دایره المعارف تاسیسات برق (اطلاعات عمومی برق)
Ohm's law is the most basic and most useful electrical equation. It's used frequently in car audio installation on both the power input and the power output side. Simply stated Ohm's law is:
Where E is voltage measured in volts, I is current measured in amperes (amps) and R is resistance measured in ohms. Memorize this equation. You'll use it A LOT in car audio. For example, if you need to figure out the current (amps) moving through a 12 volt circuit and you know the resistance of the circuit is 4 ohms, the equation would look like this:
E = 12volts
I = unknown
R = 4 ohms
I = E/R or I = 12/4 which is I = 3 amps
Another useful equation to know is the power equation:
P = E*I (power equals voltage multiplied by current or watts = volts * amps). From this we can substitute Ohm's law for any values we don't know. For instance if we need to know power but we only have amperage (I) and resistance (R) then we could substitute I*R in the power equation (because according to Ohm's law E=I*R) and get P = I*R*I.
There are two ways to wire electrical components. In parallel or in series (or a combination of the two). Both are important to understand, especially when properly hooking up speakers to amplifiers.
Parallel wiring is connecting components to a source so that they share the same voltage. To put that in a useful way, it would be connecting all of the speaker positive terminals to the positive terminal of the amplifier and connecting all of the speaker negative terminals to the negative terminal of the amplifier.
This increases the work load on the amplifier because more current will need to be supplied to this lower resistance (impedance). Parallel resistances (in this case 4 ohm speakers) will combine according to this equation:
1/Rt = 1/R1 + 1/R2 + 1/R3...
Where Rt is the total resistance and R1, R2 and R3 are the individual resistances. For our example Rt will be the resistance at the amplifier's speaker output terminals and R1, R2 and R3 will be the resistances of the individual speakers. If we connect (2) four ohm speakers (R1 and R2) in parallel to an amplifier the total resistance will be:
1/Rt = 1/R1 + 1/R2
or 1/Rt = 1/4 + 1/4 or 1/Rt = 1/2
Inverting the equation we get Rt = 2 ohms.
Similarly if we connect (3) four ohm speakers (R1, R2, and R3) we will get:
1/Rt = 1/R1 + 1/R2
+ 1/R3 or 1/Rt = 1/4 + 1/4 + 1/4 or
1/Rt = 3/4
Inverting the equation we get Rt = 4/3 or 1.33 ohms.
Series wiring is connecting components to a source so that they share the same current. To put that in a useful way, it would be connecting the amplifier's positive terminal to the positive terminal of the first speaker and then connecting the negative terminal of the first speaker to the positive terminal of the second speaker and so on. The final speaker in the chain will have it's negative terminal connected to the negative terminal of the amplifier.
This decreases the work load on the amplifier because less current will need to be supplied to this higher resistance (impedance). Series resistances (in this case 4 ohm speakers) will combine according to this equation:
Rt = R1 + R2 + R3...
Where Rt is the total resistance andR1, R2 and R3 are the individual resistances. For our example Rt will be the resistance at the amplifier's speaker outputs andR1, R2 and R3 will be the resistances of the individual speakers. If we connect (2) four ohm speakers (R1 and R2) in series to an amplifier the total resistance will be:
Rt = R1 + R2 or Rt
= 4 + 4 or Rt = 8 ohms
Similarly if we connect (3) four ohm speakers ( R1, R2, and R3) we will get:
Rt = R1 + R2 + R3 or Rt = 4 + 4 + 4 or Rt = 12 ohms
Speaker wiring is an important issue because it directly relates to the speaker resistance, or rather, speaker impedance. All speakers have resistance to electrical current. Resistance to Alternating Current (AC) is called impedance. Impedance cannot be easily measured, it is usually calculated. This is because there are three components found here: direct current resistance, capacitive reactance, and inductive reactance. Speakers usually have two components which make up the impedance: voice coil resistance, and voice coil inductance. Thus, the resistance of the voice coil is the direct current resistance, which can be measured with an Ohm meter. The inductance of the coil can be measured with special tools or calculated. This factor (inductance) accounts for the inductive reactance. These two components together form to make the impedance of the woofer.
Typically the impedance of most speakers are between 4 and 8 ohms. However, the impedance of any speaker is not the same at all frequencies. Usually, the resonant frequency of the speaker yields the highest impedance. The rated or nominal impedance is the average impedance of the speaker over the useful frequency range of the device.
When multiple speakers are connected to an amplifier, certain decisions need to be made. How will the speakers be wired to amplifier? Will this wiring present a proper impedance to amplifier? Will this wiring reduce the fidelity of the speaker system?
