Passive Bass Drum Mic Filter

Inspired by the *rthwrks* "kick pad" I want to try a passive filter to take a healthy scoop out of the midbass from my bass drum mic. The idea is to scoop out about 8dB centered at about 400Hz. As a first approximation, I came up with this-


I'm pretty much out of my depth with filter design, so
I'd appreciate your thoughts on this, especially anyone who could run this on a sim program. I'm assuming a mic impedance of 150ohms going into a mic pre with an input impedance of around 5kOhms.

I'd be grateful for any help with this.

[removed].

Not bad, a bit high in freq. Did not had time to play with other values. Your 5k load may be a bit high for typical designs.

Samuel

Cool. The mic pre I'll be using has a 4.4Kohm input impedance, so that should be pretty close. Looks like if I double up my capacitor values, I'll be close to where I want to be, but I'd like the slope of the scoop to be a little steeper so I'm going to have play with the resistor values, too.

Thank you very much for your help, at least I know I'm on the right track.

Viitalahde that is one of the more weider things I have ever seen on the net. :thumb:

Why not make the scoop variable?

"But why do I see that filter as peaking?"

Maybe been spending too much time with the acid llama?

The best thing is that this finally got me to download MacSpice and learn how to use it. Fortunately, this circuit is easy enough for me to learn the program with. Realizing that my original circuit was equivalent to this one-



I only have to model the top arm of the circuit and cut the source and load impedances in half to get the response curve. I popped the values into MacSpice, and to my delight, the result looked just like Samuel's plot. So I'll play with this and post back in a bit.

Welcome to the Dark Side of the Force muahahaha...

Thank you; my inner nerd is happy, but my musician side is horrified.
Anyway, after playing around a bit, I decided to see if I could use some of these 460mH inductors I have lying around here and get a more complex response.

Here's what I came up with-


The response plot looks like this-


Which looks a lot like the EQ I usually put on my bass drum.

I'd love it if someone could check this for me (although it's got quite a bit trickier) because I'm pretty new with this stuff. I had to model it as an unbalanced version because Spice wouldn't let me do it balanced and then translate it into a balanced version. I think I did it right, but I reserve the right to be an idiot.

[quote author="pucho812"]Viitalahde that is one of the more weider things I have ever seen on the net. :thumb:[/quote]

The link in his sig line? You want to induce a psychotic reaction w/o drugs, right click on the flash animation and click on play. Repeat. Repeat. Repeat. . . . Or maybe don't. It's not a particularly good idea, but it's particularly f***ed up.

Bear

> Am I stupid or..

Insufficient evidence.

> but isn't that actually a peaking filter instead of scooping?

These things can go either way, depending how you stack the R and C values.

In this case, working up from DC: at DC it is unity gain. At some point the 1.0uFd kicks in and loads the series resistors, causing a droop. This stops when the shunt resistor gets in the way of the 1.0uFd cap. Then at some higher frequency the 0.2uFd caps short-out the series resistors, bring gain back up, ultimately to unity again.

So it is a dip.

Actually, you'd have to rearrange things to make it a peaker.

> I'd like the slope of the scoop to be a little steeper

The RC network is about as steep as any RC network gets. Oh, you can use a bridge and get a null, but the sharpness of the corners (which is probably more important than the depth of the dip) won't get better without more tricks.

> finally got me to download MacSpice and learn how to use it.

Filter responses are one of the few things I trust SPICE for. Of course, ONLY if they give the general shape that I have already worked-out on a napkin. Then I trust SPICE's reactance interaction calculations and dB conversions, saving many slide-rule miles.

> I had to model it as an unbalanced version because Spice wouldn't let me do it balanced and then translate it into a balanced version.

SPICE needs a "zero" reference. If you must fool with fully floating circuits, you can put 1,000 Meg resistors from balanced nodes to ground. Or you can force it to hard-balanced with a couple 1Ω resistors across the voltage source, center-tap grounded. (Remember that SPICE voltage sources can't be overloaded with "impractical" loads, and you don't get the electric bill.) But that kind of stuff forces you to find the differential probes and keep your head straight.

Any balanced nework has an equivalent unbalanced network. For your R-L-C network, it is trivial: move 0.5Rs 4.7uFd 1K from the bottom leg to the top leg, and combine terms (Rs, 2.35uFd, 2K). Now you can hard-ground the bottom leg, and measure everything with simple unbalanced probes. When you go to build it for real, cut those combined parts in half and double-up again.

Another thing that will upset SPICE: a node that connects to the rest of the world only via capacitors. While I don't see that problem here, it can arise in filter networks. SPICE can't compute the DC level of a node that has no DC connection to zero, and SPICE caps have no DC leakage. While our caps have some leakage and we really don't care what the DC level is at a cap node, SPICE "has" to know the DC at every node. Again the fix is an incredibly high resistor from the offending node to the zero reference.

