Car Audio

Your automotive battery is one of the most important, and most often overlooked, electrical components in your vehicle. If your battery is not in good condition, you can be stranded, and other components in your vehicle may be damaged. This article discusses how batteries work and how to take care of them.

What is Inside an Automotive Battery?

Inside a typical car battery are six smaller energy-producing components called cells. Each cell contains a series of electrodes or plates. The positive plate of the battery is lead [eroxide (PbO2). The negative plates of the battery are pure lead in a soft, sponge-like state. The plates within each cell are arranged in alternating layers for a total of 16 components. All of the positive plates in a cell are wired in parallel, as are all of the negative plates.

Each cell produces roughly 2 volts of electricity. The six individual cells are wired in series with one another so the voltage generated by each cell adds together. The result is 12 volts.

Are You Ready for the Chemistry?

A diluted solution of sulfuric acid (H2SO4) surrounds the plates. The ratio of acid to water (H2O) is typically in the region of three parts of water to one part of acid.

When we connect a load to the external terminals of the battery, a chemical reaction starts to take place. Our diluted sulfuric acid mixture comprises H2SO4 and water (H20). As the reaction commences, the sulfuric acid splits into positive hydrogen ions (2H) and negative sulfate Ions (SO4).

When the hydrogen ions reach the lead peroxide plate, they absorb electronics from it and become a hydrogen atom. This process attacks the lead peroxide to produce lead oxide (PbO) and water (H2O). The lead oxide reacts with the sulfuric acid to form lead sulfate (Pb SO4) and water (H2O).

Negative sulfate ions move freely within the solution. As they reach the pure lead plate, they give up their extra electron and become what is known as a radical sulfate. As a radical sulfate cannot exist on its own, it will attack the pure lead plate to produce lead sulfate (PbSO4).

The action of positive hydrogen ions taking electrons from the lead peroxide plate, and the negative sulfate ions giving electrons to the pure lead plate produce an electron imbalance. These electrons flow through the external load to try and balance themselves. This process is how the battery provides power to our load (light, amplifier, heater or computer).

The Chemistry behind Battery Charging

When we apply an external DC source to the battery, we reverse the process. An external DC source such as an alternator or a battery charger feeds electrons to our positive lead sulfate-covered lead peroxide plate and the negative lead peroxide-covered lead plate. During the charging process, the density of the sulfuric acid solution falls, but we still have positive hydrogen ions and negative sulfate ions.

The positively charged hydrogen Ions more toward the negative terminal of the external DC source. Each hydrogen ion takes one electron from the negative plate to become a hydrogen atom. These hydrogen atoms attack the lead sulfate to produce lead and sulfuric acid.

The negative sulfate ions move toward the positively charged plate. When they get there, they give up their extra electron to become radical sulfates. This radical sulfate reacts with the lead sulfate, and forms lead peroxide and sulfuric acid.

We Can Simplify that a Lot!

In a nutshell, the negative terminal of a lead-acid battery has an over-abundance of electrons. When you connect a load to the battery, the electrons scramble through the load to get to the positive terminal. This electron flow is what allows the battery to provide energy to do work.

When we apply a voltage to the battery that is higher than its resting voltage the electron flow reverses. The sulfate layers on the plates are converted back to lead and sulfuric acid.

Battery Charging: Calm Down – What’s the Rush?

About the worst thing you can do to a car battery is to rush the charging process. If you rush the recharging chemical reaction, the lead sulfate will heat up and adhere permanently to the lead and lead peroxide plates. Once it is stuck there, we can no longer use that area of the plate to flow electrons, and we have reduced the effective size of the battery.

You probably have heard the expression “a battery is never the same after it has been killed.” This statement is very true if the battery is not charged gently and thoroughly.

When you want to recharge your battery properly, keeping the process slow will allow the chemical reaction to take place at a controlled rate. If you are using a high-quality, computer-controlled charger (and you should be!) there are two major charging stages. The first stage is called bulk charging. The charger will maintain a constant current flow to the battery by adjusting the applied voltage.

How do you know if you are charging a care battery too quickly? Standard flooded batteries should not exceed roughly 120 degrees Fahrenheit during charging. We suggest that slower and cooler is always better. An absorbed glass mat (AGM) or gel battery should not exceed 100 degrees.

Once approximately 80% of the used energy has been returned to the battery, the charger will switch to the absorption stage. At that stage, the charger provides a constant voltage to the battery and the current flow diminishes as the battery reaches full charge.

How to Calculate Maximum Battery Charging Rates

A relatively large car battery may have a capacity of 70 or 80 amp-hours. This specification means that under ideal conditions, you can draw 1 amp of current from the battery for 70 or 80 hours. After that time, the battery will be considered dead.

