Home and pro audio equibment interconnect

musical input

musicians typically use a mixer to connect xlr mikes to effect processors or recorders. This mike may not require phantom power, but many of the better ones do, so buy a mixer with that capability. My mixer is a unity gain Peavey, there are many other brands. They go down to as few as six inputs, that is three stereo channels. Mikes are assigned arbitrarily by you to the left or right output channels. Disco mixers and home preamps typically don't have a way of handling the balanced inputs, that is an op amp where both inputs are connected to the 2-3 of the XLR connector. For lower noise pick one that uses 4560 or 33078 or 3352 op amps on the input, not the 4558 or 741. Certain brands the schematic is not easily available online. Others, like Peavey, are fully supported by the factory. Mono mixers (one channel out) are particularly cheap now on craigslist, but the discrete ones with rotary knobs and transistor electronics are usually quieter than the early op-amp products. Anything really old will require electrolytic capacitor replacement, which is not expensive, but tedious, very DIY.
 
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First check...

to see if the wireless mic receiver has a switch for LINE/MIC output. If not you'll need a preamp with about 60dB of gain. Microphones (and wireless mic receivers) have a -55dB output. Since you need line input (+4dB) level, it will take 59 dB of gain.
Fortunately you are going bal to bal. Almost any dual opamp can be used for this. Operate each opamp separately with 30dB of gain. You'll need a bipolar power supply.
 
I'm looking for schematic of this for DIY.
Check these pcb vendors. tonepad.com the simple microamp. He is in el salvador and charges $2 shipping. Elliott Sound Products - The Audio Pages (Main Index) project 88 and others. He is in australia and charges $12 packing plus freight from there. You'll have to buy the XLR jack yourself. Make sure you pick a project where the + of the op amp is hooked to one input and the - of the op amp to the other. Tonepad references 741 op amps, but 4560, 33078 NE5532 have the same pinout in the dip package. You'll find more help on the analog input forum, lots of diy threads there. You pick the gross gain of the opamp by the ratio of the input resistors to the feedback resistor- Elliot has a popup table on it for his unit. Then you adjust with pots. Note the tonepad stuff seems to lacks power bypass caps, easily and cheaply added but you have to think.
 
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