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Tutorial - Amplifier Gain & Crossover Adjustment


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#1 Marc

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Posted 19 May 2004 - 12:31 AM

Correctly Adjusting Gain and Crossover Controls on your Amplifiers
Written by - shiny_car

This article has been written to help people tune the gain and crossover settings on their amplifiers.

Before leaping into adjusting the settings, first you should have a basic understanding of the controls and reasons behind proper adjustments.

Abbreviations used in this article:

*EQ = equaliser
*HP = highpass
*HU = headunit
*LP = lowpass
*RMS = root mean square
*W =watts


What is a gain control?

The gain control is NOT a volume control. Turning the gain up higher and higher does not produce higher full-power output. If an amp produces a maximum of 100WRMS per channel, increasing the gain will not yield more than this.

Better to think of the gain as a sensitivity setting: the higher the gain, the more sensitive the amp is to the signal fed into it.

As an example, consider three gain settings: A, B, C. A is low, B is medium and C is high. The lowest possible setting would be fully anticlockwise and highest would be fully clockwise (where the gain control is a knob or pot).

Now let us say the HU is delivering a 2V signal via the preouts and RCA cables. With the gain at B, the amp may be producing 100WRMS per channel. If the gain is now changed to A, the amp is less sensitive; in order to produce the 100WRMS as before, the HU must now deliver a higher voltage signal, say 3V. Setting the gain to C, the amp becomes much more sensitive; it now may require only a 0.5V signal to produce 100WRMS.

The purpose of the gain setting is to match the HU preout signal-voltage with the amplifier. A HU with a higher voltage signal will require a low gain/sensitivity setting; if the HU only produces a low voltage signal, the amp would need to be more sensitive to produce the same power.

What do the voltage markings on the gain setting mean?

Some gain controls have markings with voltage to use as a guide. These voltage settings suggest to you what RCA input voltage is required to make the amp produce full power. You�ll note that with a high gain/sensitivity setting, the marking may read 0.5V; this makes sense because the amp needs to be more sensitive to respond to a low voltage signal in order to produce full power. Whereas a low gain setting may have a marking of 4V; with such a high input voltage, the amp needs to be far less sensitive to produce full power.

What RCA preout voltage does my HU produce?

Unless you have sophisticated measuring equipment like an oscilloscope, you will not know what voltage the RCA signal is from you HU. The advertised voltage found in the specifications refers to the signal under special conditions: usually by playing a constant signal like a test tone at a fixed frequency. This is obviously very different to conditions of playing music. Music is very dynamic, meaning it is constantly changing between loud and soft.

It is important to realise that the RCA preout signal, when playing music is far lower than if playing a test tone. Therefore, whilst a HU may feature 4V preouts, when you play music the average voltage may in fact never reach beyond 1~1.3V. Sure, it may peak as high as 4V, but never reach that level in a sustained manner.

Of course, these voltages are only achieved when the HU volume is turned up to full. Using a low volume setting, as you would for normal listening, the signal voltage is far less again.

The implication is that the gain setting on the amplifier must be much higher than predicted. You should expect a 3~4 times overlap in voltage.

For example, a HU claiming 4V preout signals will only produce an average of 1~1.3V when playing music and at full volume, which is 3~4 times less than predicted. The gain setting remains true however; that is, if you set the gain to the 1.3V marking, the amp will produce full power when the HU delivers 1.3V.

Why set the gains properly?

There are several reasons to set the gain control properly:

1. The wider the range over which you can use the volume control on the HU, the finer the control you have. There's no point having the amps reach full power with only 30% on the dial where every little step produces a large volume difference. Therefore, you want full volume to be achieved when the volume control on the HU is full (100%).

2. To avoid overpowering speakers. Every speaker has a power handling limit; if it is fed a continuous amount of power beyond this, it will be at risk of having the voice coil overheated and permanently damaged. Gain settings can be used to limit the amount of power being delivered to within safe operating levels.

