Passive Bass Drum Mic Filter Pt. 2 "Final"

EDIT: Samuel solved my inductor problems, so I've been able to proceed with an LCR design for this. The RC circuits I tried were just not as good, especially with respect to variations in source and load impedances. Skip down a few posts to see the "final" (yeah, right) design.

Here's a link to Part 1 of this thread-
http://www.groupdiy.com/index.php?topic=7293




Well I was pretty smugly self-satisfied with my filter design until I took a look at the inductors I had here for my speaker project that I had designed my filter around. They are not 460mH as I thought, but 0.460mH. I was only three orders of magnitude off.

So it's back to the, uh, drawing board. I'm going to play with RC designs, trying a couple cascaded stages and taking a look at twin-t filters.

I'll be back.

I was thinking that 460mH is a lot - but I guessed at 800 Ohms speakers.. :razz:

Well, you can order free samples from www.wilcocorp.com...

Samuel

Thanks Samuel, but their biggest one is 10mH. I think a half henry coil is gonna be pretty darn big.

I've been getting some good simulation results with variations on twin-t filters, though. The problem is that they're quite sensitive to variations in source and load impedance. I've been able to come up with a nice design for my BD mic (which it turns out is a "rated" 500 ohms - who knows what that really means) and preamp, but it looks hard to come up with a more general solution.

I'll be posting again, but I'm going to check things out a little more carefully this time. :green:

their biggest one is 10mH

Click to expand...

No, I have 1000 mH at home... BSL series.

And it's not that big.

Samuel

Try measuring inductances on some small audio transformers you have around.

400mH is in the area where many cheap 600:600 telephone transformers are..

Jakob E.

dayvel, you could include a small phantom powered active circuit so you can calculate your filter with a fixed input impedance instead of the unknown impedance of your preamp. Take a look at PRR's ribbon booster using two bipolar transistors in the active ribbon thread. Your only unknown variable, then, would be the mic output impecance. And if you plan to use it with just one mic, it wouldn't be too hard to work it out, I guess.

Thanks guys, turns out that 500mH inductors are pretty standard in wah-wah pedals too.

EDIT: I should have poked around the Wilco site a little more; I've got samples on the way. Too cool.

They were in your pedal, you say! :grin:

Samuel

No I don't have a wah pedal, but I'll be able to build one now! Thanks again Samuel for the tip about Wilco. (See my edited post above.)

Now that my inductor problems have been solved (thank you, Samuel) here's a generalized design for the filter.



This includes LoPass and HiPass rolloff filters but you can make this as simple or complex as you want; you can leave the HP and LP filters out, you can switch them in and out, you can switch between different values of Ctune to change the scoop frequency. Whatever.

Here are the response curves for Rsource=150R and Rload=5K, without and with the rolloff filters with Rdepth set to 1K -



The circuit is well behaved with a range of source and load impedances, mostly just level shifting.

Here's the SPICE code if you'd like to play with this-


Bass Drum Filter Variable LCR With Rolloff

*input resistors
Rin1 SIG31 SIG14 1000
Rin2 SIG6 SIG16 1000

*tuning and depth
*Rlim includes Rdc of L1
Ctune SIG14 SIG41 0.22u
Rdepth SIG41 SIG20 1000
Rlim SIG20 SIG42 500
L1 SIG42 SIG16 470m

*Source and load imp Rs1=Rs2=1/2 Zsource
Rs1 SIG1 SIG4 75
Rs2 SIG2 SIG32 75
Rload SIG14 SIG16 5000

V1 SIG1 SIG2 ac 1 dc 0

*HPF and LPF - put asterix before Clp
*and Rlp lines to eliminate LP filter
Chp1 SIG4 SIG31 4.7u
Chp2 SIG32 SIG6 4.7u
Clp SIG14 SIG39 0.005u
Rlp SIG39 SIG16 470

*Spice wants these - change Rdum1 and
*Rdum2 from 1g to 1 to eliminate HP filter
Rshunt1 0 SIG1 1
Rshunt2 0 SIG2 1
Rdum1 SIG4 SIG31 1g
Rdum2 SIG32 SIG6 1g

.control
delete all
ac dec 100 10 20000
plot db(v(SIG14,SIG16)) xlog
.endc

.end