Installing a Sound System In Your MGB

As most of us know, getting a sound system in an MGB is just another one of the difficult tasks associated with our LBC's (or any open-top sports car for that mater.) But once you know the basics, you can get a good sounding audio system with out a lot of expense.

As the owner of a mobile DJ company, I have been working with audio systems for over 20 years so, hopefully, this information may be of some help to you.

We'll skip the basic basics (electricity pasing through a coil moves a magnet which moves a diaphragm which produces sound wave, etc) and move on to the practical knowledge.

Sound System Terms and Definitions

Here are some terms you need to know when choosing the components of your new sound system.

Watt

The fundamental unit of electrical power, it is a rate unit, rather than a quantity

Ohms

A measure of resistance to electrical flow. (think of it like this: watts push through a wire, ohms push back against the watts) also called impedance in the world of audio. Any car audio system you consider will probably work on a 4-ohm impedance.

SPL or sound pressure level

This is the pressure of a sound wave powered by one watt of power and measured at one meter distance from the source. It is measured in decibels (a logarithmic measurement of power or intensity)

Hertz (Hz)

A measurement of frequency or cycles. A measurement of 1 hertz means a signal is oscillating a 1 cycle per second. A measurement of 1,000 hertz (1 kilohertz or 1 KHz) means a signal is oscillating at 1,000 cycles per second.

Head Unit

No, it's not a Grateful Dead fan. It is what is also known as the stereo, the unit in your dashboard that plays the music via radio, CD, etc.

DIN Chassis

A standard size common to almost all head units produced now to allow for common sizes to be used by auto and audio manufacturers. A standard DIN chassis head unit usually comes in what is called a cage.

Cage

A metal box which fits into the opening in your dashboard that the head unit slides into and plugs into. The cage remains mounted in the car even if the head unit can be removed and it carries the wiring connections for the head unit.

Speaker

Generic term for the device that converts electrical signals into sounds.

High-end speaker or Tweeter

A speaker which reproduces sounds in the high part of the audible frequency range, usually 1,000-20,000 hertz. Sounds at this frequency are often referred to as Treble.

Mid-range speaker or Mid

A speaker which produces sounds in the middle range of the auditory frequency, roughly 500-1,000 hertz. Most human speech occurs somewhere around the 1,000 hertz range.

Low range speaker or Sub or Sub-woofer or Woofer

A speaker which produces most of it's sound in the low end of the frequency range, roughly 0-500 hertz. Sounds at this frequency are often referred to as bass.

Crossover

A device which separates frequencies in audio systems according to their hertz values. This is used for sending different types of signals to different amplifiers. In other words, the signal leave the head unit on the way to the amplifiers. Before getting to the amps, the crossover splits the signal into two or more separate signals and sends them to the appropriate amp. A two-way crossover may split the signal and send anything above 500 Hz to the high end amplifier and anything below 500 Hz to the low end amplifier (called a bi-amp system). A three-way crossover may send anything above 2,000 Hz to the High-end amp, anything below 200 Hz to the low end amp and anything between 2,000 Hz and 250 Hz to the mid-range amp (called a tri-amp system)

Crossover point

The frequency where the split occurs. In the above examples the bi-amp system's crossover point is 500 Hz. The tri-amp system's crossover points are 2,000 and 250 Hz.

Active crossover

Described above. A device which splits the signal up before it reaches an amplifier. This is what most pro and high end audio systems use since different frequencies require different power levels. Sub-woofers or bass speakers require a great deal of power to drive whereas high end and mid-range speakers require very little power by comparison.

Passive crossover

A crossover which splits the signal after it has been amplified. When you have a full range speaker (such as a typical 6" x 9" car speaker) and it has one or two additional little speakers built into it in addition to the big main speaker cone, it has a built in passive crossover to send the amplified signal to the right parts of the speaker This is because sending a powerful full range signal to the (low-power) high end part would burn out the high end very quickly.

Amplifier

A device which amplifies the low power audio signal to levels that allow it to drive a speaker and generate audible sound. Most head units have built in amplifiers.

