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Old 4th April 2006, 01:41 PM   #1
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Default Mic pad

Hello everybody,
I have an ALTEC 1591a compressor amplifier. Line input works ok but mic input is way too hot. The pre amp is putting out so much gain that it is nearly impossible to connect a mic and obtain a non distorted output. Can anybody suggest a 50dB or so pad to install right between the mic input and the preamp module? The preamp module impedance is rated at 150/200Ohm input and 600Ohm output.
I made a simple pad made of two 430Ohm and one 26Ohm in a Pi figure that seems to be working but I did not considered the input and output impedance of mic input and pre input.
Thank you.


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Old 5th April 2006, 01:55 AM   #2
infinia is offline infinia  United States
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Hi
50 dB seems too much to atten. (maybe you don't need a preamp
Your present network has too much loading for a 600 ohm output. Try a pi network with three 1K res. should be around 12dB atten depending on it's loading and much closer to 600 ohms. Add a 500 ohm shunt resistor at atten output if the compressor input is higher than 600 ohms. Keep lowering the last shunt resistor if more atten is needed.
I hope this helps
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Old 5th April 2006, 12:23 PM   #3
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Thank you,
unforytunately I need that much of an attenuation. The pre amp module is really really hot! 12dB is not enough. With the gain knob just above zero a snare drum is completely distorted...
So you suggest three 1k and a 500 shunt resistor in parallel? Did I get it right? This is like having two 1K and a 330...
Lowering the resistor between the signal wires will give me more attenuation. Is it correct?
Thank you

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Old 5th April 2006, 07:07 PM   #4
infinia is offline infinia  United States
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You got it correct,
Try this first for approx -12 dB pi network with two 1K and last 330 ohm shunt.
lower 330 ohm for more atten.
If you need alot more atten than 12 dB increase series 1K to 2-3K.
So you are basically trying to keep network total resistance seen by source as 600 ohm.
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Old 6th April 2006, 07:46 AM   #5
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thank you.
how do you calculate the impedance of the net?

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Old 6th April 2006, 01:41 PM   #6
infinia is offline infinia  United States
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Try the network first to get it in the "ballpark".
I think the network will work fine for you based on your previous experiment. I calculate around -18dB voltage loss.
regards
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