Transcendent Sound

Making Music Better

 

Amp Input Sensitivity and Gain


People are always asking me what my amplifier input sensitivity is.  What they should be asking me is “What is the gain?”  We need to look at all the major components of the audio system to determine a correct response. 

There are four issues here that need to be considered.  They are:

  1. 1.The level of the signal source.

  2. 2.The gain of the amp and preamp.

  3. 3.The efficiency of the loudspeakers.

  4. 4.The desired maximum sound pressure level.


Whatever the signal source is, it has a nominal or average output level.  Unfortunately, these levels are not regulated or uniform.  To make matters worse, CDs and LPs have their average signal level determined by the recording engineer.  Some play louder than others.  Again there are no absolute standards governing signal source levels.  Phono cartridges have widely differing outputs.  So do CD players.   Some very expensive ones have low outputs.  The way this problem is dealt with is to construct a system with surplus gain.  The volume control is used to attenuate the input signal to a desired level to compensate for varying signal levels.  This works so long as there is enough rotation in the control to achieve a desired loudness.  Too much system gain and the control is maxed out at the 9 o’clock position. 

 The system user will have to determine what the typical or nominal signal levels of the sources used in the system are.  There are no calculations that can be run to determine these levels.

 Because signal levels vary, additional gain is usually needed to compensate for low level signals.  Most people don’t realize it, but when volume controls are at the 10 o’clock position, the signal is being attenuated by at least 20 db.  This is necessary to compensate for instances of high signal levels.  The position of the volume control has nothing to do with how much reserve power the amp has left.  It just is an indication of how much the drive signal is being attenuated.

 At least gain is something that is easily determined from amp and preamp specs.  So the question becomes’ “How much signal does the amp need?” 

 Let’s say the loudspeaker has an efficiency of 90 dB for one watt at one meter and the desired maximum sound pressure level during peaks is 110 dB (that’s really loud).  This will require 100 watts.  If the signal source is 1 volt maximum, how much amp gain is required?

100 watts into 8 ohms requires 28 volts.  So the amp needs to provide a voltage gain of 28 times which is 29 dB.

 What if the amplifier only outputs 10 watts?  With a maximum input signal level of 1 volt, 9 volts will be required for the speaker.  This is an amp gain of 19 dB.  Lower power amplifiers therefore need less gain because they produce less signal. 

We also have to consider 4 ohm speakers.  Because of their lower impedance, they require less drive voltage for any given sound pressure level.  10 watts into a 4 ohm speaker requires 6.3 volts, or an amp gain of 16 dB.  Four ohm speakers always require 3 dB less signal voltage.  They are not 3 dB more efficient.  The power consumed is the same.  Many speaker specs are misleading in that they specify sensitivity at 2.83V.  For an 8 ohm speaker, 2.83V is 1 watt of input power.  For a 4 ohm speaker, that’s 2 watts of input power.  For the specs to be a valid comparison between 4 and 8 ohm speakers, always subtract 3 dB from 4 ohm speaker sensitivity specs if they are rated at 2.83V.      

What happens if the loudspeakers have a sensitivity of 84 dB and all other conditions remain constant?  The amp/preamp will need an additional 6 db of gain to maintain the same loudness.  All that is usually required is to rotate the volume control slightly to recover the lost signal. 

If the loudspeaker has a sensitivity of 100 dB, a system with a 10 watt amp will only require 9 dB of amp gain with a 1 volt maximum input signal.  Increased speaker efficiency reduces the need for amp/preamp gain.  That's why horn speakers can be driven by very low power amps that only have a few dB of gain.

 Most amplification equipment has too much gain.  Many tube preamps have gains of 20 to 24 dB.  When coupled with a power amp, that’s usually way too much.  The Grounded Grid Preamp has a gain of 12 dB which is much more practical and useful.  The T16 and T8 OTLs have gains of 20 dB giving a combined amp/preamp gain of 32 dB.

 I have found from practical experience that a combined preamp/amp signal gain of 32 dB is excellent for most systems with speakers that have efficiencies between the mid 80’s to low 90’s.  Horn speakers with efficiencies of over 100 dB can generally get by with amp/preamp gains of 20 to 25 db.  This depends of course on the level of the signal source.  A low output phono preamp could push the horn speaker system to require an amp/preamp gain over 30 dB.  A little excess gain is needed to have the flexibility to crank up the signal when desired.  That's what the volume control is for.   

As with most issues in audio, there are no set answers.  Each system has to be looked at individually.