The phantom menace
October 16, 2017 9:39 AM   Subscribe

I bought a standalone preamp to supply phantom power to a condenser microphone. But the microphone isn't working – I just get a weird loud buzz. Any idea what's going on?

The preamp is the PreSonus BlueTube (pictures here). I'm using the right power adaptor for it. The microphone is a Rode NT1-A.

I'm plugging the XLR from the microphone into the preamp input; then turning on the correct 48v phantom power button; and lastly running a 1/4 inch cable from the output of the preamp into the 4 track. (The 4 track only has 1/4 inch inputs, no XLR. Could that be the problem? But I would have thought that the XLR was only necessary between the preamp and the mic.)

Things I've ruled out: the preamp does work for guitars, i.e., when no phantom power is involved, so it's not a total dud. Also: the cable and mic themselves work elsewhere. When I use this microphone and the same XLR cable with my USB interface, the interface powers the mic.
posted by Beardman to Technology (16 answers total) 1 user marked this as a favorite
 
Does your 1/4" cable has 2 sections or 3? You want a 3 section (aka balanced, or Tip Ring Sleeve). If you're using a guitar cable no bueno. Also make sure your 4 track takes balanced audio (should say so right next to connectors)
posted by chasles at 10:13 AM on October 16, 2017 [3 favorites]


The hum is probably a Ground Loop. Does the hum occur when you're using the preamp with the guitar?

The simplest explanation for the lack of sound from the mic would be that phantom power isn't working on the PreSonus. Can you borrow another condenser mic to try?
posted by Candleman at 10:26 AM on October 16, 2017 [1 favorite]


Thanks! In answer:

Does the hum occur when you're using the preamp with the guitar?

No, the guitar sounds fine.

Does your 1/4" cable has 2 sections or 3? You want a 3 section (aka balanced, or Tip Ring Sleeve). If you're using a guitar cable no bueno.

Yes, it was just a guitar cable. However, the 1/4 output of the preamp that I used also says "unbalanced," and the 4 track inputs say nothing about being balanced. It's a Fostex from 1992 and the manual just says "Standard 1/4 phone jacks for microphones (-60dbBV) or line level signals (-10dbBV).

Newb question: if I'm running a condenser w/XLR into a preamp and then trying to take unbalanced sound out of there, is that no bueno? Does it have to be balanced all the way through from audio source to recorder?
posted by Beardman at 11:19 AM on October 16, 2017


It's less accurate to talk about XLR vs 1/4 inch than it is balanced and unbalanced audio. Both can be done on either connector, but XLRs are most often balanced. Quarter inch plugs are unbalanced when there's only two connections (the tip and the sleeve), like a guitar cable. A tip/ring/sleeve (TRS) cable could be wired to be balanced or unbalanced. A three wire cable (which most balanced cables are, regardless of what type of connector they use) can also carry stereo signals unbalanced on a single cable (like a standard headphone plug).

One other factor is that guitar cables are designed to carry high impendance signal while line level is low impdendance. You can use cables that are designed for a different impendance, but it's non-optimal.

if I'm running a condenser w/XLR into a preamp and then trying to take unbalanced sound out of there, is that no bueno?

Part of the advantage of using balanced audio is that it helps cancel out electrical interference. In general, it's better to use balanced audio when possible. The longer an unbalanced run you make, the more likely you are to get noise.

Your 4 track might have a balanced quarter inch input, though I extremely doubt it. You can try getting a low impedance unbalanced quarter inch cable and see if that makes any improvement. I don't think it will solve the mic problem, because you're not getting any proper audio from the mic, but it might improve your overall sound compared to using a guitar cable. Unless you're trying for the specific lofi sound of the old four track, I'd highly recommend trying to step up to something newer and digital, which I would not be surprised if it eliminates your ground loop as well.
posted by Candleman at 11:55 AM on October 16, 2017 [3 favorites]


Under the "One Weird Trick" category, if either/both the Presonus and the Foster have 2-prong wall warts or non-polarized 2-prong power cables (polarized meaning one prong is larger than the other), you can try flipping the power connection so each prong is in the "opposite" jack of the socket. This can occasionally fix ground loops, depending on how the device's ground scheme is set up internally.

