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Amplifier Gain Controls:
Contrary to popular belief, an amplifiers gain control does not determine the maximum power that an amplifier can produce. As long as the preamp/drive signal has
sufficient level, the amplifier will produce its maximum power output level. The gain controls are used to match the amplifer's gain to the gain of the other amplifiers in the system (in the case of a multi-amp system). The gain controls also allow you to match the amplifier(s) to the head unit. Not all head units have the same maximum preamp output voltage. Some head units are capable of producing 9 vrms out while others are only capable of 1.5 vrms out.
Please note that a head unit will reach its maximum output level (clipping) well before the volume control reaches the upper end of its range (usually at a point of 85-90% of its maximum range). This is especially true of tape decks, not because the audio section is of lesser quality but because some tapes are recorded at lower levels than others and the manufacturer must design the deck to be able to produce a sufficient output level with virtually any tape.
NOTE:
- This example uses a rotary volume control instead of the digital type pushbutton controls but the same basic concept applies.
- The output of some decks will clip at less than 50% of its full volume when the loudness is engaged or bass and treble are boosted. Many of the low to mid priced head units have a maximum output level (before clipping) of 2 vrms. If a 100hz test tone is played in the head unit and the bass control boosts signal levels at 100hz by 10dB, the output would clip if the pre bass boosted signal would have been .63 vrms. In other words, If the output of the 100hz test tone is more than .63 vrms and the bass control was turned to its maximum level, the signal would clip. This would very likely be at less than one-half volume.
Note:
The red arrows (near the volume controls) in the following drawings are showing you the position of the volume control which will produce the maximum (without clipping) output voltage. The test signal would be a tone recorded at its maximum level. This first section principally applies to tape decks.
The diagram below shows the approximate position of the volume control which will drive the amplifier to reach its rated power. Notice that the gain controls are set at 5vrms. The head that only produces 1.5vrms cannot drive the amplifier to rated power. The head unit rated for 9vrms drives the amplifier to its rated power with its volume control at approximately the '10 o'clock' position (which will make it difficult to adjust the volume precisely). This would not be an acceptable gain setting for either head unit.
This diagram shows the gain control set to 9vrms. The head unit capable of producing 9vrms would be able to drive the amplifier to its rated power. The 1.5 volt
head unit would not be able to drive the amplifier to its rated power. This would be a good setting for the 9vrms unit but would NOT be good for the 1.5vrms unit.
This diagram shows the gain control set to 1.5 volts. At this gain position, the 9vrms head unit will drive the amp to rated power at a very low volume control position. The 1.5vrms head unit can drive the amplifier to rated power and would be an acceptable gain position for the 1.5 volt head unit.
As you can see, the gain controls match the head unit to the amplifier. No single gain position is right for every head unit. There are many factors which
must be taken into account when setting the gain controls in any audio system. Setting the gain controls so that the amplifiers were never driven into
clipping with the 0dBfs 100hz test tone would likely leave you with a system which would not produce sufficient output with a typical music source. The
previous example is only to show you the relationship between the head unit output and the amplifiers gain controls.
This section applies to CD players more than tape decks.
Note:
This demo assumes that the test signal is a sine wave recorded at the highest level possible (0dBfs). It also assumes that all tone controls are set flat and the loudness coutour is switched off.
This is essentially how a system should be set up. The head unit should be able to use most of its range before the amplifier is driven into clipping. Again, if the gains
are set too low, the head unit won't be able to drive the amplifier to its maximum output level. If the gains are set too high, the amp will be driven into clipping at a very low volume level (on the head unit).
Clarification of Terminology:
Generally, when someone says that you need to 'turn the gains up', you need to make the amplifier more sensitive. When you make the amplifier more sensitive, it will take less input signal to to produce a given amount of power. The confusion comes in when you look at the gain control markings on the amplifier. If they are marked in volts, it's generally the amount of input voltage that it takes to produce full power. When you turn the gains 'up' you turn the gain control to a 'lower' (smaller) number. For example, the gains are higher at 2 volts than they are at 4 volts.
And to show it in another way...
At the risk of being redundant, the following diagram shows how the output of the amp relates to the output of the amp at different gain settings.
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