Using 2 or More Mics for The Guitar Amp Speaker(s) - Part 4

Using 2 or More Mics for The Guitar Amp Speaker(s) - Part 4 Brought to you by: guitar.com

Just Press Play Logo Gif 19824This article is on using two amps and speaker bottoms with a twist the 2nd amp and speaker bottom is a sub woofer system. To keep things simple, we will use two mics, one on the typical guitar amp speaker and another on the sub woofer speaker. If you want to use more amps and/or more mics, refer to previous articles for details.

So why use a sub woofer? In this era, many rock bands tune town to Eb, D, or lower, and if wanting to get major low end, a sub woofer is the way to make that happen.

Rivera makes a sub woofer system that is a self contained amp and speaker. I assume there are other systems as well. The way the Rivera sub woofer system works is the signal is picked off from the main amp insert effect send. I assume there may be other ways to patch.

As with this multiple mic series, I suggest reviewing the archived article on Miking The Guitar Amp (Part 3) since so much important physical set up information will not be covered here. You may also want to review the article that mentions mic choices per application (Part 1 Microphones Explained).

In this multiple mic series I have previously mentioned the following, but it bears repeating: 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 so as 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 placements. More on this as things unfold.

This will be very important because in this case since the main guitar amp and sub woofer will be producing low frequencies. As mentioned in the EQ articles (Parts 4 through 9), speaker cabinets have low end buildups and suck outs. In this case, with the main amp and sub woofer amp both producing low end on the low 5th power chords, the comb filtering issue really needs attention. Yes, the signal on both could be compressed huge time (and most likely would be), but its best to help fix the problem as much as possible in comb filtering fix land, before recording the tracks.

Btw: This is surely a dangerous area regarding the bass instrument in note and tuning land. If the guitar and bass are not playing the exact same notes, or are slightly out of tune, low end rumble will not be friendly to playback speakers. Yes, you could just squash (using major compression) the whole program when mastering. Did I say that?

OK, let's get going and deal with ways to make this new medium be as friendly as possible.

 

Mic Placement

In the Miking The Guitar Amp article (Part 3), we set the mic position to get a good balance between the upper and lower frequencies. Review the article and do that first on the main amp to get a good blend of treble, mids and low frequencies.

Now its time to mic the sub woofer. (But hey, I have never done this! I don't use subwoofers.) Logic tells me that the concept is to start with our standard mic position and experiment with moving the mic away from the cone towards the side of the speaker for more low end. I assume that with the mic about half-way between the cone and speaker edge may be the sweet spot to get the serious low end while still getting a taste of the real low mids from the speaker cone.

As mentioned before with using more than one mic, its best to record to two separate tracks so we can deal with comb filtering problems after recording. Until this era of nearly unlimited tracks with hard disk recorders, engineers dealt with mic placement manually, checking for comb filtering and moving mics ever so slightly to find the best placement and help avoid frequency comb filtering problems. 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 For Two Mics to Two Recorder Tracks

Note: this is different that 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 next article on comb filtering.

As usual, if you are the guitarist and engineer, and 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 youll be playing chords for the guitar part, play all the open strings when adjusting the level with your other hand. If youll be playing single notes for the part, simply play one string for now like the D or G string and adjust the level with the other hand. OK, in this case, the odds are very good low BUZZ SAW 5ths will be the feature and maybe some chords as well. So play the two low strings and then all the open strings. Go back and forth when setting the mic placement, levels and EQ.

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. Lets use mixer module #9 for the mic on the main guitar amp speaker and mixer module #10 for the mic on the sub woofer amp speaker. If youre 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 only one input gain trim pot per mixer module, that is used for both source line input gain and mic input gain. If using a computer hard disk recorder with outboard analog to digital inputs, use the same setting on the input level control as to start).
4. Assign mixer module #9 to bus #7 (bus #7 routes to the recorder track we are using for the main amp mic). 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.
5. Assign mixer module #10 to bus #8 (bus #8 routes to the recorder track we are using for the sub woofer amp mic). 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.
6. 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).
7. Bring up the studio control room monitor level up to a normal listening level.
8. Mixer module #9 level settings: Bring up module #7 (recorder track return) about half way up on the fader throw. 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.
8a. 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.
8b. 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 players louder when recording. For now, mute mixer module #9.
9. Mixer module #10 level settings: Bring up module #8 (recorder track return) about half way up on the fader throw. 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).
9a. 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.
9b. If using a digital recorder format, adjust the mic pre amp trim level to -4 dB on the recorder track meter for now to play it safe. For now, mute mixer module #9.

