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-   -   'Tube' preamp bad for measurement mic? also Apex mic question (http://www.diyaudio.com/forums/multi-way/48751-tube-preamp-bad-measurement-mic-also-apex-mic-question.html)

morbo 31st December 2004 07:05 PM

'Tube' preamp bad for measurement mic? also Apex mic question
 
So over the holidays I got myself the Apex mic and tried to get the Apex preamp as well. The idiots at the music store had 10 on hand, but couldnt find a power supply for even one of them :mad: I honestly don't understand how some of these 'pro' audio shops can stay in business with their attitude towards running the business. Anyhow, the sales guy 'substituted' a Behringer Tube preamp:

http://www.behringer.com/MIC200/index.cfm?lang=ENG

I asked several times if the FR on this thing was flat, if the highs were rolled off because of the tubes, etc. He swore up and down that it was fine for measurement, flat FR, and that as long as I kept the 'preamp mode' dial in 'neutral' position, it would be fine. The 'preamp mode' dial has a bunch of options, default being 'neutral', others being varying degrees of 'warm' or 'limiter'. I checked in the manual, and the specs say 10hz-47khz +/-3db.

So can I use this thing with JustMLS or other measurement software?

Also, a question regarding the Apex 220 measurement mic. I read folks on here talking about how it was comparable to the Behringer and had very flat response. So since it was a bit cheaper I went with it. However, there is an included FR graph that does not look that flat at all... its +/- .5 db from 50hz (where the plot starts.. shouldnt it start lower?) to about 2khz, then has a +1.5 peak @ 3khz, and a gradual bumpy rise to ~+3db @ 20khz. Is this normal in a good measurement mic? I seem to recall the plots I saw for the ECM 8000 looking a lot better, with only some minor HF ripple. If it is normal, is there a way to compensate for this, like maybe hand-editing a calibration file?

morbo 1st January 2005 07:53 PM

Can anyone tell me if this mic preamp is suitable for use in MLS measurements?

SY 1st January 2005 08:06 PM

I haven't used justMLS, but most software allows you to input a mike calibration file. With that in place, it doesn't matter what the frequency response is as long as it's known.

The FR you cite isn't bad at all. If you can get a copy of the first Stereophile test CD, there's a wonderful cut of Gordon Holt speaking where they switch from mike to mike. Now, all the mikes they used were top-quality pro mikes, yet each sounds utterly different from the others, and they all sound colored. Puts a lot of stuff like wire and caps into perspective.

morbo 1st January 2005 10:05 PM

thanks SY. In fact I don't have a .cal file for this mic, and haven't been able to find one on the net so far. Hence my concern over the FR. Also, the FR abberations I mentioned are actually twice what I said, I was misreading the graph. So the mic is + 8db @ 18khz. I may try to edit the cal file for some other mic to reflect my own. I guess I just assumed it would be as flat or close to the Behringer mic.

I am more concerned about the 'tube' preamp, since I have read some blanket dismissals of them for this purpose. I don't know whether that applies only to those that are meant to add a tube 'colour' to the music, or also to those which have a 'neutral' setting like this one.

SY 1st January 2005 10:44 PM

Well, it's hard to tell from the specs what the "neutral" means, but that should be easy to check. If it's not too far off flat, it'll work.

The dismissals of tube preamps in this application are based on several things:

1. Noise. A low-noise tube design is tricky and expensive.

2. Why bother? Going through the difficulties of designing and building a tube preamp won't make a measurement any more accurate. A 50 cent opamp works perfectly well.

3. Gain stability. If you want your absolute measurements to be repeatable and transportable, a tube will tend to be too drifty. Gain stability is enhanced by high open-loop gain and lots of feedback, i.e., silicon.

So... if you're doing more casual home measurements, the tube unit will do, but is unneccessarily compromised. That said, I'm again impressed at what Behringer can do for so few dollars; the bill-of-materials for that preamp can't be more than $15 or so.

bzdang 2nd January 2005 04:08 AM

The Behringer mic isn't perfectly flat either, nor is it as smooth as the curve in the spec sheet. Did you get a BK test sheet with your Apex mic? I did, and it's honest looking, not smoothed. Trends up around +3dB towards 20kHz. Perfectly good for measuring the impact of design changes on my boxes and crossovers. Not a darned thing I can do about the higher freqencies anyway, they're in the hands of ScanSpeak.
Btw, http://www.avleader.com.tw/index0.asp is where they're made. I can't say if they're making mics for Behringer. These cal. mics are too flat for performance recording, typically the industry uses mics with more character.
I will try to get a calibration done on mine this winter and of course will share it. Might even be able to do back-to-back comparison of mics if we end up at a diyToronto meeting.
The Behringer preamp is probably bypassing the tube circuit when set neutral. If you can't verify it, take it back and get an Apex apa1 preamp, they come with wall-wart in the box and as I'd posted elsewhere, have a bit of ripple at 200kHz. There's an L&M in Stratford if your dealer doesn't have them.

morbo 13th January 2005 08:16 PM

thanks for the replies. I know the behringer mic is not flat, but I've seen some 'independent' measurements of it, and they look a lot flatter than the plot I got with my Apex mic. At any rate, I've decided (well, been forced) to keep it. If you ever do a .cal file, I would be extremely interested in, and grateful for, a copy.

I ended up returning the tube mic preamp. Not worth the uncertainty, and I got a UB802 which many have used and said is good for the application.

bzdang 15th January 2005 10:28 PM

2 Attachment(s)
I've started trying out the LSPLAB [not LSPCAD] demo software, looks promising, I discovered that it comes with a mic cal. file for ECM8000, I have zipped and attached for you, hope it is valid :)

morbo 2nd February 2005 05:29 PM

Thanks for that file, but I ended up with the Apex mic. If you ever do a calibration of it let me know, and I will do the same.

bzdang 23rd March 2005 02:06 AM

My Audigy 1 soundcard is giving me fits when I try to calibrate my mic against the LMS mic, so I'm going to use the ECM8000 cal file until I can buy an Audigy 2 and do the cal. outside.


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