Pre-Amp and output voltage  - By Eddie Runner (NU5K)

 Pre amp is usually the control center.  Your volume control, tone controls, EQ (if any) and source switching is usually at your pre-amp.  The Pre-Amp is designed to be a very clean low signal voltage and take care of any signal modifications you might want to make to the signal (music) BEFORE the signal goes to the amplifier.

Pre-amp signal is usually LOW,  it was designed to be low so there is less distortion at this stage. Home pre-amps usually operate with about 1 to 1.5 volts of output, the idea was to keep the voltage down as low as possible to keep the distortion down as low as possible in the pre-amp stage and then after the signal leaves the pre-amp we let the amplifier do what it does best.

In car audio we have used this very low level for years with pretty good success.  But in many cases in car systems there might be some noise.  Some of the car stereo manufacturers have stepped up the output voltage of their headunits in an attempt to help us have a noise free system.  

Installers have always found that certain types of noises in a car system can be reduced by turning down the amplifier gain controls. But far too often the customer gets his system fairly noise free but with no gain!! The system just may not play loud like it should if the gain controls are turned down too far. Turn the headunit all the way up and it still wont get loud.  This is where the manufacturers come in with the higher level headunits! Now you can turn them higher, even when your gains are very low (and hopefully any bad noises are also very low)...

Some purists find this added amplification (and distortion) on the headunits appalling. But most of us cant really hear the added distortion and find it much better than the noise !!!

Some rookies find the added headunit voltage an advantage cause now they think their amp will play louder!  NOT TRUE!  The amp will still have its maximum output power and it will not change in most cases of the amps max drive can be achieved.  Many rookies compare a 1or 2 volt headunit with a 4 to 8 volt headunit and they only notice that with the higher voltage the system gets loud at 10 (on the volume scale) and you used to have to turn it up to 30..!!  Some rookies are fooling themselves into thinking this is GOOD! This is better!!    But it may not be.

Ideally, a system has a good swing to the volume control... Too much signal voltage into the amp the system will get loud too soon on the volume swing. Not enough signal voltage to the amp and they system volume control can go all the way up and not get loud like you might like it.  Setting the amp gain controls correctly will give you this proper swing!  

And the rookies may not know it, but if they set the gain controls correctly with a low signal output headunit and compared it to a high signal output headunit then there would be NO volume difference anywhere in the system volume swing...

But I still hear them say, "WOW, I got a 5 volt headunit!, it sure plays louder!, its worth the money!"   

I just shake my head in amazement...