Yep, I'm wrong about the out of phase signal while the arm is lowering. I now believe this short period is the very front of the record that just has a lead in groove, for some reason this produces an out of phase signal, but once the needle gets to the audio track it's back in phase. So no mystery, it's a low frequency rumble in the turntable. I've cleaned and re lubed the bearing, without any improvement, so I guess it could be in the motor. Not sure I'm game to tackle that, especially when audacity can filter it out quite well after recording.

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Ian K said..
I know I can't hear as well as I used to, but 10 Hz!

en.wikipedia.org/wiki/Hearing_range

You're down there with the whales!

"Several animal species are able to hear frequencies well beyond the human hearing range. Some dolphins and bats, for example, can hear frequencies up to 100 kHz. Elephants can hear sounds at 14–16 Hz, while some whales can hear subsonic sounds as low as 7 Hz (in water)."

I'm not worried about hearing it, it just makes processing the signal harder. And I'm guessing if it's not meant to be there, there could be a bad reason why it is.
Beside that it is a mystery, and aren't they there to be solved?

Maybe your house is "haunted" www.cracked.com/article_18828_the-creepy-scientific-explanation-behind-ghost-sightings.html any extractor fans running in the house?

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jonnulla said..
Maybe your house is "haunted" www.cracked.com/article_18828_the-creepy-scientific-explanation-behind-ghost-sightings.html any extractor fans running in the house?

Hmm very interesting new it was a good idea to get rid of that 10hz,

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Specifically, researchers found that sounds between 7 and 19 Hz it could induce fear, dread or panic.

Luckily it wasn't the brown note/frequency... then you'd have a whole other thing to worry about

Here are a few things to try...

Is the rubble there when the headshell is removed? (if it is there it is some sort of electrical problem..... If it goes then it is mechanical)

Is the 10hz rumble there when NOT in contact with the record? (motor / bearing problem) or does it start when it contacts the surface (platter / spindle problem).

Why would it be out of phase (polarity reversed)?, we know that when the needle moves one way it gives a L signal when it goes to the other it gives a R signal.... But in the pressing they are opposite to each other. So one side of the pickup coil needs to have its polarity reversed in manufacture.

If it was basic track rumble and not actually part of the recording it will appear equally to both L & R of the pickup coil and therefore be polarity / phase reversed.

The thing is that many years ago we wouldn't have even known that rumble was there, its only due to technology we can now see it.
Often DAW programs have set-able windows for snipping quiet sections out. I know ProTools has it, not that I use Protools often but its good to snip out breath pauses / noise in voice tracks for radio /TV.
Most Digital mixing desks have 'noise gates' on every input if you wanted / needed them.

No the rumble is only there when the needle is on the record. I was mistaken when I said it was there as the arm descended.
I now believe the 2 different sections at the start are the lead in groove then the start of the audio track.
Yes you're right it's only a concern because I can see it, it's probably not a problem in reality.

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If it was basic track rumble and not actually part of the recording it will appear equally to both L & R of the pickup coil and therefore be polarity / phase reversed.

Now that makes perfect sense for the out of phase lead in groove , but why does it change to in phase as soon as it hits the audio track, this is so weird?

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decrepit said..
Now that makes perfect sense for the out of phase lead in groove , but why does it change to in phase as soon as it hits the audio track, this is so weird?

Its because in the actual music recording the L & R are polarity reversed and then sorted out in the electrical side of the pickup to give an 'in phase' signal to the audio output. Where as in the leadup to the track the signals are both equal..... then if you reverse the electrical side (to suit the recorded material) the lead up then comes out as out of phase.

Using signals in and out of phase is used quite often in the Audio / Broadcasting industry, if you really want to blow your mind on the subject do a search of how 'stereo' FM transmission is done (FM Radio stations) or even Mid-Side stereo recording.. Good research for techy boffins.

I thought the Audacity program would have it..... 'Truncated silence' This should fix your problems.

manual.audacityteam.org/man/truncate_silence.html

Sorry, I didn't mean the audio, I meant the rumble, how does that change polarity when the groove changes from lead in to audio?

I think I'll try a few different records, see if they all do the same thing.
Then I'll see what happens at 45rpm, the motor stays the same speed, the belt just moves to a thicker part of the motor shaft. That could be interesting.

I started my working life as a radio tech, back then it was all valves and relays. But I was out of it way before FM came in.
Echo and reverb were just a big chamber in the basement with a speaker at one end and a mic at the other.

So, volume of rumble increases when changed to 75rpm, from about 0.04db for 33 to about 0.15db for 75 but frequency stays around 9hz

The phasing is interesting, it's always exactly 180deg out when needle touches down, but then it starts to vary, (I think when the lead in groove hits the start of the blank audio track), it can then vary between 90deg out of phase to in phase. At times it looks like the two tracks have a slightly different frequency, somewhere between 9hz and 10hz.

So I don't think this is a turntable problem, I think rumble frequency would go up as turntable speeds up if it was. I guess if I measure dia of motor shaft and dia of turntable I can work out how many revs the motor is doing, any bets on 10rps or 600rpm? or some fraction/multiple thereof

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decrepit said..
Beside that it is a mystery, and aren't they there to be solved?

How about dust in the groove of a record piles up into corrugations with a wavelength corresponding to 10Hz?
Maybe a dodgy capacitor? A rectifying/discharge effect going on somewhere in your amplifier? It doesn't look like a steady 10 Hz. Does it disappear when you short the input?

Wild guesses Ian, that much dust would make the record unlistenable, it's not in the amp, only happens with the turn table, but I think I'm getting closer as we speak.
-----going back half hour or so now, thought I'd lost this bit but it's mysteriously reappeared. -------

OK, should have done this ages ago, this old brain is getting slow.

Removed the drive belt to compare dia. and for 33rpm at the turntable, the motor is doing about 1500rpm, which is 25rps, no obvious relationship there.
But it was then I had the inspiration to check the wave form with the belt off, and here's the result.





So the first part is the residual mains hum with pickup shorted out. then you can see the needle land and cause the initial out of phase spike, then it sort of settles into an in phase aprox 9hz rumble (10cycles in 1.13s) So this is without the record spinning, and the needle resting on the lead in section.

Decided to have a look a bit further along the waveform, this is what happens just before I lift the needle off the record.





So the amplitude is way down and it's now 25hz, obviously the motor noise. not much above residual mains hum.

So what happens with the power to turntable off???
Next test coming up.
And it's also weird!




So looks like two resonances, the familiar 9hz that fades out after about 5s and a 1.5hz that seems to be modulating the 9hz and fades out after 2s. I guess when the record is playing both these resonances are being activated.
I wonder if playing with pickup pressure has any effect on either of these?
More later, ------------

Silly old fart, the 1.5hz is my hearbeat!!!!!!!!!!!!!!!!! Fades out when I let go of the lower lever. If I leave my hand there, the 1 to 1.5 hz stays there.

I've reduced the tracking weight from 1.7gr to 1gr, (the manual says that's the min)
And the 9hz has decreased, what's mainly here now is the mains hum





Now all that's left to figure out, is why my heartbeat modulates the 50hz? I can see that I act as a bigger antenna and increase the 50hz signal, but how is that affected by heartbeat? (If I'm right about heart beat, but it's about the right frequency)

Here's a look at my finger resting on the table body.




so that's needle lowered, hands off, then finger back again for a while before lifting needle off.

And I'm sure you'll be pleased to know, I finally figured out how to invert one channel in audacity, so I can now fix all the songs I've taken off vinyl over the last 18 years.