Wave pattern in Audacity LP recording

[dwk1983]

I have started to see a wave pattern in my LP recordings that had not been present before. I have a DUAL 1242 turntable hooked to a ART USB PhonoPlus Preamp and my operating system is XP. When I first started recording my (over 300 so far) LPs, the wave pattern was not there, but now it shows up for every record. I can't say I hear any distortion from it, but I don't have a real good ear for that. It appears more pronounced at the beginning of the LP and appears the moment the stylus touches the record and goes on into the sound recording. I trying to decide if it's a problem with my turntable or my computer system or the preamp. I read in other posts about problems with Skype and I may try to uninstall it to see if it maybe the problem. I tried changing the stylus on my turntable, but it did not have any affect on the wave pattern. I also plan to get my turntable serviced. What would cause a wave pattern to show up in an Audacity recording?

[billw58]

It depends on the frequency of the wave pattern. Can you post a screenshot of a portion of a typical recording that demonstrates the problem? Be sure to include the timeline so we can get a feel for the frequency.

-- Bill

[dwk1983]

I have attached an example from a recent LP showing about 11 seconds. I have been getting this same pattern on other LPs so I do not think it is the fault of the record.

[steve]

That is very strange and I can't say that I recall ever seeing such a problem. My suspicion is that the fault lies with the turntable though I'm far from certain at this stage.
Could you try a little experiment - set the turntable to play at 45rpm and record a couple of seconds from the begining of a record - any record - I'm not interested in the music recording, I'd just like to see if the wiggle changes when the turntable speed is changed.

If you had not been recording you would probably not have noticed the problem at all as the wave pattern is a sub-sonic frequency (about 7 or 8 Hz) and is too low to be reproduced by your loudspeakers. We can "repair" the problem in the recording by using a high-pass filter, but the wiggly wave should not be happening and should be fixed properly.

[billw58]

The screenshot was very useful. I loaded it into photoshop then copied and pasted a section of the waveform to compare with a following section:

rumble.jpg (40.73 KiB) Viewed 1184 times

As you can see the waveform repeats every 1.8 seconds, which is exactly 33 1/3 rpm.

This turntable is a belt drive unit so I can't see how this could be a motor or belt problem. My guess is the main turntable bearing.

Taking the turntable in for service would seem like a good idea at this point.

-- Bill

[waxcylinder]

Wow Bill - I'm really impressed with those diagnostic skills ...

Before taking it in for a service you may want to try lubricating the bearing. You will need to take the platter off the deck(carefully_ - and don't damage the arm/cartidge/stylus)- and you will need to remove the belt (consult the manual for the TT). I would recommend using a very light oil (try the stuff they sell for sewing machines) and use it sparingly. There should be a bearing for the platter itself - but while you have the platter off I would also lube the bearing of the drive capstan (the little spindle that drives the belt. Do be careful not to get any oill where the belt runs on the capstan or on the platter.

Once you have re-placed the platter and re-seated the belt then run the deck for 20 minutes or so - and then repeat Bill's tests at 33 1/3 and at 45.

WC

[billw58]

Before taking it in for a service you may want to try lubricating the bearing.

I'd also recommend that before you put new oil in the bearing that you carefully clean the bearing - both the post and the well. A lint-free cloth is essential (don't want to leave any fibres behind) - paper towels are right out. Dampen the cloth very lightly with mineral spirits. Wipe the post, then poke it down into the well with a non-metallic instrument and wipe. Hopefully you will get the gummy oil and any contaminants out. Let the mineral spirits residue evaporate then sparingly apply light machine oil to the post. Very carefully slide the post back into the well - it should slide easily.

-- Bill

[dwk1983]

I get a repeat time of about 1.3 seconds at 45 RPM, so it must be the turntable. Thanks for the info. I'm the original owner of the turntable and it has never been serviced. It sat unused for well over a decade (maybe two) when my old stereo receiver went bad and I couldn't afford to replace it. I picked up the Dual 1242 service manual off Ebay last year when I started using it again and it describes the lubrication points, but it mentions different oils for different points so I hadn't tried to do it. I may try to lube it myself, though I also found a shop in the area that will service Dual turntables. Thanks again. Great info.

[billw58]

I picked up the Dual 1242 service manual off Ebay last year when I started using it again and it describes the lubrication points, but it mentions different oils for different points so I hadn't tried to do it. I may try to lube it myself, though I also found a shop in the area that will service Dual turntables.

Since you have the service manual you could save a few bucks and do the service yourself. Turntable manufacturers often specify very specific oils for different parts of the turntable. Unless the shop has those specific oils in stock you'd do just as well servicing it yourself, using a light sewing-machine type oil as WC recommended. However, if you take it in to the shop they should be able to supply a replacement belt which may also be a good thing at this point.

-- Bill

[waxcylinder]

if you take it in to the shop they should be able to supply a replacement belt which may also be a good thing at this point

Yes the old belt may be a little stretched or a little brittle by now. If you are going to service it yourself then you should be able to find a replacement belt on t'interweb. I did a quick Google on your TT model this morning and at least one of the links seemed to be a supplier of such spares.

If you clean the stylus (and you should) don't do what I did - I used an alcohol based solvent with a fine camel-hair brush and unfortunately the solvent dissolved the glue sticking the stylus to the cantilever - an expensive mistake

WC