Almeria StringsFolk, Bluegrass, Celtic and Traditional Music

Folk, Bluegrass, Celtic and Traditional Music

September 2014

Choosing and Using a Preamp or DI Box

There is a huge amount of confusion among players on this issue. As a result, many are using ‘mismatched’ combinations that could be causing loss of tone, and could result in unwanted distortion, lack of volume, hums and other issues.  In this short article I’ll try to avoid too many technicalities and instead focus on the practicalities of deciding which external ‘box’ (if any) might be needed to bring out the best from your instrument.

The first thing to establish is whether your installed pickup system (which we’ll term ‘the transducer’) is either passive or active.The term “passive” in this context means that the unit does not require power in order to function. This is important, as it completely changes the picture in terms of the ideal ‘box’ to plug it into, or which socket to use on an acoustic guitar amplifier, for example.

Active pickup systems will normally require a battery inside located inside the instrument to function. There are one or two specialist types that will run from ‘phantom power’ supplied by the mixing desk, but these are the exception rather than the rule. The vast majority use a 9v battery in a slide-out compartment, or in a ‘battery bag’ fixed inside somewhere. Some may also have tone (EQ) controls, a volume control, or even a built-in tuner. What you need to know is that the 9v battery is in fact there to operate a built-in preamp. This preamp will offer a very high impedance input to the transducer itself, and usually sends out instrument level signal which you can plug into either the ‘active’ input jack of an acoustic amplifier, into the ‘line level’ jack socket on a mixing desk or into a DI box (which will convert the unbalanced signal into a balanced signal, more suitable for long cable runs).

Level Matching

Though we are avoiding getting too technical, a brief descent into basic technicalities is important for the sake of accuracy and understanding, so it is worth knowing the difference in output between typical ‘active’ and ‘passive’ systems. Naturally, these vary a lot, but on average:

A passive transducer may generate anything from 0.1V to 1V

An active transducer system may generate in excess of 1.75V

That’s quite a difference. If you plug that active system into a low level, high gain input designed for a microphone, for example, you may well cause serious distortion as it will overload the circuit. Conversely, if you plug a passive transducer into an input designed for line level signals, you may not hear much at all because that circuit is expecting a much higher input voltage.

For future reference, here’s some typical inputs and what they expect to see:

Mic level: Typically 2mV (0.002 volts) to about 1V (-52 dBu to +2.2 dBu)

Instrument level: Typically 0.1V to 1V for passive guitar pickups; up to 1.75V for active pickups ( -17.7 dBu to +7 dBu)

Semi-pro or consumer line level: Typically -10dBV (0.316 volts) which is approximately -7.8 dBu

Professional line level: 1.23 volts ( +4dBu)

Therefore, it is important to match the levels correctly. A pre-amplifier (whether internal or external) raises the lower level voltage generated by the transducer to a high voltage suitable for a line or instrument level input. The exact amount of gain will vary, as can output levels, which will be affected by any ‘volume’ control fitted. Check the specification sheets for both inputs and outputs to ensure a correct match. By ensuring a correct level match, you will avoid distortion, and also reduce background hiss and other extraneous interference. Many of the reported problems people have with pickups, preamps and mixing desks can be directly attributed to something as simple as not getting the levels matched correctly. i.e., plugging a low level signal into a socket expecting line-level voltage, or plugging an active pickup directly into a secondary external preamp expecting a low level signal. The result? Lots of background hiss and overall poor sound quality in the first example, overloads (‘clipping’) and distortion in the latter.

Impedance Matching

Levels are not the only thing that needs matching, however. A somewhat more complicated concept known as ‘impedance’ is also involved.

Impedance is defined as resistance to an AC electrical current, such as an audio signal. Impedance is measured in ohms. As a general guide, to achieve the best possible sound quality, the input impedance of the connector you are plugging into should be a 7 to 10 times higher than the output impedance of the source. There are actually two components to impedance: resistive and reactive. In turn, the incoming frequencies affect the values involved. Fortunately, you do not have to delve into the rather complex mathematics involved to make things work properly!

Piezo transducers perform at their best into very high impedance inputs.  As a general rule, an absolute minimum input impedance of 1M Ohms (one million Ohms) is required. Ideally, the impedance with be much higher. Many acoustic instrument pre-amps and the ‘passive’ inputs on acoustic guitar amplifiers will offer from 5-10M Ohms which is generally regarded as being an ideal rage for this class of transducer. Remember that if you have an ‘active’ transducer system, you can plug into far lower impedance inputs, as the ultra-high impedance buffering will have already taken place via the internal preamplifier.

Most ordinary guitar amplifiers, those intended for electric guitar, for example, offer nowhere near the very high impedance inputs required by passive piezo transducers. They are typically in the range of 50K to 150K, too low by a factor of several times. If you plug a piezo transducer into one of these you will certainly get sound - but it will be ‘thin’ and ‘harsh’ sounding, as much of the frequency range generated by the transducer will have been lost due to the incorrect loading.

To fully take advantage of the wide frequency response that a high quality piezo transducer can generate, you absolutely must ensure that it is used in combination with a suitably high-impedance input. This can be a special ‘passive’ pickup input on an acoustic amplifier, a special super-high impedance input on a mixer (not many have these - check) or it can be in the form of an external, stand-alone box. These are generally known as ‘Acoustic Preamps’. Check the specifications carefully. Some are very simple, impedance-buffering preamps with few other controls. Others might offer quite sophisticated EQ options. If you are running into a high quality desk, you may not require the onboard EQ (unless you like to be able to ‘tweak’ your sound from the stage). Much depends on the desk involved. While the EQ options offered by a good, standalone acoustic preamp may be superior to that of lower cost mixing consoles, they will not necessarily be superior to the EQ offered by a top-class desk. If this is the case, then you may find you would be better off without a complex preamp and instead prefer a high quality DI box.

