Sometimes we think about the good old times, when a Milonga was DJed with just a CD-Player connected to an amplifier and two speakers (are there any Vinyl DJs for Tango out there any more ?). In this case everything you have to do is turning on the sound and adjust the sound level with your ears (increase if the dancers complain and decrease if the police shows up).
Today – for big events- a DJ brings his own computer, which may be connected to a digital DJ-controller. This is connected to a DJ-Mixer, this is connected to a house mixer, which is connected to a house equalizer, which is connected to an array of amplifiers, active and passive speakers and subwoofers. All of them are equipped with a plethora of controls for sound level and equilization, so finally you may have to deal with >20 dials to adjust the sound levels and EQ. As a beginner, you may easily get lost in this situation.
A DJ has to know, what he does and what he wants. The whole chain of sound processing has in fact to be tuned like an instrument before you start the Milonga. This process is called sound check. The procedure is typically used for sound and stage engineering for concerts or performances. The sound check makes sure, that the sound for the audience is “clear and at the right level and tonal frequencies” . In most situations, everything can be done manually with just your ear as the instrument. Some cheap instruments like a transportable sound level meter (for30-50$) may help a lot to introduce some objective measurements. Sometimes, you have to deal with equipment worth >100,000 $ (but then, typically an experienced house technician will be available to give you a hand).
If you arrive at a venue with a professional sound system, make sure that you first hook up the equipment with all lines turned down (drawers, knobs or killswitches) and then start to switch on all devices from player towards the amplifiers in order not to destroy equipment. If you are at a big venue, make sure you are inducted into the use of house equipment and what to do and not to do with this.
1. Line check
We start with a “line check” to make sure, the devices are connected properly. Double check your wiring before you turn on the equipment and make sure, every cable is plugged in properly into the right connections. Turning the volumes on from player to mixer and amplifiers, you should be able to hear some music if a song is playing. If you can´t hear music, the best procedure is always to follow the signal from source to speakers and use all instruments to check whether a signal is created and transported. The best instruments for this are the level meters and headphones which you can typically connect to all mixers for monitoring. Also check all cables, just in case there is an electrical problem. Here a cable tester or a simple and cheap multimeter (for 30$) comes in handy. Again, always buy premium cables with lifelong warranty. They are a bit more expensive, but they hold forever and give you the best reliability for your money. Paying 20$ more for a premium cable is nothing in comparison to having to replace a broken cable 15 minutes before an important gig (I call you wise if you have a spare one ready). I also recommend not to save money on headphones. Cheap headphones for 10$ certainly allow to prelisten but not to check for sound quality. If you ever have tried an environment sound cancelling pair of headphones for 250$, you know what I am talking about.
Another important task during setup is to check wether all cables are secured (gaffa tape, cable channels and cable ties). Specially the speaker cables need to be double and triple checked.
2. Signal levels
For an undistorted sound, we have to make sure, the signal produced by your player is not clipped and uses the full dynamic range. All modern equipment contains electronic signal amplifiers. These are designed to have a maximum input level, which is the nominal level (reference) devices are manufacture to operate at for best dynamic range. If you increase the volume then the signal may be distorted (clipped), if you operate the device at low level, internal sound like hiss can be created. The dB scale is a logarithmic comparison between a signal (S) and a given reference (R). The formula is dB = 10xlog(S/R) . That means, we always have to look for the reference (or nominal level).
For professional equipment the reference is 0.775V, which is called dBu. The zero level of the meters (nominal level) is defined at 0VU = +4 dBu. VU (“volume units”) are displayed at the meters of consumer electronics 
For consumer electronics the reference (R) is 1Vrms for 0 dBV and the nominal level (0VU) is standardized to -10 dBV. Again, your signal should stay below that threshold for a clear signal without noise and distortion .
For the initial soundcheck, we have to understand, that as soon as a signal is clipped, all other devices in the chain will play this clipped signal and nothing can be done to correct that in a later position of the chain. So we have to start to adjust the signal chain from player to amplifier.
