Audio Power Amplifier Fundamentals continued

[GlassWolf]

Damping Factor... What is this?

The Damping Factor of an amplifier in general refers to the ratio of the amplifier's output load impedance (the speaker, nominally 4 ohms) to the output impedance of the amplifier. Ideally, the damping factor would be infinity (in other words, the ideal output impedance for an audio amplifier is zero ohms). Damping factor, like many amplifier specifications, is a function of many factors and is thus difficult to quantify with a single number. As such, "low end" manufacturers can have a "field day" with this spec, publishing fantastic numbers (however with no information as to how the measurement was made).

The damping factor of an amplifier depends greatly upon the speaker to which it is connected, the wire connecting the speaker to the amplifier, the signal frequency that the amplifier is sending to the speaker, and the power level at which the amplifier is operating, among other things. Damping factor is most critical at low frequencies, generally 100 Hz and below (i.e. frequencies that a woofer reproduces). At such frequencies, a high damping factor is desirable in order to maintain a "tight" sound. If an amplifier/speaker pair has a low damping factor, the bass response is likely to be "boomy", "uncontrolled", and "loose" sounding.

Specifying damping factor as a simple single number does not really tell the whole story. Damping factor is a ratio of two numbers, one of which (the speaker impedance) varies by a large amount depending upon frequency. This being the case, the damping factor will also vary considerably as a function of frequency. Most of the variation in damping factor is due to the characteristics of the speaker connected to the amplifier. The wire which connects the speaker to the amplifier has finite resistance which must be accounted for; basically it is lumped in with the impedance of the speaker. So, it is wise to use heavy speaker wire in order to minimize degradation of the damping factor.

As mentioned, the output impedance of an amplifier is ideally zero. In the real world, this is never the case. The next best thing would be a very low constant (non changing) impedance. Again, the real world does not allow this either. The output impedance of most amplifiers is relatively constant except for when they approach the last 10% or so of their voltage output. This is due to the nature of the waveform from which most power supplies obtain their energy (especially analog supplies) . What this means is that the output impedance of an amplifier tends to rise considerably as it approaches its output limit. As the amplifier's output impedance increases, the damping factor must decrease proportionally. In my opinion, if manufacturers specified the output impedance of their amplifiers, there would be a lot less ambiguity among the numbers.

High damping factor numbers go hand-in-hand with amplifiers that can drive very low impedance loads (these are amplifiers with power supplies capable of delivering tremendous current). If you want to "artificially" degrade the damping factor of your system (to hear the effects), a simple test can be done:
Listen to your system at a "healthy" volume (use a CD with lots of low, tight percussion type sounds); be sure to use a heavy gauge short length speaker wire. If you have a sound level meter, note the sound level at which you listened. Then, connect your speaker up through a 100 foot (give or take) wire with much smaller gauge (use #20 or higher). Play the same music as before, but make sure the volume (to your ears, not the volume control!) is the same (this is where the sound level meter comes in handy). The volume control on the amp will have to be turned up a bit to overcome the power loss in the smaller wire. You should be able to tell that the sound has changed (for the worse, in most people's opinion).

Do not be terribly concerned with damping factor when choosing quality equipment. Most of the good amplifiers and speakers available today will yield excellent sound when used together. To avoid degrading the damping factor of your system, simply follow these (easy) steps:

Don't load up an amp with multiple pairs of low impedance speakers

Use heavy gauge speaker wire, ESPECIALLY in long runs

Never wire resistors in series with your speakers (you can't change a 4 ohm speaker to 8 ohms by doing this!)

Use a heavy duty (i.e. 8 gauge or heavier) power cable wiring your amps.

Can I get a shock from the speaker connections on my Amp?

YES! Amplifiers in the 400 plus watt per channel range are not uncommon today. Such an amplifier will put out about 50 to 60 volts RMS to a speakers. While this is only about half the amount that comes out of a wall socket, it's definitely enough to be unpleasant if you are holding on to it!
Note: The US Military defines any voltage in excess of 30 volts as hazardous. Such a voltage can be generated by any amplifier in the 100+ watt per channel range.
Zapco's C2K series amplifier manuals actually state as a warning that their amps can produce over 120 volts AC at 60Hz, which is equal to the output of a wall outlet! Not the sort of thing you want to test with your tongue.

As a side note, it's not a good idea to plug in or unplug speakers when the amplifier is playing at high volume. The "make and break" of connectors can cause momentary short circuits, as well as voltage and current transients (none of which is healthy for the amp). The preferable procedure is to make all speaker connections (and disconnects) with the amp turned OFF.

What is "Bridging"?

Bridging an amplifier refers to configuring a two channel (stereo) amplifier to drive a single load with more power than the sum of the two original channels combined. For an example, a 100 watt per channel at 4 ohms amp may put out 400 watts(one channel at 4 ohms) after bridging.

There are important things to know about running an amplifier in the bridged mode:
An amplifier running in bridged mode has one output channel to which a load (speaker) can be connected. It is no longer a two channel (stereo) amp as far as input signals and loads are concerned.
If the amp you want to run in bridged mode does not have built in facilities for doing so, you should not attempt to use it in this manner (unless you are thoroughly sure of what you are doing).
If you run bridged amplifiers, you must pay close attention to speaker phasing (see next item). Otherwise, you may have "hollow" or "weak" sound.
You must pay close attention to speaker wiring. The manufacturer will state which terminal is really the "positive" connection when bridged.
The speaker output signals of a bridged amplifier are floating; such connections must never be connected to any grounded device (such as an external accessory power meter, for example). If you do make such an illegal connection, one amplifier channel is basically short circuited (worst case result is a blown amplifier!).
Amplifiers running in bridged mode are generally limited to speakers with impedance ratings of no less than 4 ohms (in other words don't use a 2 ohm speaker load unless the manufacturer specifically allows it).

