Why stereo amplifiers have gain

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In commercial audio, the gain structure is a little different. For the best signal to noise ratio, the mixer or signal source is run at full line level regardless of the venue power or speaker driver requirements. For example in an indoor room such as a reception, a lower level is required than for an outdoor field concert. The console or program source is always run at the same level so the headroom remains in place and the noise level is the same in relation to the signal so the signal to noise ratio is not altered. For the venue, different power amps require differing amounts of gain to accept the unified nominalized signal and amplify them at differing amounts based on amp power rating and adjustable for the venue requirements.

A reception with background level music so people could comfortably talk and visit often would include a guitar and singer or piano that needs to carry with the help of the sound system as well as support the emcee. A low wattage amp or a larger amp turned down is the best mix. Turning the console way down because an amp can't attenuate the input results in low signal but full amplifier noise level from the console output line driver. Attenuation of the signal at the amp attenuates the signal to performance level requirements while also attenuation of the noise level is automatic.

If the venue wishes a dance to party to afterwards the console remains at the same nominal signal level, but the amplifiers are adjusted for the new performance level. On a console the advantages are many, including uniform channel trims for cueing up music, pre fade listen for trimming levels for a new speaker or performer, and use of the VU meters to prevent volume creep during the performance.

In high end home audio, the venue does not change, but the event often changes from general atmosphere background for relaxing or working, to the high impact demo of the system with the full symphony with live cannons.
To eliminate attenuators from the amp is only practical when the system noise level is extremely low to be silent to the point the hard drive in your laptop is louder.

Another advantage of attenuation in a commercial setting is peak power limiting in feedback or other accident conditions. With a fixed gain amp, a feedback event would run to the limit of the power rating of the amp into clipping. With an attenuator, in smaller venues, a feedback event is instead limited by the headroom of the console to no more than 18 DB over program level. This saves drivers and hearing.

A good amp design instead of simply using an input attenuator could vary the amount of negative feedback to change the gain. If you design with op amps, you know reducing the gain lowers the noise and increases the frequency response as well as reducing distortion. A few commercial amplifiers have a true gain control instead of an input attenuator. These do not turn down to zero as the lowest they can go is to unity gain. These are not very common as most people in sound prefer a control that goes to zero and don't understand gain controls that don't.

isettech

According to my QSC's (adjustable) I'm running their gains at 21db. Would much rather have plenty of power while being able to pull the gains back. Then play with fine tuning the gains with my EQ.

finscreenname

What about early McIntosh SS amps for example the MC2105 it is not integrated but has Vol. gain controls

tpock

Thanks again, Paul. Great video! Learning a lot. :)

greglee

I can attest that a 40 dB MM phono preamp works quite well straight into a volume pot with no line stage.

mctous

I have a Sansui 9900z receiver with kappa 7s. And an older Dual turntable. Do I need a phone amp. Would it improve it? I know, stupid question.

chadbarker

Wonka - This is where sound and poetry meets.

MrKnutriis

Thanks again for all these excellent videos. I have Always wondered why the sensitivity of power amps is so high. Most of them give full power with 1V input. I would place more of the gain in the line preamps. Looking some lab tests a lot of high quality line preamps have no problem in putting out even 10V with very low distortion. Even 20V sometimes. In this way the power amps will act as a high current buffers. I also read the the sound is formed in the voltage amplification stages, but this is confusing me.

gino

What's going on with the DAC every manufacturer is about 6 years behind because the supply of the DAC module are withheld, I could buy a DAC module tomorrow for about $100 and it be well superior to any DAC on the market what's with the delay in supplying the DAC module to commercial manufacturers

johnsweda

Yes Paul, 40 Volts into 8 Ohms = 200 Watts. Ohms law. Voltage (40V) / Resistance (8 Ohms load) = 5 Amps. And then Power = Voltage x Amps, so 40x5=200 Watts

hugoromeyn

I am still getting my head around gain, I have been watching a lot of your talks on YouTube and they are super helpful so my thanks. I am looking at a Bluetooth only pocket amp, it has no DAC apart from the bluetooth chipset. it is 1W and all the specs show is +-9v which I assume is not RMS. It is unique in my admittedly limited experience in that it dosent have a gain switch on it or I believe any impedence based auto gain, I have used it with a 32 ohm hp's and 300 ohm hp's and the volume is near identical, should I avoid though as it may be useless without gain with hp's needing more current?

adrianmorrish