Precisely matching two pre-amp inputs

Hi - SX3 here.

I’ve got an M-Audio Omni pre-amp, which has multiple pre-amp inputs. I recorded a stereo piano track, “Left” out from my piano into one preamp input, “Right” out into another.

Playing back what was recorded and listening on headphones, I noticed that the left side was somewhat louder than the right side. This seemed to be confirmed by looking at the waveforms, the left side was “larger” than the right, and also when doing statistics, it was the same thing. I rechecked all the balance controls in the chain … all centered.

The pre-amp has a dial for the gain/trim on each channel, so it’s kind of hard to visually dial in the precise same gain in each channel - it’s not like you type in a digital amount. I was trying to think of a sound source that was 100% reliably reproducible, so I could record it once through the left channel, then once through the right, and compare amplitudes.

I couldn’t think of a sound source that was 100% reliable, so then I thought of using an insert cable - XLR at one end, split into a TS x 2 at the other end. I don’t have one of those, so I’d have to buy one (don’t solder, can’t even spell it …).

Is that a good way (using an insert cable) to do what I want - have the pre-amp gain/trim identical in each channel when recording a stereo source? For all I know, maybe it WAS recorded correctly (since I played more low notes than high ones, and low notes are mapped to the left more than the right), but I’m just looking for a way to know for sure that I’m matched well between the two channels.


Run a 1khz test signal through each channel, and using a precision meter (can be hardware or software, as long as it’s accurate) set the preamp trim control so each channel is exactly aligned to the same level. Then, don’t touch the trim pots anymore.

I’ll check on my Motif to see if it has a 1 KHz signal. If it does, I’ll flip polarity on one of them, would that make it even more precise than aligning the VU meters?

If the Motif doesn’t have a signal like that, I’ll just sing it - I used to walk around making those Theremin sounds when I was a kid, as a matter of fact now that I think about it even more recently when MY kid was a kid … should be easy to reproduce!! :smiley:

You can download calibrated test-tones from sources online (Google). Download a 1khz file, load it in Cubase, adjust level to 0 db output (no more than that, so you don’t distort the sine wave), burn to CD, and use it as your test-tone generator.

I have a couple of audio generators I use for calibrating gear and building tube guitar amps.

Hey Swamp - I know I’m missing the obvious here … :blush: … how do I get the test tone from the CD to the physical input of the pre-amp? It’s a .wav file, or some such digital thing, on the CD … how do I get it out of the digital realm to run through a T/S cable into my preamps?

I feel like such a dumkopf sometimes …

Thanks -

Use a CD player. Route the output from the player (mono) to the audio-interface/mixer input. Adjust trim. Repeat procedure for each channel.

This might help:

OK, thanks for simplifying, I understand that! I guess all that assumes the CD player outputs at the same volume reliably time after time, but heck, I understand we’ve got to make some assumptions somewhere.

That link was awesome … I’ve got Katz’s book, and read (or tried to) multiple times, never quite sure I understood all that “K-n” stuff … till now!

Thanks again -

All equipment will have “inconsistencies” preamp gains will be different, microphone outputs will be different etc
This is due to component tolerances, even after calibration of any preamp inputs the microphones gain, frequency and polar responses will be different. If you are looking to do stereo recording with a pair of microphones then a factory matched pair will get most of the way there.

And remember if calibrating mic preamp then using any insert or line level inputs will bypass the actual mic pre itself!

The more important point when recording something like a piano is to make sure the sound is balanced to the ear.
If you are using just two mics on the piano, one to cover the high/mid strings and one on the mid/low the meters will always look skewed, as long as the piano sounds well balanced the metering is unimportant.

If you are trying to balance a stereo pair (matched preferably) then using a central point sound source, maybe a radio or other noise you can visually set the gain untill both meters are in agreement.

Thanks for that Split. It actually does sound balanced in terms of how loud each part of the piano should realistically be. However, it’s a little bit off-balanced in terms of more coming out of the left speaker than the right (or headphone, in this case). I guess what I can do is … ? … send the stereo pair to a group which is panned a bit to the right, then on to the master stereo outs?

Mission accomplished!

I routed a 1KHz stereo sine wave out of Stereo Out 3,4 on my Delta 66 soundcard/break out box, then connected those into Line 1 and 2 in (where the preamp is).

I fiddled with the knobs on the preamp trim until the Delta 66 sound card mixer showed -1 dB on the Hardware In 1.2 (the sine wave track was described as being -1dB). As it turned out, the actual “unity” position was a fair amount less than the “U” position that was indicated on the breakout box.

It was also nice to see that Cubase output read -1dB, and the Cubase input read the same thing. I kind of felt that meant that everything was internally consistent.

Thanks for everyone’s help!

Hi Alexis

Without wanting to dampen your accomplishment too much, I fail to see what benefit balancing the line inputs to unity has with real life recording?

Maybe if you repeat the calibration at a number of differing levels and mark the results with a chinagraph pencil.

The problems as I see it is that a unity calibration will only be of benefit when the levels are set to unity and the source is also calibrated such that you wont need to move the gain on the line ins. Also the actual mic preamps are another gain stage before the line preamp section so they are effectively not calibrated.

Combined with the fact that even a good matched pair of microphones could still give slightly differing outputs. Are you wanting to be able to plug a pair of mics in to the preamp and record a number of sources without touching the gain controls on the Delta 66?

Maybe a better way to ensure the two mics and pres are set to the same level would be to make a test tone or three, low, mid and high frequencies, arrange for them to play from a single speaker. Plug both mics into preamp, set them up in a very close AB arrangement in a dead space in your studio (vocal booth?) and adjust preamp gains untill both channels match (or as close as possible across the three frequencies) then mark gain knobs, repeat for various levels and mark knobs, also note which mic is left/right.

Hi Split - those are great ideas, thanks. What I wanted to do this for initially was just to test the stereo outs from my Motif. I also wanted to confirm the calibration of chain - would the -1dB signal read -1 dB in my soundcard, in the Cubase ins, and all the way through the chains to the Cubase stereo outs? Of course it all did!

Thanks again!