To find the ideal charging rate for our 70 amp-hour battery, we divide this specification by 10 to get seven amps. The battery should be able to accept 7 amps of charging current without overheating. It is worth noting that, if the battery is completely discharged, it will take 10 hours to charge it. Remember, slower is better when it comes to charging batteries.

Taking Care of Your Car Battery

Some of us who are more fanatical about the care and maintenance of our car batteries will connect them to intelligent battery chargers several times a year. One rule of thumb is to charge your battery fully after each oil change, or four times a year. You should increase this frequency if you make short trips that do not provide adequate charging time. Likewise, time spent playing your audio system with the engine off can drain a battery very quickly. If you have been out with friends and your car battery has been depleted, put it on a high-quality charger overnight.

If you can access the acid solution in your battery, ensure that it is at the proper level, or at the very least, is covering the lead plates completely. A hydrometer should be used to confirm the specific gravity of the solution, but if it is low, adding distilled water is better than doing nothing. That little green “eye” included in some batteries is a hydrometer. When it disappears, the chemical balance within the battery is off and it needs to be charged.

Your local mobile electronics retailer may have a battery load tester that they use before every remote car starter they install. If you are concerned about the condition of your battery, ask them to check it. Being stranded due to a dead battery when the temperatures get cold is frustrating if you are trying to get home or to work.

Ensure the battery terminals and connections to your vehicle are clean and secure at all times. A loose connection can have a dramatic adverse effect on the functionality of your electrical system.

If you need a new battery, check with your local mobile enhancement retailer first. They often have extensive experience in upgrading batteries and can help you choose a solution that will ensure your car is ready to go every time you turn the key.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

If you spend any time reading car audio discussions on Facebook or in forums, then you will have undoubtedly come across comments involving the supposed drawbacks of using RCA Y-cables. There seems to be a lot of misconception or misunderstanding about how preamp signals works, and this misinformation leads to comments that aren’t always accurate. Let’s take (more than) a few minutes to clear things up.

Understanding Preamp Level Audio Signals

The audio signal that connects your source unit to your amplifier is both very weak and quite small. The voltage of the preamp signal is rarely above 10% of the maximum voltage capability of your source unit for several reasons. Firstly, the signal level is directly proportional to the output of the system. When the volume is low, the signal is low in amplitude.

The second factor that contributes to the microscopic amplitude of the preamp signal is known as the Crest Factor. By way of a formal definition, the Crest Factor is the ratio between the peak signal amplitude and the RMS value of a waveform. For a pure sine wave, this value would be 1.414. For music, the Crest Factor value is much larger.

We analyzed a few different songs to come up with some relatable numbers. The new song Run by the Foo Fighters has a maximum amplitude of +0.15 dB and an RMS amplitude of -12.7 dB over the entire track. To keep the math simple, let’s call it 13 dB, which is a ratio just shy of 20:1. We also analyzed Heathens by Twenty One Pilots and found that it has a Crest Factor of 10.5 dB, or just about 11.25:1.

If we think about the highest voltage possible on our preamp signal as being 4 volts, then the average voltage for the above track would be 200 millivolts and 355 millivolts respectively. The peak of 4 V only happens when the volume is at maximum. Don’t forget that.

Scotty, We Have No Power!

Another characteristic of our preamp signal is that it contains almost no current flow. As with any electrical circuit, the amount of current flowing through the circuit is determined by the voltage in the circuit and how much resistance there is. The output impedance of most head units is between 300 and 500 ohms. The input impedance on most amplifiers is about 10,000 ohms.

Using our maximum voltage of 4 volts, and a resistance of 10,500 ohms, the maximum current in our circuit will be 0.381 milliamps. If we consider that the average signal amplitude is about 275 millivolts, then we have an average current flow of 0.0275 milliamps. That is nothing.

What does an RCA Y-cable Do?

An RCA Y-cable allows you to connect a single RCA output to two RCA inputs. Typical applications for Y-cables are a single subwoofer output RCA on a source unit or processor and the need to feed a pair of inputs on a subwoofer amp. Another common application is a source unit with only a single left and right RCA output; you want to use a four-channel amp that doesn’t include a two-input/four-input switch.

Please Don’t Believe the Hype

The biggest myth about the use of Y-cables is that they dramatically reduce the signal going to each input. To prove why this is not true, we need to understand how a voltage divider circuit works. Yes, it is time for a little physics and math.

In an ideal situation, when we have a signal source and a single load, all the voltage developed by the source appears across the load.

