Time for one of those long, boring semi-technical posts that no one here reads...
There have been various posts from people who have just acquired a "new" vintage amplifier, have connected it up to their speakers and fed it with a nice clean signal from a (relatively) modern CD player, and have commented along the lines of
"I only turned the volume up to 10 o'clock and the whole house was shaking - boy that amp is powerful".
They never seem to consider that their amplifier might well be producing near to full output power even though the volume control is nowhere near maximum - a consequence of the sensitivity mis-match between vintage and modern equipment.
It seemed to me that it would be useful to go over a few basics regarding the decibel (dB) scale and how it relates to the sensitivity of the inputs on vintage amplifiers.
First a few basics about the dB scale (and a little bit of math - but nothing too difficult
) :
1. The decibel is a
relative rather than an absolute measurement, i.e. it is used to measure the
ratio of one signal to another. I am sure everyone is most familiar with it's use in representing the
signal to noise ratio of equipment.
2.
Positive dB values mean that a signal is
greater than the reference value (ratio greater than 1),
negative dB values that it is
less than the reference (ratio less than 1).
3. If a signal passes through a number of amplification (+dB) or attenuation (-dB) stages, then the
overall gain is found by simply
adding up the dB values of each stage. For example, assume a signal passes through components with the following typical gain : phono amplifier +60dB, pre amp -20dB, power amp +30dB : The overall gain is therefore +70dB.
4. To convert the ratio between two
voltages, V1 & V2, to decibels we use the formula :
dB = 20 * log(V1 / V2)
A doubling of voltage = +6dB.
Conversely, to convert a dB value to the ratio between two
voltages use the formula :
Voltage Ratio = 10^(dB value / 20)
(The symbol "^" means "to the power of").
5. To convert the ratio between two
powers, P1 & P2 to decibels we use the formula :
dB = 10 * log(P1 / P2)
A doubling of power = +3dB.
Conversely, to convert a dB value to the ratio between two
powers use the formula :
Power Ratio = 10^(dB value / 10)
So back to the original question : Just how far do we have to turn up the volume to get maximum output power from our amplifier?
Look at your amplifier's manual and find the
sensitivity value for the input you are using. The value it gives is the
input voltage required in order to produce maximum rated power when the volume control is set to maximum. If the input signal you are feeding to the amplifier is greater than the sensitivity value, then maximum output power will be produced before full volume on the control.
Let's work through a real example to make things easier to follow (using data for my Pioneer SX-1250 receiver and Marantz CD-65SE CD player - typical of many vintage amp / modern CD player combinations) :
Input sensitivity of SX-1250 "Aux" (and "Tape") inputs : 150mV
Output voltage from CD-65SE at maximum signal level : 2000mV (2V)
So the CD player output signal is 20 * log(2000 / 150) = +22dB higher than that required for full output power from the SX-1250.
What this means is that the receiver's full power of 160 watts will be produced with the volume control set to -22dB (referenced to maximum = 0dB). Fortunately, in common with much high-end equipment, the SX-1250 has a volume control marked in -dB, making it easy to see where this occurs.
Perhaps the most "surprising" thing to come out of this calculation is just how little the volume needs to be turned up to get full output : -22dB on the SX-1250 corresponds to somewhere between the 11 and 12 o'clock positions.
So the next time someone tells you how loud their system is even at low "volume" settings (and implying that it would be ten times louder if turned up fully), just pause for a minute before you get too impressed and instead consider if they may have an amplifier/source sensitivity "mis-match". :scratch2:
Finally, we can apply the same calculations to the use of graphic equalizers and bass / treble / loudness controls. If +3dB of boost is applied at some frequency, then the power required at that frequency is doubled. If +10dB of boost is applied, then the power required increases by a factor of 10. So applying high levels of equalization (for example to compensate for the falling bass response of a speaker)
massively increases both the power requirements of the amplifier and the handling capacity of the speakers.
- Richard B.
