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The A75II 24v from Rebec is a versatile 4-channel amplifier that offers excellent sound quality and high efficiency. It is suitable for both hobby audio enthusiasts and professional users and is characterized by its compact design. The Class AB amplifier has an input sensitivity of 0.9-4.5V and works with a 20-32V power supply. With an output power of 75W x 4 channels and the possibility of bridging to 240W, it supports precise music reproduction in the frequency range from 20Hz to 20KHz. Its dimensions of 323x170x50mm make it easy to install in tight spaces, ideal for modern car installations. The amplifier is equipped with internal fuses to protect it from overvoltage and offers a signal-to-noise ratio of over 95dB for optimal sound.
Technical data
Rebec A75II 24v
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From a gross order value of 300,00€: flat rate 129,99€ (incl. VAT)
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As space is at a premium in most vehicles, it is important to find the right place to install your amplifier. An amplifier needs some space around it so that it can dissipate the heat that builds up during operation, otherwise it will overheat and shut down. Our two preferred installation locations are therefore under a seat or in the trunk. Installation under the seat saves space and the amplifier remains invisible. It also allows you to run shorter cables from your receiver. An amplifier mounted in the trunk compartment requires longer power and signal cables, but is safely hidden and closer to the rear speakers and your subwoofer.
The answers to most questions about fuses include the word "safety". While most car stereo amplifiers come with their own fuses, these fuses are only meant to protect the amplifiers themselves. You need to install a fuse on the battery to protect the power cables, your car and yourself from fire in the event of a short circuit. This is explained in more detail in the next two answers.
As space is at a premium in most vehicles, it is important to find the right place to install your amplifier. An amplifier needs some space around it so that it can dissipate the heat that builds up during operation, otherwise it will overheat and shut down. Our two preferred installation locations are therefore under a seat or in the trunk. Installation under the seat saves space and the amplifier remains invisible. It also allows you to run shorter cables from your receiver. An amplifier mounted in the trunk compartment requires longer power and signal cables, but is safely hidden and closer to the rear speakers and your subwoofer.
In the event of an electrical mishap, you don't want a live cable running through your vehicle. Installing a fuse with the correct amperage rating in your power cable will protect your car and equipment from the dangers of a short circuit. A short circuit occurs when a live cable comes into contact with bare metal (such as the chassis of your car). Since the entire body of your car can be considered a "negative" or ground, you can think of a short circuit as the positive terminal touching the negative terminal. You definitely don't want this to happen, but if it does, a properly installed fuse can prevent a fire or other damage. A fuse works by "blowing" and interrupting the flow of electricity. A fuse is much easier and cheaper to replace than your car or your life. So given the alternatives, "blowing a fuse" is a relatively good thing. You can optimize the protection your fuse provides by installing it as close to the battery as possible - this increases the length of the protected cable behind it. We recommend a maximum distance of 30 centimeters from the battery.
The following list provides a guide to the correct fuse values: 80A (suitable for up to 16mm² cable and approx. 1300W amplifier power) 100A (suitable for up to 20mm² cable and approx. 1700W amplifier power) 120A (suitable for up to 25mm² cable and approx. 1900W amplifier power) 150A (suitable for up to 35mm² cable and approx. 2100W amplifier power) 200A (suitable for up to 50mm² cable and approx. 3000W amplifier power) 250A (suitable for up to 70mm² cable and approx. 3600W amplifier power) 300A (suitable for up to 95mm² cable and approx. 4300W amplifier power)
Amplifiers are not usually supplied with the cabling needed to connect them. However, we offer a range of amplifier wiring kits that include everything you need to send signals and power to your amplifier. You will need a thick power cable that runs from the positive terminal of the car battery through the bulkhead of the car to the amplifier. The required thickness of the power cable is determined by the manufacturer of the amplifier - you can look it up in the user manual or online, we will be happy to help you make the right choice here. Make sure you install a fuse or circuit breaker near the battery. Without such a fuse, an accidental short circuit can be a fire hazard and damage your amplifier (not to mention your car). You will also need another cable with the same cross-section to act as a ground wire. You don't need such a long earthing cable because the earthing point can be close to the amplifier. The point on the chassis of your car where you screw on the ground cable should be cleaned and free of paint to ensure a firm electrical connection. The last cable you need to power the amplifier is a power-on cable. It does not need to be as thick as the power and earth cables (1mm² should be sufficient). It is tapped from the remote connection on the back of your radio or from the ignition lock or fuse box. The audio signal is routed from the back of your radio to your amplifier via a cinch. Your RCA cable should be long enough to reach the amplifier, but not so long that it has a lot of slack and could get kinked over time. If you are using a radio without RCA outputs, you can get the input signal from your amplifier through the factory speaker wiring, either behind the radio or through the rear speaker wires. Many amplifiers have high level or speaker inputs to support this type of setup. Otherwise, you can use a highlow adapter as an output converter to convert the speaker signal to the preamp/cinch level required by your amplifier's input. At the other end of the amplifier you need a speaker cable. Usually 1.5mm² or 2.5mm² mm thick cable is sufficient. Remember that current flows more easily through thicker cables.
