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beenjammin

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How Matched is Matched?
« on: September 22, 2015, 11:24:36 AM »
I purchased a set of matched capsules and have noticed on my recorder a level difference. In setting up the two mikes next to one another, my DAW tells me there's about a 1-1.3dB difference between the caps. This difference is invariant amongst two sets of mike bodies -- so four mike preamps in total.

Now, in looking at a graph that shows the frequency response of the caps, everything seems visually dead on, save the total level, which is off by about a decibel.

The real question is: how do they sound? Well, the problem I'm having is that I'm new to cardioids and can't really say. I've done some tests in ORTF, which I rather like, and raising the "weak" cap sounds like the stereo image is being pulled unnaturally in favor of the newly raised side. But perhaps that's how it *should* should given the placement?

I'm not especially worried, and doing a number of tests has confirmed that I'm really digging these mikes. I guess I want to confirm that the difference in level is linear across the entire frequency spectrum so as to avoid a shifting image.

What do folks do in such a case as this: run a test tone and match the caps with that? Which frequency? 1000 kHz?

Offline aaronji

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Re: How Matched is Matched?
« Reply #1 on: September 22, 2015, 11:43:33 AM »
^ Personally, I'd call them matched if the frequency response is the same and it's just the sensitivities that are a decibel off.  What did the manufacturer promise as matched?

Offline Gutbucket

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Re: How Matched is Matched?
« Reply #2 on: September 22, 2015, 11:54:27 AM »
As long as the frequency response graphs are identical (or at least very close) you're good.   Make up any small differences in level due to differences in capsule sensitivity afterwards with a minor gain adjustment tweak to one channel.

Also consider that although preamp and recorder channels are essentially 'matched' with each other, their controls are often not linked, so level-matching in the resulting raw recording is determined by how accurately you set one knob with relation to the next.  Many professional-grade recorders and preamps have infinitely adjustable potentiometers without detents as gain controls for each input channel.  With that kind of control interface it is easy to compensate for a 1 to 1.3dB difference between channels.  And it is equally easy to unintentionally introduce a difference greater than that unintentionally!

I've always been surprised that there aren't more preamps and recorders which feature a single global gain control that adjusts the gain of all channels simultaneously, keeping them balanced relative to each other, in addition to individual channel trims.  There are a few: The Aerco (stereo preamp) has a level and balance control instead of two separate level controls.  The Tascam DR-680 allows channel linking after gross and minor gain trim adjustments to each channel, so once balanced, all linked channels can be adjusted as a group and retain relative gain ratios.

Make up any small differences in level due to differences in capsule sensitivity afterwards with a minor gain adjustment tweak to one channel.  I do that everytime for best subjective balance of the playback image in the recording, irregardless of the peak or RMS level match between channels.  It's not the numbers that count, it's the subjective listening experience on playback.
« Last Edit: September 22, 2015, 11:56:18 AM by Gutbucket »
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beenjammin

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Re: How Matched is Matched?
« Reply #3 on: September 22, 2015, 11:57:25 AM »
^ Personally, I'd call them matched if the frequency response is the same and it's just the sensitivities that are a decibel off.  What did the manufacturer promise as matched?

I agree, as level norming is easy enough at the recorder or in post. I'm just trying to figure out how to know for certain -- that is, beyond eyeballing frequency graphs -- that the frequency response is matched.

I did run a test where I lined the mikes up and then inverted the phase of one in post. Of course there's not a complete 100% null, but the levels did drop.

According to the manufacturer, they match to 0.5dB.

In these situations, I try to first look to myself as the cause of the problem. My appealing to the board is an attempt to see what, if anything, is going on, and to determine how big a difference it makes. 

beenjammin

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Re: How Matched is Matched?
« Reply #4 on: September 22, 2015, 12:04:18 PM »
As long as the frequency response graphs are identical (or at least very close) you're good.   Make up any small differences in level due to differences in capsule sensitivity afterwards with a minor gain adjustment tweak to one channel.