But "loud buzz" on a condenser says "bad cable" to me, but since the cable works with a different preamp, the next guess would be that the Presonus isn't putting out phantom properly.
posted by soundguy99 at 1:17 PM on October 16, 2017 [1 favorite]


He's using a guitar cable on a balanced 1/4" output. Thats a classic "there's your problem". Get a TRS 1/4"cable (a balanced audio cable in other words) and I'll bet it works.
posted by chasles at 1:38 PM on October 16, 2017 [1 favorite]


Ugh i meant "you're"... No idea how that typoed.
posted by chasles at 1:44 PM on October 16, 2017


Presonus tech specs says the 1/4" outs are TS, so unbalanced.
posted by soundguy99 at 1:51 PM on October 16, 2017


It's certainly possible that the phantom power isn't working, but that sounds like a ground loop.
Definitely try plugging your Fostex into a different outlet; improperly grounded outlets can do all kinds of fun stuff.

Presonus tech specs says the 1/4" outs are TS, so unbalanced.

Yup, the back of that preamp it implies that the XLRs are balanced and the TSs aren't.

Nevertheless, if you look at the Manual it recommends using balanced cables for all connections, and also mentions that the XLR and TS outputs can be used at the same time, but a TS/TRS input overrides phantom power. That implies to me that you should be running either a shielded TRS cable and/or a XRL to TRS cable for the preamp-to-4 track leg. These are both cables that are worth having around anyway, so I'd probably just get one of each.
posted by aspersioncast at 3:11 PM on October 16, 2017 [1 favorite]


In general use instrument cables for instruments. Use low impedance 1/4" for connections.
Switching guitar cables with speaker cables is a classic bad thing.
posted by bongo_x at 3:44 PM on October 16, 2017


What's your 4 track? Are you sure it's not 1/4" balanced input?
posted by bongo_x at 3:48 PM on October 16, 2017


*grumble grumble*

The manual promptly confuses the issue by listing the 1/4" outs as unbalanced TS in some places, but referring to them as TRS (which could be a balanced signal) in others. . . . .

So, I dunno. The Presonus has the VU level meters on it - without connecting to the 4-track, plug in the mic and turn up the gain knob, then make a short noise, like clap your hands. Do the meters jump up quickly and then return to zero? If the meter is always showing some level of signal that could be the buzz registering, suggesting that your problem is with the mic/cable/Presonus rather than the connection to the Fostex.

Also, what happens when you push the phase/polarity reverse button on the Presonus?
posted by soundguy99 at 7:30 AM on October 17, 2017


Should note that the ground connection on a TRS phone jack will generally just be ignored by most TS inputs. Here's a pretty great explanation of how phantom power works.

Other things to check: make sure you aren't running any non-balanced cables next to or on top of likely sources of interference (power strips/wall warts etc). Also double-check your gain-staging, and start with the gain all the way down on both the preamp and four track. It sounds like your mic isn't picking up anything, but it's possible that one or more pieces or your equipment is wired to clip if the signal's too hot.
posted by aspersioncast at 7:40 AM on October 17, 2017


Should note that the ground connection on a TRS phone jack will generally just be ignored by most TS inputs.

Um, don't you mean the ring connection is ignored? Ground is the shield part of standard TRS wiring.
posted by soundguy99 at 11:47 AM on October 17, 2017 [1 favorite]


Many thanks! I'll try getting a shielded TRS cable for the preamp-to-4 track leg, and also switching power outlets and hope that solves the issue. If not, I'd be ever-grateful if I could MeMail some of y'all, in case this thread gets too old for people to be returning to.

As for the 4-track, it's an old Fostex 280 (which, yeah, I am using for the specific lo-fi sound – we track some things on the cassette before putting them into the computer). The manual is silent on the question of balanced inputs.
posted by Beardman at 8:17 AM on October 18, 2017


without connecting to the 4-track, plug in the mic and turn up the gain knob, then make a short noise, like clap your hands. Do the meters jump up quickly and then return to zero? If the meter is always showing some level of signal that could be the buzz registering, suggesting that your problem is with the mic/cable/Presonus rather than the connection to the Fostex.

I don't have it in front of me (it's at the jam space), but I think it was registering claps but not otherwise showing a buzz-like signal. So that suggests the buzz is manifesting between the preamp and the Fostex.

Also, what happens when you push the phase/polarity reverse button on the Presonus?

Nothing, alas.
posted by Beardman at 8:19 AM on October 18, 2017


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