3. To avoid clipping the amplifier. Clipping occurs when an amplifier is pushed beyond its limits; this limit is the maximum clean power the amp can produce. Beyond this, the output signal becomes highly distorted, and is referred to as being clipped; it is very audible and difficult to mistake. If the gain is set too high, the amplifier will reach full power when the volume control has not reached full; if the volume control is turned up higher, the amplifier is now pushed beyond its limits into clipping. By reducing the gain setting, the amplifier will never reach clipping despite the volume control being turned up full. Note that clipping does not necessarily mean the speakers will be overpowered; therefore it is not always dangerous for the speakers (see later).

4. To achieve a nicely balanced system between front and rear speakers, and the subwoofer, their respective levels need to be matched. Using gains is a useful means to achieve this.

What happens if the gain is set too low?

If the gains are set too low, the amp will not be sensitive enough to reach full power. For example, if the gain is set where the amp requires an input signal of 3V to produce full power but the HU only delivers 2V even at full volume, the amp will never reach full power. This is wasting the potential of the amplifier.

However, this may still be required if you are to achieve a better balanced system, or more importantly, to avoid overpowering the speakers or subwoofer.

As such, a low gain setting can be used to cap power output from the amplifier.

What happens if the gain is set too high?

This has been discussed to above. The problems encountered include the risk of overpowering speakers and causing clipping.

What is distortion and overpowering speakers and subwoofers?

In audio, distortion is a music signal (electrical or acoustic) that is less than 100% perfect from the original. Where the CD recording is the original, distortion may occur when this data is converted from digital to analogue, as it passes through the controls (preamp) of the HU, along the RCA leads, into the amplifier, out to the speakers, and in the reproduction by the speakers themselves. Some is audible and this should always be considered bad, if not only to our ears, but also to the components.

Important forms of distortion to be aware of include:

1. Over-excursion of the cones of speakers and subwoofers from being pushed beyond their physical limits. This typically occurs when small speakers (eg: 4~7" diameter) that are part of the front-speaker setup are fed too much bass, especially when delivered a fullrange signal. By using a HP filter to remove bass, it will greatly improve the power handling of the speaker and avoid over-excursion.

2. Clipping of the amplifier. Discussed above, clipping occurs when the amplifier is pushed beyond its limits of producing 'clean' power and becomes distorted. This transition is typically sudden and harsh, but some amps have soft clipping, which is far less noticeable. Other amplifiers features a clipping warning system, which is commonly an LED warning light.

Overpowering of speakers occurs when either they are pushed to over-excursion or the power being delivered exceeds their power handling. Every speaker and subwoofer will have a power handling designation specified by the manufacturer, and usually marked onto the speaker itself. You should always refer to the RMS power specifications.

It is possible to cause over-excursion without exceeding power handling. For example, a 6" speaker may have a power handling of 50WRMS, but if fed a signal with subbass it may over-excurt and distort with only 30WRMS of power.

Usually a speaker or subwoofer audibly distorts if it is overpowered. This is a warning to back off or risk permanent damage!

Clipping can cause overpowering and is therefore dangerous for speakers. It is a common cause for burning out voice coils. A clipped signal is typically far more powerful than a clean signal, quickly reaching double the power of the clean signal. It is because of this rapid transition that people are caught out. Setting gains properly can avoid clipping!

Note that clipping per se is not dangerous for the speaker. It is the actual power of the signal that causes the overpowering. For example, an amplifier may produce only 100WRMS of power, beyond which it will clip. If clipped, the signal may reach around 200WRMS. However, if the subwoofer it drives has a power handling rating of 300WRMS, it will not be overpowered or at risk of overheating. Therefore, the subwoofer will happily reproduce a clipped signal of 200WRMS all day long; it will just sound terrible and the amplifier may overheat!

So understand that clipping is a sign that the amp is being pushed beyond its limits; that the onset of clipping causes a rapid increase in power output, albeit highly distorted; clipping does not necessarily mean the speaker or subwoofer will be overpowered.

How do I best utilise crossover filters?

Crossover filters are defined by the frequency at which they are set and how dramatically they filter out the signal (slope). Nearly all amplifiers include a crossover filter, and most are variable. This means you can adjust the frequency from where the filter takes effect. Most slopes are fixed, commonly of 12dB/oct effect. Detailed explanation of filters is beyond the scope of this article.