Also consider that although preamp and recorder channels are essentially 'matched' with each other, their controls are often not linked, so level-matching in the resulting raw recording is determined by how accurately you set one knob with relation to the next.  Many professional-grade recorders and preamps have infinitely adjustable potentiometers without detents as gain controls for each input channel.  With that kind of control interface it is easy to compensate for a 1 to 1.3dB difference between channels.  And it is equally easy to unintentionally introduce a difference greater than that unintentionally!

I've always been surprised that there aren't more preamps and recorders which feature a single global gain control that adjusts the gain of all channels simultaneously, keeping them balanced relative to each other, in addition to individual channel trims.  There are a few: The Aerco (stereo preamp) has a level and balance control instead of two separate level controls.  The Tascam DR-680 allows channel linking after gross and minor gain trim adjustments to each channel, so once balanced, all linked channels can be adjusted as a group and retain relative gain ratios.

Make up any small differences in level due to differences in capsule sensitivity afterwards with a minor gain adjustment tweak to one channel.  I do that everytime for best subjective balance of the playback image in the recording, irregardless of the peak or RMS level match between channels.  It's not the numbers that count, it's the subjective listening experience on playback.

Super helpful post and spot on. My recorder happens to have a pot ganging feature that I love. I'm an omni AB freak, and am used to placement and AB width; level matching has never been a concern until now. I usually set my mikes up, crank up the gain to 11 and hope no helicopter ruins my bird recording.  ;D

But seriously, I think one of the issues I'm having is being new to ORTF and other near coincident techniques. I think the answer to my question is to simply get more experience with this setup and listen.

Offline bombdiggity

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Re: How Matched is Matched?
« Reply #5 on: September 22, 2015, 12:20:57 PM »
Depends on who is matching them I think.  Matching is a bit dicey.  Those manufacturers who do, match to their own tolerances. 

I'd agree with others the frequency characteristics are the critical element.  Output matching seems to be much more difficult for manufacturers. 

My Schoeps (even after two decades? give or take) still seem spot on level wise, but the SP's I got seem to be roughly 1 dB off out of the box. 

With cards you are looking for a bit of a difference between channels and should see some natural variance during a performance anyway (unless you're in the back).  Depending on how you point them and where you are you may see lots of dB's of level difference in the channels at different points in a performance (at least if you're close to dispersed sound sources).  I wouldn't worry about a consistent 1 dB or even maybe 2 since you can easily compensate in post.  My SP's seem to be about 1 dB different but the difference in the average of the signal track by track in any given show may range from near 0 to up to 2 dB off, which, with the mics in fixed position, would have to be due to the relative strength of the musical output varying. 

I tend to play it by ear.  Sometimes it sounds best to match them up and apply that 1 dB baseline or a little more or less.  Other times it sounds best to leave it as it is.  If operating close to the bandstand with directional mics and a small ensemble you can utilize the relative volumes of the channels to adjust the relative mix a bit if the sound wasn't necessarily balanced to your taste at the show. 

A dB is certainly within the range of error for matching and less than the natural delta in many settings. 

Cards have different applicational aspects than omnis.  Omnis will smooth it out and make things more an even wash.  The natural differences in directional mics can be used to advantage (though again back of hall that particular potential advantage disappears because the distance is smoothing the sound out to an even wash). 

« Last Edit: September 22, 2015, 12:40:25 PM by bombdiggity »
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Offline acidjack

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Re: How Matched is Matched?
« Reply #6 on: September 22, 2015, 12:23:29 PM »
Also, depending what recorder you have, the inputs of the recorder may not "match" perfectly either. Agree w/ others that frequency response matters more.

That said, my matched Schoeps have always been pretty dead-on.
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Offline Gutbucket

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Re: How Matched is Matched?
« Reply #7 on: September 22, 2015, 12:35:20 PM »
I did run a test where I lined the mikes up and then inverted the phase of one in post. Of course there's not a complete 100% null, but the levels did drop.

That's a good test.  Make sure the two mics are parallel and as close to each other as possible.   Then carefully adjust gain to find the maximum cancellation achievable.  They can't be perfectly coincident, so the cancellation will not be a complete null, but the difference in gain between the two channels equals their sensitivity mis-match.  Listening to the difference signal can be quite informative, since you will be hearing the frequencies which are not canceling as much as others, allowing you to subjectively asses the mismatch.

