Using 2 or More Mics for The Guitar Amp Speaker(s) - Part 2

Using 2 or More Mics for The Guitar Amp Speaker(s) -  Part 2 Brought to you by: guitar.com

Just Press Play Logo Gif 13557This article is on using two or more mics on one or more speakers miking in front of the speaker(s). I suggest reviewing the archived article Part 3 Miking the Guitar Amp since so much important physical set up information will not be covered here. You may also want to review Part 1 Microphones Explained, which describes mic choices per application

When using more than one mic in a close mic proximity amp miking situation, the key is to position the two (or more) mics the exact same distance from the speaker to eliminate comb filtering phase problems. Yes, even with careful placement, since the speaker is not flat (its kind of bowl shaped), it's hard to measure the exact distance with one mic looking at the cone and the other looking about half way from the speaker center and side (typical positions). More on that as things unfold in this multiple mic article series.

In this era of hard disk recorders with many tracks available, if you're using such a format, I highly recommend recording each mic to a separate track since this will allow fixing possible comb filtering problems after recording. This will also save time with mic positioning. If you need to record on one track, pay close attention to mic placement as explained in the next article in this series on Comb Filtering.

 

Mic Placement

In the Miking The Guitar Amp article, we set the mic position to get a good balance between the upper and lower frequencies. The following set up allows more control since one mic is used for the upper mids and treble and the other mic is used for the balance of mids and low frequencies.

For an amp with one speaker, position one mic approximately an inch back from the speaker grill pointing directly towards the center of the speaker cone (the exact center of the speaker). This mic will most likely not need to be moved. Position the other mic approximately an inch back from the speaker grill pointing directly towards the speaker, around half way between the cone and the edge of the speaker. The closer to the outer edge, the less sweet the mids will be. Note that if you position the mic too close to the outer edge, you will loose punch and the sound will get kind of muddy in the low end frequencies. As usual, experiment. Play around with the mic positions on both mics for the desired sound.

If the speaker cabinet has two or more speakers or if you're using two or more separate speaker cabinets find the best sounding speaker using the single mic technique (explained in the Miking the Guitar Amp article). After doing so, position one mic approximately an inch back from the speaker grill pointing directly toward the center of the cone (the exact center of the speaker). On the second best sounding speaker, position another mic approximately an inch back from the speaker grill, pointing between the cone and the side of choice.

With a single speaker or two speakers, try all side positions to find the best sonic spot for the mic positioned between the speaker cone and outer edge.

As mentioned with using more than one mic, its best to record to two separate tracks so we can deal with comb filtering problems after recording. Yes, until this era of nearly unlimited tracks with hard disk recorders, engineers had to deal with mic placement and checking for comb filtering manually, and had to move their mics ever so slightly to find the best placement. This time-consuming work was all they could do to avoid frequency comb filtering problems. Now, with hard disk recorders, much of that work can be done electronically and rather quickly after laying down tracks. If you're recording both mics to one track, mic distance placement is very important. We will deal with that after the two mic tracks setup.

Signal Path Routing For Two Mics To Be Recorded to Two Recorder Tracks

Note: This is different than the routing we have been using in previous articles.

Important! For all of the following, do not use any effects that alter pitch and tone meaning pitch shifters, wah-wah, etc. If you will use such effects, plug in after reading the article on comb filtering.

(As usual, if you are the guitarist and engineer, if the amp is in a separate room or closet, you will be monitoring over the control room speakers. If you have no guitar amp isolation, meaning the amp is in the same room, you will be using headphones to monitor. In any case, if you're playing chords for the guitar part, play all the open strings when adjusting the level with your other hand. If you're playing single notes for the part, simply play one string for now, preferably the D or G string, and adjust the level with the other hand.)

For our example, we are recording the guitar on recorder tracks #7 and #8 and using mixer modules #9 and #10 for the two guitar mic inputs. Let's use mixer module #9 for the mic placed in the center of the speaker cone and mixer module #10 for the mic placed in-between the center and side of the speaker. If you're using a digital recorder with a built in mixer, simply adapt with the same layout.

