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LINE CONDITIONERS, PREAMPS AND D.I. BOXES

 

close-up of part of the front panel of a Neve 1073 DPD dual channel preamp

close-up of part of the front panel of a Neve 1073 DPD dual channel preamp

 

Today I’ll look at the preamp, the line conditioner and the D.I. box, all useful tools in a home studio.  A line conditioner is less common in home situations but is still very handy.

You will certainly use DI boxes and preamps, whether standalone units or built into a mixer or console or DAW interface.

Let’s take a quick look at different levels a signal may have en route to the recorder, which is the primary reason we use these items.

BALANCED AND UNBALANCED CONNECTIONS

A signal can be balanced or unbalanced.   Unbalanced cables have two wires where balanced use three.

Unbalanced wires can fall prey to hum and noise and radio and taxicab bursts!  Not ideal.   They are excellent at certain things, but you won’t really need them at home too often.

Balanced cables are your friends.  They are basically what the general public think of as a typical (stand-alone) microphone cable.

One end will be a female XLR (with three sockets in the cable connector) and that attaches to the three pins in the end of the microphone.

If the other end of the cable ends in a ¼” jack, it might have one black ring around it near the tip.  If so, then it is an unbalanced cable.  This is called a TS cable (tip, sleeve).

If instead it is a balanced cable, it will have two black rings around the jack.

The third wire in a balanced cable carries a phase-inverted version of the second wire.   The equipment connected to the cable at each end has to be balanced too for this to be helpful.

The two (non-ground) wires are summed and the remaining wire goes to ground.  The summing of the two mirror audio channels phase-cancels out any stray hum or electro-magnetic interference the cable picks up.

Most of the time, you’ll use balanced connections.  The idea is to make it less likely that hum and electrical radio interference will be amplified along with the wanted signal.

A balanced cable has three pins, hot, cold, and common (a.k.a. ground).  They are also called Ground, Live, & Return by some folks , especially in Europe.  This is a low-impedance cable.  It’s not actually the cable that is balanced – it’s the system itself.

Where balanced connections are used, the mic cable typically ends in an XLR connector (rather than the TRS jack type), which should mean, assuming proper wiring (which is always the case with commercially made cables in my experience) that it is wired as a balanced connector, and it will have an impedance of 75 ohms.

Please use decent cables!  You’ll be glad you did.  Cheaper cables can cause a lot of noise and hum, and will normally fail at the worst moment.

Look along the side of a mic cable to see if it says in writing on it at regular intervals that it is Balanced 75 ohms.   It will often say Balanced Twisted Pair, or perhaps Co-Axial Twisted Pair.  Signal flows in the direction of the pins, males sending, females receiving.

I recommend Radial or Mogami or Canare cable.

These three brands I can recommend without reservation for home studio cables, but they are not the cheapest cables.  Cheap cables are a false economy, you will find.

THE KEY DIFFERENCE BETWEEN
BALANCED MIC CABLES AND DIGITAL AES/EBU CABLES

Digital AES cables look very similar to mic cables, and are wired the same way (they even work perfectly well as mic cables in a pinch) but they have an impedance of 110 ohms instead.

Digital audio devices will not pass audio if you try to use a mic cable of 75 ohms to connect it’s AES inputs and outputs to another digital device.  Look on the cable to see – it will usually state either 75 ohms or 11 ohms, or specifically say AES or digital audio on it’s side at regular intervals along the cable.

Summing up…

1      Don’t use 75 ohm balanced mic cables for AES connections

2     You can use an AES 110 ohm balanced cable for a mic if you need to

LINE LEVEL, MIC LEVEL and INSTRUMENT LEVEL

You will find signals at all of these three levels.

Both MIC level and INSTRUMENT LEVEL need to be boosted to LINE LEVEL before they can be recorded into the DAW or recorder.

This is because line level signals are what we deal with when we are mixing signals together at mix-down, and line levels are what we actually record, regardless of the original level of the signal to be recorded.

