SYSTEM NOISE

I’m frequently asked about “hiss” or “hum” emanating from speakers. Put your ear close to one of your speakers and you’ll be able to sample the sounds we'll be talking about; Their causes and potential remedies are the subject of this article.
       

Two types of extraneous noises are common: hiss and hum. Hiss is high frequency in nature, hum being lower frequency, most commonly 60Hz.

First we should understand that some amount of noise is normal. A system that is totally silent is probably off! To determine whether we are in the realm of normalcy, I’ll ask the question “with volume set to a normal listening level, how close to the speaker must you be to hear the noise.” If the answer is a few feet (or less), the noise level is likely normal. If you can hear it from the listening position (or, especially if it intrudes upon the listening experience), we have a problem that warrants further investigation. Be advised that raising the volume to maximum is not the way to check for noise levels. Set the volume control to a normal listening position to understand whether the amount of noise generated will be noticeable while listening.

Where do these noises come from?

Self Generated Noise -

        The flow of electrons though a conductor creates thermal noise. The active and passive parts of a component create thermal noise in performing their normal operations. The total amount of noise generated by a given component depends on a wide number of factors including: the type of parts and materials used, circuit topology, the quantity of parts and the amount of gain in the circuit. The power supply, in particular, is likely to create noise, some of which leaks into nearby low-level circuits. The sum total of the noise generated by a component we’ll refer to as quiescent idling noise; the amount of noise when no signal is present. We'll call this quiescent idling noise.

Signal to Noise -

        Simply put, signal to noise ratio is the amount of noise present in a signal at a given level. The amount of self-generated noise compared to a given signal level gives us a signal to noise ratio (expressed in dB). As an example, a component whose signal to noise ratio is measured at -90dB has a noise floor ninety decibels below the signal. All things being equal, the lower S/N ratio (higher number) the better, but like any specific design criteria, tradeoffs exist. Circuit designers carefully balance the pro and cons of any potential topology, as choices made can impact sound quality.
        Ultimately, concessions are made to optimize a particular characteristic deemed important. In some cases S/N ratio may be an acceptable tradeoff if performance gains can be realized in other areas. Thus, it is important not to take this specification out of context; overemphasizing it or any other performance parameter. The bottom line is some components are “quieter” than others, but that in and of itself is not a measure of quality.
        Further, the signal to noise ratio of a given component does not necessarily tell us how much noise we’ll hear though the speakers. That is determined by all of the components in the system along with other influences.

        What are some of the influences that affect system noise?

RFI/EMI -

        Our audio/video systems are constantly bombarded by a plethora of radiated signals. Radio, television and radar are just a few examples of the cacophony of possible disruptions ready, willing and able to invade the relative silence of our audio systems. Radio Frequency Interference (RFI) is so pervasive that I doubt any system is unaffected by it. RFI can manifest itself in the form of general degradation, added noise or even direct translation of the broadcast.
        Electromagnetic Interference (EMI) can also induce noise into the system. Radiated EMI fields are both common and detrimental. In an audio system these fields are usually induced by transformers and can manifest themselves as hum or low frequency noise.

Gain -

        A high gain device is more likely to be a bit noisier than a low gain design. All things being equal, a preamp, let’s say, with more gain is likely to have a bit more noise.  So, if we are talking about a very high gain application (e.g. phono), there will almost certainly be a bit more noise, or his, apparent.

Ground Loops -

        The single most common "noise" problem we encounter, ground loops are a subject broad enough to demand an entire article, we’ll skim the topic here as it relates to our theme at hand.
        Manifested as a constant 60Hz tone that does not vary with volume, ground loops are an especially common source of noise and complaints. Checking for ground loops can be a bit challenging. Following the suggestions mentioned below under Possible Remedies would be a good place to start. Contact your dealer for more specific help in solving ground loops.

Speaker Sensitivity -

        Speaker sensitivity is certainly the most important (and most often overlooked) factor determining whether noise is an issue. Sensitivity (expressed in Decibels) is the amount of loudness produced by the speaker at a given distance (usually 1 meter), for a given input signal (typically 1 watt). The higher the resulting number, the higher the sensitivity. A speaker rated at 90dB produces a louder volume than one rated at 85dB, with the same input. Speakers with measured sensitivities below 85dB are considered low sensitivity, moderately sensitive speakers range from the mid 80’s to the low 90’s, and high sensitivity speakers above that. There are very efficient speaker systems (typically horns) rated at 100dB and above.
        From this information, it is not hard to see that the potential for noise becomes much more of an issue as speaker sensitivity increases. A given set of electronics mated with a low sensitivity speaker may produce noise levels barely audible just a few feet away, but substitute a very high sensitivity speaker and noise will be easily heard at the listening position.

Possible Remedies -

        Ok, so now we have an understanding of the problem, how can we reduce or eliminate the noise? The first step is to determine the origin of the noise. It is a simple matter of checking the components one by one to find the trouble maker. Here's what to do.
        Turn off all the components, and then disconnect the inputs to the amplifier (or all the inputs from an integrated amplifier or receiver). That will leave us with just the speakers connected to the amplifier. Turn the amp on and listen for noise. If all is quiet, we can assume that the problem lay upstream. Onward.
        Now turn the amplifier off and connect the preamp (be sure all other components are disconnected from the preamp). Turn on the preamp, then the amplifier. If the noise returns, we can assume the preamp is at least part of the problem, if things are silent, then the problem is further upstream.
        The next step is to begin connecting your other sources, one at a time, listening for when the noise returns. Since system noise is the sum total of the noise generated by all the components in the chain, it may be that you’ll find the noise level increases slightly with each component you add to the mix. If this is the case, there is no simple way of reducing noise levels, short of changing equipment.
        If you find hum to be a problem, check for ground loops and experiment with cable layout and locations. Be sure to route interconnects to keep them away from AC cables. If they must overlap, cross them at right angles to minimize hum pickup.
        Grounds loops (constant 60Hz hum that does not vary with volume setting) are the result of two or more components being at different AC ground potentials. They can occur in any system and often pop up when a new component is added to the system. Many people feel there is "problem" with the new component, or that it is a mismatch. Neither is true and it is simply happenstance that the grounding scheme of the new piece of gear is different from those currently in play. Follow the step by step process outlined above. At some point your 60Hz hum will appear. Floating the ground on that component (and maybe others in addition) should cure the problem. A common source of ground loops are cable and satellite TV connections. There are specially designed in-line isolation transformers designed specifically for this problem.

The Bottom Line -

        To sum up, remember that a certain amount of noise from the system is to be expected and is quite normal. As long as the noise does not intrude on the listening experience, most people are satisfied. Do remember that the quality of a system is not determined by the noise level it generates but by the musical experience it imparts.