs, and to the final results. It ought to have somebody next to it, and a visual indicator of its status, unless it's remote controlled. So it is neither cheap, nor simple.

Between the two, decisions about what's useful, and outside them things like power supplies (not trivial) while a good transformer, while not cheap, leaves you only wondering about the layout of the box.
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Old 13th January 2012, 03:51 PM   #3
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Look into Deanne Jensen Transformers in Evanston Illinois. He died recently, I hope the company went on. Best mic splitter transformers in the business.

Otherwise the quality of the pre-amp is truly critical and a very personal issue of much debate.
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Old 13th January 2012, 09:40 PM   #4
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Actually, you can "split" without a special transformer. You wire one path stright-thru and the other isolated via the transformer, with the transformer primary in parallel with the straight-thru. You usually keep on line wired straight-thru. If you have a transformer with 2 secondary windings, you have 1 input and 3 outputs.

This is probalby the link you want:
JENSEN TRANSFORMERS, INC. - COMPANY BACKGROUND
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Old 13th January 2012, 09:43 PM   #5
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Jensen transformers aren't cheap, but their shields are excellent, low RF pickup, good isolation, etc. Even if you make a good active splitter, it's still best to get tru balance and really good DC rejection via one of his input transformers, and you can get great performance by using one that doesnt have too much of a turns ratio. Some then would even recommend one of his "BM" or "Big Mac" output transformers for each of the dual outputs, again giving the best floating balanced operation and optimal output impedance. For real pro PA work transformers are good things, not bad. They're essential if the parking lot has big trucks with big CB linears, and the lighting dimmer packs are putting square waves on all the AC lines which are crude generator or inverter outputs when at their best. They can even tell you exaclty how to make the perfect network to receive or drive long lines of your choice. "low impedanc" has severl alternate specs. "Balanced" also has different performace parameters.

If you split it with a really good transformer, you're probably not going to overload it and maybe a T-pad switch will suffice on the input side, and it leaves all the voodoo of the input stage of the mic preamp to the various destinations. If you go active splitter, everyone enjoys or suffers the benefits or detriments of your pre-amp.

If you're serious, look at Jensen's 990 discrete op-amp or build one yourself to use with his transformers. The design is gettiing old but still excellent.

Last edited by cyclecamper; 13th January 2012 at 09:58 PM.
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Old 22nd January 2012, 01:05 PM   #6
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If your mic splitter is intended to split for FOH and a on stage monitor board you can make it easy on yourself by just using a hardwired split. My 24ch. split snake is hardwired, I did put pin 1 lift switches on the monitor side of the spit but in the past 12 or so years using that snake I can only remember two times where I had to lift any channels because of ground noise. It does help that that any time I have a system out large enough to need the split for on stage monitors everything is powered off of the power distro. If the split is going to be used feeding something in a complete remote location off of a different power source then a transformer isolated out on each channel could be a good idea. Like has been mentioned an active splitter will incense become your mic preamps, any gain adjustments, pads and phantom power will have to be applied at that point. Also keep in mind that an active splitter is another point of failure and if it does fail you loose a lot of channels at one time!
Goog transformers like Jensen, Sowter, Lundhal will be expensive and start to get heavy and bulky if you talking about many channels.
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