MIC PREAMPS

UA Solo/610 microphone preamplifier and DI box

UA Solo/610 microphone preamplifier and DI box

 

If you start with a mic level, you will need to boost it with a device called a microphone preamplifier, or, simply, a preamp.  There’s one of these built into the head of every mixer channel or DAW input that says MIC IN or MIC/LINE on it, which allows you to simply connect the mic straight to that console, where it will be amplified to reach line level.

You can also buy a great many varieties of preamp as stand-alone units, in 19” rack format, or in the API Lunchbox 500 Series Module format, or in various different sizes of “tabletop” boxes.

There are multichannel mic preamps, which may have as many as eight separate mic inputs, each feeding it’s own preamp internally to the box.  These are usually in 19” rack format, but not always.

Engineers often carry their own favourite preamps from project to project, and there are always heated debates raging around which preamps sound best at which tasks, and with which mics.  P

The choice of preamp does affect the sound, although at a budget level many of them sound extremely similar, and sometimes indistinguishable from each other.  It’s the more expensive preamps that attract most attention from engineers and producers.

The qualities you should look for if you go shopping for stand-alone preamps are:

    Transparency or, alternatively, Character
You may want to be invisible or obvious, but rarely can you do both with the same preamp, so pick one or the other with each preamp you buy. The tube preamp is typically characterful, adding certain pleasing qualities to the sound, and the transistor preamp is often very clean and transparent, virtually transparent in operation.

    Low Self-Noise at High Gain levels
You want to see quoted statistics of better than 90dB signal-to-noise ratio (S/N ratio)

RFI/EMI Shielding
Make sure there is no EMI or RFI interference being picked up due to improper shielding from electro-magnetic interference of any kind.

Flat Frequency Response
It’s good to have a flat response up to 30kHz or more.

MORE ON PREAMPS

Preamps can be built using vacuum tubes or transistors (a.k.a. solid-state) but there are quite a few that have a switch that lets you choose between either type.

You should expect that a quality (i.e. expensive) sstandalone outboard preamp is going to sound better than a built-in preamp in a mixer channel, or the ones in your DAW interface.   They do make a difference.

Obviously, tubes are warmer and transistors are cleaner, in very general terms, and most people seem to prefer the sound of tubes, given the choice.

Tubes can be noisy but sound full and rich and have tonal character, where solid-state is often so clean as to be almost sterile.   Either is useful, and having both available would be a great option to have.

DI BOXES

UA Solo/610 single-channel preamp and DI

Close-up, UA Solo/610 single-channel preamp and DI box

Many preamps have a DI input (or Instrument Input), which will be a ¼” high impedance unbalanced guitar-type jack socket.

This input is used to convert high-impedance instrument signals into low-impedance signals a DAW or console can work with.

This feature appears on many microphone preamps, to give you further functionality in your studio work.

This will usually be a High-Z (high-impedance) connection, since that is the output of most guitar pickups.  A guitar amp input, for example, is very high impedance.

There are also standalone DI boxes, that only do DI inputs, and these may or may not accept mic or line level signals as well.  They may be passive or active in nature, and each type has advantages.

Passive units do not need power, so they have no battery compartment, and no power connection.  Active units will need a battery, or power supply.

As a signal travels down a guitar cable, it loses high frequencies over distance.  You don’t want to go over fifteen feet in length, and the best length is the shortest one you can use and still play!

It’s not just a loss of highs over distance.  The transients in the signal (the instantaneous attack peaks) are also rounded off and damaged in the process of going too far down a high impedance cable.

Highs are lost more than lows are at a given distance, and this preferential loss is called capacitative resistance.

If you need to run further than eighteen feet away from your instrument, then you should connect to a DI box (or preamp with a DI In) before continuing on the journey, so that the signal will be low impedance for the rest of the way.  Low impedance signals, on the other hand, can run for hundreds of feet without significant audible losses occurring.  This is all very handy to know if you are gigging on larger stages.

