Pete (AC7ZL) wrote in to tell us about his latest project: building a flat response microphone and channel amplifier. You may remember his previous project: building a crystal radio from modern junk. Sounds are “colored” by their surroundings; things like furniture, wall coverings, drapes and building materials all affect the way something sounds. To measure the effect that a space has on sound you need a microphone with a flat frequency response. The core element of Pete’s mic is a modified Panasonic WM61A condenser capsule. He rewired it so that it had a broader dynamic range and could handle a higher SPL at the cost of reduced gain. To boost the signal to a usable level he built a preamp with three stages of amplification. He’s got schematics and a more detailed description on the site.
Is this “flat” in theory or in actuality? It would make it easier to calibrate for other programs if we knew more about what we had. Can you provide a frequency plot (MLSSA or equiv?) Those Panasonic capsules arent what they say in datasheets, and Im a wee bit dubious of anything based on TL084 being pristine by any means.
Pardon my ballbusting, but audio is a sneaky b*tch, and without real-world data I woulndt believe any claim made by any mfg.
The mic looks damn cool. Ala Dave Royer pipe-bomb.
Any time I see a mic that claims to be “flat”, I am extremely skeptical. In this instance, the guy does seem to know a decent bit about audio, but I agree with the above statement that without any actual measurement letting you know IF the mic really is flat, it’s pretty much an issue of perception. :)
Just because the spec for the capsule says it’s “flat” (I wonder what the tolerance is in their definition of “flat”), and even if those specs really reflected real-world use, once you add in all the other components you STILL have to graph the response, as the coloration most certainly would have changed.
A few thoughts:
a) Guys- Lighten up…this isn’t rocket science…it’s simply a project for somebody interested in acoustics who cannot justify the expense of “pro” gear.
b) Panasonic published a data sheet for the capsule which includes a very flat response plot from the low 20’s to over 20khz. If you choose not to believe the published specs, well, ok. What can I say? Now, it’s entirely true that the response could be altered by the capsules (mis)application, but a fair bit of attention went into addressing that. The data sheet is here:
http://www.hpfriedrichs.com/downloads-projects/wm60a.pdf
c) Somebody used the phrase “pristine” with regard to the tl084 capabilities or lack thereof. Input bias current, input offset current, input offset voltage, open loop gain, gain bandwidth product, slew rate, settling time, CMR, PSR, output impedance, et al, are among the parameters used to characterize an op amp. Note that “pristine” is an audiophile word, not an engineering term. I chose the ‘084 because it performs reasonably well, and is both cheap and ubiquitous. But if you don’t like the TL084, pick a different op amp! That’s the nice thing about homebrew gear. It’s what you decide to make it. :)
d) Someone drew my attention to a minor error in the amp schematic. This has been corrected and uploaded to my site.
Pete
AC7ZL
Pete,
I tried this with a crummy mic element and I seem to have screwed it up. The issue was soldering to the aluminum case of the mic. How did you handle that with the modified version? Did you ground the case?
Also, Digikey is currently out of the WD-61A. I’m wondering if the 62b will work. Do you know?
This microphone element has been used in Speaker Builder magazine in the Mitey Mike a system to measure speaker performance. If anyone is curious, they can look at the frequency response of the microphone in this PDF article: http://www.audioxpress.com/reviews/media/kd4review.pdf.
eric,
see:
http://www.hpfriedrichs.com/images-projects/audio-mscheme.gif
The drawing does not illustrate the principle as well as it should, but here’s the trick: when you cut the pcb trace linking the ground terminal to the case, make the cut toward the terminal end of the trace, (not the case end).
this will leave a very tiny pcb “nub” on which you can solder your new ground wire.
I admit to having worked this under a low-power stereo microscope, using a dremel bit with a real fine point on it.
If you try to solder directly to the microphone case, you probably won’t be successful.
eric,
see:
http://www.hpfriedrichs.com/images-projects/audio-mscheme.gif
The drawing does not illustrate the principle as well as it should, but here’s the trick: when you cut the pcb trace linking the ground terminal to the case, make the cut toward the terminal end of the trace, (not the case end).
this will leave a very tiny pcb “nub” on which you can solder your new ground wire.
I admit to having worked this under a low-power stereo microscope, using a dremel bit with a real fine point on it.
If you try to solder directly to the microphone case, you probably won’t be successful.
If you study the frequency response of the Panasonic datasheet, you notice the following on the diagram:
* the y-axis has got large values, upto +20 dB and -30 dB, that’s why it shows flat for most of the spectrum!
* if they reduced their scale for the Relative Response, it would show the minor variations!
* if you guys are concerned about flat response mics, there will always be slight variation, there is no perfect mic!
* if it’s really absolutely flat, it won’t be cheap!!
it is very intresting but i want to know how to build a remote or some kind of trigger that would mess up any kind of radio frequency
Hi,i need a good flat response mic for audio testing.Can someyone help? eg Make and model.Preferrably in South Africa.Wilhelm
Hi, Wilhelm,
I hope it’s not too late…I would suggest the Panasonic WM61A mic capsule..it cost only about $2.50 or so,but performs like a $1000,00 mic!!!
I’m not sure how much you’d be willing to spend on a mic…but
You can get this very cheap on EBay(I did)or http://www.digykey.com.who might still have a few.But then you’d need to build it into a suitable housing like a 3.5mm mini stereo jack plug or similar.you’d also need a pre amp to drive it.
If you’re not really geared up for this then you cold perhaps take a look at the Behringer ECM8000 measuremnt mic.
Good luck.
Aus