If you have an amplifier with an 8 ohm output jack, and you have one speaker that is rated at 8 ohm impedance, you simply connect the two devices together. But if you have two speakers, you now have a decision to make. Should I wire these in parallel or series? What will the resulting impedance be? What if the speakers are wired out of phase? These are the questions that need to be answered.
First, most transistor amplifiers can tolerate a range of impedance loads. Typical is 8 or 4 ohm loads. See diagram below for a typical speaker wiring.
Some amplifiers are rated for 8, 4, 2 ohm loads. However, tube amps are very particular about loading issues. If the amplifier says 4 ohms, then you better have a 4 ohm load. Transistor amplifiers operate on a completely different basis than tube systems. A tube amplifier is really a current source device, this is why it requires a load to operate. A transistor amplifier is really a voltage source device, and does not require a load. No load is viewed by the amplifier as infinite resistance. For tube a amp, no load (no speaker) is a disaster, and appears as an internal short to the system.
If you wire two 8 ohm speakers in parallel, the resulting impedance is 4 ohms. If you wire two 8 ohm speakers in series, the resulting impedance is 16 ohms. The 4 ohm load will draw more power, because the impedance is lower, but the amplifier will not have as good of control over the speaker as an 8 or 16 ohm load. The series load of 16 will provide the best control for the amplifier, but the power will be reduced, and the high frequencies will also be reduced. This is because the voice coil for each speaker is acting as a low pass filter to the other speaker. See the wiring diagram below for these two examples.
If you have 4 speakers, they are typically wired together in series-parallel. This is where two of the speakers are wired in series with each other. The other two speakers are wired in series too. Then the resulting systems are wired in parallel. If all four speakers are 8 ohms, then the result from this wiring practice is 8 ohms. See the wiring diagram below.
This speaker system pictured below is four 10 inch aluminum cone bass guitar woofers wired in series parallel configuration.
Notice in the wiring diagrams that the polarity is consistent. Where the + sides are connected to the + sides, and the - sides are connected to the - sides. This provides "in phase" operation of the woofers. If one of the woofers is connected backwards, it will be pushing air, when the others are retreating, and visa versa. This will cause a noticeable decrease in the bass, since one woofer is increasing the air pressure next to one that is decreasing the air pressure, with the net effect being almost zero!
A Simple Test for Phasing
There is, however, a simple way of double checking the wiring (Phasing) in complex systems. Simply connect a "D Cell" battery (1.5v) to the speaker jack and observe the motion of each cone. All of the cones should move in the same direction when connected, and then in the opposite direction when the battery is disconnected. If one of the speakers moves in the opposite direction from the others for same connection to the battery, it is out of phase and your system will not sound that great.
Wire Sizes to the Speakers from the Amplifier
For modern Power amplifiers, the wire size becomes more important as your system develops more power. This is especially true with Solid State power amps. This is because in reality the output impedance of most Solid State amplifiers is around 0.005-0.020 ohms. You can generally find out this factor by taking the recommended speaker impedance and dividing it by the Dampening Factor of the amplifier. The Dampening factor tells you how much control or leverage the amplifier has over the speaker system. But this factor assumes that you have zero resistance in your wiring to the speaker. The reality is that amplifier and wiring to the speaker are one unit. As your wire size becomes smaller, and your wire resistance increases, your actual dampening factor drops. Thus, if your amplifier had a dampening factor of 400 for an 8 ohm load, the real internal resistance at the output stage is 0.02 ohms. But if you are using a long length of 24 gauge wire that has 0.1 ohms total length (there and back), you have just dropped your dampening factor to (8ohms/(0.02 + 0.1)) = 66.7. A similar thing happens when you decide to reduce your speaker impedance, either by replacement or by adding speakers in parallel.
This is why you want to use as large of a wire as possible and as short as possible from the power amp to the speaker. It really makes a difference.
The was a debate a while back on a forum on which way to wire a 4x12 cab the best way for the best tone. Since "Best" is an opinion. I wired my 4x10, loaded with EVM 10m Speakers, both ways and made recordings with my Traynor amp (pictured above). I'll let you decide.
مدرس دانشگاه و نظام مهندسی.
طراح،مشاور و ناظر برق و BMS.
نویسنده کتب تاسیسات الکتریکی.
مدیر گروه برق خانه عمران و موسسه مهندسان.
قصه های قبیله
معرفی نویسنده و مدیر وبلاگ
جذابیت های برق
در ستایش معماری
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فکر اقتصادی، کارآفرینی
تکنولوژی های روز دنیا
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اطلاعات علمی ساده و خلاصه
خلاقیت های ناب ناب
کافه کتاب،فرهنگ و ادبیات
رسانه (ایران، بین الملل)
آسیب های اجتماعی
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