Plug one guitar amp mic cable into mixer module #9s mic input. Plug the other guitar amp mic cable into mixer module #10s mic input. On mixer modules #9 and #10, set the mic pre-amp trim to 20 dB. (If you have only one input gain trim pot per mixer module, that is used for both source line input gain and mic input gain. If you're using a computer hard disk recorder with outboard analog-to-digital inputs, use the same setting on the input level control to start). Assign mixer module #9 to bus #7 (bus #7 routes to the recorder track we are using for the mic positioned in the center of the speaker cone). Note that if mixer module #9 has a direct output, instead of using bus #7, patch the direct output into recorder track #7s input. Make sure that module #9 is not sent through the monitor chain and is only routed (bussed) or patched directly into record track #7. Mute this module for now. Assign mixer module #10 to bus #8 (bus #8 routes to the recorder track we are using for the mic positioned somewhere between the cone and speaker side). Note that if mixer module #10 has a direct output, instead of using bus #8, patch the direct output into recorder track #8s input. Make sure that module #10 is not sent through the monitor chain and is only routed (bussed) or patched directly into record track #8. Mute this module for now. Set recorder tracks #7 and #8 into input mode so we can monitor the guitar signal through the recorder, which routes to mixer modules #7and #8. (In digital land, you may want to monitor the mic-input signal on modules #9 and #10 BUT only do so if you notice a delay when monitoring through the digital mixer and or recorder path. I will get into this subject in future articles). Bring up module #7 (recorder track return) about half way up on the fader throw. Bring up the studio control room monitor level up to a normal listening level. Mixer module #9 level settings: Ask the guitarist to play the part for the song. While the guitarist is playing, un-mute mixer module #9 and slowly bring up the fader to zero (unity gain). This level setting is typically around 3/4ths up on the fader throw Look at the etching next to the fader to find the zero mark. a. If using an analog mixer and recorder (hardly used these days) adjust the mic pre-amp trim level on the mixer to average zero dB on the recorder track meter for now. For now, mute mixer module #9. b. If using a digital recorder format, adjust the mic pre amp trim level to -4 dB on the recorder track meter for now (-4 dB is safe in case the guitarist plays louder when recording the odds are good that will happen!) Always remember that going into the red (past zero) on a digital format meter will definitely clip the A to D converter (analog to digital converter) which is not advisable! Digital distortion sounds terrible! But there are no rules, so this is a good time to slam the level to see what digital distortion sounds like. For now, mute mixer module #9. Mixer module #10 level settings: Ask the guitarist to play the part for the song. While the guitarist is playing, un-mute mixer module #10 and slowly bring up the fader to zero (unity gain). This level setting is typically around 3/4ths up on the fader throw Look at the etching next to the fader as to find the zero mark. a. If using an analog mixer and recorder adjust the mic pre-amp trim level on the mixer to average zero dB on the recorder track meter for now. For now, mute mixer module #9. b. If using a digital recorder format, adjust the mic pre-amp trim level to -4 dB on the recorder track meter for now (-4 dB is safe in case the guitarist plays louder when recording). Again, watch out for digital distortion, its ugly.

Note that if you want to use a third mic (or more), simply follow all of the above instructions using another mixer module and another recorder track.

EQ Settings: OK, the path and basic levels are set so its time to EQ both mics separately. Refer to the articles on EQ per the application (Recording the Guitar Parts 4 through 9). Mute mixer module #10 and un-mute mixer module #9. EQ the signal on mixer module #9 to taste. When you are happy with the EQ settings, mute mixer module #9 and un-mute mixer module #10 and EQ to taste. In all of the following steps, you may want to tweak the EQ on both mixer modules #9 and #10.

Adding the Compressor: Refer to the articles on compressors (Recording the Guitar Parts 10 through 13). Note that since you are using two mics, it's best to use two compressors. You may also want to use a stereo compressor in this case, to have either mic control the overall compression. In any case, work with one mic path at a time starting with mixer module #9s path, muting mixer module #10 for now.

The Blend: Keep in mind that we are recording to two separate recorder tracks so the blend is a simple monitor blend that is not permanent.

To set the blend, pull down both mixer module #7and #8 faders to the bottom of their throws. Make sure both mixer module #9 and #10 are un-muted. While the guitarist is playing (as always when dealing with electronic settings), slowly bring up both faders and experiment with different level settings to achieve a good blend. When needing to change the overall guitar monitor level when listening to the full band, simply move mixer module #9 and #10 faders together keeping the same relative levels. If you have mixer automation, sub-group.