If we have multiple loads, the voltage produced by the source is divided among the loads when they are wired in series. In the image below, we have two loads in series with our single signal source.

If the resistance value of the two loads is the same, then the voltage produced by the source is divided equally across the loads. Half the voltage can be measured across each load. Using our 4 V preamp example, we would see 2 V across each load. However, what happens when the load resistance is not the same? We have to do some math to determine how much voltage is across each.

Let’s label the loads. The load on the left will be called Rs. This is the resistance of our source. For this example, we will use a value of 500 ohms. The load on the right will be our amplifier input resistance of 10,000 ohms, and we will call it Ra1.

We have 4 volts being produced by the source and a total circuit resistance of 10,500 ohms. We can calculate that the current flowing in the circuit is 0.0381 milliamps using Ohm’s law. Knowing the current in the circuit allows us to determine how much voltage is dropped across each resistance. For our source load, we have a resistance of 500 ohms with a current of 0.381 milliamps to produce 190.476 millivolts. The rest of the 4 V source signal or 3.809525 volts appears across the load.

Let’s wire another amplifier in parallel with our first amplifier. This is the same effect as using a Y-cable. Our second amplifier will be called Ra2.

Now it is math time again. This time, our circuit has a total resistance of 5500 ohms, and as such, has a current of 0.7272 milliamps flowing in it. The voltage dropped across the source has increased to 0.363636 volts, and each amp is seeing 3.636 volts. That seems like a noticeable difference, doesn’t it?

The Decibel Scale Changes Everything

Between the two examples above, we have seen a decrease in voltage at the amplifiers by 4.772%. Does that mean our music is almost 5% quieter? No. When we talk about the ratio of voltage to volume, we need to take into account the decibel scale. Our decrease of 4.772% percent in voltage works out to -0.405 dB less output.

Before you get your knickers in a knot, you can fix that by turning the gain on your amplifier up by that amount.

A Worst-case Mathematical Example

This example was a worst-case scenario. What if you have a source unit with a lower output impedance? Some head units have an output impedance of 300 ohms. For that head unit, with the same 10,000 ohm input impedance on the amplifiers, the change in output by using a Y-cable would be -0.2493 dB. If you have a premium line driver in your system, the output impedance may be as low as 50 ohms. In this scenario, the loss is a paltry -0.0431 dB.

What did we learn from this? If you need to connect many amplifiers to a single source, then choose a source with a low output impedance.

RCA Y-cables as a Solution are Not Evil

If your system requires that you use a set of Y-cables to distribute the audio signal to multiple amplifiers, then go right ahead. Once your installer sets the sensitivity controls on your amps, you will never, ever know they are there.

If you have any questions about the design of your audio system or what to know about how your installer will be wiring it, talk to the salesperson and your local mobile electronics specialist retailer – they would be happy to explain things to you.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

In the 80s and 90s, it seemed like car audio installers were often having to resolve noise issues after installing a system that included an amplifier or signal processor. In many cases, the problem was called a ground loop. However, in some cases, the noise was present because one of the signal cables passed by a source of noise. The only solution was to reroute the wire. No matter what the problem was, or what cure was applied to resolve the problem, ultimately it wasted time and frustrated installers.

Noise problems are not as common these days. They do crop up from time to time, but certainly not with the same frequency as before. One simple technology that has helped to reduce noise issues is the inclusion of differential inputs of modern amplifiers and processors. In this article, we will explain what differential inputs are, how they work, and what benefits they offer.

Amplifier Input Circuitry – Single Ended Inputs

The RCA input connections on your amplifier have a relatively easy life. They do not need to pass any significant amount of current. They do not get connected and disconnect very often. They just sit there and do their thing.

Behind the connections, where the circuitry of the amplifier starts to do its job, there are two circuit designs commonly used. The aforementioned differential input, and single ended inputs.

In a single ended input design, the shield of the RCA cable is connected to ground through a resistor. This resistor is often around 1000 Ohms. The center pin of the RCA cable, which contains our audio signal, is connected to the preamp stage of the amplifier. The preamp stage includes the sensitivity control, crossovers and so forth.

The image below shows an RCA with a sine wave and noise on both the shield and the signal conductor.

When we feed the signal shown above into an amplifier with single-ended inputs, the noise on the ground wire is ignored, and the sine wave and noise on the signal wire are amplified. The unfortunate and unwanted result is below:

Amplifier Input Circuitry – Differential Inputs

In a device with a differential input design, the shield of the RCA cable is completely isolated from the chassis. Before the audio signal passes to the preamp stage of the amplifier, it goes through an op-amp. An op-amp (or operational amplifier) is a small IC that has two inputs and a single output. One input is labeled as +, and the other as -. When we put a signal on the + input, it is passed to the output of the device with little to no change. When we put a signal on the – input, the polarity of the signal is inverted and sent to the output. In a differential input configuration, the – input of the op-amp is also connected to the shield of the RCA.