Your car amplifier draws a lot of current from your vehicle's electrical system and therefore needs a sufficiently thick power cable so that the current can flow freely and without resistance. This is important, as otherwise your amplifier may underperform or even overheat and switch off. For the installation of a single amplifier, the required cable cross-section is specified by the manufacturer of the amplifier and can be found in the operating instructions or online. We are happy to help you choose the right cable! If you use thicker power and earth cables, your amplifier can draw the required current from the battery more easily. However, the cables must still fit into the amplifier connections. If you do not have the manual or are planning a system with several amplifiers, you should look at a 12V cable thickness table to determine the correct cable cross-section. 16mm² cables are suitable for approx. 1300W amplifier power 20mm² cables are suitable for approx. 1700W amplifier power 25mm² cables are suitable for approx. 1900W amplifier power 35mm² cables are suitable for approx. 2100W amplifier power 50mm² cables are suitable for approx. 3000W amplifier power 70mm² cables are suitable for approx. 3600W amplifier power 95mm² cables are suitable for approx. 4300W amplifier power
The thin blue cable is called the power-on cable as it transmits the signal to switch on the amplifier. The power-on cable runs between the amplifier and your radio. You don't want your amplifier to be on all the time and drain your battery every time you park. The power-on signal triggers an electronic switch in the amplifier that turns it on as soon as the receiver is turned on. The switch-on cable must be connected correctly so that the amplifier can be switched on.
The amperage of your battery determines how powerful the amplifier is that you can install. This will give you a rough idea of how much power reserve your car's system has. Next, you need to calculate the approximate power consumption of the amplifier you want to install. To calculate the current draw of an amplifier, add the number of channels to the RMS watts per channel (a 2-channel amplifier with 100 watts RMS per channel would draw 200 watts), add the power of other amplifiers. If these figures are too much for you, you will find a simpler solution here: A battery with a capacity of 60Ah is normally sufficient for a music system with 1000 watts RMS total power, an 80Ah battery for a system with 1500 watts RMS and a 100Ah battery for a system with 2000 watts RMS total. A capacitor (called a powercap) or an additional battery can help if your system has more power. We will be happy to advise you on whether you need a powercap or an additional battery.
The amperage of your battery determines how powerful the amplifier is that you can install. This will give you a rough idea of how much power reserve your car's system has. Next, you need to calculate the approximate power consumption of the amplifier you want to install. To calculate the current draw of an amplifier, add the number of channels to the RMS watts per channel (a 2-channel amplifier with 100 watts RMS per channel would draw 200 watts), add the power of other amplifiers. If these figures are too much for you, you will find a simpler solution here: A battery with a capacity of 60Ah is normally sufficient for a music system with 1000 watts RMS total power, an 80Ah battery for a system with 1500 watts RMS and a 100Ah battery for a system with 2000 watts RMS total. A capacitor (called a powercap) or an additional battery can help if your system has more power. We will be happy to advise you on whether you need a powercap or an additional battery.
Your ground cable should have the same cross-section as your power cable and must be in direct contact with the metal body of the vehicle. Look for a bolt or screw that is in contact with the vehicle frame near the amplifier. Remove the bolt or screw and scrape off the paint or dirt. A washer will help your ground cable maintain a firm contact with the body. Use a ring terminal on the end of the ground wire to securely attach it to the bolt. If you can't find a suitable grounding screw or bolt, you can drill a hole for it. Be careful not to drill into the gas tank, a gas line or a brake line. If you need to earth several components, try to earth them all to a single screw. How much power do I need for my subwoofers or speakers? The correct answer to this question depends on which subwoofer or speaker you choose. In the technical data of each subwoofer and speaker you will find a recommended power range (in continuous or RMS watts). As a general rule: more power is better, because bass and higher levels are very power-hungry. To ensure that your system really delivers, we recommend that you choose an amplifier that is in the upper third of the maximum RMS output of your speaker and subwoofer. As a rule of thumb, you can assume that an amplifier should offer 1 to 1.5 times the RMS power that a subwoofer or speaker is specified with in the RMS specification. The RMS power of your amplifier can even exceed the maximum RMS power of the speaker, provided you pay attention to the sound quality - if the sound starts to distort or "break up", you have reached the power limit of your amplifier/speaker combination. If you turn down the volume control a little, your woofer will be safe; clean, loud sound will not damage your speaker. This RMS power of the amplifier is only available when the volume control of your radio is turned up and the gain control of your amplifier is set correctly.