By the way, you may want to pan mixer modules #7 and #8 slightly off center or pan in any position in the stereo spectrum (if using the 5.1 format for monitoring and mixing, there are so many pan possibilities!) In any case, it's always best to pan both in the center to start, so as to check for phase and comb filtering problems, which well get into in the next article in the series.

Important! When listening to the blend, if the signal happens to cancel out with the levels near or the same, there is an electronic phase problem. If so, and if you're using two speakers, one speaker may be wired reverse phase, though this is unlikely since you would hear the problem when listening to the guitar on its own. If you're only using one amp speaker with two mics, the phase problem is somewhere from the mic through the recorder path and the odds are good a cable in the chain is wired incorrectly.

For now, if you're using a compressor, bypass it. If you're using EQ on either module, switch it out. Switch this stuff back in after the following hunt and fix.

If you are using a hard disk recorder format, record a few bars and look at the initial waveform attack on both tracks. If you see one waveform dipping down at initial attack, that is the path that has incorrect wiring somewhere in the chain. You should find out where the problem is, using the following concept:

Mute the mixer module that is showing the reverse phase. Also mute the studio monitor speakers. After each of the following changes, un-mute both and listen to the two mic tracks to see if the cancellation goes away.

1. First swap out the mic.
2. If that's not the fix, swap out the mic cable.
3. If that's not the fix, plug the mic into another mixer module and duplicate the set up on the mixer module to the recorder.
4. If that's not the fix, route the mixer module to another recorder track.
5. By now you should have found the problem so use a piece of red tape on the incorrectly wired component to let you know a tech needs to fix the wiring.

OK, without a hard disk recorder that can view waveforms, you do not know which of the two tracks in the chain is phase reversed. Simply perform the above 4 steps on both of the two mic paths one at a time and you will find the problem. Again use a piece of red tape on the incorrectly wired component to let you know a tech needs to fix the wiring.

We have not gotten around to comb filtering problems caused by mic speaker placement distances as of yet since we need to deal with combining the two mics to one recorder track as keep a flow. Comb filtering will be covered in the next article in full.

Signal Path Routing For Two Mics To Be Recorded on One Recorder Track

For our example, we are recording the guitar on recorder track #7 and using mixer module #9 and #10 for the two guitar mic inputs. If you're using a digital recorder with a built in mixer, simply adapt with the same layout.

1. Plug one guitar amp mic cable into mixer module #9s mic input.

2. Plug the other guitar amp mic cable into mixer module #10s mic input.

3. On mixer modules #9 and #10, to start, set the mic pre-amp trim to 20 dB. (If you have only one input gain trim pot per mixer module, that is used for both source line input gain and mic input gain. If you're using a computer hard disk recorder with outboard analog to digital inputs, use the same setting on the input level control to start).

4. Assign mixer module #9 to bus #7 (bus #7 routes to the recorder track we are using for the mic positioned towards the center of the speaker cone). Make sure that module #9 is not sent through the monitor chain and is only routed (bussed) to record track #7. Mute this module for now.

5. Assign mixer module #10 to bus #7 (bus #7 routes to the recorder track we are using for the mic positioned somewhere in-between the center of the speaker and the outer edge). Make sure that module #10 is not sent through the monitor chain and is only routed (bussed) to record track #7. Mute this module for now.

6. Set recorder track #7 into input mode so we can monitor the guitar signal through the recorder, which routes to mixer module #7. (In digital land, you may want to monitor the mic-input signal on modules #9 and #10 BUT only do so if you notice a delay when monitoring through the digital mixer and or recorder path. I will get into this subject in future articles).

7. Bring up module #7 (recorder track return) about half way up on the fader throw.

8. Bring up the studio control room monitor level up to a normal listening level.

9. Mixer module #9 level settings: Ask the guitarist to play the part for the song. While the guitarist is playing, un-mute mixer module #9 and slowly bring up the fader to zero (unity gain). This level setting is typically around 3/4ths up on the fader throw Look at the etching next to the fader as to find the zero mark.