It's not the numbers that count, it's the subjective listening experience on playback.

There are some technical exceptions to that, but for most recording done around here that statement holds. Here's a couple interesting ones which come to mind-

The differential microphone technique used for the Grateful Dead's famous Wall Of Sound used two closely arranged omnidirectional microphones with opposite polarity at each vocal mic position.  These microphone gains needed to be very closely trimmed before summing.  Once gain-matched, most environmental sound reaching both microphones became electrically cancelled out when the two mic signals were summed together, including that from the big wall of sound PA located directly behind the performers, projecting directly into the microphones.  An announcer or singer would do their thing directly into just one or the other microphone, causing that part of the signal to differ between the two mics, and that part would not cancel out.  If those microphones were not very well matched in both frequency response, as well as having the gains well matched at the input of the differential amplifier, severe feedback would have occurred.

Today, a couple relatively inexpensive ambisonic microphones such as the Soundfiled SPS2000 and the Core TetraMic rely on a computer to do the necessary matrix decoding after the recording has been made.  Those mics require very close gain matching between four recorded channels for that complex matrixing to work correctly.
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beenjammin

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Re: How Matched is Matched?
« Reply #8 on: September 22, 2015, 12:44:53 PM »

A dB is certainly within the range of error for matching and less than the natural delta in many settings. 


Another really helpful reply, I'm glad I posted.

I agree, a dB is very minor and is much less than the delta introduced in sound settings. This I find especially true with a cardioid.

For what it's worth, my capsules are switchable and the difference carries over between the omni and cardioid settings.

By the way, I don't mean to be coy in not naming the manufacturer, I want only to be extra careful with my tone in order to prevent coming off as indecorous. I have a deep respect for the maker of these mikes and even with the small level difference, I find the caps to be especially lovely. 

The only thing that gives me pause is that given the high tolerance in making these caps, I want to be sure nothing else is going on. For example, if they were set to 0.5dB when they were matched and a ~1-.1.3dB difference has been introduced between their making and my purchasing them, would this indicate something problematic?

I've spoken with the maker and they think all's ok, but that it may be a bit odd to have a slightly larger difference between the caps. They're happy to check them out, but unless someone thinks there's something going on here, I'm likely to hold on to them.


Offline aaronji

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Re: How Matched is Matched?
« Reply #9 on: September 22, 2015, 12:54:48 PM »
^ If you're going to worry about it, it's perhaps easier just to send them in for a check-up.  I'd leave it alone, myself, unless something else cropped up...

The Aerco (stereo preamp) has a level and balance control instead of two separate level controls.

Not all of them.  Mine has a left and right gain knob (detented, so easy to set the same).  I think that is the stock option, although Jerry will make them with the level and balance on request.

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Re: How Matched is Matched?
« Reply #10 on: September 22, 2015, 01:45:34 PM »
The Aerco (stereo preamp) has a level and balance control instead of two separate level controls.

Not all of them.  Mine has a left and right gain knob (detented, so easy to set the same).  I think that is the stock option, although Jerry will make them with the level and balance on request.

Noted, thanks.  The only Aerco I've used had the level and balance controls.  I was pleased to find it setup that way and didn't realise that was not the standard arrangement.  Seems a far more useful control configuration for stereo recording, which is pretty much what everyone here is doing.
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Offline DSatz

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Re: How Matched is Matched?
« Reply #11 on: September 24, 2015, 08:33:28 PM »
> I've always been surprised that there aren't more preamps and recorders which feature a single global gain control that adjusts the gain of all channels simultaneously, keeping them balanced relative to each other, in addition to individual channel trims.

Like it or not, you're asking the manufacturers to choose among (a) the convenience of ganged controls, (b) the accuracy of their gain settings and interchannel balance, and (c) an affordable selling price. You can have one, or at most two of those things--but you definitely can't have all three at once. And since the consumer or "prosumer" equipment market is so heavily determined by price, you're basically asking NOT to have accuracy when you look for a control scheme like the one you describe.

