A cord question

[RAK]

I'd bet you a weeks salary you'll end up with two results in terms of sound quality by doing this:

1) Absolutely no change

2) horrible badness/poor performance due to bad wiring.

And I'm only refering to sound quality here, not the possibility of frying gear (which could also happen I suppose)

There's nothing in this scenerio that makes any sense as to why the audio quality would change other than that. You're not affecting the signal chain in anyway.

Now if you wire one of the cables to be 45 degrees out of phase or delay the signal of one of the mics or something like that, you might come up with something funky.

sjjohnston is right on.

 

[SouthSIDE Glen]

originally i thought the same thing, however in this post https://homerecording.com/bbs/showpost.php?p=2041769&postcount=16

both farview and glen explained that two of the same thing is still mono, soooo, why bother effecting the two different, just take two mics (sources) and amplify them equally. Maybe i'm missing a finer point of what they said, but it seems like an interesting thing to try.

Whoa whoa whoa. First, the two mics in a coincident pair are not the same thing. If they were there'd be no point in having two mics! The mics are pointed to different sources; everything on the left, and everything on the right

Wiring two mics to a single preamp is not a great idea, and there *is indeed* a risk of damage along the way. Both the amperage and the resistance of the circuit will be affected by running the mics in parallel, whether one is using phantom power or not. While the risk may not be huge in a well-built preamp and well-buit mics, it is nevertheless there. If you're going to experiment, do it with equipment you don't mind frying.

If you want to know what it would sound like, that's easy; just mix the two signals mono signals together into a single mono out either in your mixer or in an editor. That's all there is to it.

If you're really doing it because you're trying to save on preamp costs by recording two sources thorugh a single preamp simultaneously, I wouldn't recommend it. Taking the chance of having to pay for the repair of the first preamp will just delay the acquisition of a seond preamp just that much longer.

G.

 

[brendandwyer]

cool. thanks glen...and thanks for the clarification on the other post

 

[xstatic]

RAK, I am curious. How do you wire a cable 45 degrees out of phase?

 

[sjjohnston]

For some reason, two situations occurred to me in which normal equipment does mix two mic-level signals before hitting a preamp. Further explanation (and some further digressions) below.

Example 1: Mid-Side Stereo Mics

As most of you probably know, the way you produce a stereo signal with a mid-side (aka MS) technique is to use two coincident mics:
- bidirectional (figure 8) mic, positioned sideways to sound source, with "front" pointing to the left;
- cardiod (typically) mic, pointing directly at sound source.

The left channel is a mix of the cardioid and the bidirectional mic
The right channel is a mix of the cardiod and the bidirectional mic, with the polarity of the bidirectional signal inverted.

There are some stereo mics (i.e. single units which output a left and right stereo signal) which use an MS array. Obviously, they're mixing both signals into both outputs, and doing it at mic levels. Sometimes they have a little switch or knob that "widens" or "narrows" the stereo effect, by changing the mix of the bidirectional and center mic: relatively more bidirectional to be wider, less to be narrower.

When recording MS with two separate mics, you'd normally record them to two separate tracks, then mix (or "matrix") them appropriately after the fact. There are some decoding boxes out there that you plug two mics into, and they produce the matrix themselves and output a L and R signal. Typically, the "matrixing" would be built into a preamp, but I think there are some which do it at mic level.

Example 2: Your Run-Of-The-Mill Dual Diaphragm, Multi-Pattern Mic

This one's a very common design, which goes back to the U47. Most of the brand lines include at least one. The mic has two cardiod diaphragms back-to-back, and by mixing their output into a single channel, can simulate the standard mic patterns, and variations thereon.

Of course, the dual-diaphragm mics mix the two signals with a carefully-designed active circuit in the mic (the active part is what you need the phantom power, or big power supply in the case of a tube mic, for). They don't just have a Y-cable inside of them. Same for the stereo mics (I guess ... I think at least some of them are unpowered, though). Plus, the stereo mics matrix the signal as discussed above.

Something You Might Want To Do

If you don't have a bidirectional mic but you do have a handful of cardiods, and want to experiment with MS technique, you can do it with three mics:

- 1: cardioid positioned sideways to sound source, pointing left.
- 2: cardioid " " ", pointing right.
- 3: cardioid pointing directly at sound source.