Line-level output

This is an un-amplified electrical output from a head unit which is sent to an amplifier. Most better-quality head units have both an amplified output to hook directly to speakers and a line-level output to hook into an amplifier. Line-level outputs generally put out a cleaner signal with less distortion.

Channels

The number of lines coming out of an amp/head unit. A two-channel system can drive two speakers (left and right) a four-channel system can drive four speakers (front left, front right, rear left, rear right)

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Choosing your Sound System

Now that I've overwhelmed you with so may technical terms let's discuss what you want in your sound system.

First of all, you have to decide on the head unit you want and the features you want in it. You can get the standard AM-FM with CD player or even the older cassette player. You can get a High-definition radio receiver, satellite receiver (XM or Sirius) MP3 player, with inputs for data cards or external audio sources. You have to study up on these thing and decide what the right features are for you.

Then you have to decide if you want your amplifier to be in the head unit or outside of the head unit. As I stated above, almost all modern head units come with some type of amplifier built into them. The power (wattage) of them vary greatly. In my reality, wattage levels do not really matter at all since I use an external amplifier, not the one built into the head unit.

The reason I do this is because the sound from a good external amp always sounds better than the internal one, it allows me greater power from the sound system, plus more and better choices of speaker systems. But do remember, if you go with the internal head unit amp, there is a limit to how much wattage you can get pack into a compact head unit. The greater the wattage, the larger the unit will be. In addition, the larger the amp in the head unit, the greater the amount of heat that will be generated which means the more heat in and under your dash and in the head unit itself. Electronics are very temperature sensitive and they don't like a lot of heat. I have seen many head units which have failed because of the waste heat that they themselves have generated.

While we are on the subject, let me share this little nugget with you. A unit's listed wattage is generally a bunch of B.S.! if you see a head unit listing something like 500 watts per channel then you are, more times than not, being misled. For years manufacturers have used wattage figures to trick consumers into thinking they are getting more power than they actually are. These figures usually quote the theoretical maximum peak wattage capabilities of the amplifier in ideal circumstances, not the average maximum usable power available to drive your speakers. If the amp in the head unit was run continuously at this theoretical maximum the result would be burned out speakers and amp in a very, very short time.

In a typical system with full range speakers that you do not want to drive to excessively high volume levels, 25-75 watts per channel (RMS, not peak) will be more than sufficient for the vast majority of users. This does not apply to sub-woofers which require a great deal of power to drive.

Now let's look at speakers. I am going to touch on what, from an audio point of view, is the most important thing to consider: the SPL rating of a speaker. What the SPL number basically represents is the efficiency of a speaker or how adept it is at converting electricity into sound. This is important because the more efficient the speaker is, the less powerful an amp has to be to drive the speaker.

As an example, a 200 watt amplifier driving 200 watt speakers with an SPL rating of 89-dB will sound okay but will strain the amp if any appreciable volume is desired. The same amp driving 200 watt speakers with an SPL of 94-dB will sound much louder, clearer and will allow the amp to run at lower settings consuming much less electricity and generating much less heat.

Amps cost more than speakers so you get more bang for your buck if you buy an average amp and efficient, high quality speakers. In my mobile DJ service I can run inexpensive amps because I use ultra-efficient JBL speakers. The speakers cost about $400 a piece and the amps only cost about $300. If I did the reverse, the speakers would cost about $200 a piece but to get comparable sound I'd have to get $1000 amps. And neither would sound as good or last as long since they would all have to work harder to produce the same sound levels.

If you have amps (separate or in the head unit) which produce 200 watts peak power per channel, then get speakers that can handle 200 watts peak power per speaker, just make sure they have and SPL level as close to or exceeding 100-dB. Its hard to find an SPL number that high in car audio equipment so aim for at least an SPL level of 92-94-dB or higher.

Amplifiers are the next stop on our little audio tour. They come in a bewildering variety of sizes, power levels and even colors. Some have built in crossovers, some have 2, 4, 5, 6 or even more channels and some have multiple channels with different power levels for each channel. Amps can be tricky things to pick out so you have to make your decision based on a huge variety of variables. Internal or external amps, that is the question. If you aren't looking for a super sound system, a typical head unit with an internal amp powering two full-range speakers in the door and two full-range speakers in the tail will probably be sufficient for you. If you want a little more oomph from your system, you'll probably want to go with an external amp with multiple speakers and sub-woofers.