DI Boxes

One of the key functions of a DI box is to convert unbalanced high impedance signals to balanced low impedance signals suitable for direct connection to the XLR inputs of your mixer’s microphone channel. Unbalanced connections transmit a signal via a single connection, with a ground.   These unbalanced connections are highly susceptible to background noise and electrical interference, particularly in the case of longer cable runs. Typical unbalanced connections include mono jacks and RCA connections.

Balanced audio connections, however, have a ground, a live and a return. This results in the cable rejecting interference as the signal is carried. Typical balanced connections are via 3-pin XLR cables (such as microphone connections) or via balanced stereo (TRS: Tip-Ring-Sleeve) jacks.

Most instruments offer only unbalanced outputs, typically on a 1/4” mono jack socket. To connect these instruments to a mixer with the lowest possible noise levels over the long cable runs required to reach the console, a DI box is employed. It takes unbalanced inputs on one side and coverts them to a balanced output on the other.

DI boxes come in two main varieties. Passive, which rely solely upon a transformer to convert unbalanced to balanced, and typically to reduce levels slightly, and active which use either the phantom power from the desk or a battery to alter levels and convert signals from unbalanced to balanced.  DI boxes (unlike a preamp) are not intended to raise the signal level from the pickup significantly, rather, in most cases, they reduce it. This does not matter much as they are intended to be plugged into the balanced microphone level input of the mixing console, which normally has plenty of gain in reserve. If you wish to run your signal into a line level input, you will need a preamp that offers line-level signal strength output (see above).

Passive, transformer based DI boxes typically offer from 50K to 150K input impedance. This is absolutely fine for an instrument with an active transducer system, but is too low for a passive piezo transducer. Instead, look at active DI boxes for this application. These are available with much higher impedance inputs than their passive relatives. For a piezo transducer, you will need one with a minimum of 1M Ohms on the 1/4” jack input side. Quite a few are available with input impedances of up to 10M Ohms, which is ideal. Bothy types typically put out a balanced microphone level signal to the mixer in the 200 Ohm range.

It should be pointed out that passive DI boxes are in no way inferior to active models. They are just different. Do avoid cheap passive models, however, as their performance depends upon the quality of the matching transformer inside, and top class transformers remain expensive to manufacture.

When using an active DI box, it is worth noting that most perform far better when powered from the +48v phantom supply from the desk than they do when running on batteries. In particular, headroom (resistance to overload on the input side) and distortion figures are far superior on the higher voltage supply.

One drawback to some active models is that they do not provide true galvanic isolation (a state where electrical current cannot flow directly from one winding to the other as they are not in direct electrical contact. However, an audio signal can still flow via electromagnetic coupling between the two windings). This can be a very important safety feature in certain circumstances (faulty wiring on the stage, for example). Hybrid models are available, however, that incorporate both a high impedance active input stage and a true isolating transformer. As always, check the specifications carefully.

It may be an over-simplification, but it remains a useful guide, that with an active transducer system a passive DI should be considered, while with a passive transducer, an active DI box with a suitably high impedance is preferable.

Other Features

Many DI boxes incorporate a few extra switches. Mostly often seen are some form of input attenuation, a  ‘ground-lift’ switch, and a polarity or phase reverse button. The input attenuators can be useful if dealing with very high input levels (from active preamps or line level or even speaker-level sources in some cases),  and the polarity or phase reverse switch can assist in controlling feedback, or when using a ‘blend’ of microphone and DI’d signal, ensuring that the phase is correctly aligned. If the instrument’s acoustic vs amplified sound waves are out of phase, there may be a poor bass response, and an overall ‘thin’ sound, as the out-of-phase vibrations attempt to cancel each other out. The polarity (or phase) reverse button ‘flips’ the phase 180 degrees which can resolve the problem in many cases.

The ‘Ground Lift’ function is particularly important, but often confuses users. As a general rule, if you are using a guitar, mandolin, or bass plugged exclusively into the preamp or DI box you should leave the ‘Ground Lift’ button in the ground-connected position as this is used to provide the ‘ground’ to the instrument cable’s shielding. If this cable is not grounded, you may get stray radio signal pickup (RFI) or buzzing. If you are also using a stage-based personal amplifier, however, or certain mains-powered effects units in addition to the feed to the console (normally connected from the DI boxes’ THRU jack) then you might experience hum from a ‘ground loop’. This occurs when a second ground (or ‘earth’) connection is made (via the amplifier’s power cord, in example) and slight discrepancies in ground resistance allow a small current to flow via the signal cables, producing that characteristic 60Hz (US) or 50Hz (Europe) humming noise.

What you should absolutely never do - because it is potentially lethal - is to disconnect the amplifier or mixer’s own ground connection. Instead, flip the ‘ground lift’ switch on the DI box. This will safely break the ground loop and eliminate the hum.

Both full featured stand-alone preamps and straightforward DI boxes have their various advantages and disadvantages, as we see from the above. Which one suits your particular situation will vary according to the instrument’s pickup installation and with the characteristics and facilities offered by the sound system in use. Understanding the various functions on the units will also play a big part in obtaining optimum sound quality.