Subsonic audio Filter

Posted on Aug 6, 2012

The circuit shown is completely conventional. 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. a traditional Butterworth filter). Although in theory the tolerance of both resistors and capacitors should be 1% or better, in reality it is not that important. 1% metal film resistors are recommended (as always) but only for lowest noise, and capacitors are standard (i.e. 5% or 10%) tolerance. Yes, this will cause the response to deviate from that shown below (see Figure 2), but compared to other errors in the system (recording EQ, room LF node problems, etc.) these may be considered minor.

Subsonic audio Filter
Click here to download the full size of the above Circuit.

At least one circuit that the I know of 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 (IMO) are not acceptable. The subsonic frequencies generated by record warp are by nature out of phase. The mono component of a vinyl disc is lateral, whereas warp signals are vertical. Stereo signals are at 45° The summing method was examined in great detail before deciding that it should not be used if the overall frequency response of the disc is to be preserved. Frequencies 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. 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. Any warp in a vinyl disc will cause large outputs in the subsonic region, typically well below 20Hz. For 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). This is a signal that will cause significant cone movement, but is undesirable in the extreme. Not only will vented subs be completely unable to handle such a signal linearly, but sealed subs will also be stressed. As can be seen from the above, below 2Hz the overall response is...

Leave Comment

characters left:

New Circuits



Popular Circuits

Responsive Wearable Technology
capacitor Choosing components for a triacs snubber
propeller receiver
Automatic Night Lamp Circuit PCB
JCM800 2204 bias problem
Using tap potentiometer loudness circuit
As current feedback circuit of four b