It is very difficult to build ganged analog level controls that maintain accurate balance between channels across a wide range of settings. By "difficult" I really mean expensive and time-consuming (which again means expensive). The manufacturer has to keep wiring potentiometers into the circuit, testing them, and rejecting them until the desired degree of matching is found. For any given parts and assembly/testing budget, all other things being equal, you will get better results with separate, unlinked gain controls in each channel.  So I wish I could persuade you to stop wanting what you have described, or else accept a version of it that uses digital gain controls, which don't have these limitations.

Among analog equipment, I don't know whether you're familiar with the Grace Lunatec V3, but the gain control arrangement of its preamp section seems good. The basic gain for each channel is set in 5 dB increments with stepped controls that use discrete resistors (i.e. each setting of the knob selects a different discrete resistor); the basic gain of each channel can thus be set to within a small fraction of a dB of the nominal value, and the gain of the two channels will match almost exactly at each given setting. Then there's a smaller, second knob for each channel that allows continuous adjustment within ±5 dB around that basic setting per channel.

And back to the original question in the thread: For coincident or near-coincident recording with directional microphones, even matched pairs of condenser microphones from a good manufacturer aren't necessarily matched as well as one might ideally want. If you have a stereo recording that you've made using X/Y or ORTF or the like, try bringing it into your favorite editing software, selecting your favorite 30-second passage, and listening to it a few times. Then reduce the gain in one channel by, say, 3/4 dB and listen again. It's different. Perhaps it will be equally pleasing or maybe even more so--but it won't sound the same, because our brains are extraordinarily sensitive to differences between what our left and right ears are hearing. Try the same thing by restoring the balance to what it was originally, but this time try adding 1.5 dB at 100 Hz in just one channel and not the other, then listen again.

The fact is, no manufacturer will guarantee that a matched pair of their microphones will match within such close tolerances that the difference between them (i.e. switching which microphone is used to record which channel) could never be heard in precisely the kinds of stereo recording that most of us do. When you order a matched pair from some manufacturers, you may get two microphones that are within that limit, or you may not, although your odds are definitely improved by preferring matched pairs (particularly of capsules, since that's where the main variations occur).

Finally, since there is no standard definition for "matched", an unselected pair of brand "X" capsules could very well match better than a selected, matched pair of brand "Y" capsules if company "Y" has less consistency in their production to begin with--and there really are major differences among manufacturers in that respect, and even among different microphone types from the same manufacturer. It's not just a matter of competency. Not every manufacturer strives equally hard for consistency in manufacture, since most market segments place less value on it than we do. Customers who think in terms of pairs of microphones don't dominate the marketplace.

--best regards
« Last Edit: September 24, 2015, 09:12:17 PM by DSatz »
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Offline Gutbucket

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Re: How Matched is Matched?
« Reply #12 on: September 25, 2015, 10:26:50 AM »
Thanks D,

I understand the "one or two but not all three" thing in general.  Hower, I don't think that it's necessarily the case in that we can't have all three at reasonable cost.  And I am unwilling to stop wanting that kind of control since it just a better fit with the needs of stereo recording, and a far better fit for the needs of multichannel array recording where more than just two channels need to be adjusted simultaneously and by the same relative amounts to maintain their signal level relationships.  Two counter examples:

I not only accept but prefer digital gain controls.  Ganged stereo control is de facto on most all small modern digital recorders that I know of commonly used by members here- one control adjusts gain for both channels simultaneously either with buttons or a control potentiometer, one exception being to that being the M-Audio Microtracker.  And the digital gain controls of the Tascam DR-680 allows multiple channels to be 'ganged' together and controlled as groups with one knob.  Adjust the each channel gain independently, then gang the channels together so that further adjustment maintains the relative differences between them while adjusting the entire group globally.  The digital control provides excellent tracking and accuracy channel-to-channel during those global group adjustments.  Besides the capability of recording more than 4 channels, that feature is one of the primary reasons I'm using that particular recorder.  The lack of that feature is why I'm not overly interested in the new Zoom 8 channel recorder.