You should set the filter to suit the speakers and subwoofer, and also to achieve a nice blend between front, rear and subwoofer. If a small speaker is fed too much bass, it will distort prematurely from over-excursion, therefore reducing its power handling; this may limit how loudly you can run the system.

The choice of frequency where the HP filter is set is a compromise: whilst it is ideal to have more midbass (70~150Hz range) from the front speakers, asking too much of them will diminish their power handling. It may also muddy their sound because they struggle to maintain the clarity of the higher frequencies whilst reproducing the midbass. Setting the HP filter slightly higher will improve power handling at the expense of midbass.

Setting LP filters is also a compromise. The subwoofer may also be able to reproduce midbass and as high as 200Hz quite well. However, these high frequencies beyond 100Hz are directional to our ears: they can usually detect where in the car these frequencies are coming from. Having our ears detect midbass being reproduced from the rear of the car adversely affects the soundstage. You are aiming to have all the sound appear to arise from infront. Subbass frequencies (well below 100Hz) are non-directional; therefore it works to have a subwoofer in the boot because our ears cannot determine this. Note that if you can hear a subwoofer's location, it will be because the subwoofer is either reproducing higher frequencies or there are rattles and vibrations that localise it.

The actual best setting for your setup will depend on many things, including quality of the speakers and their installation, acoustics in the car, and your tastes. However, as a rough guide, the HP can be set depending on the size of the speakers:

*6~7" speakers: 70~100Hz with 12dB/oct crossover

*5.25" speakers: 100~150Hz

*4" speakers: 150~250Hz

A steeper sloped filter will remove the bass more dramatically. Therefore you can afford to set the filter at a lower point without delivering excessive low frequencies.

Most people recommend setting the LP filter near that of the HP for the front speakers. Note that filters are not a brick wall; they do not suddenly cut out frequencies below/above their setting. Therefore, any underlap between filters does not leave a gap. An example of underlapping would include a HP setting of 90Hz and a LP of 70Hz. Underlapping is favoured by most enthusiasts for best results. However, many systems sound best with equal settings or indeed overlapping crossovers where the subwoofer and front speakers share more of the frequencies around the overlapped range.

Setting the gains

You are now hopefully armed with some useful basic knowledge to help tune the system to good potential. The following is a recommendation for the steps required to properly set gain and crossover adjustments.

You will want to perform these settings in an environment where loud music is not a nuisance. Also protect your ears with earmuffs as necessary to avoid hearing impairment.

In general, adjust front channel gains first, then rear (if applicable), and finally subwoofer. Then adjust the balance in sound between front, rear and sub, which may require turning the gains 'down' for some amps (eg: subamp).

Use of any EQ or processor can be left till the end and gains readjusted as necessary.

Steps to set gain levels:
1. Play music that is well-recorded and you are familiar with, featuring the entire frequency bandwidth from subbass to treble.

2. Start with only the front speaker channels; disconnect all others by pulling out the RCA leads or disabling them via the HU (if applicable). If using a bridged 4-channel amplifier to provide 2 channels, you will need to set the gain for each side, left and right, separately. Do this more easily by using the �balance� function to concentrate on one side at a time.

3. Turn gain right down (full anticlockwise).

4. Turn HP filter up higher than required (eg: 150Hz for 6" splits).

5. Turn off loudness and set any EQ to flat.

6. Now play the CD and turn the volume as high as it goes without distortion.

Some HUs allow 100% volume without distortion via the RCA preout; others go only as high as ~90% before distortion; set it just below any distortion.

7. Gradually turn the gain up: keep going until you now hear distortion. Any audible distortion should be considered 'bad', so turn the gain down a little, just below its onset.

8. Finally adjust the HP filter: lower it until your front speakers play adequate midbass without loss of power handling. Alternatively, lower the crossover setting further and expect diminished power handling which will require a lower gain setting accordingly. This may be preferable for some listeners who don't want the volume to be so loud and prefer added midbass.

So now the front gains and HP filter are set properly. They are at a level where you will have maximum usable power when the HU volume control is turned up to full (or to pre-distortion level).

