I (fairly) recently splurged and bought a DPA 4011ES for instrument recording. You can sort of view this as the big brother of the DPA 4099 I’ve been using previously.
I’ve used the 4011ES for a few sessions now and I’m still getting the hang of it (for one thing it’s very bass heavy). But I have had problems with electrical noise. For example, on this video of Grace Gillespie, if you turn up the volume at the end you’ll hear the sound of electrical interference (and this is after I’ve run a filter to remove it).
And here it is isolated:
This is obviously no good at all.
The first test
My initial thoughts on the source were the air conditioner, the studio lights, the PC power supply or possibly even the Wi-Fi router.
However, through a process of elimination in turns out the noise was all being generated by an Aukey PA-U33 5-port USB charger. Unplug the charger the noise goes away. Also the pattern of the noise is affected by how many devices are plugged into the charger at the time.
Also from this test I found the following:
- The noise was present when using the Focusrite ISA2 preamp and RME ADI-2 ADC
- The noise was present when using the Scarlett 18i20 interface directly
- The noise was not present using the Audient ASP800 preamp over ADAT
- The noise was not present or only slightly present using the DPA 4018VL (d:facto) mic on any amplifier
- The noise was not present when using a dynamic microphone
First conclusions
My initial hypothesis was electrical noise from the charger being passed down phantom power.
The 4011ES is the smallest of the 4011 family. The capsule is the same for each variant, but they come with different amplifiers. The 4011A variant has an amplifier around 15cm long where for the 4011ES it’s around 0.5cm. In that smaller package there’s going to be less room for adding filtering capacitors etc., so it would make more sense that it was susceptible to this cause (if we ignore the idea of putting the filtering in the XLR end).
The 4018VL, being a much larger microphone, has room for more filtering of the phantom power and the dynamic mic had no dependency on phantom power at all. So that all makes sense.
But that still leaves the question of why the ASP800 wasn’t affected. My guess was that for whatever reason the Audient has better internal filtering on the phantom supply. The Scarlett being affected is perhaps less surprising since it is pretty cheap but it did suggest the ISA2 was falling short.
Fixing
The obvious thing to do is to unplug the USB charger during recording, and I intend to do that. But this issue made me concerned about similar things that could occur. Perhaps something else will put some noise down the power during a recording and I don’t realise until everyone has gone home and I’m left with a comprised recording.
One option was to stop using the ISA2 for this mic, but that would be a shame as it is a better amp than the ASP in other ways, and replacing it would be a roll of the dice as I doubt anyone is really testing for this case.
Cleaning up the power to the ISA2 is another option. There are various rack mounted power conditioners which might have worked, but I also heard of them producing their own audible whine. From the hi-fi world there are various products, although you are skirting the islands of Woo. I did like the idea of the PowerPlant products which convert to DC, smooth, and then convert back to AC. These are effectively power-amps with a fixed output and a signal generator. Very nice, also very pricey.
In the end I took a gamble on a TritonAudio True Phantom.
True Phantom
The True Phantom is a high-end dedicated phantom power supply. It works on a different principle to most phantom supplies which is supposed to give advantages to microphones even in good conditions. Sound-on-sound tested it and said, I paraphrase, “you can see it’s better in the lab, but in the real world you’ll probably never notice”. I have to admit, I heard the demo files on Triton’s site and struggled to hear the difference.
But for me, it’s not about the incremental difference to a well-behaving supply, but a way of getting a clean supply in bad conditions with a sensitive microphone.
The True Phantom arrived and I set up to test again and was unable to replicate the issue. The charger, unfortunately, doesn’t always cause this problem and I haven’t definitely worked out what the trigger for it is, but I now suspect it’s when the charger is providing a low, but not zero, load.
I kept the True Phantom anyway and just carried on. That is, until I forgot to unplug the charger during a session recording and the problem reoccurred.
The second test
Obviously not pleased I’ve bought the world’s flashest phantom power supply for no reason, but here we are. But regardless, I set up another test and this time managed to get the USB charger to misbehave.
I ran various tests, but the more I tested the less sense anything made. Everything from the previous test remained true, but also:
- The True Phantom made no difference
- Recording the ISA2 via the ASP800 (instead of the RME ADI-2) with the 4011ES was fine
- With the DPA 4011ES and ISA2 the noise exists when the phantom power is turned on, but not when turned off
- …but with the DPA 4099 and ISA2 the noise exists when the phantom power is turned off, but not when the power is on
I couldn’t run the ASP800 preamp output through the ADI-2 as I don’t have the right cable, but I did use the combination of 4011ES, Behringer MIC100, and ADI-2 and that was fine.
Second conclusions
So I’m left with the situation where I can’t just blame the mic because it works fine in some setups, I can’t blame the ISA2 because it works fine via the ASP and I can’t blame the ADI-2 because it works fine when used with a different amp or mic.
There are three components in play, and changing any one of them solves the problem but together they’re vulnerable.
I won’t do anything significant for now and will just keep remembering to unplug the charger. But I might come back to this later. I have long term plans to get another two ‘good’ mic channels and that will require replacing the ADI-2, perhaps breaking the bad combination effect.