Gain An Advantage: Amplifier

Anyone who has dabbled in car audio will have come across an amplifier at some point. They come in many shapes, sizes, and outputs but from the cheapest eBay special to eye wateringly expensive European exotica they all have one purpose, to get the musical signal to the speakers. The amplifier is, therefore, sat between the CD player and the speakers accepting the low-level input signal and converting it to a high power speaker output. I don’t want to get too bogged down in technicalities but let’s discuss the different types available and the controls we are given.

The JL Audio RD900.5 is a perfect example. This 5 channel, D class amplifier is capable of running a complete car audio system with front speakers, rear speakers, and a subwoofer. Alternatively, it could run a pair of front speakers bridged for enormous power plus a subwoofer once again. So, the RD900.5 is a very high performance, a flexible amplifier with a pretty small footprint making it easy to install.

The input stage is on the far left of this amplifier and comprises the usual red and white RCA connectors. There are 3 pairs that equate to the front, rear and sub input. Above these are three switches that allow for hi or low-level input, auto turn on mode and input channel selection (2, 4 or 6). Not all amplifiers are equipped with these selectors but these add to the RD900.5 flexibility.

The next stage is what interests us here, the frequency filter and the gain controls. The RD 900.5 is nonsymmetrical meaning that all channels are not identical. Channels 1 to 4 are for main speakers while 5 and 6 are for subwoofers. Therefore there is a variable high pass filter for 1 to 4 and a low pass filter for 5 and 6.

What is the filter for you ask? It is used to remove frequencies that are not required by the speaker on that channel. If we know we have a subwoofer fitted then it makes no sense to send the very low frequencies to the door speakers. Sub frequencies require a speaker cone to move a great distance and most door speakers simply cannot achieve this without distortion, I’m sure you have all heard a speaker trying too hard causing the classic ‘burping’ sound. In extreme cases, the speaker coil can be heard crashing into the back plate creating a clapping noise. Either situation is undesirable so to prevent these from occurring the frequency filter is applied to restrict the low tones.

The cutoff frequency is variable to suit different speakers and installations and setting it correctly requires the use of a signal generator. If you do not have access to one of these then it is possible to do roughly by ear, simply play music and slowly raise the filter frequency until there are no very low sounds in the speaker. The result will be a much clearer sound with no distortion. The mid definition should be greatly improved meaning vocals will cut through better. Typically the filter control is graduated and a very rough initial setting for a typical door speaker would be around 80Hz but you should experiment either side of this to find the best setting.

The subwoofer (low-pass) filter is the opposite of the high pass filter as it removes the higher frequencies sent to the sub. This ensures it only gets useful frequencies and we do not waste amplifier energy trying to make it play the higher tones.

Again the cutoff frequency is variable but what we want is to blend the sub with the main speakers so it is important to find a filter point that complements the high pass filter for the speakers.

So now we come to the input sensitivity, perhaps one of the most misunderstood controls. First and foremost it is not a volume control. Sure, it does increase the volume as you turn it but that does not mean it is increasing the amplifier power. Let’s consider what an amplifier is doing, it is taking a small signal and making it bigger. That signal is very dynamic meaning it is made up of loud and quiet parts, an extreme example could be Tchaikovsky’s 1812 Overture. Not something we listen to every day but it serves our example very well. If you are not familiar with the piece it starts very quietly and builds to a crescendo of orchestra and cannons. A more modern piece would be Warriors Dance by Prodigy but with fewer cannons!

The most important thing to consider when setting the sensitivity is that the amplifier only has a finite amount of power, let’s say 100Watts. That is represented by the upper dotted line on the graphs above. Our goal is to get the maximum, undistorted output.

If we look at a typical input sound wave it is obvious that there are loud and quiet parts. As we stated the amplifier is there to make this signal louder. Now let’s add the maximum amplifier power to the input signal graph.

As we increase the sensitivity we increase the size of the input signal at the input stage to the amplifier and remember there is a finite amount of power available. So what happens if we go too far?

As can be seen the nice smooth curves at the top and bottom of the loud spikes have been flattened which is clipped distortion. So yes we have a loud sound but it will be fuzzy and unpleasant. It will also damage speakers as the flat tops of the wave are a DC signal that will only induce heat into the voice coils. So if we take our example, the 1812 Overture, with the sensitivity set as above the quiet passages will sound good but the loud sections will be badly distorted and this will get worse the louder we play it.

For the best sound and maximum output, it is imperative that the loudest sounds are not clipped. Normally this would be set up with an oscilloscope but it can also be done with a voltmeter by measuring the output for maximum unclipped voltage as long as you know the true output power.

Fortunately, the JL Audio RD amplifiers have a brilliant feature, a clip indicator. This will light up if the signal is distorted by over ambitious sensitivity meaning setting up is made much easier.

At Car and Home Stereo, we want all our installations to give the best possible sound and that is why we carefully set up each and every one.

If you would like to have your system checked for any problems and to find solutions to these we would love to measure your car using our state of the art test equipment. These can highlight deficiencies in the sound stage that are difficult to locate just by listening.

As usual, I have run out of space so I’ll leave amplifier types and their best application for another time. But please don’t hesitate to call on 01625 432707 if you have any questions.

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