On the analog side, I use a Grace Lunatec V3, and I understand and like it's control interface.  However, I see no reason why it would be more costly to make the primary stepped gain control a single rotary switch instead of two separate ones.  It would still use the same discrete resistors for gain selection of each channel, only the two separate switches would be ganged onto one knob axle.  The two smaller, continuous ±5 dB adjustment knobs would remain the same, and allow for control over fine balance between the two channels.  Yes, that scheme would exclude applications where one needs more than a 10dB offset between the two channels.  So perhaps better still may be a co-axial rotary switch, in which the switch knobs for both channels are separate but turn together unless the user specifically turns one while holding the other still.

That's been a common gain control scheme for stereo recorders for years, the old Sony D-5 casset, the Sony D1 & D100, the Tascam HD-P2, the Fostex FR2 and FR2-le all come to mind as having co-axial control which turn together.

Yeah, it's easy enough to turn two knobs simultaneously on something like a V3.  It becomes a more significant challenge maintaining interchannel gain relationships when more than two channels need to be adjusted together.
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Offline goodcooker

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Re: How Matched is Matched?
« Reply #13 on: September 25, 2015, 11:08:31 AM »


On the analog side, I use a Grace Lunatec V3, and I understand and like it's control interface.  However, I see no reason why it would be more costly to make the primary stepped gain control a single rotary switch instead of two separate ones.  It would still use the same discrete resistors for gain selection of each channel, only the two separate switches would be ganged onto one knob axle.  The two smaller, continuous ±5 dB adjustment knobs would remain the same, and allow for control over fine balance between the two channels.  Yes, that scheme would exclude applications where one needs more than a 10dB offset between the two channels.  So perhaps better still may be a co-axial rotary switch, in which the switch knobs for both channels are separate but turn together unless the user specifically turns one while holding the other still.

That's been a common gain control scheme for stereo recorders for years, the old Sony D-5 casset, the Sony D1 & D100, the Tascam HD-P2, the Fostex FR2 and FR2-le all come to mind as having co-axial control which turn together.

Yeah, it's easy enough to turn two knobs simultaneously on something like a V3.  It becomes a more significant challenge maintaining interchannel gain relationships when more than two channels need to be adjusted together.

On something like the V3 even though it is a "stereo" preamp not everyone uses it for stereo pairs and the control scheme you propose would make it more difficult to use in some scenarios.

I used my V3 when tracking , for instance, to mic a guitar cabinet with a condenser and a dynamic which required radically different settings of the two channels. Another use was a cardioid condenser in front of a mandolin and an omni pointed straight down over the shoulder of the player from near the right ear. Also requiring totally different settings on each channel. That in particular sat in the mix really well.

Sorry for the off topic post.
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Offline Gutbucket

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Re: How Matched is Matched?
« Reply #14 on: September 25, 2015, 11:27:01 AM »
Yes I realize that.  Which is why I suggested a coaxial control, which would still provide the flexibility for what you are doing, only moving simultaneously as a pair unless specifically adjusted individually.  If a bit less convenient for the kind of use you describe, it would be more convenient for control of stereo pairs like most are using around here. 
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beenjammin

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Re: How Matched is Matched?
« Reply #15 on: September 25, 2015, 12:40:49 PM »

And back to the original question in the thread: For coincident or near-coincident recording with directional microphones, even matched pairs of condenser microphones from a good manufacturer aren't necessarily matched as well as one might ideally want. If you have a stereo recording that you've made using X/Y or ORTF or the like, try bringing it into your favorite editing software, selecting your favorite 30-second passage, and listening to it a few times. Then reduce the gain in one channel by, say, 3/4 dB and listen again. It's different. Perhaps it will be equally pleasing or maybe even more so--but it won't sound the same, because our brains are extraordinarily sensitive to differences between what our left and right ears are hearing. Try the same thing by restoring the balance to what it was originally, but this time try adding 1.5 dB at 100 Hz in just one channel and not the other, then listen again.