First we start with the player itself. Typically a digital player has a level meter. Every player (whether it is a pure digital one or a CD-Deck has a dial to adjust the output level. In my example, I use a digital DJ deck with four players and an internal mixer. So in this case I start to play an example with a uniform high sound level (Tango Techno music with a good punch). I first adjust the player so that when the respective drawer of the mixer is at top level, the meter shows around 0 VU, means that the red LEDs are only flashing rarely. This is a worst case setup and will not be changed if you play different music.
Now the internal Mixer of the deck has to be adjusted in a way that the output level stays always below 0 VU. From there, the signal is transferred into a DJ-Mixer which is used to mix the output of other external devices like Ipods, other computers and microphones. Here the input channel has to be adjusted so that at maximum drawer position no clipping is indicated.
The output of this DJ-mixer is fed into the house mixer. Here again the channel is adjusted to avoid clipping with drawer fully open.
The output drawer is used to adjust maximum volume. House Mixers can be small like this 12 line mixer (Picture 6) or real monsters like this with 24 inputs (Picture 7). Don´t care about the size of the house mixer, you just need to operate one single channel. Again – please get a full induction until you hook up your devices to professional equipment of big venues.
You also have to be aware of the connection between consumer electronics and professional sound systems. Consumer electronics use unbalanced cables with one hot signal in the centre and a wire braid shielding around that, which serves as ground level. Professional equipment typically uses balanced wirings with a pair of hot wires (+) and (-) and a wire braid as ground level. Typically they use XLR type connectors .
Usually unbalanced signals can be fed into professional equipment (with adapters, feeding the signal into the (+) input and connecting the (-) input to ground level), because these inputs have an adaptive electronic circuit. Older equipment may require a special active adapter to convert unbalanced signals into balanced signals.
If you are unsure, please ask the venue technician, he typically helps to connect your equipment to the house mixer, by providing the proper adapters. I personally do not rely on adapters, so if I operate frequently at a certain venue, I provide my own cables.
3. Maximum sound level.
As a DJ you normally set up the maximum and intended level before the gig. There are two three problems we have to deal with:
During a Milonga people are present. People are generally a problem for sound engineering. They not only create a decent background sound which will make a carefully set volume level typically sound too low. Above that, people change the sound quality of the room, so the EQ may have to be adjusted as well.
Second problem is, that the DJ at his desk (or booth) is not in the best position to acoustically monitor sound levels and frequency distribution. So generally, the DJ has to move occasionally to the dance floor to check sound levels. On the other side, there are maximum sound levels that should not be exceeded in order to not damage the ears of the people. Here we need to measure the sound level, which is the local pressure deviation created from a sound wave compared to the ambient atmospheric pressure. The unit for this is dB(SPL), where 0 dB(SPL) is the lower level of audibility. According to official recommendations, the exposure time to noise before damage occurs can be assumed like this:
8h 85 dB(SPL)
4h 88 dB(SPL)
2h 91 dB(SPL)
However, these values are defined for noise. In regards to music, humans can stand an additional 5 dB(SPL) .
So for a 4h Milonga, a maximum SPL of 93 dB(SPL) should not be exceeded. For this, a little SPL-meter comes in handy. I typically measure the SPL at the dancefloor next to the speakers. In the centre of the dancefloor, a different reading can be obtained. My SPL meter can be mounted to a tripod and hooked up to my secondary computer using a long 20m USB cable. The software allows for a constant monitoring of the SPL and a datalog of the complete gig. Typically, I use it manually for initial checks and later I keep it well off the dancefloor in the audience area.
The third problem is the adaptation of the ears to high sound levels. So after some time your ears are adapted and then sound levels may appear subjectively too low, so you are tempted to increase gain.
For the final soundcheck, I adjust the level of the amplifier or the internal amplifiers of active speakers for a soundlevel of 93 dB for a 4h Milonga or 90dB for a longer Milonga in front of the speaker (worst possible place). If the venue is critical (neighbourhood etc.) I reduce the maximum level. It may be also a good idea to program a sound compressor (if available) to that maximum SPL. Please note, that this is not the level I will operate the sound system at, but definitely the maximum level, which should not be exceeded.
