Friday, February 22nd, 2013 | by matthew mcglynn
You’ve probably seen engineers hang tube condenser microphones upside down. Do you know why they do it?
Maybe it’s just because of old studio photos of Frank Sinatra.
The explanation I’ve heard, probably from some totally reliable source like one of the audio forums, is that microphone capsules will last longer if they’re not continually baked by the heat rising from the tube. Hang the mic capsule-end-down, and the capsule won’t be in the in the proverbial line of fire. It’s an easy thing to do, and if it was good enough for Sinatra’s U47, it ought to be good enough for all of the cheap tube mics I own that “sound just like the U47.”
The problem is, this explanation doesn’t really make any sense. The capsule deck in every tube mic I’ve seen is sealed off from the chamber where the tube is. Plus, the headbasket is vented to the outside air. Does the capsule really get any warmer due to tube heat?
I don’t believe so, but as they used to say in Chicago, that and a dollar will get you on the “L.”
There’s one other factor worth considering: the electrolytic capacitors found in most audio gear have a useful life span that is inversely proportional to operating temperature. The hotter they run, the sooner they die. So, maybe we shouldn’t be asking so much about the capsule temperature as the PCB temperature.
(Who am I kidding? Of course we’re going to ask about both. But jump to the TL;DR (summary) if you’re not up for a nerdy elaboration on microphone component temperature.)
Tube Mic Temperature Test
I hung a temperature probe in an Apex 460. The sensor was laying alongside the PCB. I let the mic warm up for several hours in each position.
PCB Temperature – with mic inverted (tube above PCB)
Ambient temperature: 72.5° F PCB temperature: 90.0° F Delta: 17.5° F
PCB Temperature – with capsule at top (tube below PCB)
Ambient temperature: 70.9° F PCB temperature: 96.1° F Delta: 25.2° F
What about the capsule? I didn’t expect much of a delta, for the reasons stated previously. The capsule deck has small holes drilled into it to allow passage of the capsule leads to the PCB. Otherwise, the deck seems like a pretty effective firewall. Nonetheless, let’s test and see:
Capsule Temperature – with mic inverted
Ambient temperature: 68° F Capsule temperature: 78.4° F Delta: 10.4° F
Capsule Temperature – with capsule at top
Ambient temperature: 67.3° F Capsule temperature: 82.4° F Delta: 15.1° F
Inverting the Apex 460 keeps the capsule ~5°F cooler.
Will these results be consistent for every tube mic? No, of course not — it depends on the heat output of the tube, the presence of ventilation ports, and the interior structure of the mic’s circuitry and chassis.
Regarding the last point: some mics put the tube between the capsule and the PCB. With such mics you have to choose whether to cook the circuit or the capsule.
The experts weigh in
I asked some microphone designers for their thoughts on whether tube mics should be used upside down.
Dave went on to analyze the power output of various tube models — and to suggest a much more practical explanation for why Frank’s U47 is usually pictured upside-down:
With the Mojave microphones, the power being dissipated inside the microphone is just under a watt.
With a microphone using a 12AX7 or similar dual triode, with both halves of it in operation, the power dissipation would be just under two watts.
As for the Neumann U 47, the VF 14’s heater and its dropping resistor … dissipated 4 watts between them. With the U-47, it made sense to have them upside down.
But I think that an even more important reason for U-47’s being used upside down was to keep them out of the way, and well out of range of a singer turning the pages of a score.
I also asked Dr. Charles Chen, the lead microphone designer at Lauten Audio.
The tube will heat the air; as the hot air rises, it will increase the temperature of the air [surrounding] the capsule if the capsule is above the tube. Theoretically this will change the tension of the diaphragm a little bit, and therefore its frequency response. But the slight frequency change due to the temperature change might not be detected, and even if it is noticeable, we cannot say whether it is good or bad.
Dr. Chen noted that inverting the mic will change the recorded signal due to changes in reflection and refraction of the incoming audio waves. But, as with the diaphragm tension, we can’t predict whether this would generally be perceived as a beneficial or detrimental change.
As if reading my mind, he also mentioned the issue of capacitor temperature:
The life of electrolytic capacitors, compared to the other components on the PCB, is most sensitive to high temperatures.
But how sensitive?
The lifespan of electrolytic capacitors
We’ve come this far. We might as well get our geek fully on. We already know that the lifespan of electrolytic capacitors is determined significantly by operating temperature. Arrhenius’ Law of Chemical Activity suggests that the lifespan of a capacitor doubles for every 10 degree Celsius decrease in temperature.
For the Apex 460 specifically: the PCB measures 8°F cooler when the mic is inverted. According to the capacitor lifespan calculator linked above, there is a nontrivial extension of usable life due to this 8°F drop when the mic is inverted — something like 20,000 hours more, given a 4°C drop in operating temperature.
That said, even at a higher temperature, the capacitor for which I ran the equation will live 60,000 hours. That gives you more than 20 years of daily 8-hour sessions before you have to find a replacement capacitor.
Should you worry about the orientation of your tube mic?
As in so many cases, the answer is: it depends. But in general, probably not.
If your mic uses a subminiature tube, as found in the Mojave mics, orientation probably makes no difference to lifespan, because the tube isn’t putting out enough heat to matter.
If your mic uses a hotter tube, and the tube is not isolated from the PCB or capsule, then orientation might help the mic last longer. But you won’t live long enough to notice.
Unless you keep your mics powered up 24×7, I think the mic’s orientation makes no practical difference to lifespan.
Regarding the capsule, I think it is even less likely to be affected by tube heat. The test results show a ~5°F delta depending on the mic’s orientation. I can’t prove it, but I suspect one “spitty” singer does more damage to your mic’s capsule than a 5°F temperature change.
In summary: although your mic’s components might theoretically live longer if you orient them so the tube heat goes in the other direction, those same components will probably already outlive your recording career. Therefore, you should mount the microphone so that the artist is unimpeded in delivering an inspired, energetic performance. This outweighs technical considerations.