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Tigz™

Setting gains on an amp

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I thought Id start with this topic as this is one of the easiest things to get wrong.

A common belief is that the amp ‘gains’ determine the maximum power from the amp.

The maximum power an amp can produce is determined by the signal entering the amp.

The best place to start is at the source unit (in this case we are talking about the headunit.)

You have probably heard that you should set your headunit to two thirds its maximum volume.

This isn’t always the case.

As far as I know, the two thirds volume came about at the time of tape deck headunits. This was not because a tape deck produces a less quality signal, but because cassette tapes are normally recorded at a lower level than other forms of sources. The manufactures will try to produce a high output with virtually any tape.

CD decks have come along since the days of cassette players, so it is possible to have a headunit that can play at full volume without clipping (the maximum signal the source can produce.)

You may also have heard people saying a bass boost shouldn’t be used.

I will try to explain the reason for this.

Some decks may clip at 50% its maximum volume when the bass boost is used.

Let’s say for example, that a headunit can play at 30/35 before it starts to clip.

And the headunit gives out a clean signal of 2v rms.

If you added +10db of bass boost, the headunit would clip at 0.63v rms.

Basically, you’re looking at the headunit clipping at around 10/35 volume.

So you can see why its important to do any adjustments to your bass and treble controls before you set up your amp gains.

We now move onto the amp.

As said, the clean signal from the example headunit is 2v rms.

We are assuming that the music was recorded at the maximum 0db, and the headunit is playing the maximum 2v signal. We are also assuming that the maximum 2v out put is at 75% of the headunits power. All bass and treble boost are switched off.

When someone says ‘turn up the gains’ what you are really doing is making the amp more sensitive.

People sometimes get confused when the gains are marked in volts.

It is normally the amount of input volts needed to make maximum power.

For instance, when you turn the gains up, you turn the gains to a smaller number.

The gains are higher at 2v than they are at 4v.

What you are trying to do with the gains is match the signal of the headunit.

Your headunits birth sheet should tell you the pre out signal at maximum output, all you have to do, is match this on the amp.

Heres a 50hz test tone to help you set the sub amp.

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The best way to do it is to use an oscilloscope. you need to record tones at -6dB to -3dB, 1khz tone and 40 or 50hz for components and sub respectively. You then increase the output until the signal clips. that is the max you should have the volume. You then do the amp. the problem is it doesn't seem like you are turning it up much but it will give the cleanest output without distortion (which is death for speakers).

When i did it, it made it sound a bit quiet. so i adjusted them up a tiny bit because you never really drive round listening to tones and music recordings can be alot lower.

You should NEVER just turn them up to max cos all that money you spent on you speaker will be wasted when you fry your voice coils.

Edited by tshirt2k

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