Excerpt from "A Clean Audio Installation Guide": System Frequency Response

In 1986, the founder of Benchmark Media Systems, Inc., Allen Burdick (since-retired), wrote an application note titled "Clean Audio Installation Guide". The paper began circulating among engineers at broadcast facilities and recording studios. Eventually, engineers at major facilities were personally thanking Mr. Burdick for this comprehensive guide to audio system deign and installation, and began regarding the paper as "required reading" for audio engineers. More then 22 years later, engineers are finding the topics in this paper more relevant then ever.

Here is an excerpt of the "A Clean Audio Installation Guide." This section explains the importance of wide bandwidth components in an audio system. To download and read the entire Clean Audio Installation Guide", click here.

Excerpt from "Clean Audio Installation Guide":

3.5 System Frequency Response

All of the (previous discussions) assumes a 30 kHz interconnect full voltage (slew rate) capability. This is our recommendation for minimum system performance. It is also our firm recommendation that a 200 kHz small signal interconnect bandwidth capability be the design goal to achieve the flattest response and minimum phase shift at high frequencies for good overall system performance.

It is important to recognize the difference between the small signal bandwidth of an interconnect and its slew rate limitations.

Small signal bandwidth sets the 3 dB cutoff of the interconnect filter, which in turn describes the flatness and phase response back at 20 kHz. It must be remembered that every element of an audio system will contribute it's 3 dB cutoff and associated phase shift to the overall performance of the system. Every element must be viewed as one section of a large multi-pole low pass filter; and while one element may have adequate response to 30 or 40 kHz, it is the cumulative effect of these filter sections that is of major concern. At first glance, the proclaimed need for a wide bandwidth of 200 kHz in both the equipment and the interconnect may seem outlandish. However, when you realize that the audio may travel through five, ten, twenty or even more pieces of equipment in the audio chain, each contributing its cutoff characteristics, we begin to realize the magnitude of the problem in achieving adequate high frequency performance through the entire system.

If a piece of equipment or an interconnect has a 100 kHz bandwidth and has a simple single-pole, 6 dB per octave roll off (often only true for interconnects), the following chart can be used to estimate the system roll off.

Manufacturers of audio equipment have for years mistakenly considered an upper bandwidth of 20 to 30 kHz to be totally adequate for their equipment. This narrow viewpoint, of course, fails to see their equipment as an element in a long chain, and potentially the limiting element. While we will actually never "use" - that is, put a signal into that upper portion of the 200 kHz bandwidth - it must exist to achieve the necessary 30 to 40 kHz system bandwidth.

We have often been told that "an audio chain is only as strong as its weakest link." However, in the case of audio systems, each additional link makes it weaker then its weakest link. In fact, a better analogy may be several layers of tinted glass. Even a highly transparent layer of glass will further reduce the total light emission.

Slew rate limitations, on the other hand, are large voltage swing limitations and again, have to do with the actual current output required of a stage that is driving a capacitance. It is important that no amplifier be allowed to slew limit. To do so produces high frequency intermodulation distortion. If an amplifier can provide adequate current to a cable to allow full output swing to 30 kHz at low THD, the chances are practically nil that it will ever slew limit with normal audio.

"Helpful How-To's": How to Properly Connect Balanced Outputs to Unbalanced Inputs, and Vice-Versa

When connecting audio equipment, it is important to understand the differences between various types of 'inputs' and 'outputs'. It is especially important to understand differences in electrical specifications. Connecting gear without proper electrical considerations could degrade the system's performance, or even damage the equipment. This article will discuss the necessary considerations of connecting balanced and unbalanced connections (e.g., XLR-to-RCA).