If we have an audio signal on the + input, and nothing on the – connection, the audio signal is passed through to the output of the op-amp, then on to the pre-amp stage of our amplifier.

The image below shows an RCA with noise on the shield and the signal conductors.

If we have noise induced on the shield and signal conductor of the RCA, then the noise signal on the shield is inverted and passed to the output. The inverted noise signal adds to the non-inverted signal and cancels out.

We call the system a differential input because it is looking for the difference between the shield and the signal conductors. More accurately, it subtracts any signal that is common to both conductors from the output signal.

How does Noise get into the System?

When our interconnect cables run through the vehicle, they come close to all sorts of electrical noise devices. High-current wiring, computers, sensors, electric motors and even the alternator can produce radiated noise in the form of a pulsing magnetic field. When a conductor passes through this field, a current will be imposed on the wire.

When this imposed current (or signal) reaches our amplifier, it gets amplified. If the noise is a high-frequency buzz, then that buzz is amplified, and we hear it through our speakers. Yuck!

Another source of noise is a ground loop. In a ground loop, current is flowing on the shield of our interconnect to equalize the ground potential between two devices.

Let’s look at an example. Perhaps someone has installed a radio in the dash of a truck and grounded that radio to the dash support. In some cases, that dash support is isolated from the truck chassis with big rubber bushings. When we connect the RCA cables from our radio to an amplifier in the trunk that has single-ended inputs, the radio seeks an improved ground location through the RCA shield. The resistor in the amp prevents massive amounts of current from flowing, but there is often enough to induce noise in the signal conductor.

A quick test for a ground loop is to pull the RCA out of the amp until only the center pin is touching. If the noise goes away, you have a ground loop. A remedy is to install a ground loop isolator. A ground loop isolator is a device with a transformer in it. The audio signal is transferred through the magnetic interaction of the transformer windings. There is no direct electrical connection between the input and output cables. With no electrical connection present, current cannot flow. The drawback of a ground loop isolator is that it can detrimentally affect the frequency response of the signal passing through it.

Do the Interconnects Matter?

For differential inputs to work, we need the noise signal to be equal on both the signal and shield conductors of our RCA cables. In a coaxial cable design, the shield can block noise from getting to the center conductor. If you use coaxial interconnects with differential inputs, the noise on both conductors is not equal and can induce noise into the signal path.

The easiest way to ensure that any induced noise is equal on both conductors is to use a twisted pair interconnect. In a twisted pair configuration, both conductors have the same effective amount of shielding and subsequently, the same amount of noise.

Differential Inputs are Your Friend

The next time you are shopping for an amplifier or signal processor for your car, remember that one of the questions you should be asking is if it has differential inputs. While you can certainly build a great sounding audio system around an amp with single-ended inputs, there is no point of taking the risk of having noise or installation headaches. Your local mobile electronics specialist retailer would be happy to assist you in finding an amp or processor with differential inputs.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

Awhile ago, a client asked us what you get when you buy a “better” headunit. The usual answer is that you get more features and improved functionality. Those of us with a drive for the ultimate in sound quality and realism from our mobile electronics systems choose products in hopes of them receiving improved sound quality. Beyond frequency response and noise, what else makes one radio sound better than another?

We invited two head unit specimens to the Best Car Audio test bench for a little head-to-head battle. The first subject is a modern multimedia station that is equipped with navigation, smartphone integration and the general Bluetooth goodness. The challenger is a veteran, but premium, CD receiver. It has never heard of MP3 files, doesn’t understand the concept of satellite radio and thinks Bluetooth is the result of eating blueberry pie. However, in its day, it was one of the best. We do not need to mention names here, but we will call this the bench battle of features versus performance.

The Equalizer – Premium Test Equipment

Measuring the frequency response of a car audio product is relatively easy if you can feed a known signal into the device. When it comes to measuring a signal source, that is responsible for generating the signal, so all we can do is measure the output. We have a high-end digital interface on our bench. It offers a flat frequency response from 5 Hz to 92 kHz with a tolerance of 1 dB. The signal to noise ratio is an amazing 116 dBA and distortion is specified at less than 0.00032%. These specifications exceed those of both source units we are going to test.

