Preamp Clipping

Preamp Clipping

A quick survey of preamp specifications reveals little attention to the input level the preamp can take without clipping. Our measurements have revealed that most preamps provide little headroom on the inputs for audio peaks typically found with analog sources.

According to Wikipedia, "Consumer audio equipment [nominal] line levels are rated in dBV, and the most commonly used nominal level for such equipment is - 10 dBV, which corresponds to a sine wave of about 0.3162 root mean square volts (VRMS). The absolute reference voltage is 1 VRMS. "

"Professional audio equipment [nominal] line levels are rated in dBu, and the most commonly used nominal level for such equipment is +4 dBu, which corresponds to a sine wave of about 1.228 Vrms." These of coarse are nominal levels, not peak levels.

Now to complicate things, we have two types of preamps, passive and active. A passive preamp has no gain, so the maximum output level is the input level. A passive preamp cannot clip. The higher in input level, the more attenuation is applied and the lower the noise on the output, so high input level equals better sound.

Active preamps add gain. Signal levels can exceed the preamps capability in two ways. If the output clips, sound is distorted at high volumes. If the input clips, sound is distorted at all listening levels. A clipping output is an obvious preamp design failure and is usually avoided, but a clipping input opens up the question of how large an input level should be accommodated. Again, the higher the input level accommodated, the lower the noise on the output and the better the sound, UNLESS the input clips.

Line Level Specifications

Before digital sources, nominal audio levels were specified and the quality of the product determined just how high a signal could be accommodated before clipping would occur. With the advent of the CD player, suddenly a source was available that had a maximum output level that could not be exceeded. That level was specified to be 2 Vrms. Of coarse an assumption was made that music would fall about 12 dB down from the maximum, and that level would match the old standard pretty well. Along comes better compression in the studio and music levels have been creeping up. The market also went to work, and as we all know, louder is "better", so companies just keep upping their output level just a little higher than their competition and voila, it sounds just a little better. So now we have audio sources with a range of output levels. Most digital consumer products provide an output that is always under 2 Vrms and most professional levels are under 4 Vrms. BUT, analog sources are still with us, and can produce much higher output peaks. A truly good active preamp will still accommodate these higher levels, but most have found great cost savings by clipping any input level much over 2 Vrms.

Problems are also found with passive preamps. Because high-end products in particular vary greatly, and amplifiers also vary in gain, and speaker efficiencies vary, passive preamp designers also run into a problem, but one they can do nothing about. With this 2 Vrms maximum output level, and a passive preamp, there are cases where a particularly inefficient speaker and amplifier are put together and the system is just not loud enough. MSB addressed this problem in our DAC design.

MSB DAC Outputs

The MSB DACs have always accommodated passive preamps, by providing a higher than average output level. Many customers use the internal passive volume control modules with the DAC with excellent results. The MSB DAC III consumer RCA output is 3.6 V rms and the professional balanced XLR output in 7.5 V rms, a little less than twice the "2 V common " output level. This level is perfect for all passive preamps, but too high for most modern active preamps.

MSB has provided a solution for the active preamp user in the form of a switch which provides a 6 dB level cut (50% cut) which brings our output level down below the "common " output maximum.

It is very important to pay attention to this effect, as clipping begins with subtle loss of sound quality. You may not be getting the most from your DAC if your preamp is not optimized for our output level. Current MSB products all ship with this 6 dB cut feature. Older DAC IIIs and Power DACs that do not have this feature can obtain it via our latest Extended Frequency Filter Upgrade which can be purchased online and installed by the user. This upgrade also provides a significant improvement in sound. It can be used with any DAC using the 16x or 32x MSB digital filter. Older Platinum DACs can also have a 6 dB cut with an attenuation header installed.