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monty78pig 30th August 2012 11:06 PM

The ultimate rumble filter - far more effective than just a high pass filter!
 
1 Attachment(s)
Rumble is something that has always irritated me and I have always been dissatisfied with the standard cascaded high pass filter type designs.

One of my many hobbies is restoration, mostly dead mono 78s, but sometimes the odd LP. If the disc is even slightly warped, then the rumble immediately shows up on the waveform and it seems impossible to reduce it to a good level without affecting the audio. Even with a 160dB/Decade filter at 20Hz or so, I can still see the bass drivers in my RX6's quivering, this is (to me) intolerable :mad:. The fluttery effect on the stereo image that it causes is horrible :(.

However, when using mixing the stereo track down to mono, this dissapears almost completely (all thats left is some very low frequency noise <8Hz), along with the low frequency 'road noise' sound. 95% of this crap seems to lie in the stereo separation (vertical plane of the stylus' movement).

By experimenting with the filters and effects in Audacity on some quick transfers I have found that:

Firstly, virtually no discs have any real stereo separation below about 200Hz or so at most, this means that I can lose the separation at, say, 150Hz without any change to the original audio. By splitting the channels at this frequency with 40dB/Decade filters and then mixing the two low frequency tracks together the rumble can be almost completely eliminated with far greater effectivity than just using a high pass filter. It will cancel rumble of far far higher frequencies than just a high pass filter ever could. No more 'road noise' :D!

Secondly, after this has been applied, the remaining rumble is of very low frequency, and a simple 40dB/decade filter at 12Hz does the job wonderfully. After this has been applied, the rumble becomes invisible on the waveform (and more importantly inaudible), as opposed to simply having the subsonic elements attenuated when using a high pass filter. This filter really works wonders at reducing low frequency surface noise well above 100Hz.

I have incorporated my findings into a circuit that will sit between my phono preamp and control amp (see attached) although a little more complicated than a steep subsonic filter, the extra complexity is well worth it. The circuit applies an 60dB/Decade filter at 12Hz, and collapses the stereo image below 160Hz. When using these filters on Audacity the rumble is reduced by about 24dB at 20Hz.

I'd be interested to hear peoples ideas on this concept, so tell me what you think :). I'm still experimenting so will keep this thread posted with my results.

andyr 31st August 2012 02:43 AM

It's a very impressive circuit but I'm surprised you were driven to research it ... as I have never experienced rumble with my LP12. Maybe (for better musical enjoyment) you need to get a different TT as the problem I see with this device (sitting as it does between phono stage and line stage) ... is that it inevitably must be degrading the signal, compared to if it wasn't there? (IE. yes, it fixes your rumble issue but it does that at a cost. :eek: )

Regards,

Andy

blakkshepeaudio 31st August 2012 03:00 AM

I think the answer here is in the quote
'One of my many hobbies is restoration, mostly dead mono 78s, but sometimes the odd LP.'
Turntable rumble is not the issue, I think, more the LF noise inherent in the recordings and the physical limitations of the discs themselves.

In the studio world, the idea of 'mono-ising' low frequencies is very common and the technique you describe is one that many people (myself included) use as a matter of course when digitising very noisy recordings. It is normally done using processing plugins in a DAW. Your attempt to implement it in hardware is admirable.

And I am sure you can take it out of circuit when you don't need it. :)

Regards,
blakkvelvet

andyr 31st August 2012 03:18 AM

Yes, good point. :)

Thanks,

Andy

Pano 31st August 2012 03:57 AM

How's your tone arm resonance? Causing any problems?

andyr 31st August 2012 04:13 AM

Quote:

Originally Posted by Pano (Post 3146527)
How's your tone arm resonance? Causing any problems?

If you're asking me ... I have absolutely no problems with tone-arm resonance. :)

Regards,

Andy

monty78pig 31st August 2012 09:51 AM

I didn't know that this was done so much in restoration, although with some of the CDs out there I have very little faith in analogue restoration practice at the moment :( (CEDAR being overused), although I've been doing it in the digital domain for a while, I would like to put it in a box and use it when playing back some LPs through my hifi.

I don't think I have any problem with tonearm resonance as the LF noise doesn't seem to peak anywhere on an FFT on most recordings, it seems rather flat (within it's limits) to me. The turntable setup I have is a Project Debut 3 with a Shure M97XE and is meticulously, almost to the point of becoming obsessively, adjusted for optimum performance.

I only have rumble problems with some discs, mostly early stereo LPs, but I can't see any reason why putting a well constructed version of this circuit in the signal chain would hurt anything, OPA4134's have a distortion of something like 0.000008% or so at unity gain (look it up if you don't believe me) and are very low noise, so I don't think that theres any chance of degradation as any distortion/noise introduced would be immeasurable when connected to even the very best phono preamps :D. I think it's well worth any LF inter modulation that any rumble whatsoever will inevitably cause in the speakers. The rest of my setup goes all the way down to 2Hz, so having a cutoff of 12Hz is not a bad thing (especially as the response of the cartridge only goes down to 20Hz and then rolls off increasingly steeply after that). Seeing as the process is transparent (no LPs have stereo separation at below 200Hz anyway, and I can't hear below 12Hz, and its extremely flat till then), I see no problem in having this circuit permanently between my preamp and control amp. If you don't agree with me then you can ABX test it (with a very flat disc, of course :)).

oshifis 31st August 2012 10:29 AM

I recommend a simpler topology: convert X-Y to M-S, HP filter S (use Bessel filter for minimum group delay), then convert back to X-Y.

DF96 31st August 2012 10:48 AM

The idea of 'mono-ising' LF resurfaces from time to time. The record itself will have had this treatment, so if done carefully most of what you are throwing away will be noise rather than signal.

A simpler version can be achieved by adding a cross-channel resistor at the right point in the RIAA network in a phono preamp. This only gives 6dB/octave LF mixing, but it might be enough for warped LPs.

andyr 31st August 2012 10:52 AM

Quote:

Originally Posted by monty78pig (Post 3146764)

The turntable setup I have is a Project Debut 3 with a Shure M97XE and is meticulously, almost to the point of becoming obsessively, adjusted for optimum performance.

With respect, while you may well have "obsessively adjusted t for optimum performance", a Project Debut 3 with a Shure M97XE is at the low end of the vinyl delivery scale.

Quote:

Originally Posted by monty78pig (Post 3146764)

I only have rumble problems with some discs, mostly early stereo LPs, but I can't see any reason why putting a well constructed version of this circuit in the signal chain would hurt anything, OPA4134's have a distortion of something like 0.000008% or so at unity gain (look it up if you don't believe me) and are very low noise, so I don't think that there's any chance of degradation as any distortion/noise introduced would be immeasurable when connected to even the very best phono preamps :D.

IMO, you are confusing a distortion rating with "degrading the vinyl signal".

Let me give you an example: at one stage, I decided it would be a nice idea to have a "mono" switch on my phono stage. This phono stage is not a bad piece of kit - it's seen off an EAR 834P, for instance. :)

So I installed the phono switch - simply connecting the 2 input "hots" together (possibly with a series res on each side - I can't remember) ... and it significantly reduced the width of the sound stage - this is when the switch was "off" (ie. the switch was the problem - which has zero distortion! :D). This is not something that would be picked up on any oscilloscope - or, indeed, any piece of measuring equipment! :eek:

Regards,

Andy


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