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Author Topic: Are Preamp Channels Always Matched?  (Read 2374 times)

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stevetoney

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Are Preamp Channels Always Matched?
« on: April 30, 2009, 08:52:31 AM »
I was designing a DIY test last night to check how well my mics are matched and it occurred to me as I was figuring out how I wanted to do it that I should keep the amplification path constant when doing this.  That led me to wondering if there are ever issues with a preamp and whether the two channels are always matched.  We talk so much about matching microphones, but I don't think I've ever seen anyone talk about whether their two (or four or eight) channels are matched perfectly on their preamps. 

So my question is, is preamp channel matching ever an issue or is the performance of electronics consistent enough that this is a moot issue for preamps?
« Last Edit: April 30, 2009, 08:55:36 AM by tonedeaf »

stevetoney

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Re: Are Preamp Channels Always Matched?
« Reply #1 on: April 30, 2009, 10:34:49 AM »
Thanks for the response ms...

If you learn that both mics and recorder vary, then mark one of the mics as right channel so you can make a consistent adjustment in post.

Yes, this was essentially the reason that I was setting up the test to check my mics.  I also wanted to be able to get to the point where I have a fairly high level of confidence that I've got everything pre-set pretty close when the music starts so that tweeking is held to a minimum in the first 30 seconds of the show.

Offline it-goes-to-eleven

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Re: Are Preamp Channels Always Matched?
« Reply #2 on: April 30, 2009, 10:55:25 AM »
Some thoughts:

If pots are involved, it seems unlikely the channels will be matched.  Maybe in good designs with the pots all the way down.

Lab/instrumentation pre-amps are probably very close and have test/calibration results.

Temp changes tend to cause drift/variation.  Some designs have thermal compensation.  What tolerance are the internal components?  My RMOD pre-amp uses Vishay resistors. They're expensive, large, and hard to package.   Many good pre-amp designs separate the left/right signal path...  But does one side tend to warm up more than the other due to proximity to heat sources like the power supply?

Grace ships a custom calibration sheet with each v3.  I'm sure it isn't instrumentation accurate but it is a good sanity check.

If you want to compare pre-amp channel balance as a baseline, I'd suggest using line out from something (sine wave, white noise, etc).  And as ms suggests, moving the same output to each input.

Since a lot of gear tends to distort at high input levels, especially at low frequencies (even before "clipping"), I'd expect the behavior to be somewhat inconsistent under those circumstances.

What about the awkward question of whether the sine waves being close at 2Khz means the mics will sound the same overall on music?

I have always advocated taking "noise floor" baselines of gear, especially more fragile stuff like mics and cables.  That way, if you're wondering "hey, do I hear a buzz in this channel?", you can go back and re-test against your baseline to see if it is something new, etc.  It also gives a sanity check in deciding whether you need to send mics to Germany for service, etc.

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stevetoney

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Re: Are Preamp Channels Always Matched?
« Reply #3 on: April 30, 2009, 11:50:26 AM »
Interesting thoughts and comments. 

Notwithstanding the concept that it's nice to get as close as possible, do the comments you guys make support the concept that matching microphones is fairly unnecessary because you're gonna have unmatched preamp channels and compensations need to be made all along the signal chain to get the signal matched at the output of the signal path?

Offline it-goes-to-eleven

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Re: Are Preamp Channels Always Matched?
« Reply #4 on: April 30, 2009, 12:00:47 PM »
I think matching the electronics is a lot easier than the mics.  And the electronics are a lot less likely to change over time.  Though I'm not sure about capacitor issues as they age, etc.  Or whiskers.

Mics in general are delicate, and hard to mfg consistently.  If they were easy, we'd all be buying chinese mics for $50 each.  The good mfgs seem to have very good consistency.  But they're delicate and subject to moisure issues, etc, in the field..  So I think it is well worth it to baseline your mics.

Drumsticks are matched.  The pistons in your engine are matched.  It makes sense to match mics.

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stevetoney

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Re: Are Preamp Channels Always Matched?
« Reply #5 on: April 30, 2009, 12:29:19 PM »
Good points mshil.  When I posed my question about matched mics a couple of posts up, I was asking from the perspective of of mic sensitivity rather than with respect to frequency response. 

stevetoney

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Re: Are Preamp Channels Always Matched?
« Reply #6 on: April 30, 2009, 12:36:28 PM »
Drumsticks are matched.  The pistons in your engine are matched.  It makes sense to match mics.

LOL...while I think pistons makes sense becuase of the speeds and stresses that are probably in an engine, I had no idea that drumsticks are matched, but it makes sense!  I guess a drummer would want sticks the same relative length, weight and wood density, eh?

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Re: Are Preamp Channels Always Matched?
« Reply #7 on: April 30, 2009, 12:47:05 PM »
I saw it on Modern Marvels - how drumsticks are made. Since they're wood, the density varies.   So the natural frequency when they strike something varies.  So they're actually paired by frequency.

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Offline Chuck

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Re: Are Preamp Channels Always Matched?
« Reply #8 on: April 30, 2009, 02:18:22 PM »
Getting back to testing your mics though...
When I do that, I don't use two channels. I use one channel of the pre-amp, the same gain setting, one cable, one stand etc... and just swap the mics in and out. That way, I know, for sure, that any differences are just in the mics.
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stevetoney

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Re: Are Preamp Channels Always Matched?
« Reply #9 on: April 30, 2009, 05:32:32 PM »
Getting back to testing your mics though...
When I do that, I don't use two channels. I use one channel of the pre-amp, the same gain setting, one cable, one stand etc... and just swap the mics in and out. That way, I know, for sure, that any differences are just in the mics.

Right-o.  Exactly what I was gonna do.  Thanks.

Offline DSatz

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Re: Are Preamp Channels Always Matched?
« Reply #10 on: May 03, 2009, 09:21:28 AM »
Technologically it's a question of the weakest link in the chain. Continuous analog level controls (potentiometers) never match precisely; the resistance at the "10:00" setting (for example) of any two chosen at random won't be the same. And there is no such thing as a precision capacitor (or inductor) in the sense that there are precision resistors.

Practically it's a question of economics. Pay more money and as a manufacturer, you can get closer tolerances and closer matching between/among channels and units--in many cases via pre-selection and discarding rather than through inherently more precise manufacturing of the parts. But does any customer ever choose preamp "A" over preamp "B" because the channel gains and the level pots on preamp "A" match each other better in all possible settings?
music > microphones > a recorder of some sort

Offline hugh

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Re: Are Preamp Channels Always Matched?
« Reply #11 on: May 07, 2009, 05:08:26 PM »
...Does any customer ever choose preamp "A" over preamp "B" because the channel gains and the level pots on preamp "A" match each other better in all possible settings?

For matrixed recordings it's important: things like the SoundField and TetraMic microphone systems need really well-matched channels.  When I started recording with a TetraMic, my pre had analog gain pots, so before each take I needed to record a calibration tone to each channel.  Extremely laborious and painful.  Now I have a pre with stepped controls, which is vastly easier.  If I ever replace that, it'll hopefully be something with digitally-controlled gain and good channel matching (at the high end that includes Orpheus, 788t, etc).

 

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