The M signal will, of course, be from mic 3. The S will be from mic 1, combined with the polarity-inverted signal from mic 2. The easiest and tidiest way to do this would be to record the three mics to three separate tracks, then matrix them after the fact. If you were trying to save tracks, you might record 1 and 2 together. The preferable way to do this, of course, is to run the two S mics through two mixer channels, and combine them to one track with the mixer.

One other pretty irrelevant note: it occurs to me that the Symetrix 202 dual-channel preamp, has three output jacks on the back: one is one mic channel, one is the other, and one is the two combined. Presumably that last is there for some purpose (though I'm not totally sure what).

 

[sjjohnston]

there *is indeed* a risk of damage along the way. Both the amperage and the resistance of the circuit will be affected by running the mics in parallel, whether one is using phantom power or not.

I don't see the risk, at least so long as you limit yourself to two unpowered (let's just say dynamic) mics.

Consider what would happen if you didn't use the preamp at all, and just connected two mics to each other (which you could do, e.g. with a F-F XLR coupler, if you have such a thing). Yell into one all you want ... it's not going to kill the other one or, for that matter, do much of anything. It will drive the other diaphragm like a speaker ... sort of: a very inefficient speaker being driven by a very low-powered source. Which will, in practice, not be very much like a speaker at all. (Or consider what would happen if you just took two speakers and connected them together. Anything bad? No. Anything at all? Not really.)

It's hard to see how introducing the preamp input into your two-microphone circuit poses any danger. Of course, if there's something wrong with the preamp, it could. Then again, if that were the case, it would introduce the same danger when you connected one mic to it.

If you want to know what it would sound like, that's easy; just mix the two signals mono signals together into a single mono out either in your mixer or in an editor. That's all there is to it.

In the non-hypothetical world, that's really the bottom line, though.

 

[sjjohnston]

RAK, I am curious. How do you wire a cable 45 degrees out of phase?

Yeah, I was kind of wondering about that too.

Presumably with a high (and fixed) frequency signal and/or one cable that's a lot longer than the other.

 

[SouthSIDE Glen]

It's hard to see how introducing the preamp input into your two-microphone circuit poses any danger.

When plugging the two microphones together in a very refined version of what Alexander Graham Bell did a long time ago one is creating a series circuit (all resistance occrus to the overal circuit in series - ignoring any parallel curcuitry that may be within the microphones itself.)

When plugging two mics into a sigle preamp via a simple Y jack, one is now creating a parallel resistance circuit which changes the calculations. In effect, the total resistance of the circuit at the preamp input is now cut in half.

One SM57 plugged into a lowZ preamp input presents a rated load of 150 ohms. This is not uncommon for dynamic microphone design. Two of them plugged in parallel will present only a 75 ohm load. Here's where one needs to check the preamp design; many more robust (but lesser quality) preamps may be rated to handle loads as low as 50 ohms or lower, whereas other preamp designs (usually the "better sounding" ones) are often not rated any lower than 150 ohms and often are designed to perform better at higher impedances than even that.

I'll grant you that we are talking low voltages here, which is one of the reasons why I say the risk of damage is not super-great, but the risk is there.

The spirit of curiosity/expirimentation is admirable, no question. But when one is expirimenting on something already known (it's a hell of a lot easier and cheap to look this simple stuff up than it is to make a Y cable), one should sseriously question whether disrespecting the equipment - which is designed and meant to be used as performance gear, not test equipment (yes there is a fundamental difference) - by pushing or exceeding their intended design and design specs is really that great of an idea.

G.

 

[RAK]

xstatic. I just made that up. Obviously there are barrels that will flip the phase 180, so maybe you can tweak something to do a 45 degree phase shift, but I don't have the knowledge of how those are actually built.

This thread got split in two because of the initial confusion over the first post. As has already been clarified, he's not interested in a stereo recording of any kind, he just wants to experiment with two mics in one cable because it's something to experiment with I guess.

If you want to experiment with two mics (or 100 mics for that matter) on one source, then just do it independantly, and record them on their own tracks, or mix it together to 1 track. You'll get the same effect without the danger or frying anything or getting poor performance

 

[RAK]

Consider what would happen if you didn't use the preamp at all, and just connected two mics to each other (which you could do, e.g. with a F-F XLR coupler, if you have such a thing). Yell into one all you want ... it's not going to kill the other one or, for that matter, do much of anything. It will drive the other diaphragm like a speaker ... sort of: a very inefficient speaker being driven by a very low-powered source. Which will, in practice, not be very much like a speaker at all. (Or consider what would happen if you just took two speakers and connected them together. Anything bad? No. Anything at all? Not really.)