Thanks, DSatz! I've done just as you suggest and the differences are immediately noticeable if I have a loop running and make the gain adjustment while running. The effect is much less audible to my ears if I stop playback, adjust, and restart.

Yesterday I set up an ORTF with the caps to capture an afternoon ambience in my backyard. This included birds, a highway, an alley and some construction work. While I could notice differences in the stock matching and a 1.3dB offset, the difference wasn't especially meaningful in that setting. That is to say: each setting was as pleasant as the other. (Well, in fact, neither setting was better or worse, as the recording was pretty boring!)

I'll set up ORFT and DIN this weekend and record some cello to see how much difference I can detect in a less ambient setting.

Offline MIQ

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Re: How Matched is Matched?
« Reply #16 on: September 26, 2015, 12:44:58 AM »
The effect is much less audible to my ears if I stop playback, adjust, and restart.

Auditory short term memory only lasts a few seconds.  To hear subtle differences, an instaneous switching arrangement works better than one with a delay between samples. 

And our brain's ability to hear even fractional dB level differences over wide freq bandwidths is pretty good (like full bandwidth channel differences).  Narrowband differences are much harder to hear.

Miq

Offline DSatz

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Re: How Matched is Matched?
« Reply #17 on: October 03, 2015, 01:43:40 PM »
I agree with the last two posts; I should have recommended some way of listening to the results that would get around the lack of reliable auditory memory longer than a few seconds.

The experience is instructive; it's very good to realize how limited our perceptual abilities are in that respect. Plenty of self-styled "golden ears" render judgments as if they had that ability, when (being human) they don't. And for those of us who live at ordinary altitudes, it reminds us to make careful comparisons, not just "microphone X sounded better last Thursday than microphone Y sounded the week before in another location." I mean, that's a perfectly valid statement, but it says nothing one way or the other about the quality of either microphone.

--Back to the topic of microphone matching: I'd like to quote some bullshit that I just read on a manufacturer's Web site, but I don't want to give them any publicity by identifying them. This is, however, a manufacturer of equipment intended for use by professionals, who should instantly know better if they read any such claim:
Quote
Manufacturing tolerances have been minimized thanks to highly precise calibration of the capsule and the electronics – which means that any two microphones from this series, set identically, will always form a matched pair at +/- 0 dB!
« Last Edit: October 15, 2015, 08:34:40 AM by DSatz »
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Offline aaronji

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Re: How Matched is Matched?
« Reply #18 on: October 03, 2015, 02:59:06 PM »
Yes, and some of their other marketing claims are a bit over the top, too...

beenjammin

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Re: How Matched is Matched?
« Reply #19 on: October 03, 2015, 03:22:26 PM »
Well, I confess that I lack the courage and experience to keep the mikes as is and have sent them in so that the experts may have a look at them.

I have two other sets of mikes, one matched and one non matched, and each pair is spot on. The manufacture set me the measurement charts of the pair I sent in and they are dead, dead on. It was the discrepancy between these charts and what I'm seeing on my end that motivated me to send them in to see if anything is going on.

What's most likely the case is that I'm being overly neurotic and am thinking about this too much. At least when I get them back, I'll have peace of mind. And in the case that there is something the matter, then I'll get that sorted.


 

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Re: How Matched is Matched?
« Reply #20 on: October 13, 2015, 12:09:11 PM »
I just posted PDFs of the calibration charts for a DPA matched pair of their 4061 omnis (measured with the ~ +5dB 'low-boot' short grids installed on them), over in TS Knowledge Base / Microphones & Setup.  These provide a good example of what actual measured response charts look like, for a well-matched pair.

Link- http://taperssection.com/index.php?topic=175013.msg2163073#msg2163073



[Edit-]
And a few years ago I posted calibration charts for a well-matched pair of Microtech Gefell cardioids provided by the factory when I sent the mics back for service and repair.  A single PDF showing the response of both mics can be found attached to this post in the Team Gefell thread-  http://taperssection.com/index.php?topic=144869.msg1974932#msg1974932
« Last Edit: October 13, 2015, 12:11:23 PM by Gutbucket »
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