10. If you're using an analog mixer and recorder adjust the mic pre-amp trim level on the mixer to average zero dB on the recorder track meter for now. For now, mute mixer module #9.

11. If you're using a digital recorder format, adjust the mic pre-amp trim level to -4 dB on the recorder track meter for now (-4 dB is safe in case the guitarist plays louder when recording). Digital distortion sounds terrible, so be careful not to distort. For now, mute mixer module #9.

12. Mixer module #10 level settings: Ask the guitarist to play the part for the song. While the guitarist is playing, un-mute mixer module #10 and slowly bring up the fader to zero (unity gain).12.

13. If you're using an analog mixer and recorder adjust the mic pre-amp trim level on the mixer to average zero dB on the recorder track meter for now.

14. If you're using a digital recorder format, adjust the mic pre-amp trim level to -4 dB on the recorder track meter for now. Dont distort!

Note that if you want to use a 3rd mic (or more), simply follow all of the above and just add in another mic and mixer module assigned to bus #7.

EQ Settings: Let's EQ both mics separately. Refer to the articles on EQ per the application (Recording the Guitar Parts 4 through 9). Mute mixer module #10 and un-mute mixer module #9. EQ the signal on mixer module #9 to taste. When you are happy with the EQ settings, mute mixer module #9 and un-mute mixer module #10, then EQ it to taste. Now its time to blend the two mic signals. When doing so, you will surely want to revisit the EQ on both mixer modules.

The Blend: To set the blend, pull down both mixer module #9 and #10 faders to the bottom of their throws. Make sure both mixer module #9 and #10 are un-muted. While the guitarist is playing (as always when dealing with electronic settings), slowly bring up both faders and experiment with different level settings to achieve a good blend. Make sure to look at recorder track #7s meter as to achieve proper level settings as mentioned in either articles #10 and #11 or #13 and #14.

Adding the Compressor: Refer to the articles on compressors (Recording the Guitar Parts 10 through 13). Note that since you are using two mics, if you need to change the overall levels of the two mics, and if the compressor does not have an input level control, the odds are good that your mixer has a bus sub master volume control. We are using bus #7 in our example so use to set the necessary level to the compressor.

Important! When listening to the blend, if the signal happens to cancel out with the levels near or the same, there is an electronic phase problem. If so, and if youre using two speakers, one may be wired reverse phase, though this is unlikely since you would hear the problem when listening to the guitar on its own. If you're only using one amp speaker with two mics, the phase problem is somewhere from the mic through the recorder path and the odds are good a cable in the chain is wired incorrectly.

For now, and if you're using a compressor, bypass it. If you're using EQ on either module, switch it out. Switch this stuff back in after the following hunt and fix instructions below:

If you are using a hard disk recorder format, mute mixer module #10 for now. Record a few bars and look at the initial waveform attack on recorder track #7. If you see the waveform dipping down at initial attack, that is the path that has incorrect wiring somewhere in the chain. If not, the phase reversal is in the chain on mixer module #10. You should find out where the problem is using the following concept.

Mute the mixer module that is showing the reverse phase. Also mute the studio monitor speakers. After each of the following changes, un-mute both and listen to the two mics to see if the cancellation goes away.

1. First swap out the mic.
2. If that's not the fix, swap out the mic cable.
3. If that's not the fix, plug the mic into another mixer module and duplicate the set up on the mixer module to the recorder.
4. If that's not the fix, route the mixer module to another recorder track.

By now you should have found the problem so use a piece of red tape on the incorrectly wired component as to let you know a tech needs to fix the wiring.

Without a hard disk recorder that can view waveforms, you do not know which of the two tracks in the chain is phase reversed. Simply perform the above four steps on both of the two mic paths one at a time and you will find the problem. Again use a piece of red tape on the incorrectly wired component as to let you know a tech needs to fix the wiring.

That takes care of the path and settings for both mics to one recorder track.

You've Done It!

Now youve got working signal path instructions for recording with two or more mics to one, two, or more tracks. Before recording, there is one more issue to do deal with, which is to fix any nagging comb filtering. The tests and fixes will be fun!

Like It!
Subscribe

There are no comments yet.

You must login or register to comment.