Posted by Gluke
Wed Jan 05, 2005 3:28 pm
Why use a mixer in a guitar rack system
The main reason to use a mixer in a guitar rack system is to preserve the dry (unaffected) signal. If we run all our signal through a digital effect the dry signal will most likely have to pass through the analogue/digital and digital/analogue converters inside the effect. This is not a good thing and degrades the quality of the dry signal. This is very easy to try out. Just compare the sound of running through the effect without any effects on to the sound when you remove the effect altogether from the circuit. The way a signal passes through a common digital effect is shown in figure 1.
This can be avoided by introducing a rack mixer into the signal path. Figure 2 shows how a mixer can be connected. Most guitar system mixers are unity gain mixers. That means that the different inputs are added at the same level which is unity gain (level out = level in). To adjust the level between signals one have to adjust the level of the signals going into the mixer. There is no panning function. Left input is hard left and Right input is hard right. The CAE mixers we use and I think also the DMC mixer have an internal connection between the inputs, so when feeding only the left (mono) input the signal will automatically be bridged to both inputs (So the signal is panned centre).
Here the dry signal goes straight into the mixer and through to the power amp. The dry signal is also split and fed into the effect. The effect is then set to 100% wet (no dry signal passing through the effect) and the effect level is adjusted with the output level of the effect. Running the effects this way is far superior to running the dry signal through the effect. Most of us do not play a stereo guitar so out dry signal is mono. There is also no need to feed most effects in stereo so most effects can be used mono in/stereo out. So the connections would normally be done like shown in figure 3.
There are many ways to split the dry signal and switch the effects in/out. A lot of us use a loop switcher to switch on/off the input to the effects. The effect outputs are left going straight into the mixer. This lets the effects fade out naturally when the input is switched off. Some loop switchers like the CAE 4×4 will automatically split the signal to all loops of you do not use the returns. Figure 4 shows an example of how a switcher like the 4×4 can be connected to switch on/off 4 effects. Note that the output from the effects goes straight into the mixer allowing delays and reverbs to decay naturally.
There are many different way to do the splitting/switching. Some effects like the TC1210 chorus can be remote bypassed so you don’t need a loop to switch it in/out.
Series or parallel
So far we have been running all the effects in parallel. A very common question is should I run my effects in series or parallel? The answer is for the best result you will probably end up doing a combination of both. Some effects you would want to run in parallel and others in series. A common way to achieve this is to use a dual mixer. The effects in each stage of the mixer is in parallel, but the two stages are used to run effects in series. I’ll try to illustrate with an example.
So what is happening here? First the signal from the pre amp goes into the loop switcher where it gets split to loop1 and loop2. The dry signal goes into mixer 1 from loop1 out. Loop1 and loop2 activates FX1 and FX2 that are connected in parallel into mixer 1. This has all been explained above, so nothing new about this. The new thing introduced here is that the output of mixer1 is then tapped and fed into loop3 and loop4. So into these loops there goes a mix of dry signal/FX1/FX2, which is then fed into FX3 and FX4 when loop3 and loop4 is activated. At the same time the output of mixer1 is fed straight through mixer2 through the internal connections inside the mixer. Usually this requires that we do not use input1 left and right in the mixer2. Using these inputs will break the internal connections and the two stages will work as two totally independent mixers (which we don’t want). This is a common way to run effects in both parallel and series. Normally the effects connected in mixer1 would be things like chorus, pitch shift and reverb and in mixer2 mostly delays. The key thing is that the whole time none of the effects will degrade/affect the dry signal. And when an effect is switched in it’s just adding the effect sound. Note that mixer1 out usually is sums both sides of the stereo signal so you don’t have to feed the delay in stereo (if you don’t want to that is. It is off course possible to do it that way as well).
Using the expand out functions
Most mixers have an expand function on the inputs. These are hardwired to the input so the splitting is done directly at the input jacks before the signal enters the mixer. If only one input is used the signal is expanded to both expand outs. The expand outs we can use to split the signal to different loops/effects as shown in figure 4.
Another way to use the expand function is to use it as another way to run effects in series.
Here the dry signal (Out loop 1) is sent into mixer IN 2. Mixer IN 1 is instead used for the FX1. It does not matter which input is used for dry sound and which for effects. What this does is letting us expand using the EXP OUTs from the outputs of FX1 and into loop 2. Loop 1 functions just as in the figure 2, but when loop 2 is engaged we get the sound of running through both FX1 and FX2. Note that it’s only one of the outputs of FX1 that is feeding FX2. Also we have both the output of FX1 and the output of the FX1 left into FX2 going into the mixer. This is a cool way to get the thick 80’s chorus sounds.
Figure 6 and 7 is common ways to do this with a SPX 90 or an Eventide H3000. Here the TC1210 is bypassed with a control function instead of using a loop for that unit. To really go over the top replace the TC1210 with a Dyno My Piano Tri Stereo Chorus.
Putting it all together
Combining a loop switcher like a 4×4 with a dual stage mixer leaves us with a lot of different ways to connect our gear. The key thing to decide is the order you want to run your effects. A normal setup would be use the expand function for the 80s thick chorus and mixer2 for delays. An example of that is shown in figure 8. I have added some names to the effects just so you can recognise the stuff if you compare to pictures and what not you’ll find of different rigs.
Key thing is to remember that all effects are running at 100% wet the whole time, so no dry signal is going through any of the effects. The effects are just added to our dry signal and with all the effects off we should have the original unaffected dry signal.
Comment: I have personally found that rack mixers will add some colouration to the dry signal. It’s rather minor and something you might not ever notice if you haven’t A/B tested running through the mixer and not. The way to get around this problem and even further preserve the dry signal and tube amp feel is running a wet/dry/wet 3 cabs setup, but that’s a whole other article.