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Topic: Filtering the Vinyl Signal between a Phono Preamp & Digitizer? (Read 5275 times) previous topic - next topic
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Filtering the Vinyl Signal between a Phono Preamp & Digitizer?

My basic question is do I need a separate low-pass filter between my phono pre-amp and digitizing soundcard?

I have been tinkering with how best to rip vinyl when using a phone preamp (I have an Emotiva XPS-1) between the turntable and an external sound card (I have a Focusrite USB Scarlett 2i4).  My current plan is to digitize at 24-bits/96kHz (the best the Scarlett can do), and capture that using Audacity on a fast gaming computer (I have an Alienware laptop).  I already know the sound card/computer/software combo is capable of grabbing that signal without gaps or burps.  With Audacity (which does 32-bit floating point processing), standard stuff like DC removal, normalization, channel balance, and maybe high pass if necessary to remove turntable rumble can be done before spitting out 16-bit/48kHz FLAC files (good enough for my ears).  I like that frequency better than 44.1 kHz because it is more likely compatible with Bluetooth as a couple of mine will not deal with 44.1 when using WASAPI on a Windows computer unless the music player (I use foobar) is set up to re-sample on the fly.  The sole reason for digitizing at 96 kHz assumes the signal bandwidth is no higher than 20kHz and thus well below the Nyquist frequency of 48 kHz and therefore minimize aliasing of any artifacts.

The main issue is whether what comes out of the preamp is essentially limited to 20 kHz,  My preamp has good specs over 20-20,000 Hz, but is it also acting as the necessary low-pass filter so 96 kHz can do its thing?  Or do I have to find another piece of gear to do the low-pass filtering (which I have looked for without success)?  I understand one of the biggest criticisms of Redbook (CD's that used 44.1 kHz) is they required quit steep low-pass filtering that could generate artifacts since the Nyquist frequency in that case is 22.05 kHz, i.e. uncomfortably close to a desired 20 kHz bandwidth.

Re: Filtering the Vinyl Signal between a Phono Preamp & Digitizer?

Reply #1 –
Quote from: coyotearms on
My basic question is do I need a separate low-pass filter between my phono pre-amp and digitizing soundcard?

There is already a strong one built into your sound card (the antialiasing filter), so most likely not.  

Quote from: coyotearms on
The main issue is whether what comes out of the preamp is essentially limited to 20 kHz,  My preamp has good specs over 20-20,000 Hz, but is it also acting as the necessary low-pass filter so 96 kHz can do its thing?

Amplifiers do not and usually should not provide any filtering at all. 

Quote from: coyotearms on
I understand one of the biggest criticisms of Redbook (CD's that used 44.1 kHz) is they required quit steep low-pass filtering that could generate artifacts since the Nyquist frequency in that case is 22.05 kHz, i.e. uncomfortably close to a desired 20 kHz bandwidth.

This is not actually a problem with equipment made in the last 10 or 20 years.  You also are very unlikely to benefit from recording at 96khz vs 48khz, although it will merely do nothing, not actually hurt. 

Re: Filtering the Vinyl Signal between a Phono Preamp & Digitizer?

Reply #2 –
No LP needed, use HP to remove content below 20Hz (I normally use steep > 36dB/oct HP @20Hz).
Check the frequency responses for Scarlett (probably its not flat for 44.1kHz). Also, if the 'base frequency' is 48kHz then 96kHz is a good choice.

BTW, is Audacity still suffering from 24-bit recording under Windows unless you have the ASIO support version installed?

Re: Filtering the Vinyl Signal between a Phono Preamp & Digitizer?

Reply #3 –
What's the benefit of removing sounds you can't hear?

Re: Filtering the Vinyl Signal between a Phono Preamp & Digitizer?

Reply #4 –
It makes the signal compress more easily? Psychoacoustics.

Re: Filtering the Vinyl Signal between a Phono Preamp & Digitizer?

Reply #5 –
Quote from: nastea on
What's the benefit of removing sounds you can't hear?

Have you ever dealt with vinyl playback?

OP, if you use HP filter, remember check that the cutoff frequency really goes by your settings. I have noticed that many HP (VST) plug-ins works correctly only at 44.1kHz.

Re: Filtering the Vinyl Signal between a Phono Preamp & Digitizer?

Reply #6 –
What's the benefit of removing sounds you can't hear?

Re: Filtering the Vinyl Signal between a Phono Preamp & Digitizer?

Reply #7 –
Quote from: nastea on
What's the benefit of removing sounds you can't hear?

In the case of vinyl playback, there can be subsonic rumble, as high as ~15Hz on some setups. Without a subsonic filter, this can cause the woofers to "pump" trying to create the very low frequencies. This also puts unneeded strain on the amplifier.

Re: Filtering the Vinyl Signal between a Phono Preamp & Digitizer?

Reply #8 –
Quote from: nastea on
What's the benefit of removing sounds you can't hear?
What's the benefit of keeping sounds you can't hear?

Re: Filtering the Vinyl Signal between a Phono Preamp & Digitizer?

Reply #9 –
Quote from: nastea on
What's the benefit of removing sounds you can't hear?

You probably do not want digitize large amplitude subsonic content.  The DAC will probably filter out most of it, but whatever gets through will waste dynamic range and could possibly introduce clipping. 

Re: Filtering the Vinyl Signal between a Phono Preamp & Digitizer?

Reply #10 –
When playing a vinyl 12" with an unmodulated groove at 33⅓ RPM, on my Technics SL-1200M3D, the RMS is about -40 dB near the outer edge of the platter, dropping to about -54 dB about halfway in, and holding steady to the end of the groove.

Applying a HPF to roll off the range from 20 Hz down to 0 Hz takes the outer edge down to -54 dB, and it very quickly drops to a steady -60 dB.

I recall reading in the instructions for the old Sonic Foundry noise reduction plugins that DC offset correction and rolling off the low frequencies both helped their declicker to work better. In fact, such a filter was an option, a checkbox I think, in the plugin. I don't know if the rolloff also helps other declickers, but it's something I got into the habit of doing even as I moved on from Sound Forge. For example, it does help, visually, when comparing left and right channels with overlapping display in Adobe Audition's waveform editor.

Re: Filtering the Vinyl Signal between a Phono Preamp & Digitizer?

Reply #11 –
Alright, that makes sense. Thanks guys.

I sometimes digitize vinyl, my phono pre-amplifier is the Cambridge Audio Azur 651P, and it has a Subsonic Filter switch on the backside. Would it be best to use this filter or would it be better to remove the subsonic noise later in a wave editor?

Re: Filtering the Vinyl Signal between a Phono Preamp & Digitizer?

Reply #12 –
Quote from: nastea on
Alright, that makes sense. Thanks guys.

I sometimes digitize vinyl, my phono pre-amplifier is the Cambridge Audio Azur 651P, and it has a Subsonic Filter switch on the backside. Would it be best to use this filter or would it be better to remove the subsonic noise later in a wave editor?


Depends on what kind of filter 651P has (I couldn't find specs for its subsonic filter). Digital domain allows better control of filter parameters. http://sound.westhost.com/project99.htm