how do you make a bass rolloff pad for a condenser mic?

[tempus]

Hey all;

I use a condenser mic mounted inside my guitar for amplification. As you can imagine, the bass is HUGE in there. I've looked into adding some type of bass rolloff, but I can't see how to implement it since any cap I use would be in series with the DC power supply (thus blocking it) going to the mic itself. I know it can be done, because I have a (much bigger) mic that has a bass rolloff.

Does anyone know how to accomplish this?

[Steve Conner]

The pad in a condenser mic is part of the mic circuit, before the phantom power gets mixed with the audio. You'd have to open those mics up and mess with them.

Or get a preamp with tone controls on it.

[tempus]

Well, that's inconvenient.

I've got ample tone control when the signal hits the mixer, but there is a ridiculous amount of excess bass in the everything-below-250Hz range. Like I said, I can get rid of it at the mixer, but it would be better if the inputs weren't getting hammered by all that excess bass.

Another part of the issue is that the mic requires 48v. As I'm sure you know, many mics can get away with more like 15v, which is what my mixer-amp combo offers for phantom power. The mic works with 15v, but is very distorted. I was kind of hoping that rolling off all that extra bass might make it workable at 15v so I don't have to lug around a phantom power supply to every gig.

Do you have any other suggestions?

BTW, is your signature quote from RG Keen?

Thanks

[J M Fahey]

What mic are you using now?
Any technical data on it,links, etc.?
Although the original Phantom concept is based on 48V , *most* modern ones accept much less, I've seen as low as 12V, so yours puzzles me.

[Steve Conner]

Yes, it is from RG Keen.

The problem is, that any filter you put on the output of the mic will be after the internal preamp, so won't prevent that from clipping.

In which case, I would bust the thing open and replace the 1.21 Giga-ohm resistor that biases the capsule with a smaller one.

If it's not the mic that's clipping, but the front end of your amp, well, this thread http://www.dvinfo.net/forum/all-thin...tom-power.html states that the Shure A15AS in-line attenuator will pass phantom power.

Most other attenuators should, as they're balanced networks, they can attenuate the differential mode signal without touching the phantom power, which is common-mode. Find the schematic for that kind of attenuator and add a capacitor in series with one of the resistors, to make it a high-pass filter instead of an attenuator.

[Enzo]

And just where in the guitar is the mic mounted? it matters a great deal. Ther are resonances in the guitar box. perhaps at the edge of the sound hole might be better than down in the middle inside.


Imagine you are micing a guitar amp for the PA or recording. It would sound a lot different putting the mic inside the speaker cab rather than out in front of it.

God, it has been 40 some years ago now, but I once needed to mic my big old Gibson acoustic - J50 - I took an EV lavalier mic and fashioned a coathanger wire and hung the thing on the edge of the soundhole pointing across the strings. This was before the tiny things they clip on your collar these days, the mic was about the size of my finger. But the result actually sounded pretty good. Earlier we had tried just chucking the mic into the hole - didn;t work so well.

[tempus]

First, I'd like to thank everyone for their replies.

What mic are you using now?
Any technical data on it,links, etc.?
Although the original Phantom concept is based on 48V , *most* modern ones accept much less, I've seen as low as 12V, so yours puzzles me.

Agreed. Here's a link to the mic - unfortunately it's all I have: K-Micro Silver Bullet Matched Pair :: Condenser Mics K-35, Prophecy, K-10, K-micro, Silver Bullet :: Karma Mics - Dynamic mic - Condenser Microphone - Recording Microphone - San Jose, CA. I'm wondering if an easy solution might be to pop it open and lower the resistor in there to make it accommodate 15v.

In which case, I would bust the thing open and replace the 1.21 Giga-ohm resistor that biases the capsule with a smaller one.

Funny, I was thinking the same thing. My line of reasoning is that if there's a really high value resistor in there for proper biasing at 48v, perhaps I could scale it down for 15, thus eliminating some of my problems. Does this seem viable to you?

Enzo, I've tried a number of different arrangements over the years, and this current (inside the guitar) setup is the best compromise between good tone and rejection of unwanted background noise. And the tone really is good - no comparison between it and a pickup. If I could eliminate the bass and phantom power issues I'd be all set.

Thanks

[Steve Conner]

Well, I wasn't thinking of the resistor altering the bias point. The resistor forms a high-pass rolloff with the tiny capacitance of the capsule. If you make it smaller, you'll roll off the bass.

[J M Fahey]

Given the price, $29 for *two* of them, there's no way that they are anything else than regular electret mics, altered to be properly fed by phantom power.
*Maybe* the designer tried to follow official phantom power specs "by the book" (which nobody else does) and he designed them strictly for 48V, not considering that most real-world mixing desks provide what they already have available to power the rest of the electronics, usually from +24V, down to +12V.
If you dare, open one of them and take a peek, maybe with a couple sharp well illuminated pictures we can guess the schematic.
I think it won't be much different to what's explained in:
Powering microphones
Build your own following what's posted under "Balanced electret microphone circuit" (*horrible* ASCII art but you can redraw it ) but lowering those "10u" coupling capacitors to 1uF or .47uF to cut bass below 160 or 320Hz, assuming the Mixer input impedance is around 1K. If it's different (might be as high as 4K7) adjust to suit.
In practice, use a dpdt to switch between those 10uF (flat response) to the new much lower (bass cutting) ones, let your ears be the judge.
Switching will pop loudly, but it's meant only as a temporary measure until you find (by ear) a value that suits you; then you solder the definitive value.
Your Silver Bullets can't be much different from this.
The designer might have used large value dropping resistors which work only with 48V.
Being electrets probably they don't use a 1.21 GOhm resistor, all I have butchered had the Fet gate just pressing one face of the electret capsule, no resistor at all. (I thinj air humidity alone will be enough)

[Steve Conner]

Oh, that is a good point JM. I had two cheap Yoga cardioid electret mics like that.