Frequency Response Measurement

Testing the frequency response of a source unit requires some trickery. We have developed a reliable method that has proven itself time after time. The image below shows the frequency response of the reference signal. It has a slight incline in the high-frequency region, but everything is within a tolerance of about 1 dB. We converted this 192 kHz, 24-bit test track down to the CD standard of 44.1 kHz and 16 bit. Response to 22.05 kHz remains ruler-flat.

Frequency Response Results

Before we get into the results, we want to explain how to use the measurement graphs. The test track uses random noise as part of the test procedure. We process that after the test is complete. What you want to observe is the trend of the charts. A small peak or valley is not an anomaly in this scenario. Average the curve in your mind to see the overall trend.

The CD receiver: We played our test track from a standard CD audio file to make sure both source units were given the same information. The frequency response of the unit showed a typical response. The high-frequency filter kicks in around 17 kHz, which is normal for consumer products.

The DVD receiver: We played the same CD in the DVD receiver to see how it responded. The manufacturer of the DVD receiver has included a 2 dB boost on the top end that starts at 7 kHz and peaks at just over 2 dB at 15 kHz. The high-frequency filter response is similar to that of the CD player.

The high-frequency boost is not a big deal in terms of how a system sounds. Most of us have some high-frequency attenuation in our hearing, so this helps put some of the sizzle and air back into our music. It would be worth checking whether the source unit output clips when a 0 dB 15 kHz tone is played. This article is not a product review, so we will save that for someone else to tackle.

Bring the Noise

Our next challenge for the new versus old shootout is a little more technical: We wanted to see how each unit performed regarding background noise. This test is often reserved for lab environments, but can quantify the effort put into the component selection and system design.

For this test, we used a 1 kHz test tone recorded at -90 dB relative to full scale. Because the two source units have different pre-amp capabilities, we adjusted them so the output of the 1 kHz tone was equal in amplitude. This would best depict the noise imposed on the signal.

The CD receiver: We can see that the background noise relative to the signal is very quiet. There is a little bump at 60 Hz that was created by the power supply on our test bench. Otherwise, the test was impressive.

The DVD receiver: The background noise relative to the 1 kHz is 10 to 15 dB louder than that of the CD receiver at higher frequencies. There are also some spurious harmonic distortions in the output signal, mostly above 1 kHz. The large bump in noise in the low-frequency region could be caused by our 60 Hz 120 volt power supply causing some harmonics. The bandwidth is really wide, so it is hard to determine for sure.

A Distorted Perspective

We decided to repeat the test with a full-amplitude 1 kHz sine wave to see what harmonic content would be created at higher internal levels. The limits of the FFT analysis in our software starts to show up here. The flat horizontal line on the left of the chart and the angled line on the right are due to the analysis software and don’t represent noise.

The CD receiver: We noted two small harmonics at 14,750 and 165,000 Hz, with a slight harmonic at 12,000 Hz. Otherwise, the signal was very pure.

The DVD receiver: Harmonic distortion was clearly present at 1 kHz intervals starting at 2 kHz. It is worth noting that the first resonance is 45 dB quieter than the reference signal. If you were just playing the test tone, you might be able to hear it, but only just barely.

The Grand Finale – Intermodulation Distortion

Testing for intermodulation distortion is, well, mean. Out intermodulation distortion test is comprised of a CD test tone with 19 and 20 kHz sine waves played simultaneously. The spectral response of the test track can be seen below.

The CD receiver: When you are looking at an intermodulation comparison test, you are looking for frequency content that wasn’t in the original file. In the case of the CD receiver, we can see a very small bump at 1 kHz. This is significant because it is the difference between 19 kHz and 20 kHz. This would be considered an excellent result. A few other spikes show up at 3,500, 4,500 and 9,500 Hz, but they are still quite low – peaking at -93 dB relative to the test signal level of –15 dB.

The DVD receiver: We cannot really explain what happened here. There is 1 kHz content only 20 dB down from the 19k Hz and 20 kHz tones. Then harmonics upon harmonics of this up to 21 kHz. This test shows why some source units sound accurate and pure, while others do not.

Are Headunit Features Worth the Trade-off?

A few months ago, we published an article about harmonic distortion. That served to establish the basic understanding of how distortion creates content that wasn’t present in an original audio file. Our intent in this comparison is not to put down the modern DVD receiver, but to show what happens when manufacturers forego the bells and whistles and simply focus on all-out performance.

The same tests that apply to these source units are also common in amplifiers and speakers. We will subject an amplifier to the same mean and nasty tests in the coming months.

Don’t ever let price, perception or age dictate how you think a product sounds. Work with your mobile electronics specialist retailer to listen and compare for yourself. You will be amazed at what you hear.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.