- 26 May 2008
- 834
Με αφορμή κάποιες διατύπωσεις εδώ που ήταν λάθος, ας έχουμε υπόψη μας την αρχή λειτουργίας:
http://sound.westhost.com/project14.htmBridging Principles
For those who have not used bridging or who do not understand the principles, a short explanation of how the adapter is used and how this almost quadruples the output power is called for.
The adapter is connected between the preamplifier and the power amps. The power amps must be the same - power rating, minimum impedance rating, etc. Generally, a stereo power amp is used, so when connected in bridge mode we are assured that the amplifiers are more or less identical.
Normally, the speaker is driven from the amplifier output to ground, and the AC swing is limited by the supply voltages in the amp. Consider a 50 Watt per channel power amp - 50W into 8 Ohms requires a signal voltage of 20V RMS:
P=V2 / R = 202 / 8 = 400 / 8 = 50 WattsTo achieve this, the peak voltage is just over +/-28V (20 * 1.414), and a power supply voltage of about +/-35V will generally be used to allow for losses and mains voltage variations.
The same amplifier into 4 Ohms will deliver close to 100W - provided the power supply does not collapse under the load.
For both these examples, only one side of the loudspeaker is driven, and the other is grounded.
Now, if a second amplifier is connected so that its output is exactly 180 degrees out of phase (inverted) with the first, and connected to the normally grounded side of the speaker, as one speaker terminal is driven positive, the other is driven negative by the same amount.
Figure 3 - Voltages Applied to the Loudspeaker Using BridgingFigure 3 shows this, with the waveforms at each speaker terminal shown. As you can see, as one terminal is driven positive, the other is driven negative by the same amount, and although a sine wave is shown, the principle is not affected by the signal waveform. (Note that both waveforms should be viewed from left to right, otherwise the diagram would indicate zero output from the speakers - which is exactly what you will get if the adapter is not used.)
At maximum power, the 8 Ohm loudspeaker now 'sees' double the voltage that it would receive from one amp alone. Using the formula above, we get:
P=V2 / R = 402 / 8 = 1600 / 8 = 200 WattsSince the voltage across the speaker is doubled, naturally the current through it is also doubled, and that is the reason that each amplifier must be capable of driving 1/2 the normal speaker impedance. This technique is very common in car audio systems, because the nominal 13.8V of a car's electrical system is too low to obtain much power except into very low impedances. Loudspeakers are very difficult to make if the impedance is too low, because there are too few turns of wire in the voice coil, and efficiency is lost. Four Ohms is a reasonable minimum, but even with this impedance an non-bridged car amplifier is still only capable of a maximum of about 5 Watts. By using bridging, close to 20W is now possible, with each amplifier driving the equivalent of 2 Ohms.
This is the reason for all the dire warnings about not grounding either speaker lead of a car audio system - because each lead is the output of an amplifier, shorting it to ground or another speaker lead will destroy the power amp.
The same principle applies to the bridged connection shown here - no connection other than to the speakers is possible without damaging the amplifier.
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