The impedance is the electrical resistance of the voice coil of a loudspeaker or subwoofer to the voltage output by the amplifier. In contrast to a fixed resistance value of a resistor, the impedance of a coil changes with the frequency of the signal. The "nominal" impedance of a particular loudspeaker or subwoofer is the value assigned to it by the manufacturer for meaningful calculation and application of power. This impedance represents the load against which the amplifier must work to produce sound. The lower the load on the amplifier, the more power it can deliver. Unfortunately, there are limits to how low a speaker's impedance can be before the amplifier tries to deliver more power than it can. The minimum impedance that most car amplifiers are stable for is a 2-ohm load on each channel, or a 4-ohm load for bridged channels, and 1 or 2 ohms for subwoofers. Almost all full-range speakers in cars have an impedance of 4 ohms. There are different impedances for subwoofers. You can combine multiple subwoofers in different configurations and achieve a total impedance that your amplifier can handle, so dual voice coils are often found here. When the voice coils of speakers or subwoofers are connected in series - one after the other, a plus from one to a minus from another - you add their impedances to get the total impedance. Two 2-ohm voice coils in series give a 4-ohm load. If you wire speakers or sub-coils in parallel - each terminal is connected to the same pole, plus to plus, minus to minus - you take the impedance value of one coil and divide it by the number of coils. Two 2-ohm speakers connected in parallel result in a 1-ohm load. For more information on impedance, please contact our customer service.
One option is a 3-channel amplifier with two channels for your front speakers and a separate channel with more power for your sub. But this leaves no room for future expansion. To get the most out of your investment, it's important that you choose a device that will serve you well now and won't be too small when your system grows. If you opt for a 4-channel amplifier, driving a pair of front speakers and a subwoofer is a breeze. You simply need to operate your amplifier in what is known as 3-channel mode. To do this, you bypass the rear channels to drive your subwoofer, while the front channels drive the two normal stereo speakers. When you bridge the rear channels, you combine them into a single channel in mono mode. Choose an amplifier that has a built-in low-pass filter for this bridged channel. The crossover and higher output power of the mono channel make this an ideal solution for driving your subwoofer. As your system grows, you can use a separate amplifier for your subwoofer. Then you could add another pair of stereo speakers as a rear fill and run your 4-channel amplifier in 4-channel mode.
Amplifiers are divided into different classes depending on how their internal circuitry works. In the world of car hi-fi, most amplifiers are either Class AB or Class D. Class A amplifiers always have current flowing through their output transistors (or tubes). This is an extremely inefficient way to increase power and Class A amplifiers are large, heavy and get very hot. The advantage of Class A operation is its linearity - Class A amplifiers produce the cleanest power with the best fidelity and lowest distortion of any other amplifier class. Class B amplifiers operate in such a way that each output transistor only carries current half the time and switches off when the signal is not present. Class B amplifiers are very efficient, but distort the signal by constantly switching on and off at the output. Class AB is the traditional amplifier design used in car radios, home theaters and stereo receivers. The output transistors are turned down a little when they are not needed. The result is good sound quality and reasonable efficiency (often 50%). With Class D amplifiers, the transistors are switched on and off quickly, which significantly increases the efficiency of the amplifier, sometimes reaching up to 90%. A Class D amplifier takes the input signal and transfers it to high current pulses generated by the power supply. This creates a slight distortion in the high frequency range, which is above the audible range and can easily be removed with a low pass filter at the output. Class D amplifiers can be very small in relation to their power. This is why they are increasingly being used in the mobile audio world, where space is at a premium.
If you connect a pair of speakers in parallel to an amplifier, connect the positive cables (+) of both speakers to the positive terminal (+) of the amplifier and the negative cables (-) of both speakers to the negative terminal (-) of the amplifier. If you wire two 4-ohm speakers in parallel, the amplifier will see a 2-ohm load. This lower load (lower resistance) allows the amplifier to deliver more power, but it gets hotter. Amplifiers that can cope with this additional heat development are considered to be 2-ohm stable. The series connection works in the same way as with flashlight batteries: Connect the positive terminal of one speaker to the negative terminal of the other speaker. Connect the positive terminal of the amplifier to the positive terminal of one speaker. Then connect the negative terminal of the first speaker to the positive terminal of the second speaker. Finally, run a cable from the negative terminal of the second speaker to the negative terminal of the amplifier. If you connect two 2-ohm speakers in series, the amplifier will be loaded with a 4-ohm load. This higher ohm load (higher resistance) inhibits the current flow from the amplifier. You get less power, but the amplifier runs cooler and is more stable.