Bridged amplifiers work basically as follows:
A single input signal is applied to the amplifier. Internal to the amp, the input signal is split into two signals. One is identical to the original, and the second is also identical except it is inverted (sometimes called phase-flipped). The original signal is sent to one channel of the amp, and the inverted signal is applied to the second channel. Amplification of these two signals occurs just like for any other signal. The output results in two channels which are identical except one channel is the inverse of the other. The speaker is connected between the two amplifier speaker output terminals. In other words, one channel "pulls" one way while the second channel "pulls" in the opposite direction. This allows considerably more power to be delivered to a single load.

If we had our perfect amplifier, upon bridging it we would have a single channel amplifier with exactly four times as much power as any one channel of the amplifier in "normal" stereo mode, assuming a 4 ohm speaker load. This is because the effective output voltage available to drive the speaker has doubled as a result of bridging. A doubling of voltage on a given load results in a fourfold increase of power delivered to that load. If we used a 4 ohm load on the perfect bridged amplifier, the output power would be a very substantial eight times the normal stereo single channel 4 ohm output! These numbers should give some clues as to why real world amplifiers cannot meet such expectations. Once again, we are back to limitations of the power supply. In reality, most amplifiers in bridged mode will put out about 3 times the power as any one channel of the amp in normal stereo mode. The fourfold increase cannot be achieved because the power supply is unable to provide the current required for such performance. With 2 ohm loads, the situation is compounded. The amount of current required to drive a 2 ohm load when in bridged mode will tax the amplifier’s power supply to its absolute limits. Not to mention, the output stage may not be able to safely handle the extra heat that will be dissipated.
Bottom line: stay away from 2 ohm loads if you are running an amplifier in bridged mode!

Maximum Power Transfer Theory and Efficiency

Note: This section is intended primarily for engineering students or those with a deeper technical interest. The purpose is to provide a "real world" explanation of the Maximum Power Transfer theory and why it is NOT used in amplifiers designed for stereo systems.

Second year electrical engineering students have most likely covered the theory that basically states "maximum power is transferred to a load when the output impedance of the source is identical ("matched") to that of the load." The connection that some people fail to make is that maximum power transfer doesn’t mean maximum efficiency! At best, if the maximum power transfer theory is used, efficiency will be only 50% (not such a good figure for an audio amplifier.) In other words, if an amplifier is designed for maximum power transfer to a load, fully one half of the energy required by the amplifier's output stage will be dissipated (i.e. wasted) in the source impedance.

For amplifiers used in stereo systems (audio amplifiers), the goal is to have the amplifier output impedance be as low as possible (ideally zero, but this is never achieved). If an amplifier were to have an output impedance of 4 ohms (a common value for speakers), maximum power transfer would occur. However two other bad things result. First, the efficiency of the amplifier is at best only 50%, meaning that the amplifier will generate a lot of heat. Secondly, the amplifier/speaker system will have a terrible damping factor. Damping factor basically refers to the ratio of speaker impedance to amplifier output impedance; high numbers are better. A low damping factor will not damage anything but it will tend to louse up the sound considerably. To maintain a "tight" sound, it is important to have the output impedance of the amplifier be as low as possible with respect to the speaker. Otherwise, the amplifier will not have as much control over the speaker. Speakers, being highly complicated electro-mechanical devices with reactive impedance properties, behave better when they are connected to an amplifier with an extremely low output impedance. Speakers tend to electrically "buck and kick" an amplifier when in operation; the best way to tame this behavior is to put a heavy "load" (i.e. an amp with a very low output impedance) on the speaker. An amplifier/speaker combination with a low damping factor will tend to have a "boomier" sound and poorer transient response, (such a sound is not always bad, some people actually prefer it!).

There is a quick test anyone can do to get a feel for what effect the damping factor has on a speaker system. Disconnect your speakers from the amplifier, remove the grille, and gently tap on the woofer cone. You will hear a low frequency sound, this is the "resonant frequency" of the speaker (in it's enclosure.) Note the characteristic of the sound as you tap the cone. Now, connect the speaker up to the amplifier, and turn the amplifier ON (but leave the volume at zero). Now tap on the speaker cone as before. You will observe that the sound has changed considerably. The sound will be much "tighter", and the cone will seem harder to move. This is because the amplifier has in effect "loaded" the speaker. The case where the speaker was disconnected from the amplifier represents the worst possible damping factor (zero).

Anyway, back to the topic of this section. Although there are many applications where maximum power transfer is desired, audio amplifiers are not one of them. Audio amplifiers generally deal with a considerable amount of power, so high efficiency is a more important design consideration. In addition, to maintain high quality audio, an audio amplifier ideally has an output impedance which is VERY small compared to the impedance of the speaker it will be driving. Note that using 2 ohm speakers on an amplifier will degrade the damping factor as compared to using 4 ohm speakers (total load.)

portions of this article courtesy of Joe Roberts