About the only signals at high impedance in a studio setting are those from guitar and bass pickups.

A guitar pickup is much like a microphone, being a transducer, and it will pick up sound around the player as well as from the instrument.

The problem can be worse with hollow-bodied guitars, since they can resonate and amplify the ambient sound of the room (and things in it resonating or making noises) before the pickup captures it and sends it to the preamp/DI box.

This is the reason why feedback is possible with guitar amps when you stand in front of them playing, after all, so it’s not so surprising.

Single-coil pickups are much more susceptible to hum than double-coil, or humbucker pickups.  The humbucker rejects hum in a similar way to the method using with balanced cables – summing two signals side by side and removing the common elements by phase-cancellation (in this case, hum and stray electro-magnetic interference).

Single-coil pickups are so susceptible to hum that it is often necessary to adjust your playing stance to face in different directions while playing in front of an amp or speaker, in the attempt to remove or at least minimize any hum and buzzing noises the pickups may be amplifying from nearby sources like lighting dimmer switches and the like.

A DI box will have an Input and a Thru connection.  The Thru lets you continue to a guitar amp with the guitar signal still at high impedance, so that the guitar amp can be miked up and recorded along with the DI signal.

The DI signal itself goes on to the recorder or DAW from a LINE OUT on the back of the DI box.  There may be a second LINE OUT enabling you to duplicate (or split) the low-impedance output signal from the DI or preamp and send it to an additional recorder or console input.

That could be handy for splitting a DI to a monitor console at a gig for the band to hear onstage, as well as to a FOH console for the main PA to feed to the audience.   In a home studio, you might want to record the exact same performance as a DI, through a guitar amp, and through an additional guitar amp, blending up to all three signals in your mix as desired.  This leaves options open in the mix.

DI boxes can have EQ onboard as well, and filters, although it’s not that common, since it’s easy to EQ elsewhere, and you typically want to do that at line level whenever possible.

It’s also true that unbalanced gear has a susceptibility to ground loops.

If you connect two pieces of equipment with an unbalanced cable, and there are two separate grounds, one for each end of the connection, then you can create a ground loop.  This causes hum and interference to become picked up and amplified in a loop.  The only way to break the loop is to break the connection or power both units from the same AC outlet so that the ground is common between them.

If a loop is physically large enough, around a room, it can pick up quite high levels of noise very effectively.

On a DI box, you will find a Ground Lift switch that toggles ground lift on and off.  This allows you to deal with these types of grounding issues at the DI box itself by lifting the ground for that connection if necessary.  Think safety first, as always.  It is NEVER a good idea to operate ungrounded electrical equipment that carries high voltages, such as an amplifier or preamplifier.

An AC power isolation transformer is a great thing to have around for studios or gigs to keep power clean and safe, and stop these issues arising.

Clean, balanced power is the ideal supply for a studio, and Furman and others make inexpensive units for this purpose.  Put one into your home studio when you have enough equipment to worry about this, and you will protect your expensive gear from power surges and the like.

PADS

Preamps almost always offer PADs and/or FILTERS.

There will often be a PAD switch that attenuates (lowers) the gain at the input to prevent distortion by the preamp when the mic input is too “hot”.

The PAD often allows -10dB reduction in mic level entering the preamp, but it may be -20dB in some cases.

Most microphones offer a wide dynamic range of signals before they distort, usually much more than the preamp   The sound entering the mic may prove too loud for the preamp (either the mic’s internal preamplifier in the case of condensers, or the preamp you connect your mic to) and in this case, a PAD will be needed to prevent unwanted distortion.

Singers are extremely unlikely to be loud enough to require a pad on their mic, but drums and loud brass or guitars may well do, since they can emit really high SPLs (Sound Pressure Levels).

You can also buy “in-line” external pads, which are little barrel connectors that get plugged into the mic cable to drop the level passing through it.

