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Post by davidpedd on Aug 30, 2019 15:42:34 GMT
I need to build a pad to reduce a +4 balanced output to a 3.5mm TRS connector for a camcorder mic input. I’ve used your info in the past (Thank you!) to build a balanced-to-balanced pad, but I’m not sure on this one. From my understanding, I will need to unbalance the XLR output by not using pin 3. Pin 3 and pin 1 will go to the sleeve on the TRS. Pin 2 of the XLR will go to both TR on the 3.5. To reduce the +4 level to mic level, I’ll use the 470 ohm resistor going to pin 2. But there is no need of R2 in this configuration. Would that be correct? Below is a photo of the cable. We just need to knock the level down.  |
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Post by Ethan Winer on Aug 30, 2019 19:10:36 GMT
A single resistor probably won't do what you want because (I assume) the input impedance of the camcorder is unknown. Though if you do know that, you could calculate the needed drop with a single resistor. It's also not a good idea to connect pins 1 and 3 together. If the source +4 device has active outputs for both pins, you'll be shorting one of the outputs to ground. So I'd take the output from pins 2 (hot) and 1 (ground), and use two resistors to better predict the attenuation amount. Though the camcorder's input impedance is still a factor. This figure from my Audio Expert books shows a 20 dB unbalanced pad:  You probably need more attenuation than that, so you'll drop the 1K to half that or even less. So unless the source +4 device has an output transformer, just take the output from Pin 2. |
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Post by ruffrecords on Sept 12, 2019 22:57:57 GMT
If the driving source is transformer balanced then using either pin 3 or pin2 will not work. A balanced signal exists between pin2 and pin3. Pin1 is a shield and doesn not carry signal. The safest way to achieve what you want is to build a balanced attenuator. You could connect a 4K7 resistor to pin2 and another 4K7 resistor to pin 3. Then connect a 150 ohm resistor across the free ends of these 4K7 resistors.Connect one end of the 150 ohm to the tip and sleeve of the 3.5mm jack. Connect the other end of the 150 ohm resistor to the sleeve of the 3.5mm connector. This should work with and kind of output (electronc or transformer), it won't damage outputs and present a typical microphone impedance to your camcorder.
Cheers
ian
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Post by Ethan Winer on Sept 13, 2019 15:18:17 GMT
^^^ This is good advice, though as you say it's needed only if the driving source is transformer balanced. Most modern device outputs are active, and many drive only Pin 2, with nothing on Pin 3 except an identical impedance to ground.
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