0
\$\begingroup\$

I have more than 10 XLR microphone cables from different manufacturers (some of them quite cheap, some of them more expensive), and as I often do recordings (with condenser microphones) of music / audio samples that have a low SNR (signal to noise ratio), I'd like to do my best to pick the right audio microphone cables.

How to measure the quality of such cables, and have a non-subjective quantitative measure of its quality?

Here is the workflow I'm using for now:

  • Generate a 10-second long 20 Hz - 20kHz frequency sweep (something like this)
  • Play it with Soundcard 1 (that is reliable)
  • Plug the XLR audio output of Soundcard 1 to XLR audio input of Soundcard 2
  • Record the sound with Soundcard 2
  • Analyse the spectrogram of the recorded signal

enter image description here

I get such results:

enter image description here

It looks good, but:

  • Can I be sure that an XLR that is ok to transmit audio signal from Soundcard 1 to Soundcard 2 at "line level" will also be good for microphone signals?

  • I don't want to involve the microphone in my workflow because everything would be dependent of the ambient noise in the room where I do the tests, and then it won't be very reliable (I tried, and even quiet footsteps from the guy on the floor above changed the low frequency charts!)

Any recommendation?

Note: I sent the audio swipe at various levels to Soundcard 1: -12 dBfs, -24 dBfs, ..., -80 dBfs, but what would be an audio level/voltage that mimics "microphone signals"?

\$\endgroup\$
2
  • \$\begingroup\$ You would have to simulate a microphone output impedance as well. What you are testing is between two line level devices. A microphone would generally have a really high output impedance, though condenser mics have some amplification and driving circuitry in them. \$\endgroup\$ – Justme Nov 9 '20 at 19:36
  • \$\begingroup\$ @Justme How to simulate this mic impendance? I don't want to use a real microphone here, because it would change the value I'm measuring with out-of-topic values (ambient noise of the room, specs of the mic, etc.) \$\endgroup\$ – Basj Nov 9 '20 at 19:41
2
\$\begingroup\$
  1. No. Pad the line level signal down by 40 to 60dB, and amplify it after the cable in a decent professional mic amp (which you have previously tested) set to the same gain as your pad, not the "mic in" on your soundcard.

The pad should present low (maybe 20 or 100 ohm) and 600 ohm impedance to the cable.

In addition to your prescribed test:

  1. Repeat with 48V DC common mode to represent phantom power.

  2. Check for microphonics. Especially with phantom power applied, and no signal, jump on the cable, flex it around, and hit it with a hammer. You shouldn't hear any thumps or noises : the cable itself isn't meant to act as a microphone.

  3. Run a long cable from a thyristor dimmer at half power to a 5 kW studio spotlight, twisted several times around the cable under test. Note the level of buzz picked up in the cable. Probably only "star quad" cable (allegedly developed at the BBC where it's known as "blue quad") will be satisfactory on this test. That may or may not matter depending on your recording location.

\$\endgroup\$
8
  • \$\begingroup\$ About 2., I could enable Phantom power 48V on Souncard 2 (that has a very good preamp inside, with Phantom power), but then it would send 48V to Soundcard #1, which probably won't like this! Could you add more details about how to do this properly? \$\endgroup\$ – Basj Nov 9 '20 at 19:43
  • \$\begingroup\$ About 1., I played the audio in Soundcard 1 with -60 dBFS and -80 dBFS too, so this has probably the same effect than a pad of order of magnitude of 60 dB, is that right? (of course, it depends on Soundcard 1's DAC) \$\endgroup\$ – Basj Nov 9 '20 at 19:47
  • \$\begingroup\$ No, lowering the amplitude lowers the amplitude, impedance stays the same. I would not enable the phantom power unless I know the output circuitry can survive it. \$\endgroup\$ – Justme Nov 9 '20 at 19:57
  • \$\begingroup\$ @Justme you say "No" about which comment? (I didn't say it will lower the impedance, did I? If so, my bad!) \$\endgroup\$ – Basj Nov 9 '20 at 20:01
  • \$\begingroup\$ @Justme how to lower the impedance, to simulate a mic? (but without using a mic) \$\endgroup\$ – Basj Nov 9 '20 at 20:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.