Please be sure to read through, Recording the Guitar Part 5, before continuing onto this next segment.
If the direct box and mic sound are totally different, meaning the direct box signal is very dark sounding and the amp is very bright sounding, you may not notice a comb filtering effect in the upper frequencies because the two signals would produce waveforms that are not very similar. In any case, there would still be some comb filtering happening in the upper frequencies, so its always best to fix it.
In other comb filtering fixes using two or more mics on a guitar amp, one option was to move one of the mics. In this case, moving the only mic to eliminate comb filtering is a possibility but a last resort if youre recording to one track. We will get to that fix later in the article.
The deal is no matter how close the mic is positioned to the speaker, the signal will arrive at the mixer later in comparison to the direct box signal. Physics of the speaker alone causes the delay in comparison to the electronic direct box path. We will delay the direct box signal to match the attack of the miked guitar amp speaker to get rid of the comb filtering effect.
As mentioned in previous articles in this series, sound travels approximately one foot per millisecond. With the mic about one inch from the guitar amp grill cover, and adding around two inches behind the grill cover to the actual speaker cone, the mic is approximately 3 inches away from the speaker sound source. This gives us approximately .3 (point-three) milliseconds of delay.
Important! For all of the following, do not use any effects that alter pitch or tone, meaning pitch shifters, wah-wah, etc. If you will use such effects, plug them in after completing the comb filtering tests.
The Comb Filtering Fix
Remember that we are recording the direct box and guitar amp mic to two separate recorder tracks. The main reason to do this is to have full control over the blend of the two sounds when mixing. OK, let's say you do not have the extra track and must record to one track. I did not mention that routing in this direct box mini series but its simple to adapt. Review articles on recording two guitar mics on one track as well as articles 27 and 28 and instead of recording to tracks #7 and #8, simply record to track #7 (as typically used in our one recorder track examples).
Direct Box and Amp Mic Recorded to One or Two Tracks
This fix is before recording! The key to this comb filtering fix is to add a delay line on the direct box path. The delay line will need to allow for 10ths of milliseconds. If not, meaning only single millisecond steps are available, pass on this if recording to two tracks and in two weeks, my next article will cover the fix. If you're recording to one track, there is a possible fix that will be explained as you read on.
The routing has been set up for either one recorder track or two recorder tracks. For the direct box and mic routed to one recorder track, mixer module #9 is the direct box and #10 is the guitar amp mic, and the recorder track is track #7. Recorder track #7 is monitored through mixer module #7.
For the direct box and mic routed to two recorder tracks, mixer module #9 is the direct box and #10 is the guitar amp mic, and the recorder tracks are track #7 and track #8. Recorder tracks #7 and #8 are monitored through mixer modules #7 and #8. Pan mixer modules #7 and #8 to center and set their faders to around 10 dB on their fader throws.
Pan mixer modules #7 and #8 to center and set their faders to around 10 dB on their fader throws. Mute these mixer modules for now.
If you have set up the path in full and have gotten a good guitar sound, and if a compressor is in the chain, bypass it for now. Also switch out the EQ for the time being. We simply want the paths unaltered.
You also need to mark mixer fader settings and EQ settings since we will change levels and EQ. Put a piece of artist or masking tape next to the mixer faders used in the direct and mic path and recorder return path(s). Mark the tape (using a Sharpie pen or any marker that can easily be seen) next to the little pointer on the fader to return to those settings later. The same goes for the EQ settings, etc.
If you have set guitar amp levels, tone, etc., we are going to move the settings for the test. Put a piece of artist or masking tape next to the controls and mark those with a Sharpie pen or any marker that can easily be seen as well. If the controls do not have a little pointer also put a piece of tape on top of the control and mark both pieces of tape with a little arrow pointing toward each other.
The guitar could be used as the sound source but since the signal dies off in sustain land fairly quickly, its better to use a non-periodical waveform sound source such as your favorite artist CD or cassette tape.
1. Plug the CD or cassette player output into the direct box input (keep in mind that the direct box 1/4 output is a mult and is plugged into the guitar amp input).
2. Since the CD or cassette player is a line-level source, it's most important to make sure the guitar amp input volume is off to start.
3. If the direct box has an input level control, mark it using artist tape and a Sharpie. If the direct box does not have an input volume control, you must use a CD or cassette player has outputs that allow its output volume control to be active to not overload the direct box input. We do not care about the stereo output, meaning use either the left or right output only.
4. On a scale of 1 to 10, with 10 being the loudest, set the CD or cassette player output level control to 1.
5. If the direct box has an input level control, on a scale of 1 to 10, set to 1.
6. Play the CD or cassette and slowly bring up the guitar amp volume.
7. In any case, all we need is a volume level that is around an average listening level on the guitar amp.
8. You may want to adjust the guitar amp tone control settings if the sound is too dark. The idea is to make sure to hear a wide range of frequencies.
9. Back to the mixer in the control room (if two separate rooms): Un-mute mixer modules #7 and #8 (just module #7 if recording to one recorder track).
10. Creep up the mixer monitor level to a normal monitoring level and you should be hearing the CD or cassette signal through both the direct box path and guitar amp mic path on mixer modules #7 and #8 (or just mixer module #7 is recording to one track) over the control room monitor speakers.
11. Now mute the mixer monitor speakers and mute mixer module #9 since time to patch in a delay line.
The key to this comb filtering fix is to add a delay line on the direct box path to the recorder.
Important! If the delay line is digital, this will only work if the mixer is also digital, allowing for a digital insert for the delay line on mixer module #9 (typically only a 3 sample delay out of 48,000 samples per second which is not a problem in this case). If not, pass on this since, if youre using a digital delay and the mixer is analog, there will be around a 2 millisecond delay before any delay is dialed in! (Note the 2 millisecond or so delay is caused by the A to D and D to A converters).
