Convert interface mic pres to line level inputs

W

Warwick720

New member
Hello! I have an M Audio Omni I/O soundcard interface, and it's a good little dude, but it's got some serious limitations. It has two mic preamps and two line level auxiliary ins. I use four mics to record drums. I have better outboard preamps than the built in ones, but I know better than to feed a line out from a pre into another mic pre. Is there any way I can convert these mediocre preamps into line ins so I can just use four of my own outboard preamps?
 
In practice you can do the level matching with a DI box, but the limit will be the internal pre-amp s/n - which will still impact on the quality. Assuming your external preamp is completely transparent (which is isn't), the drop from line back down to mic, and then up again will make it worse. Cascading preamps is never going to improve things.
 
Warwick720 said:
Hello! I have an M Audio Omni I/O soundcard interface, and it's a good little dude, but it's got some serious limitations. It has two mic preamps and two line level auxiliary ins. I use four mics to record drums. I have better outboard preamps than the built in ones, but I know better than to feed a line out from a pre into another mic pre. Is there any way I can convert these mediocre preamps into line ins so I can just use four of my own outboard preamps?
Click to expand...

Do the outboard pre amps have "proper" (as against impedance) balanced outputs? If so you can buy or build some 20dB XLR attenuators although for drums (cap mics?) 20dB might not be enough, depends upon the M-A pre's headroom.

As Rob says, attenuating then amping up again will compromise noise performance but this is how balanced line inputs are achieved in 99% of AIs and mixer and nobody seems to notice!

I would be wary of using guitar type DI boxes on line level signals. Guitars run around neg 10dBV and from a fair source Z. Preamps can hit +26dBu and from bugger all Z!

Dave.
 
I have just come across a very simple 30dB pad.
10k in each series "arm" and all in series with a centre 820R. The output to the mic input is picked up from across the 820R.
All the resistors should be 1% metal film and the 10ks matched (on a DMM) to the best of a batch to give maximum CMRR.

If this is not totally clear I could scrawl a schematic later.

Dave.
 
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