I'm currently designing a digital audio mixer. One common thing found on audio mixers is a optional 20dB pad on the input. I've seen some attenuator circuits but all of them require you to know your source impedance. However, with mixers, it's impossible to know the impedance of the connected device. How could I create a 20dB pad then? If I try to design a circuit that has large enough resistance values that the source resistance doesn't matter, it gets REALLY noisy.
Electronic – Audio Mixer Passive Attenuator Circuit
attenuatoraudiomixerpassive-networks
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To make life a bit easier use the switch wiper connected to the first resistor and the other switch contacts to individual resistors that connect to ground. Only one resistor gets grounded therefore there is no interdependence: -
simulate this circuit – Schematic created using CircuitLab
I'll assume this is not actually for use in an aircraft- if you are actually mucking with stuff in the cockpit, see your DAR for official advice that conforms to FAA etc. airworthiness requirements.
If you use 10 ohm resistors and the impedance of the phones is really 10 ohms (often that's pretty variable) your av audio power will be reduced by about 12dB, which is quite a bit. Also the headphone impedance your MP3 player is expecting is typically more like 35 ohms.
I would suggest trying more like 27 ohms on each MP3 output and 5.1 ohms on the av output. That will reduce the av power by less than 6dB. The MP3 sound may not be loud enough, but it's probably not really designed to drive a 10 ohm headset.
Best Answer
If you take a look at existing mixer designs, you'll find that they typically use a passive resistor attenuator, and that the values aren't necessarily huge. Because it's just a pad, the 20dB needn't be exact (the trim control will sort that out).
So, determine the typical source impedances that your mixer is going to be handling, and pick resistor values that give you approximately 20dB of attenuation. If it varies depending on the source, that's not a huge issue.
Most of the sources you are dealing with are going to be 600Ω +/- 300Ω (ish). Don't bother with trying to design for instrument pickups and other Hi-Z sources: that's what DIs/preamps/active pickups are for. Yes, there are microphones in the 5kΩ and up range which you may want to consider. However, any microphone past 15kΩ or so ought to be considered Hi-Z, and I would argue that microphones past 5kΩ are atypical enough to warrant a preamp of their own.
I'm a big fan of Allen & Heath because lots of their mixer schematics/service manuals are easily available (also, the consoles sound great). If you take a peek at some of these, you'll get an idea for design and component values of a typical mic pre.