So we assume that if youre using a digital delay line, the mixer is also digital and has a digital insert patch point on mixer module #9 (the digital mixer may also have a built in delay line). The other option is an analog delay and an analog mixer.
We want to delay the direct path to make up for the .3 millisecond (or so) delay caused by the guitar amp mic. The delay line will need to allow for 10ths of milliseconds. If not, meaning only single millisecond steps are available, and you're recording to one guitar track, you could move the mic back on the guitar amp speaker to around 10 inches and use the technique on moving the mic as explained in article #16 (and others on comb filtering regarding mics). In this case, you would set the delay to one millisecond and move the mic to find the cancellation spot. This will surely affect the guitar amp sound since the mic will be positioned back, but as usual, there are no rules. If it sounds good to you, no problem.
Adding the Delay
12. OK, its time to add the delay line. If you're using a digital delay and a digital mixer, patch into the digital insert patch point on mixer module #9 (direct box input path). There are a few patching possibilities depending upon the format such as light pipe [Editors note: Light Pipe is a type of cable.], etc. Check your mixer manual for details.
13. If you're using an analog delay line, and if the analog mixer module #9 has an insert patch point, patch the insert output into the delay line and patch the delay line output into the insert return. If there is no insert patch point, and if you're recording to one track, this will not work. So if you're recording to two tracks, you either assigned mixer module #9 to bus #7 or used mixer module #9s direct output to patch into recorder track #7. In either case, patch the delay line in-between.
14. Set the delay line inputs and output to unity gain.
15. Un-mute the control room monitor speakers and un-mute mixer module #9.
Setting the EQ
17. Going back and forth between the direct box signal (mixer module #9) and the guitar amp signal (mixer module #10) using the mixer mute button for each, EQ the direct box signal to get in the EQ area of the guitar amp sound (the guitar amp will surely sound much brighter). Try adding like 6 or more dB at 3 kHz and maybe the same around 5 kHz. (If not clear on this, mute the direct box module and listen to the guitar amp mic. Now mute the guitar amp mic and un-mute the direct box signal and try to match the frequencies using EQ.)
Invert the Phase on One Mixer Module
18. Now reverse the phase on either mixer module #9 or #10. We could use either module but lets reverse the phase on mixer module #9.
(Important: if your mixer modules do not allow for phase reversal, the fix is to wire up or alter a mic cable inverting the phase. If you're using a two-wire mic cable, on one end of the cable at the connector, reverse the hot and ground wire. If you're using a three-wire mic cable, on one end of the cable at the connector, swap pins 2 and 3. Make sure to label the cable inverse phase!!!)
Finding the Delay Setting that Cancels the Signal
The idea is to experiment with the delay line setting to find the point of the most signal cancellation between the direct box path and the guitar mic path.
19. Start by setting the delay line to its shortest delay. Again, we want to use a delay line that offers 10ths of millisecond steps.
20. Play the CD or cassette player (as usual in this test).
21. Set both mixer modules #9 and #10 to around 10 dB on their respective fader throws. Very slowly move mixer module #9 up and down to listen for the cancellation spot.
22. While doing so, change the delay line settings in single steps. Keep doing that and go past the point of the most cancellation and then work backwards, meaning back off on the single delay settings one setting at a time. Keep going back and forth with the delay time setting narrowing down the delay time area until you find the most signal cancellation.
Now flip the phase on mixer module #9 back to normal. Reset all mixer module settings, amp settings, etc. You may want to revisit EQ settings on both the direct box and mic signal since you should now have gotten rid of the comb filtering effect!
Working with a Less-Expensive Delay Unit
Note that if you are recording to one track and you are using a delay line that only offers single millisecond steps (keep in mind all of the above information on digital delay lines), here we go.
Perform all steps through step 17 and then,
Set the delay line to one millisecond. No need to change that setting when performing the following. Position the amp mic around 10 inches back from the amp speaker to start. Play the CD or cassette player (as usual in this test). Set both mixer modules #9 and #10 to around 10 dB on their respective fader throws. Very slowly move mixer module #9 up and down to listen for the cancellation spot. After finding the level setting with the most signal cancellation, its time to move the mic regarding the distance to the guitar amp speaker.
If you are working by yourself and can get to the guitar amp wearing headphones while monitoring the mixer signal, simply move the mic until you hear the most signal cancellation. Now go back to the mixer and slowly move mixer module #9s level up and down to hear further signal cancellation. Repeat this over and over until finding the point of the most signal cancellation as possible.
If you are working with a 2nd engineer, or someone to help you, all the better. The 2nd engineer or helper needs to wear headphones hearing the mixer signal you are hearing. The 2nd engineer or helper should hear what to do, meaning hearing the point of cancellation or more build up when moving the mic (if the guitar amp is not too loud in the studio). While the 2nd engineer or helper is moving the mic slowly, keep moving the level on mixer module #9 very slightly up and down looking for the spot of the most signal cancellation. Use the mixer talk back to communicate with the 2nd engineer or helper. When hearing the most signal cancellation, ask the 2nd engineer or helper to stop moving the mic.
The mic needs to be secured as usual so refer to Part 3 Miking The Guitar Amp. Make sure the mic is not moved when doing so!
Now flip the phase on mixer module #9 back to normal. Reset all mixer module settings, amp settings, etc. Since the mic was slightly moved, you may want to revisit EQ settings and a slight level change with that mic.
In conclusion, most engineers do not take the time to perform the test. I have blown it off when in a hurry and have ended up with a sound that could have been better.