It is common knowledge load impedance (Z, as in fig. below) to the amplifier drops rapidly towards a short circuit with parallel connections:
e.g. (ignoring wiring resistances)
A 10 tweeters in parallel reduce its load impedance to 10%;
20 tweeters in parallel reduce load impedance to 5%;
50 tweeters in parallel reduce load impedance to 2%!
What happens?
The amplifier has its output impedance (Zout) too. So together with the load impedance, the combined behavior is like a voltage divider.
What then?
If Z is high relatively to Zout, things are pretty normal. But when the external load impedance, Z, is at the same level as Zout, sound level is halved: i.e. only half power is delivered to the load, other 1/2 power dissipates within the amplifier as heat! We then tend turn up volume higher close to its limits.
So the amplifier heats up even more until thermal runaway occurs leading to amplifier failure.
What are the solutions?
1) Employ series-parallel connection can improve the situation a little and hence not favored. Connecting adjacent tweeters in series doubles the load value - but still limited by the overall parallel connection. Besides when one tweeter fails the other in series will not work too, then extra effort to trouble shoot.
2) The proper solution is to choose an amplifier with lower output impedance. This is the reason I chose amplifiers with 100W r.m.s. per channel. I have 40-50 tweeters to a channel. Once I turned up volume close to max. during endurance tests, the amplifiers did heated up and failed too after 2 days!
So just beware - I normally touch the amplifier heat sink when I visit the farm house. Best should be just warm.
2 comments:
piezo tweeters have impedence of K ohms so having them in parallel does not cause a virtual short. On the other hand some mix piezo with coil type tweeters and therin lies the problem. I use 100 tweeters in parallel without any overload.
Hi Tuck Fook, it is not true piezo have such high impedance. From the DC standpoint, one may see a high impedance due to its construction - a piece of piezo crystal with 2 metal plates attached, not unlike a capacitor. But as it operate under signal conditions, it converts electrical energy to acoustic sound energy as mechanical vibrations. There is an equivalent AC impedance in the order of several ohms similar to normal speakers. The standard equation of power conversion still holds:
P = V^2 / Z
where Z is the AC impedance,
V is the signal voltage applied by power amp.
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