Definition of Terms and Concepts

Before discussing the techniques, a few concepts should be explained. The following concepts are true for the large majority of equipment (although exceptions do exist):

• Balanced connections have three signal conductors, referred to as: hot, cold, and shield.
• The hot signal conductor carries the audio information.
• The role of the cold signal conductor varies depending on topology (more on this to follow). Often, but not always, the cold signal conductor carries an inverted (polarity-reversed) copy of the hot signal.
• The shield conductor is sometimes referred to as 'ground', and it usually connects the cable shield to chassis ground for the purposes of providing a signal shield and eliminating ground loops.
• For XLR connectors, pin-2 is the hot conductor, pin-3 is the cold conductor, and pin-1 is the shield conductor.
• For ¼" TRS connectors, the tip, ring, and sleeve respectively carry the hot, cold, and shield conductor (equivalent to pin-2, pin-3, and pin-1, respectively, of an XLR connector)
• Unbalanced connections (RCA; ¼" TS) have two signal conductors: hot and shield (the hot signal conductor carries the audio information).

Connecting Balanced Outputs to Unbalanced Inputs

Balanced outputs are very common on professional products. Unbalanced inputs are very common with vintage equipment and consumer electronics. Establishing proper inter-connections between balanced and unbalanced equipment is imperative for the performance of the system (and may also prevent damage to the equipment).

There are three types of balanced outputs: 'impedance balanced', 'transformer balanced', and 'active balanced'. Each type of balanced output requires different considerations when connecting to an unbalanced load. With each case discussed, we will assume that 2-conductor (unbalanced) wire is being used, as 3-conductor wire will offer no advantages when driving unbalanced loads.

Devices with impedance-balanced outputs actively drive the hot output only. The cold output is tied to ground via a resistor that matches (or balances) the output impedance of the hot signal conductor. In other words, there is no audio signal on the cold conductor, but, in a full balanced system, common-mode rejection will be maintained since the impedance is balanced between the two conductors.

Impedance-balanced outputs can connect to unbalanced loads with the cold output 'floated' (unconnected) or connected to ground. There will be no performance or other differences between a floated or grounded cold conductor (pin-3) with impedance-balanced outputs. Using an XLR connector, this corresponds to pin-3 floating or tied to pin-1. The hot signal conductor will be connected to pin-2, as usual, and it will carry the audio information. The shield conductor will be connected, as usual, to pin-1.

Devices with transformer-balanced outputs actively drive the primary winding of the output transformer. The secondary winding of the output transformer delivers a symmetrical signal to the hot and cold signal conductors.

Transformer-balanced outputs must have the cold output tied to the shield conductor when connecting to an unbalanced load. Using an XLR connector, this corresponds to tying pin-3 to pin-1. The hot signal conductor will be connected to pin-2, as usual, and it will carry the audio information. The shield conductor will be connected, as usual, to pin-1.

Devices with active-balanced outputs actively drive both the hot output and the cold output. The signal on the cold output is an inverted (polarity-reversed) version of the signal on the hot output. This creates symmetrical signals between the hot and cold outputs. This is the topology of all Benchmark equipment.

When connecting an active-balanced output to an unbalanced load, it is necessary to leave the cold output (pin-3) floating. Using an XLR connector, pin-3 should not be connected to anything. If a cable is used with pin-3 tied to pin-1 (shield), the output amplifier could be damaged. This type of connection establishes a low/no impedance path to ground. This means the amplifier will drain current, unimpeded, into ground, which is stressful to the output amplifier.

Connecting Unbalanced Outputs to Balanced Inputs

Unbalanced sources are very common, especially in consumer electronics (phono pre-amps, cassette decks, etc). Also, many keyboards, samplers, and other electronic sound sources have unbalanced outputs. When these devices need to be connected to a balanced input (such as the ADC1 USB), it is important to understand the proper method of cabling.

An unbalanced-to-balanced connection should use a 3-conductor (balanced) wire. The explanation behind this technique is somewhat complicated, and it is beyond the scope of this article. For an in-depth look at the technical explanation for this solution, read section 5.4, "Shield Wires", of The Clean Audio Installation Guide.

The connecting cable should have the cold signal conductor of the cable tied to ground at the source-side only (the unbalanced end of the cable). The load-side of the cable will be connected in a typical balanced configuration.

A more sophisticated connection may be needed for aggressive common-mode rejection, which may require modifying the equipment. For information about this type of setup, refer to the "Shield Wires" section (5.4) of The Clean Audio Installation Guide.