Next:

9. Repeat the above for rear speakers where applicable; leave the front and subwoofer channels disconnected.

Next:

10. Start to tune the subwoofer channel gains by disconnecting the front and rear channels.

11. Consider setting the HU sublevel control to about 1/3~1/2 (eg: 6/15). Whenever you listen to music you can lower the subbass for various tracks yet still set it higher for others; it gives good flexibility. Otherwise, set the sublevel to '0'.

12. Turn the gain right down (full anticlockwise).

13. Turn the LP filter up higher than required (eg: 150Hz).

14. Leave loudness and EQ off.

15. Again, play the CD and turn the HU volume to the level used to set the other gains.

16. Gradually turn the gain up: set it just below where any distortion becomes audible.

17. Finally adjust the LP filter: lower it to a setting close to where the HP is for your front speakers.

Now you want to achieve some form of balance between all your speakers without either the fronts, rears or subwoofer dominating.

18. Disconnect or disable the subwoofer when setting front and rear balance (ie: 'fading'). Use the fader control to achieve the balance between front and rear that you like.

19. Reconnect the subamp channels.

20. Now determine if the subbass is too much, which is usually the case. If so, do NOT increase the gain on the front/rear amp channels but reduce the gain on the subamp by turning it anticlockwise. Set the subamp gains for a nice balance between sub and front/rear. Most people like a little sub-heavy action which is fine if it suits your tastes.

Bingo, you're about done!

You can if you want, adjust the EQ to your liking. Note that if you boost any frequencies (although most people prefer to cut), you are pushing the amp a little closer towards clipping. This means, if you now turn the volume up to what was previously the maximum level, the amp may infact clip because the signal is stronger than before

For this reason, you need to either trim the gains down a little to compensate or never turn the volume up as high as you used when setting the gains. Most enthusiasts using a subwoofer never switch any loudness setting on; it is unnecessary.

Finally, realise that some CD recordings vary in quality. Very occasionally you will encounter a recording that is excessively high in level. Bear in mind that such recordings will push the amplifier closer towards clipping and you may not be able to utilise the volume control up to full.

Importantly, playing test tones (constant frequency), as could be the case during SPL competitions, makes the RCA preout signal much stronger compared with music. This too will push an amplifier closer towards clipping so you must limit the volume control to below full.

When all the gains and crossovers are set correctly, you will use the stereo knowing that you have all the usable power on tap and can turn it up as loud as it will go with limited risk of damaging your components from excessive power. So enjoy!

#2 Bassaholic

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Posted 28 October 2004 - 05:04 PM

Some extra notes:

When tuning a system for sound quality, one of the aims is "up-front" bass, ie you want it to sound as if all of the bass is coming from your front speakers, rather than noticing that it is coming from the subwoofer at the rear of the car.

The trick is of course to filter the subwoofer at a low frequency, so that your ears cannot tell which direction the sound is coming from and tune the system, so that the subwoofer blends in well with the speakers covering the midbass in your front stage.

There are two important points to consider:

First, make sure the subwoofer volume is level matched correcty. By this, I mean make sure the subwoofer is at a similar volume to your front stage. Otherwise, if the subwoofer overpowers the front stage, the bass will not blend in well and it will not sound that great.

Secondly, it is important to set the high pass (on your front stage) and the low pass (on your subwoofer) filters correctly.

The actual crossover points, depends on how low your front stage can play, without distorton.
For example, some speakers, when playing midbass may not play all that low, or they may be limited in linear excursion, so that they distort if you try to play them too loud at a low frequency.

(How high the subwoofer can play will technically be a factor too, but there are very few subwoofers that have such a high inductance that they cannot play to 100hz).

The installation will also be a variable, particularly the subwoofer enclosure design and location, as well as the installation/location of the front stage drivers, as well as the filters themselves. For example, the alignment (Q) of the filter (if it is a butterworth or linkwitz-riley etc), as well as the slope.

When sound quality is a priority, I would suggest to start tuning the filters at fairly low points (some SQ cars have their subwoofer low passed below 50hz!) and then slowly raising the points, as well as varying how close the high pass (on the front stage) and the low pass (on the subwoofer) filter points get. As Shiny_car mentioned, some underlap can often work well. The aim is to simply try out the various points and listen to what sounds the best.