Although in theory the tolerance of both resistors and capacitors should be 1% or better, in reality it is not that important. The Q of the filters has been optimised to allow a higher input impedance than would otherwise be possible, with the final Q of the two filters being almost exactly 0.707 (i.e. The circuit shown is a conventional Sallen-Key filter, but some simplifications have been made so that the number of different value components is minimised. PCBs are available for this project, which makes it very easy to put together. It can also be used with instrumentation/ measurement systems where low frequency energy 'pollutes' the results. It's also very useful with vented speaker enclosures, and prevents excessive cone excursion at frequencies below the box resonance. As noted above, it's essential for 70V and 100V line PA systems, or anywhere that a transformer is driven from a power amplifier. The project as presented here can be used anywhere that you need a rapid rolloff to prevent infrasonic signals from causing havoc. Summing also cannot be used with a mono signal, and that would limit the usefulness of the filter. Stereo signals are at ±45° The summing method was examined carefully before deciding that it should not be used if the overall frequency response of the disc is to be preserved. The mono component of a vinyl disc is lateral, whereas warp signals are vertical. The infrasonic frequencies generated by record warp are by nature out of phase. With 70V and 100V public address systems, there is usually no reason to reproduce anything below 80Hz, even for background 'music'.Īt least one published rumble filter circuit uses a method of summing the channels below 140Hz, and although this is effective in removing the low frequency rumble (or sub-rumble in this case) component, it causes frequency response aberrations that are unacceptable. An infrasonic filter is absolutely essential in these systems, but even some amplifier manufacturers don't seem to appreciate the risks.
See High Voltage Audio Systems for the details. Any very low frequency signal that gets through the amplifier and saturates the transformer is likely to cause either amplifier failure, gross distortion, or commonly both. Even 24dB/ octave is likely to be marginal, especially when driving a transformer load.Ī steep infrasonic rolloff is essential is driving a distribution transformer - typically for 70V or 100V public address systems as used in offices, shopping centres, factories, etc. To be effective, an infrasonic filter has to be very steep - this allows all wanted frequencies to get through, and rejects those that will only cause problems. Large amounts of available power will be wasted trying to reproduce a signal that was never intended to be there in the first place. Not only will vented subs be completely unable to handle such a signal linearly, but sealed subs will also be stressed. This is a signal that will cause significant cone movement, but is undesirable in the extreme. While often referred to as 'subsonic', the correct terminology is 'infrasonic' ('subsonic' means slower than the speed of sound, 343m/s).įor example, a 33 1/3 RPM album with a single warped section will create a signal in the pickup at 0.55 Hz (33.3 RPM / 60 = 0.555 Hz). Any warp in a vinyl disc will cause large outputs in the infrasonic region, typically well below 20Hz. This is especially troublesome with phono systems, since many of the vinyl discs you treasure (or wish to transcribe to CD) will be warped to some degree. Click the image for details.įrequencies below 20Hz are usually not able to be reproduced, and with the exception of synthesisers and pipe organs, are not a wanted part of the audio spectrum. Please Note: PCBs are available for this project.
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