Mic Attenuator

[BeniRose]

I know this is a little douchy on my behalf, but I'm having trouble finding an attenuator tutorial that I can understand. I was hoping someone might be able to give me direct instructions on building the attenuator I need to lower the signal from a 7 inch Sub speaker that I'd like to turn into a microphone for my kick drum. I'm not even sure the specs on the speaker, so I'll just tell you everything I know, starting with the markings on the speaker itself:
W65A04HRX30A-00

I'm going to be plugging it either into my firepod or into my ART Digital MPA, so I'm not sure how many dB I need to lower, but it seems most people who've turned sub speakers into mics say that -20 is a good number. So if anyone could tell me what ohm resisters I need and how to wire them to the + and - of the speaker, and then to the pins on the end of the XLR cable I'll be plugging into my interface/pre, I'd really appreciate it. Again, I'm sorry I'm not taking more initiative on this one, but I've been struggling with it for a while and I'm tired of looking at this speaker that's just sitting there unused. Thanks!!!!

 

[mshilarious]

The DMPA can tolerate very loud inputs and can be set for zero minimum gain, so I'd be surprised if you really need a pad. But, for -20dB, use 470-100-470. That's 470 from each leg of the speaker to pins 2 and 3, with the 100 between pins 2 and 3.

 

[BeniRose]

I'm sorry mate, I really appreciate the attempt, but I still don't follow. Can you break it down into laypersons terms for me? I attach one 470 ohm resistor to the + and attach it to the negative pin (which I can't remember is 2 or 3) and then do the same for the - to the positive pin, and then bridge the two pins with a 100 ohm resistor?

 

[mshilarious]

Speaker + --> 470 --> pin 2
Speaker - --> 470 --> pin 3
pin 2 --> 100 --> pin 3

You might want to check polarity on me, because for some reason I'm thinking a mic and a speaker would be backwards Probably somebody else knows that. I've played around with speakers as mics but never bothered to measure that.

 

[BeniRose]

Thanks mate. I know that I'm supposed to run the positive from the speaker to the negative pin, and I know that pin 2 is hot and pin 3 is cold, but I don't know if hot is positive or negative. Also, I forgot to ask, do I need to worry about wattage rating? Thanks for all your help!

 

[BeniRose]

I got 1/2 watt resisters, should this be ok?

 

[mshilarious]

Thanks mate. I know that I'm supposed to run the positive from the speaker to the negative pin, and I know that pin 2 is hot and pin 3 is cold, but I don't know if hot is positive or negative. Also, I forgot to ask, do I need to worry about wattage rating? Thanks for all your help!

Hot and cold aren't the best descriptors for a mic cable. Technically, pin 2 should be positive with respect to pin 3 when there is positive pressure on a microphone's diaphragm. That's a fancy way of saying that pin 2 is positive (hot) and pin 3 is negative (cold). Hot and cold are unhelpful because in a phantom-powered circuit (which of course a speaker doesn't need), both pins 2 and 3 have +48VDC. The hot and cold labels make some people think that only one pin is powered.

The resistor rating doesn't matter here,

 

[lassoharp]

" . . -20db a good number . . "

Go to this site:

http://www.giangrandi.ch/electronics/attenuators/attenuators.shtml

Choose the type of attenuator you want to build by selecting one of the circle boxes - pi, T, O, etc

Type in the db amount of attenuation you want - 20 - in your case. Then type in the two impedance values(in/out). The calculator gives you the resistor values. If they come out unusual just round to the nearest common value. Note the diagram beneath the calculator that shows an exact diagram of how to wire the resistors.

It's hard to say what amount will work best. For the impedances (Z values in the calculator) , try using 1500 and whatever your speaker impedance is - 8 ohms for example.

I've never tried a sub box, but from my experience, when using say a 12 guitar speaker for a mic you often wind up needing extra gain, not attenuation

But just for further reference, the best way to build them for a real world application is to use a breadboard for easy resistor substitution. Build the circuit on the breadboard and use alligator clips for the leads. Plug in your calculated resistor values and listen to the mic/attenuator/preamp/interface setup with someone out there kicking the drums. That way you'll know exactly how well it's gonna work with your particular setup.

Best of Luck