DI boxes are nothing new. So you would think everybody knows how to operate one and where it can be utilized. However, a specialist magazine went so far as to call a DI box a “hum killer” in a 19“ rack. We would therefore like to give you some brief details of the capabilities and uses of Palmer DI boxes.
The simplest way to give you a clear picture of how DI boxes work is to look at how they developed. In the “electronic Stone age”, only acoustic instruments were recorded using a microphone. When electronic instruments first appeared on the scene, you simply placed a microphone in front of the sound source, i.e. the loudspeaker. Then somebody hit on the idea of cutting out electric/acoustic signal conversion using a loudspeaker/microphone. This was achieved by feeding the electrical signal produced by the electronic instrument directly into the mixing console. However, as the electrical signal from a musical instrument is not necessarily compatible with a microphone output signal, a special signal converter box was required. This was where the DI box came in.

A DI box usually has three functions: 1. It reduces the line output level from electronic musical instruments to microphone level, so as not to cause overloading at the mixing console input. 2. Almost all musical instruments have an unbalanced output level. Microphones, on the other hand, have a balanced level. This means that an unbalanced signal must be converted into a balanced one. 3. Instrument output signal levels are in the mid to high impedance range, whereas microphone levels are in the low impedance range producing a nominal approx. 200 Ω. A DI box must therefore also be able to convert impedance levels. It basically consists of a transformer, which also isolates the electronic instrument from the mixing console. This in turn suppresses ground loops and any associated humming noise. It is obvious that a DI box cannot provide a 1:1 transfer ratio. However, in some cases, it may be possible to do this with an active DI box (PAD in 0 dB position), but only just within the DI box performance range. This is because the DI box output level is designed to operate within microphone level ranges and not at +22 dB line levels, e.g., required by radio stations.

People often claim that “Active DI boxes are always better than passive DI boxes”. It is true that you can use cheap electronic components to tweak up a “doorbell” transformer and so reduce its ear-piercing frequency response. The question is whether such an active DI box actually produces a better sound than a passive one. We believe that even inexpensive electronic musical instruments have adequate output level ranges to produce satisfactory, if not excellent, results using a passive DI box. A passive DI box is therefore the right choice for most purposes. However, some instruments without electronics, such as passive bass guitars and acoustic guitars fitted with a pick up but no preamp, require very high input impedances which can only be supplied by an active DI box. Experienced professionals also use active DI boxes when transmission routes are severely distorted. The DI box produces higher levels, which can be reduced at the microphone input on the console by pressing the PAD key. This improves the signal-to-noise ratio. If possible, active DI boxes should be connected to a +48 V phantom power supply. This produces a better dynamic range compared to using a 9 V battery power supply.

These are also known as “line boxes”. With regard to impedance and level, line boxes (in contrast to DI boxes) have an input/out-put transformer ratio of 1:1. Line boxes are used to solve problems relating to ground loops.
To provide protection against electric shocks, many devices have metal housings and a power plug fitted with a grounding pin. This ensures that the device housing is grounded. If a fault occurs, the grounding prevents users from coming into contact with dangerous voltages. If you connect up two such devices using a screened audio cable, this may cause a ground loop, which produces a (50/60 Hz) humming noise. The reason for this is as follows: In an ideal situation the ground potential should always be 0 Volts. Cable routes with different lengths and many other complications can cause the ground potential to deviate slightly. By connecting up two devices with different ground potentials the screening allows an equalizing current to flow between the devices. This current superimposes itself over the audio signal and causes the humming noise. Here, it is important that you do not disconnect the ground contact. It is your only guarantee that high voltage short circuit current can be grounded if a fault occurs. Screened audio cables cannot be used here because of their cross-sectional size and connector type.
The safest way of preventing a ground loop is to isolate the two devices galvanically, i.e. to ensure that the devices are not DC-connected in any way. The best way to do this is by using a high quality audio transformer. Here, the signals are passed through the transformer by inductive coupling. There is no DC-connection from the primary to the secondary winding.
This transformer must be designed for the intended purpose. Use of an incorrectly matched transformer can have severe effects on the frequency response and distortion of the signal. In the field of sound engineering, two types of line levels have become technical standards:
Professional line level varying between 0 and 6 dBu (0.775 V to 1.55 V), max. +20 dBu at 7.75 Volts with 600 Ω source impedance and a line level for semi-professional (consumer) equipment which is at -10 dBV nominal (approx. 0.3 V) lower, but which has a nominal impedance of 10 kΩ. As a transformer can operate in both balanced and unbalanced mode, line boxes are also suitable for converting unbalanced lines routes to balanced ones and vice versa.

Sound engineering not only involves combining signals, but also directing them to different channels. The simplest way to do this is by using a so-called hardware split. The basic design is called a Y cable. But linking several mains-powered devices increases the chance of a ground loop occurring. A passive splitter box eliminates this danger by using so-called “split” transformers. These audio transformers have one input winding and several output windings. This isolates the devices from each other. Nevertheless, it is important to remember that the signal from one source must act as a driver for several loads.