I agree that doing that won't harm either of the mics, in fact nothing whatsoever will happen. As I stated in an earlier post, and southSIDE glen also just talked about, the issue with this is experiment is what it might do to the pre-amp. At least that's how it seems to me.

Sure a dynamic microphone and dynamic speaker are really just the same thing going in opposite directions, but plugging one mic into another isn't going to drive one of them, but I know what you're saying.

 

[xstatic]

MS recording and dual diaphragm are not at all like plugging two mics simultaneiously into a preamp. First, MS is a type of summing algorithm and is not actually multiple mics on the same pre. Second, dual diaphragms are the same. With every mic there is a certain set of electronics that the signal goes through BEFORE it gets to the preamp so that the preamp can see the kind of signal it wants to see. Connecting two mics to two preamps is akin to making your preamp drive two complete loads at the same time. Sometimes preamps (especially more affordable ones) have a hard time handling one mic, so I would not want to feed that poor thing two of them. I don't think you would damage your microphones doing these tests (unless you did have some sort of phantom power issue....), but i would be worried about poosibly damaging or at the very least putting unnecessary stress on your poor little preamp. Especially if your preamp is part of a cheaper mixer. With cheap mixers, often times a problem in one area can greatly affect other areas as well.

 

[RAK]

HERE HERE!!! (or is it HEAR HEAR!!!)

 

[sjjohnston]

MS recording and dual diaphragm are not at all like plugging two mics simultaneiously into a preamp.

Yes, which I think I made clear in my posts, e.g.

Of course, the dual-diaphragm mics mix the two signals with a carefully-designed active circuit in the mic .... They don't just have a Y-cable inside of them. Same for the stereo mics

First, MS is a type of summing algorithm and is not actually multiple mics on the same pre.

An MS stereo mic (there used to be lots of them ... there are still a few, e.g. the Shure VP88) does mix the output of two diaphragms before it hits the pre. It matrixes the signals internally. Which is what I said.

Second, dual diaphragms are the same.

Which is exactly what I've already said. Twice, now.

Connecting two mics to two preamps

I think you mean two mics to one preamp ....

is akin to making your preamp drive two complete loads at the same time.

I think you're confused who's driving what load. The mic is driving the preamp. The preamp's not driving the mic. The preamp is driving whatever's next in the signal chain after it (an A/D converter, a tape deck input, something irrelevant to this discussion). The preamp input presents a load to the mics, not the other way around.

Unless you're really hooking things up backwards.

Sometimes preamps (especially more affordable ones) have a hard time handling one mic, so I would not want to feed that poor thing two of them. I don't think you would damage your microphones doing these tests (unless you did have some sort of phantom power issue....), but i would be worried about poosibly damaging or at the very least putting unnecessary stress on your poor little preamp.

I'm having difficulty seeing how you stress the preamp in this situation. The mics are going to have great difficulty generating enough current to drive each other and the preamp. The result may be a crappy, distorted signal. But it's not going to blow anything up. You can (and people sometimes do) drive input stages way over what they're designed to handle ... the result is distortion and clipping, not explosions. The voltages the two mics could possibly create are miniscule: orders of magnitude short of what could hurt a preamp input. Plus, if they were big enough to hurt the preamp input, they'd hurt the other mic.

 

[xstatic]

Given the nature of this thread and your opening statement in your longer post, in now way did you make anything clear concerning plugging two mics into the same pre. MS and dual diaphragm are very different since the mics electronics process the signals before sending the balanced and transformed signal to the mic preamp. This is nothing at all like having two seperate mics with two seperate sets of electronics plugged into the same preamp. Sorry if you took my post offensively (which it certainly sounds like you did) but I just wanted to clarify that for all of the other people reading who may not understand the intricacies of dual diaprhagms, ms decoding, and multiple mics on the same pre. Basically your long post was right on the money and correct. However, in the context of this particular thread and what was already being discussed, it"s perceived implication was actually wrong. So, in the end, my reply was sort of necessary to many readers to distinguish the difference, so I was not really repeating what you said, just placing it in a context to which applies specifically to this thread.

I also never made any guarantee that any mics or preamps would get damaged if the OP tried his experiment. I did however point out that the inherent risk compared to the very likley useless results may not be worth the time, effort or possible money.

