I was testing out some cables the other day when I discovered something that I did not know: Cables will pick up more interference the closer the interference is to the source.
Here's what I did:
I was trying to test some new unbalanced cables that I made for both tone and noise pickup, comparing a two conductor cable with the shield connected on both ends vs. the shield connected at the source vs. the shield connected at the destination. I also threw in some old guitar cables that are probably just one conductor.
Here was the signal path (I tested each cable with two preamps, an A&H CMC, and a Presonus TubePre.)
AT2020 - Redco LoZ1 mic cable - preamp - cable X - Fostex VF160 digital multitracker.
For the tone tests, I played acoustic guitar. For the interference tests, I left the mic plugged in with the same gain, and put my cellphone on top of the unbalanced cable between the preamp and recorder. I then sent a text message to myself, and recorded the results for each cable.
To my surprise, the cable that "won" was an old 20' AXL guitar cable. I could barely hear the annoying cellphone noises! It even beat out a balanced cable!! (from the balanced output of the tubepre to the balanced input of the recorder) The worst cable was a 3' guitar cable, and the noises clipped the input on the recorder!
I then began to wonder if cable length had anything to do with the pickup of interference, in a way that would be contrary to what I thought before. I was going to test a few different lengths of the same cable once I made some more unbalanced, but first I got the idea to test placing the cellphone at different distances along one cable.
Sure enough, the noise clipped the input on my recorder when placing the cellphone closer to the preamp along the AXL cable that previously I hadn't heard any interference from. It got quieter and quieter as I moved the phone along the cable towards the recorder.
I don't know why this is, but it sure is interesting. What surprised me the most was that the balanced cable didn't seem to reject the noise any more than the unbalanced. Then again, I'm going to have to test all the cables again, keeping in mind that I have to place the cellphone the same distance from the preamp now.
distance on cable vs. interference pickup
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TheStevens
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Professor
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I'll offer three things to consider.
1- It's possible that your preamp is picking up the interference from the cell phone and not the cable or the recorder. If the interference showed up on the short cable but not the long cable... until you moved the phone closer to the preamp along the long cable, then that's probably the case.
2- Is the shielding on the balanced cable foil while the shielding on the instrument cable is braided, or vice versa? Foil and braid shields reject different kinds of noise, and indeed the best shield material is both.
3- It's also possible that the output on the preamp is "semi-balanced". To save money and hit a target price on equipment, some manufacturers don't use actual differential amps on the outputs of their equipment. Instead, they send the positive signal down the positive terminal, the ground connection along the shield, but instead of having the negative lead carrying an inverted copy of the signal on the positive side, they load it with a similar impedence but without the signal. At the other end, the balanced input still flips the negative and adds it to the positive to achieve common-mode rejection. And while I'm not sure of the exact reasoning behind this, my understanding is that it's not as effective at eliminating noise through CMR. Mackie did this on all their old small-format mixers, and I'm not sure if they have changed that design on the Onyx line. I wouldn't be surprised if the Presonus used a similar approach on a smaller device like the TubePre.
But that's still a good job on the testing - let us know how the next round turns out.
-jeremy
1- It's possible that your preamp is picking up the interference from the cell phone and not the cable or the recorder. If the interference showed up on the short cable but not the long cable... until you moved the phone closer to the preamp along the long cable, then that's probably the case.
2- Is the shielding on the balanced cable foil while the shielding on the instrument cable is braided, or vice versa? Foil and braid shields reject different kinds of noise, and indeed the best shield material is both.
3- It's also possible that the output on the preamp is "semi-balanced". To save money and hit a target price on equipment, some manufacturers don't use actual differential amps on the outputs of their equipment. Instead, they send the positive signal down the positive terminal, the ground connection along the shield, but instead of having the negative lead carrying an inverted copy of the signal on the positive side, they load it with a similar impedence but without the signal. At the other end, the balanced input still flips the negative and adds it to the positive to achieve common-mode rejection. And while I'm not sure of the exact reasoning behind this, my understanding is that it's not as effective at eliminating noise through CMR. Mackie did this on all their old small-format mixers, and I'm not sure if they have changed that design on the Onyx line. I wouldn't be surprised if the Presonus used a similar approach on a smaller device like the TubePre.
But that's still a good job on the testing - let us know how the next round turns out.
-jeremy
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TheStevens
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