The big problem with MGB's is the convertible top. When it is closed, you have a less-than efficient acoustical area because of the soft material of the top. When the top is down you basically have no acoustical area at all! Practically all of the sound your audio system generates goes straight up into the air with nothing to reflect it back to your ears.

Here's my preferred set up for an open-top sports car:

1. A head unit (1x) with line-level outputs
2. The largest, high SPL, full-range speakers (1 per door) that will fit in the door panels
3. Ten-inch, high SPL sub-woofers (2x)
4. An external amplifier (1x) with a built in crossover, two channels to drive the 2 full-range speakers and two channels to drive the 2 sub-woofers.
5. OR---An external amplifier (1x) without a built in crossover, two channels to drive the 2 full-range speakers and two channels to drive the 2 sub-woofers with passive crossovers between the head unit and subs.

To install the above system you either have to cut out two holes in the rear bulkhead for the sub woofers or cut out the entire rear bulkhead (like I did) leaving a 1-inch lip all the way around the perimeter and mounting a ¾" birch board to the lip with the sub woofers mounted in the board. Or, placing a pre-made sub box on the rear deck with the sub-woofers installed in the box. Note: don't try to build your own box for the sub-woofers. These boxes are specially "tuned" and home made ones never sound good unless you know the proper building techniques and have mastered the complicated mathematics necessary for making them. In other words, home made boxes always sound like home made boxes.

Another option is to install speakers behind the MGB's seats

A second system set up would be a head unit powering at least two, full-range speakers in the doors and (if you want) two full-range speakers in the rear bulkhead. For the rear speakers, two 6" or two 6" by 9" speakers would be about as small as you'd want to go. You can also put two 8" or two 10" sub-woofers in the rear bulkhead provided that there is a passive, in-line crossover between the head unit and the subs to allow a signal with only the proper hertz range to the subs.

The system I will be installing will be a concealed head unit behind the dash with a 6-channel external amp. Two channels will power two 5" speakers in the doors, two channels will power two 5" speakers in the rear side panels behind the doors and the final two channels on the amp will power two 10" sub-woofers in the rear bulkhead. This is more sound than the average person might want or need but I feel it is justified in an open-top roadster. I will not be using a massive, over-powered amplifier so my aim will be to be able to increase the volume without loss of clarity. In audio lingo this is called "headroom," or the ability to increase the volume levels of a system to much higher than usual levels without loss of clarity or overdriving the system.

When installing any system with a high power, internal amp head unit or a system with an external amp, a separate, dedicated, fused power line should be run for the power supply. New sound systems draw a great deal of power and the decades-old wiring in an MGB will not be up to the task of handling it.

Where to get the system? Well that is up to you. I would not recommend places like Walmart or K-mart unless you want the most basic, generic components available. Mail order can be a good source though places like J.C. Whitney usually stock low-quality items that couldn't be sold at regular retail prices.

Online sources, of course, have become very popular and you can usually find what you want via the internet after some searching and usually at the price you want. Be sure that you purchase anything online from an established seller who's website ordering and payment acceptance system use encryption software to protect your credit card information (look for the "s" in the "https" part of the web address on the site's payment page.)

The best source for equipment is from a local, independent dealer and installer of custom car audio systems. They are usually very knowledgeable and have a much greater variety of equipment than you'll find at larger national chain retailers. While we're on that subject, national chains like Circuit City and Best Buy are NOT good choices for car audio. They usually stock only the most popular items or those which have the highest profit margin for them. Additionally, their installation departments are usually incompetent at best and more often than not will overcharge you, install something incorrectly or both. And the sales people can usually do little more than read a spec sheet back to you on the item you are considering and, more often than not, have little real knowledge or understanding of the products they sell.

If you are going to install the sound system yourself it is important that you fully understand the components you select and how they relate to each other. Simple and avoidable little mistakes can make a great sound system sound like a portable AM radio or worse, burn it out. The Internet is a great place to research the topic and learn more. And if I can be of any help please do not hesitate to contact me.