First of all it would be helpful to understand just how much voltage gain is required between a source component (CD player for example) and a pair of loudspeakers. Although power amps are specified in terms of how many watts they deliver, its more useful to view a power amp as a voltage amp, but one which can maintain its intended output voltage when driving low impedances. (The output of CD players is typically 2 volts rms, and 2 volts rms is enough to get most loudspeakers up to a reasonable sound level, but if you were to try to drive a pair of loudspeakers directly from a CD player you would find that it wouldn't be able to drive the speakers. That's because the output level will be 2 volts only as long as it is driving a high impedance - it won't maintain its nominal 2 volt output into an 8 ohm load.)
A voltage swing of 30 volts RMS into an 8 ohm load will dissipate over 100 watts (112.5 to be precise). So to raise the output voltage of a CD player (2V) sufficiently to drive a loudspeaker to 100 watts requires amplification by a factor of 15. A factor of 15 expressed in terms of decibels is just over 23dB. (In this instance decibels are being used to express gain - the ratio of output to input. It has nothing to do with sound pressure level (SPL). When used to express SPL, the dB figure is the ratio of the SPL to a fixed international standard level of sound which is nominally the minimum which can be heard.) Most power amps have voltage gain of about 30dB. Some have a bit less (27dB for most Naim amps) and some have a bit more (40dB for Audio Synthesis), but 30dB is quite typical. 30 dB is the same (very nearly) as a factor of 30, so amplifying 2 volts by 30dB gives 60 volts. 60 volts RMS into 8 ohms will dissipate 450 watts. If your power amp will deliver that much power into 8 ohms, and your speakers will handle it, then everything's fine. Otherwise, you've got a bit too much gain.
If you use a pre-amp with voltage gain, the problem is made worse. Its not uncommon to find pre-amps with 20 dB voltage gain. When used with a power amp which has 30dB gain, the total gain is 50dB. That's a factor of just over 300. Amplifying the 2 volt output from a CD player by 300 gives 600 volts, and applying 600 volts to an 8 ohm load will dissipate 45000 watts. Yes, forty five thousand watts. (Into 4 ohms the power dissipation would be 90000 watts!) If the power amp is not capable of delivering 45000 watts (quite likely) it will simply go into clipping and sound awful.
So, to summarise, the voltage gain of the power amp alone is usually sufficient and there is usually no need for any further gain in a pre-amp. However, a little bit more gain than is strictly necessary is not a bad idea, because you can always turn down the volume control if its too loud.But, of course, that's where the problem starts. If the extra gain is excessive the volume control ends up never getting beyond the 10 o'clock position.
The Rothwell In-Line Attenuators cut the signal by 10dB with a potential divider. The resistors which form the divider are carefully chosen so that they will work well when used with power amps whose input impedances are in the range of 10k to 1M, and with pre amps whose output impedances are in the range of anything up to about 10k. Alternatively, the attenuators can be used between a source component and pre amp or integrated amp with input/output impedances in the same ranges. The exact level of attenuation will, to some extent, depend on the input and output impedances of the equipment with which they are used. However using a potential divider with two carefully chosen resistor values ensures optimum performance with a wide variety of equipment. If the attenuators contained only one resistor (as some people seem to think is adequate) then performance would be very hit and miss.
The change in volume control position is easy to understand - less signal requires more amplification to achieve the same volume - but the improvement in signal to noise ratio isn't quite so obvious. This is how it works.
There are many ways to design a pre-amp, but the way which seems to be most common in specialist hi-fi is to have the volume control followed by the voltage gain stage. Since any piece of electronics will generate some noise its inevitable that the gain stage will output some level of noise. The ratio of the signal to the noise will be optimised when the signal is at its maximum (this will be the figure specified by the manufacturer) but turning down the volume control will reduce the signal whilst leaving the gain stage noise untouched - hence the ratio gets worse. If you operate the system with the volume control turned down around the nine o'clock position most of the time, the signal to noise ratio will be much worse than the manufacturer's spec. Operating the volume control further up its range will improve the signal/noise ratio, but the actual volume of the music may be just too loud. However, if the signal is reduced by 10dB at the inputs to the power amp, then the volume of the music will be reduced, but the volume of any noise generated by the pre-amp's gain stage will be reduced too. This allows you to operate the pre-amp's volume control farther round its range and the signal to noise ratio will be improved by 10dB.
If the pre-amp is based on IC op-amps, there are quite likely to be other sonic benefits in addition to the improved signal/noise performance. All op-amps have a push-pull class B or class A/B output stage which will inevitably suffer from crossover distortion. Negative feedback is used to correct this distortion, and at full output voltage swing the specifications will be very good. But if the voltage swing is nowhere near maximum, then crossover distortion may be audible as subtle masking of low level detail. The In-Line Attenuators will allow the op-amps to work with a higher signal level without the music becoming too loud, so the crossover distortion will be reduced.
In short, if you have a 45000 watt amp and you like it extremely and unlawfully loud, you don't need any attenuators.
87-89 Lever Street, Bolton, Lancs, BL3 2AB, United Kingdom Tel. +44 (0)1204 366133
VAT No. 823 9079 14 - Company Reg No 4936788
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