A low impedance mixing console output can easily drive a dozen power amplifiers. In most cases, this can be handled by a passive splitter. But do not forget that a short circuit at one of the outputs will be transmitted to the other outputs by the transformer, interrupting or, at the very least, weakening the total signal. Decoupling resistors can reduce this effect but will also cut down the signal level.
Microphone signals, which operate at extremely low levels, are more susceptible to interference. Active splitters are preferable where longer cables are used and where professional standards are required. There are obvious advantages here. The “plug box” is located on stage near the microphones, so only short cable lengths are required. The active splitter can also boost low microphone signals. This considerably improves the quality of the signal before it is transmitted along the multi-core cable.

With short cable lengths up to around 6 m, a guitar can even be connected directly to the line input. With longer cables, the use of a DI box is recommended; the output of the DI box is connected to the microphone input. A rule of thumb is that a passive box works for all guitars with built-in preamplifiers. However, in the case of electric guitars, it should be kept in mind that the guitar amplifier makes a significant contribution to the sound characteristics.

Mains-powered instruments such as keyboards are grounded via the mains connection. Since the DI box is also grounded, via the mixing console to which it is connected, this can result in the dreaded hum caused by ground loops. The ground lift switch solves this problem by isolating the ground connections of the two devices. In practice it is not necessary to be concerned about the complexities involved; if there is a hum, simply use the ground lift switch.

Transformer balancing always provides galvanic isolation, so as to guarantee reliable isolation of the ground connections of the various devices. This is not the case with electronic balancing. In addition, with appropriate wiring it is always possible to use transformer-balanced inputs and outputs for unbalanced operation. In the case of electronic balancing, this can lead to level reduction. However, it should also be pointed out that only a high-quality audio transformer is able to maintain the sound quality in this situation. For this reason, Palmer manufactures all required transformers itself at its own facilities.

Balanced cables largely eliminate noise interference, whereas in the case of unbalanced cables, noise is superimposed on the signal. In addition, because the shielding of unbalanced cables is used for signal return, it cannot be readily isolated via a ground lift switch.

We have previously manufactured passive versions of the PAN03 in response to special requests. If the demand increases, we will also offer this as a regular item.

At present we do not have an active dual DI box in our product portfolio. However, the PAN03 provides a four-channel solution in 19" format.

With the appropriate cables, this is certainly possible. However, the PAN05 lacks galvanic isolation, so that the phantom power can be fed through. Thus, if the split signal is supplied to different mixing consoles, there is a risk of hum due to ground loops. Our PMS02 provides a technically more reliable solution for splitting microphone signals.

Phantom power is conducted through the PAN05. However, when the PAN05 is used to mix 2 condenser microphones, this doubles the phantom power requirement of the mixing console input. If the mixing console cannot supply the power required by the condensers, the power can alternatively be supplied by a PAN48. In the case of the reverse application, splitting, it is essential to avoid the situation where both microphone inputs are supplying phantom power.

Due to a multitude of details, the PDI01 is a higher-quality product than the PAN01. To mention only a few examples: The die-cast housing of the PDI01 is much more elaborate than the sheet metal housing of the PAN01; the XLR socket of the PDI01 is a classic all-metal design; the PDI01 transformer can process even higher levels without distortion; the PDI01 level attenuation is switchable with two settings; and the ground lift switch includes a soft ground setting.

A ground lift switch normally connects or isolates the ground connections of two devices. The soft ground is an intermediate setting. High-frequency noise signals, which often originate from computers or other digital electronic devices, are conducted to ground and thus suppressed. However, at the same time hum caused by ground loops is eliminated.

As is the case with the PDI01 and PAN01, there are many details which make the PDI02 a more professional piece of equipment. To mention only one example, the discrete structure of the PDI02, with high-quality transistors, permits absolutely first-class signal processing with very low noise. The quality of the electronics in the PDI02 meets requirements that are otherwise fulfilled only by top-quality microphone preamplifiers.

With ordinary non-continuous use (as opposed to continuous operation), an alkaline manganese battery can last for a total of approximately 100 hours. Of course this is heavily dependent on the quality of the battery.

The PDI03 load can convert up to 200 W into heat, i.e. the amplifier should not produce a power output exceeding 200 W.

Yes. The regular impedance is 8 ohms. For tube amplifiers that cannot operate with this impedance, upon request Palmer also supplies the PDI03 with an impedance of 2, 4 or 16 ohms. Subsequent conversion of the PDI03 is also possible.

No, since the PDI03 serves as the equivalent of a miked loudspeaker. (An amplifier is usually required between a guitar and a loudspeaker.)

No. Since the output of a PDI03 is typically connected to a mixing console, and every mixing console has a built-in headphone amplifier, we feel that an additional headphone amplifier in the PDI03 would only increase costs unnecessarily.