Finally, I balance the left and right speakers for an adequate stereo impression. For this, all effect units have to be turned off, which interfere with the true stereo signal (like stereo base expanders or XPQ surround processors). These checks are mainly done acoustically with the ear as reliable instrument. Balance is set at the house mixer.
Al last, the subwoofer is adjusted to the intended level of punch. For Tango music, I try to keep that more unobtrusive, for a Neolonga this may change.
Rule: All adjustments that deal with the individual setup and acoustics of the venue should be made late in the sound chain. All adjustments that deal with the gig should be made in an early place of the sound chain. Never adjust the same parameter at different devices.
For a fine adjustment, I check the sound quality and uniformity at different sound levels with different music. For this, the best instrument actually is your ear.
If the position of your DJ-table is very much off the dance floor, the acoustic impression for you may be misleading. In this case I usually set up monitor speakers to clearly listen to my music. They are connected to the DJ Mixer and adjusted with a “booth” dial. This avoids undeliberate adjustments of gain and EQ on basis of misleading impressions. On the other hand, your ear as the DJ becomes uncoupled to the venue sound, so again, sound checks during the gig are recommended.
An equalizer is used to adjust the frequency output of the speaker systems to the hall acoustics . As this is a constant, typically a house equalizer is adjusted and should not be changed by a DJ, because it can take some time to find the perfect adjustment (typically house sound technicians are not really happy if you change the adjustment of devices between house mixer and amplifier). Adjustments during soundcheck generally should be done close to the end of the chain and during the event at the beginning of the chain of sound devices. As the frequency response of the venue may change, when people are dancing, adjustments may be necessary later. For mixing and blending the equalization section of the DJ-Mixer or deck are very important for a quick correction or use during blending.
I personally do not recommend using a graphic equalizer during a gig to adjust the individual sound quality of songs, because songs are very different and it takes too much time to adjust a big GEQ, which then you would have to do for each song. Anyway, there are instructions how to use the GEQ to adjust for golden age songs coming from shellac recordings . If a song has a well known problem, you should find another version of that song or manually correct that problem using software. For other common sound problems a quickly set up parametric EQ may be a quick and dirty solution if the necessary gear is available.
If you are providing your own sound system, a graphic equalizer is generally a good idea. I use a computerized device (Ultracurve Pro DEQ2496, Behringer) with additional measuring microphone to automatically adjust the equipment to the room acoustics. In adjustment modus, the device produces a pink noise and automatically sets up a graphical equalizer (GEQ) to flatten the response spectrum.
For this, I set up the measurement microphone into the centre of the dance floor and adjust the EQ. As the device is also a spectrum analyzer, stored spectra of different areas of the dancefloor can be helpful. During the gig, I keep the microphone working, because it also works as SPL meter. Of course, I have to set it up at a safe position close to the dancefloor. It is a good idea to keep an eye on the acoustical spectrum (picked up by the microfone) in comparison to the electrical input spectrum to identify an imbalances.
5. Putting it all together
All general adjustments are done during soundcheck and set to forget. During the gig, I only use the master drawer of my mixer to adjust the sound level having an eye on the SPL meters. For equalization during mixing, I use the three dials of my DJ deck. They can easily be restored to centre position. For mixing, the drawers of the players can be used to full level. I personally do not like the crossfader, but that is a personal habit.
Even with the best possible adjustment, a DJ has to do on the spot sound checks during the gig to make sure everything is fine. If you are just volunteering for your local club, nobody cares, if you dance some tandas during the evening. If you are payed for the gig, dancing is a no go! In this case just proceed to the dancefloor and get your impression or maybe a short break to stretch your legs. That is, what autocruise modus of your digital deck is made for. During the gig, you should do the real DJ work and not simply watch the computer doing an autocruise of your playlist.
-Richard (DJ Ricardo)