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 (Parts 4 through 9) per the application. 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. Yea, EQ to taste. In all of the following steps, you may want to tweak the EQ on both mixer modules #9 and #10.

OK, the above paragraph on EQ is dealing with our standard guitar amp set ups. For the sub woofer, theres some new thinking here. The odds are good that the speaker is not producing frequencies beyond 1 kHz. Also, the speaker is surely producing frequencies lower than the typical guitar amp speaker. You might try adding a few dB around 60 to 100 cycles. Keep in mind the main function of this speaker is to get serious low end but you might also want to slightly open up the harmonics. If so, try adding around 600 to 800 cycles.

Adding the Compressor: Refer to the articles on compressors (Parts 10 through 13). Note that since you are using two mics, its best to use two compressors. In this case, I would not use a stereo compressor since each amp will have different frequency bumps and suckouts. As usual, work with one mic path at a time meaning if starting with mixer module #9s path, mute 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 #7 and #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 #7 and #8 faders together keeping the same relative levels. If you have mixer automation, group them together so one fader moves both or assign to a separate group fader.

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, its always best to pan both in the center to start so as to check for phase when we get into comb filtering problems.

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. In this case, concentrate on low frequencies for the possible problem. If hearing a cancellation problem, first listen to the guitar amp and the sub woofer standing in the middle of both of them. If you are hearing cancellation, the problem is that the wire connecting the main amp and sub woofer amp is reversed phase (hot and ground are reversed on one connector end) OR a speaker wire on one of the amps has the same wiring problem. There are other possible problems but these are usually the problem.

If the above is not the problem, 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 youre 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.

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.

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 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 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 to keep a flow. There is no need for me to do a separate article on comb filtering here since this would be the same basic concept as article #17 Comb Filtering (also see Recording the Guitar Part 16: Using Two or More Mics)

Signal Path For Two Mics to One Recorder Track

Note this is different that 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 archived article on comb filtering.

As usual, if you are the guitarist and engineer, and 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'll be playing chords for the guitar part, play all the open strings when adjusting the level with your other hand. If you'll be playing single notes for the part, simply play one string for now like the D or G string and adjust the level with the other hand. OK, in this case, the odds are very good low BUZZ SAW 5ths will be the feature and maybe some chords as well. So play the two low strings and then all the open strings. Go back and forth when setting the mic placement, levels and EQ.

For our example, we are recording the guitar on recorder track #7 and using mixer modules #9 and #10 for the two guitar mic inputs. Let's use mixer module #9 for the mic on the main guitar amp speaker and mixer module #10 for the mic on the sub woofer amp speaker. 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 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 main amp mic). 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 sub woofer amp mic). 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.
9a. 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.
9b. 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 players louder when recording.) 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!!! For now, mute mixer module #9.
10. 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.
10a. 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.
10b. If using a digital recorder format, adjust the mic pre amp trim level to -4 dB on the recorder track meter for now to play it safe.

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

EQ Settings: EQ both mics separately. Refer to the articles on EQ per the application (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. Now its time to blend the two mic signals and when doing so you will surely want to revisit the EQ on both mixer modules.

OK, the above paragraph on EQ is dealing with our standard guitar amp set ups. For the sub woofer, weve got new thinking here. The odds are good that the speaker is not producing frequencies beyond 1 kHz. Also, the speaker is surely producing frequencies lower than the typical guitar amp speaker. You might try adding a few dB around 60 to 100 cycles. Keep in mind the main function of this speaker is to get serious low end but you might also want to slightly open up the harmonics. If so, try adding around 600 to 800 cycles.

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 to achieve proper level settings.

Adding the Compressor: Refer to the articles on compressors (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 the sub master volume control for bus #7 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. In this case, concentrate on low frequencies for the possible problem. If hearing a cancellation problem, first listen to the guitar amp and the sub woofer standing in the middle of both of them. If you are hearing cancellation, the problem is that the wire connecting the main amp and sub woofer amp is reversed phase (hot and ground are reversed on one connector end) OR a speaker wire on one of the amps has the same wiring problem. There are other possible problems but these are usually the problem.

If the above is not the problem, 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 youre using a compressor, bypass it. If youre 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, 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 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.

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 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.

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