Mono amplifiers with one channel are intended for subwoofers. They are designed for a wide range of impedances and have tone controls and filters specifically designed for bass reproduction. As mono amplifiers are usually class D amplifiers, they are a good choice for driving subwoofers - class D amplifiers have a high power to power ratio and excellent efficiency, which is exactly what you need when dealing with power hungry low frequency signals. Most mono amplifiers are designed for operation at 2 ohms, but some are also suitable for 1 ohm. Multi-channel amplifiers, on the other hand, are usually designed for 2-ohm loads on the individual channels, but must be able to handle at least 4 ohms when bridged. This is an important distinction if you are using your amplifier to drive multiple subwoofers, because you cannot bridge your multi-channel, 4-ohm stable amplifier to drive multiple subwoofers that have less than a 4-ohm load. Instead, use a mono amplifier to drive a 2-ohm load - for example, two 4-ohm subwoofers or two 2-ohm dual voice coil subwoofers. This will allow you to drive your subwoofers at the maximum power of the mono amplifier without driving them at a dangerously low impedance.
The advantages of connecting two subwoofers to a mono amplifier are the same as any other number of subwoofers to a mono amplifier: you can drive the subwoofers with more power at lower impedances. Because low frequencies are less directional (i.e. it's harder for your ears to tell where low frequencies are coming from than high frequencies), bass is often transmitted in mono. Mono here refers to a single channel (as opposed to stereo or two channels), not a speaker. Most mono amplifiers have two sets of speaker terminals to make installation easier: If you connect two subwoofers to the amplifier and use a thick cable, you can attach the wires there without having to cut them. In reality, however, these terminals are connected together in the amplifier - both positive terminals are connected to the same point in the amplifier, as are both negative terminals. If you are using more than two subwoofers, simply connect them in parallel or in series (or a combination) to get as close as possible to the minimum impedance of the amplifier.
One of the last steps in installing an amplifier is to set its gain correctly. The gain setting matches the input level of your amplifier to the output level of your receiver, ensuring maximum distortion-free music and minimum background noise. This process is also called "gaining in", as it is used to correctly adjust the "gain" or "level" knob. If your amplifier has a clipping LED, disconnect all speakers and only pay attention to the clipping LED instead of the distortion of the sound. Start with all EQ presets and tone controls on the radio turned off or set to low. Turn off all filters on your amplifier, set the bass boost to zero and turn the gain to a low value. Play some familiar music and turn up the volume on the radio until you hear the music start to distort, then turn it down a little to make it sound clean. If you don't hear distortion even at full volume, turn the volume of the radio to ¾ full. Slowly turn up the gain on your amplifier until you hear the music start to distort or the clipping light comes on, then turn it down a little so it sounds clean again and the clipping light goes out. Reduce the volume of the receiver to a comfortable level. Play the familiar song over and over while you continue to adjust your system. Adjust the EQ presets or tone controls on the receiver to how you want to hear your music. For a 2 or 4-channel full-range amplifier, turn on the amplifier's high-pass filter and adjust it to remove the lowest bass tones from the full-range speakers. These low tones would likely be the first to distort through the speakers when you turn up the volume later. With subwoofer amplifiers, you turn on the low pass filter and remove everything that is not bass from the sound of the subwoofer. The bass boost is another type of EQ or tone control, usually related to a particularly low tone. Experiment with it carefully and listen to how it affects the sound of the bass and adjust it to your liking. Readjust the gain control on the amplifier - first turn down the gain on the amplifier and then repeat steps 2 and 3. Turn up the volume on the receiver until you hear the music start to distort and then turn it down a little to make it sound clean. If you don't hear any distortion even at full volume, turn the receiver volume up to ¾ full. Slowly turn up the gain control on your amplifier until you hear the music start to distort, then turn it down a little to make it sound clean again. Re-adjusting the gain (the last two steps) is important to compensate for any EQ or boost effects so that the gain of your amplifier is set exactly the way you hear your music.
The RMS wattage is a measure of power capacity that is used to compare and match components with each other. The peak power, which is often twice as high as the RMS value, is only achieved during short-term peaks. You should only use RMS values when comparing and matching devices.
