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Old 28th June 2007, 10:15 PM   #1
bkspero is offline bkspero  United States
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Default Inline pad sounds muddy

I need help diagnosing problem with muddy sound when using a homemade inline microphone pad on the input to the preamp of my recording setup.

Have to turn preamp gain to its lowest setting to avoid clipping of the kick drum mic (AKG D112). I would like to drive the preamp harder, so I built an 18 dBv inline mic pad using two 560 ohm resistors on the input lines and a 160 ohm bridging resistor. It provides a nominal input impedance of 1280 ohms and an output impedance of about 160 ohms.

The pre is listed as having 1200 ohm input impedance and the mic says 210 ohms. So these resistors seemed like a pretty good match. Assembled the resistors to the male end connector of an XLR barrel connector.

All resistances check out correctly and the signal attenuates as it should (I need 3 more 5 dB steps of gain on the preamp control). Problem is that the sound of he kick is muddy/indistinct with the pad in the circuit whereas it is much clearer without the pad.

Otherwise the preamp is normal. Sounds great with the other drums and when mic'ing guitars.

I am sending the preamp signal thru a compressor, but that shouldn't be affected by the pad, should it?

What could be causing this symptom? Could poor solder joints be the issue? Did I select improper values for the resistors? Please help?
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Old 1st July 2007, 05:14 PM   #2
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For a start I would suggest turning the compressor OFF (or better still removing it entirely), and see what difference the pad makes then?. With the compressor ON you're altering the sound, and the alteration is level dependent, and the pad alters the level.
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Old 1st July 2007, 06:36 PM   #3
bkspero is offline bkspero  United States
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Default Thanks for the responses

I will try it without the compressor. I should have done that already. I didn't think that it was going to affect the compressor other than to possibly add a little more of the preamp flavor to the signal. The pad is on the input to the preamp, and compressor is after the preamp. So all I am doing with the pad is dropping the signal into the preamp and then adding amplification with the preamp to bring the preamp output level (compressor input level) to the same value it was prior to using the pad. BTW, I have experience with the preamps at the higher gain level, and they sound fine there, so I don't think that adding gain is the cause.

Hartono, I'm not sure that I fully understand your response. I thought I understood the parallel/series resistance calcs. This was my thinking:

1)The mic sees the 2 series resistors in series (1120 ohms) with a parallel arrangement of the preamp (1200 ohms) and the 160 ohm resistor. The resistance of the parallel preamp and resistor is about 140 ohms (1200*160/(1200+160)). So the mic sees 1260 ohms (1120+140). That's pretty much what it expected to see with the 1200 ohm input impedance of the preamp.

2) The preamp sees the parallel arrangement of the 160 ohm resistor and the series arrangement of the 2 larger resistors (total of 1120 ohms) and the mic (210 ohms). Or a parallel arrangement of 160 ohms and 1330 ohms. That's equal to 143 ohms (160*1330/(160+1330)). Which is, I thought, in a reasonable range of impedances for microphones.

Did I do the calculations incorrectly? Or have I interpreted them incorrectly?

Thanks again for your time and responses. Have a good day.
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Old 1st July 2007, 06:40 PM   #4
Hartono is offline Hartono  Indonesia
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oops....

Hi Bkspero,

I've just deleted my post, did you reply when I deleted it ? I'll look into it further and let you know, I guess probably I make a mistake in my explanation, so I deleted it.

Hartono
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Old 1st July 2007, 07:29 PM   #5
Hartono is offline Hartono  Indonesia
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Hi bkspero,

your calculation is pretty much correct, sorry for my mistake.

Hartono
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Old 2nd July 2007, 02:57 AM   #6
bkspero is offline bkspero  United States
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Default I appreciate your input

Hartono, thank you again for responding. I appreciate your input and double checking my calculations. Hope you are enjoying yourself while in China.
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