I ended up taking them apart, pulling the two capsules out, and mounting them at 90 degrees inside the windshield of an old SM58 clone, to make a stereo mic for my portable recorder. Because they were just electret capsules, the 3.3V phantom power from it worked fine.

[tempus]

Thanks for your reply JM.

Given the price, $29 for *two* of them, there's no way that they are anything else than regular electret mics, altered to be properly fed by phantom power.
*Maybe* the designer tried to follow official phantom power specs "by the book" (which nobody else does) and he designed them strictly for 48V, not considering that most real-world mixing desks provide what they already have available to power the rest of the electronics, usually from +24V, down to +12V.

This is exactly what I was thinking. I used to use a cheap ($3 or so) electret mounted in the guitar, and it also worked very well, but I wanted a balanced out and these were small and cheap enough to provide that for me. Plus they're cardioid which my element wasn't. I've got no false illusions that this is anything but a $3 capsule.

I wondered the same thing about the 48v phantom power - maybe it's been designed to work with 48v, and can be modded to work with 15. I think I might crack one open and see what's going on in there. If that's the case, I should be able to change the bias resistors to make it work with 15v shouldn't I? And from Steve's suggestion this should also provide a bass rolloff as well?

[J M Fahey]

Quite probably yes to both.
I think the shortest path is just building the "Balanced electret microphone circuit" from scratch, on Proto or perfboard, the capsule dangling from a 3 ft wire, taped to the guitar, don't worry about the enclosure yet.
The point is to make it sound good, only after that you either mount it in a suitable enclosure or reform your "Bullets" to suit.
If your Phantom power is too low you may lower those 22K resistors; the low frequency cutoff is given by the series capacitors (today 10uF) vs the actual input impedance from your mixer in parallel with the phantom resistors (today 22K).
Out of the top of my head, probably 0.47uF electrolytics will be in the ballpark.
Don't worry about the "cardioid" capsules, if you are inside the guitar or less than, say, 20 inches from it, it's irrelevant, and in fact Omni will work best.
Good luck.
PS: I've made *tons* of such microphones for our Tango orchestras and Folkloric players, all of which use purely acoustic instruments (Spanish guitar, violin, accordion, wood and raw hide drums, various Andean Pan flutes, etc.) and for the crazy "Les Luthiers" (Google them)which use the weirdest instruments, made out of typewriters, coconut "xylophones", "brass" made out of gas piping, Danish ham can violins, you name it.

[tempus]

Thanks again JM.

I may end up reverting to 1 of the old electrets that I have from previous installations, and using the balanced circuit in the end, but I'd like to give the Bullet a crack first. It has given me my best sound to date with the fewest negatives. The cardioid does make a big difference in rejecting the unwanted background noise. Often in between songs a patron may come up and chat with me. With the omis (which I found also added a lot more reverby sound) I had to be careful not to broadcast someone's conversation about how much they hated their date/mother-in-law/fill in your own embarrassing conversation here to the whole room. The cardioid is much better at preventing that.

Albeit at the expense of a little fun at someone else's expense...

I'm sure I'll also have to deal with surface mount stuff in the bullet which I'm not looking forward to. I'll open one up and see.

[km6xz]

I am surprised they are unidirectional, in fact testing polar response might surprise you also. Those sorts of electret elements are so cheap, you could make your own for less hassle than modifying theirs. Regardless of the advertised polar pattern, stuck inside a kick will nullify that characteristic.
Yes, they will run just fine on anything over 3 volts with. The only ways to reduce low frequency sensitivity is to do it acoustically as in placing it in a trap, or by filtering(varying the coupling cap, or adding a high pass 1 or 2 pole RC filter. It is hard to overload one of these in a proper housing, the diaphragm has such a small cross section, as a result they have naturally a very flat frequency response. Their big weakness is self noise, made worse by the low output(due to small diaphragm). But for a kick, low output is exactly what you want. A attenuator is seldom needed on one. Of all the kicks I've recorded on all the albums I don't think it once crossed my mind to try a little electret....except pressure zone configuration.
What is fun about electrets is that cheapies for $4 sound very similar to much more expensive electret mics, probably the best bang for the buck in sound. In true condensers, ribbon, or other dynamics, you get what you pay for.

[tempus]

Well, I opened it up last night - it was easy. Just one screw and the whole thing slid out just like on a normal XLR plug.

I'm going to send a photo and I'm signal tracing through so I should have a schematic soon as well, but it's basically a few resistors, 2 diodes, a cap and some T092 encased device. I initially thought that was the FET, but remembered that the FET is usually inside the capsule itself. I did some more testing and it appears to be a voltage regulator. From my 48v supply, 26v shows up at one leg, 7 in the middle and 6 on the other leg, which also shows up at the capsule.

I need to figure out if it's fixed or variable. If it's fixed, I don't see why the 17v or so from my mixer isn't adequate. If it's variable, I suppose I could change the adjustment resistor to make it work with the 15v. I could always just yank it but I'm not sure the capsule will tolerate 17v. Most I've seen have been 3-10.