> check this for me

Without actually firing-up SPICE, slide-rule, or even a napkin... it looks like the right bends in the right sequence. The 2*4.7uFd cut the bass. The 0.01 shelves the treble. The L-C tank sucks-out the mids. I did use a calculator to get "-12dB" suck-out, but there are other things I ignored so 9dB might be right. (But you might want to plot more than two points per octave: looks like you don't have a point at the very bottom of the dip. Such labor would choke my 8o88, but on a Pentium {or G3} you can plot 100 points per octave in a zip.)

Hmmm. That's not quite how I did the transition from balanced to unbalanced.

I took the original circuit-


redrew it like this-

And ran the sim on the top half-


Which, I think gives the same result, if I'm not mistaken.
Of course I could be mistaken; it's been known to happen.

On thing to consider: typical bass drum mics (dynamic ones in general) are far away from beeing a simple 150 ohm resistor; probably too far to trust the curves you get to more than +-5 dB.

Samuel

Another thing that will upset SPICE: a node that connects to the rest of the world only via capacitors. While I don't see that problem here, it can arise in filter networks. SPICE can't compute the DC level of a node that has no DC connection to zero, and SPICE caps have no DC leakage. While our caps have some leakage and we really don't care what the DC level is at a cap node, SPICE "has" to know the DC at every node. Again the fix is an incredibly high resistor from the offending node to the zero reference.

Click to expand...

Lots of info in PRRs post as well as multiple instances of good humor :thumb:

What I wanted to add: yes, better add that 'incredibly high resistor' yourself. I don't know what Spice does when it encounters a floating node, but the 'in-house-sim' we use at work automatically adds a //-resistor to solve it. But its value is depending on the cap value, so unless the default R*C-setting is altered it might influence circuit behaviour (in simulation).

And making the DIY-resistor high in value is OK but don't make it higher than necessary. I mean, making it many TeraOhms will drive the simulator nuts because it has too much, ehh let's call it 'dynamic range' to cope with since there might also be some very small numbers present (like 10pF caps or even smaller when you have some device-parasitics included).
But again, I don't know what Spice does here.

Regards,

Peter

Inspired by the *rthwrks* "kick pad" I want to try a passive filter to take a healthy scoop out of the midbass from my bass drum mic.

Click to expand...

On an aside note, what are the benefits of using such a filter as you can easily tune & toy around before and/or afterwards with an EQ ? Please note I don't want to spoil the fun here, but am just wondering. I've heard their demo-CD and wasn't overwhelmed - could figure that having some tuning-possibilities to adjust for the situation would work better than inserting a fixed EQ-shape.

Regards,

Peter

The virtue of the dingus is supposed to be that you don't need specialized mics, so they hope you'll take the savings and buy their mics.

I actually like the concept, and am watching this thread. I have general purpose dynamic mics I use on kick which I pad and eq anyway, so I see great merit in the idea. Maybe just used with a different mic. I do see the point about why only settle on one preset eq scoop. Well, do more than one and have it switchable. If we aren't using this with phantom powered mics, there should be no issue of toggling around to get the sound

Bear

This morning I realized that SPICE had driven me mad with power, and I was just making this too complicated, trying to do too much at once and losing track of my design goal. Also, Samuel made an excellent point about the mic impedance not being a simple resistance. I figure the simpler the filter, the less likelihood of wacky interactions. Since my latest iteration only works at 400hZ where the mic's impedance *should* be fairly well behaved, its behavior should be pretty predictable.
I've also incorporated a variable resistance to vary the depth of the scoop. Here's the circuit and some response graphs for several resistance values. Note that these plots assume a source impedance of 150 ohms and a load impedance of 4400 ohms and that the bottom plot is on a vastly different scale than the others.




As to why you would want to use something like this, here's what I think. Typically, one always needs to EQ out mud/boxiness on a bass drum track. This simultaneously emphasizes the desirable "whoomp" and "smack" frequencies on either side of the mud/box freqs. This little gizmo lets you do this with some nice passive EQ without the necessity for an additional box for make-up gain. And of course, the inductor adds the magic of iron.

dayvel, I think the latest plan is the best so far, that could work very well. Of course the two 1k add some noise and you don't want to use that with 60 dB gain, but that's not need with most drumers!

A simple addition would make a switchable cap for some choice of frequency. In addition to this, I would add a resistor (say 500 or 1k) in series with the pot. High Qs will not be of much musical use, IMO.

Edit: The inductor might have a rather high DC resistance, so a series resistor is not needed.

Samuel

Thanks Samuel, those are both excellent ideas, I think a 330 or 470 ohm resistor in series should be about right. The 460mH inductors I have sitting around here are left over from some speaker crossovers , so they have really low resistance.

I don't think noise will be a problem; the huge signal from a mic in a bass drum is what makes this idea work in the first place. There's a lot of signal to throw away without running into noise problems.