 

[RAK]

An MS stereo mic (there used to be lots of them ... there are still a few, e.g. the Shure VP88) does mix the output of two diaphragms before it hits the pre. It matrixes the signals internally. Which is what I said.

The end of a Shure VP88 has a 5 pin (I think it's 5 pins, but not absolutely positve, more than 3 though) connector and with a Y cable (1 Female end into the mic, and 2 male XLR ends. This is how most (not counting little sony stereo mics for mini disc which just have 1 1/8" trs jack, for example) Stereo mics work so you actually can have a 2 channel stereo recording (I know saying 2 channel stereo is redundant). You actually use 2 pre-amps to capture each capsule in the mic. That's a true stereo mic.

Slightly different than an MS configuration which can easily combine into a mono signal (The S gets cancelled out, I got this last bit from Wikipedia by the way, since I wanted to refresh my memory) The S capsule is added to the M for 1 channel, and subtracted (phase reversed) for the other channel

 

[sjjohnston]

The end of a Shure VP88

Yes ... it wouldn't be a "stereo mic" if it didn't have two separate signals coming out of it, now would it?

BUT "it matrixes the signals internally."

Inside it are two capsules. If you switch it into ordinary stereo mode:

One output is a mix of the mid capsule and the side capsule
One output is a mix of the mid capsule and the side capsule, with the side inverted.

In this respect, it (and other mics with a similar design) are different from the stereo mics that use an X-Y design: in those (generally), one capsule goes to one output, period.

At the risk of being repetitive (but clear), the way it mixes the signals is with a properly designed circuit which, at least in the VP88, is active. It's not just a Y cable.

 

[sjjohnston]

For the sake of clarity:

It generally seems like a bad idea,

* * *

"Y"ing two outputs into one input is usually bad practice. It strikes me as more so in the case of microphones.

* * *

Mixing two microphone signals together is what mixers are for.

* * *

Y-cables are not so good for connecting two outputs into one input.

* * *

Y-cables call for great hesitation when connecting anything other than regular line-level audio.

* * *

When recording MS with two separate mics, you'd normally record them to two separate tracks, then mix (or "matrix") them appropriately after the fact.

* * *

Of course, the dual-diaphragm mics mix the two signals with a carefully-designed active circuit in the mic .... They don't just have a Y-cable inside of them. Same for the stereo mics.

* * *

The preferable way to do this, of course, is to run the two S mics through two mixer channels, and combine them to one track with the mixer.

 

[RAK]

Yes ... it wouldn't be a "stereo mic" if it didn't have two separate signals coming out of it, now would it?

BUT "it matrixes the signals internally."

Inside it are two capsules. If you switch it into ordinary stereo mode:

One output is a mix of the mid capsule and the side capsule
One output is a mix of the mid capsule and the side capsule, with the side inverted.

In this respect, it (and other mics with a similar design) are different from the stereo mics that use an X-Y design: in those (generally), one capsule goes to one output, period.

At the risk of being repetitive (but clear), the way it mixes the signals is with a properly designed circuit which, at least in the VP88, is active. It's not just a Y cable.

You realize you just said everything I just said. Maybe you misunderstood. If you read the rest of my post you'd see I also explained the two outputs the same that you did (1 is M+S, the other is M-S (S out of phase))
My point was that it was sending out two signals on two cables to two inputs. I think everyone knows that an XY pattern is entirely different from an M/S configuration. I don't know the XY pattern stereo mics you have come in contact with, but the ones I have used have two capsules again with two outputs to two inputs. Just like putting up two individual mics in an XY pattern, except they're all in the same housing.

Of course the VP88 is not "just a Y cable," but again, I was just pointing out that you end up with two different signals.

 

[sjjohnston]

Yeah. It's all exactly the same as what was in my post from 5 days ago ... which people seem to be disagreeing with, for some reason. Or maybe not. I don't know if I can tell anymore.

 

[RAK]

I think the point of all of this is he didn't care about making a stereo recording (I know this has been pointed out already) so this post has morphed into lots of other things, although it is full of interesting information. I think the general consensus was it seemed like a bad idea to wire two mics into 1 cable. There's probably cause for a good explanation of how MS recording works, if that's not already on here somewhere. The post you re-posted with your bullet points still makes sense to me, as it did when you originally posted it, so I don't think there was much cause to disagree with that.

I just didn't get what you said about with XY stereo mics generally having one capsule going to one output. Did you mean each of the two capsules goes to its own output? That's not how I read it initially, but if that is what you meant, that's right on.