Condenser mics often have a PAD switch built right into the microphone body, because some of them are capable of putting out more level than the internal preamp in the mic can handle, causing distortion right at the top of the signal chain if the PAD is not switched into the circuit.

HIGH AND LOW PASS FILTERS

There will usually be an HPF (High Pass Filter), whose job it is to throw away frequencies below the chosen HPF cut-off frequency.

With a vocal, for instance, you would usually want to discard the lowest frequencies in the signal because there is virtually no required content below about 80Hz in a vocal.   There could well be low-level rumble in the signal though, such as footfall noises or distant traffic or fridge noises, which you will want to get rid of.

In the digital world, it’s easy to capture those frequencies and throw them out later (with more time for caution and accuracy) during the mix session, but you will then be working with the build-up across all of your tracks of unwanted low frequency content as you build the recording.  Over the course of tracking sessions, you may find your song getting muddy.

The available headroom of your mix’s gain structure (how loud you can make your mix relative to 0dBFS, the digital maximum) is strongly affected by the amount of low frequency content.

Lows have way more power than highs and consequently take up lots more space in the available mix headroom.

If you throw away any unwanted lows first, either at the mic with a roll-off filter switch, or at the input channel of the preamp or console or DAW interface, then things are easier to hear in the mix, and the mix itself can be louder at the end of the day.

Some mic preamps will also have an LPF (Low Pass Filter), which allows you to discard frequencies ABOVE a cut-off frequency, at the high end of the sound.  Usually the filter will let you pick any frequency, all the way to 20kHz or more, as the point above which you will discard sound, keeping the rest below that frequency.

There is less advantage to this technique than using HPFs to clean up a mix and increase headroom, since the highs don’t have much energy, but it is a very useful technique for “bracketing” sounds.

Bracketing means discarding all lows AND highs that are not going to be needed, so as to make more room for other sounds, and often gets done in order to unmask sounds that otherwise would be obscured, or masked, by the sound being bracketed.  You’re just keeping the frequencies in the sound that you need, and no others.

You’ll also find dynamics processing on many preamps, as well as switches (with fun names unique to the manufacturer) whose actions may or may not be clearly revealed to you.  Things like Fat, or Squeeze, or Vibe.

LINE CONDITIONERS

A line conditioner is simply a line-level input device that can be used to change the character and gain of a signal already at line level.  It’s very rare that these are mono, as they are typically two-channel stereo units, and  intended to go across the main outputs of a mixer, in most cases.

In mastering, one might send a stereo line level signal (your mix) into a line conditioner to add a certain tonality to the result and set a specific level.

An example might be any preamp with line level inputs.  Although it seems odd to put line level into a line level device, it’s actually a large range of possible levels at line level.  A typical preamp might have 60dB or more of available gain for increasing levels.  This allows for tweaking sounds already at line levels so that they are further improved in level or tone.

A lot of people enjoy teasing the perfect amount of distortion out of preamps by setting the various parts of the gain structure just so.

DAW or RECORDER INPUT LEVELS

For a good quality recording of a sound, you want the incoming signal to fill as many of the available bits as you can at the DAW, which means nice high levels.

On the other hand, you do want to prevent any unintentional analog distortion being introduced.  Certainly, no digital clipping is wanted under any circumstances.

This is pretty easy to do these days, since there is no significant noise penalty any more for recording at too low a level, as there used to be in the days of analog tape machines where tape hiss and noise floor were a very serious issue.

Not taking levels too high avoids clipping, while not being too low avoids creating excessive empty data bits.

Anywhere between, say, -12dB and -3dB levels on your input metering for the loudest peak should easily be squeaky-clean and good enough quality for any purposes, but the nearer your loudest peaks are to a few dBs shy of the 0dB  mark on your DAW/mixer input metering, the better.

That’s it for today’s blog.  I’ll be talking about Microphone Stands, Tuners and Accessories.  See you there!

 

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