Why is my turntable's frequency response now dropping off?

[board]

I brought this up in the Mofi StudioPhono review thread, and I'm hoping you all can help me with figuring out this issue. Sorry for the long post.

My turntable is a Rega RP3 from 2014 with a RB303 tonearm.
My cartridge is the moving coil Audio Technica AT33PTG/II. I have two of those: One I've used for a while, and one which is almost new.

So, I moved to a new house recently, and I recorded some records onto my computer that I had also previously recorded before moving, and I noticed that when I use the plugin CurveEQ, where an orange line shows the difference in EQ between two songs, the frequency response drops in the high frequencies in the new recordings compared to the older recordings. I tried this with more than one recording, and they show similar results.

First I thought my cartridge was broken, but I mounted the identical, almost new, one instead with similar results; then I thought my Mofi StudioPhono preamp was broken, but apparently it's not, because I just bought a Cambridge Audio Duo that gives almost identical results.
So, the difference between a recording from before moving and after moving looks like this, done with the same phono preamp, the Mofi (the recording after moving has less high frequency energy):

To avoid boring you with too many charts, I'll just say that comparing recordings done with the newer and older cartridges in my previous two houses (also recordings done with one cartridge in one house compared with recordings done with the other cartridge in another house) essentially shows a completely flat line. I also had a third AT33PTG/II, now a bit broken, and they all had almost identical frequency responses.

So, I figured that my Mofi phono preamp was broken, so I just bought a Cambridge Duo phono preamp today, and the difference between the Mofi and the Cambridge looks like this:

So, the Mofi has slightly more energy around 40 Hz and at the very top (or, conversely, slightly less energy between 40 Hz and 700 Hz), but it's negligible - especially compared to the difference in the first chart.
I have ruled out my A/D converter, a Focusrite Scarlet 2i2, as I did two things: One was run a cable from my DAC into the Focusrite to record a linear sweep, and the result was complely flat. The other thing was to do the same thing with a piece of music and then load "before" and "after" in CurveEQ, and I got a flat line. I used the same cables for recording from my DAC into the A/D converter as I did when recording from the phono preamp into the A/D converter, so it can't be that cable either.

The turntable is placed on top of the same rack as in my previous house, although I didn't have this rack in the first house where I lived, but comparing recordings between those two previous houses didn't show anything noteworthy. I've checked with a spirit level that the rack and the turntable are level.
On the Mofi preamp I've used 100 ohms, and I tried changing it to 1000 ohms but with no difference. The Cambridge preamp can't be adjusted.
I also tried changing the tracking weight from 2,0 grams to 1,8 grams and 2,2 grams. No noticeable difference.
As for the recordings I've made, the only difference I hear is a lack of high frequency content. I used CurveEQ to apply the reverse of the curve in my first picture to the newest recording, boosting the high end, and then the recordings from before and after moving sound identical to my ears.
I can of course send recordings to anybody interested.


So, I'm thinking that the issue could be one of a few things:
* My tonearm
* The cable that comes out of my turntable. It's fixed, so I can't replace it.
* The temperature in the room. As unlikely as this sounds, I was reading about it, but I'm very skeptical. My current room is a bit colder than previous houses, but I usually do heat it up while I'm here, but I think the difference to previous houses would at most be a few degrees celsius. Also, I haven't seen differences like these before between recordings done in different temperatures in previous houses.
* Something else that I haven't thought of (?)

Does anyone have any ideas to what the explanation could be? I don't have any measurement gear at all, which is one of the reasons why I use CurveEQ, so hopefully one of you can help me figure out what the issue is.

 

[Chester]

You sound like you know what you’re doing so I assume things like the turntable is as level as it was previously etc?

ie. The basics are all correct and as they were previously?

 

[Balle Clorin]

And you are recording from the RiAA output , not a mic in the room?

 

[board]

And you are recording from the RiAA output , not a mic in the room?

Yes, a cable directly from the phono preamp into the A/D converter.

 

[board]

You sound like you know what you’re doing so I assume things like the turntable is as level as it was previously etc?

ie. The basics are all correct and as they were previously?

It's not 100 % identical but quite close. So I'll try putting something under the feet to change how level it is and then record again to see if that makes any difference. But do you really think this could cause such a big difference?

 

[ChrisMag2099]

Since it's exclusive to high frequencies, my first guess would be that you're observing a change in the damper's response. Could be temperature related, either room temp or could be that you recorded the first time after playing several records and the damper had been heated up to a small degree, making it a bit more pliable. Try warming the room up several degrees, or, play several records beforehand and then re-test.

 

[board]

Since it's exclusive to high frequencies, my first guess would be that you're observing a change in the damper's response. Could be temperature related, either room temp or could be that you recorded the first time after playing several records and the damper had been heated up to a small degree, making it a bit more pliable. Try warming the room up several degrees, or, play several records beforehand and then re-test.

By "damper" do you mean the rubber suspension in the cartridge?
In any case, I will try your suggestion.

 

[ChrisMag2099]

By "damper" do you mean the rubber suspension in the cartridge?
In any case, I will try your suggestion.

Yes. Dampers aren't linear in their behavior. It's entirely possible for temp to change their damping characteristic.

 

[DSJR]

Back in the day, the OC9 model (bear with me), needed 1.7g for a few sides to 'bed the suspension in' and then 1.5g thereafter. No idea though if the current OC9 range (different tips but similar mechanics I gather) are the same.

The 33PTG was a favourite of the forums a few years back as it had mnore 'flesh' to the sound than the basically truthful but subjectively leaner OC9 tones (the ML tip may well have played a part here I subsequently discovered). I seem to remember the 33PTG having a slight response dip up top compared to the OC9 models which rise slightly if anything. I have no idea now but I wonder if you're now seeing the true response of the PTGs now they have a few hours on them.

Again a guess, but is the mean temperature at your new place different to your last one? Cartridge suspensions can be temperature sensitive, quite definitely so in some cases (such as my vintage B&O MMC20 models) - the colder it is, the duller the hf response gets..

 

[Thomas_A]

Speed change between moves?

 

[mhardy6647]

Occam's Razor guess: Time for a new stylus. They do wear out (especially if misaligned or at non-optimal VTF), as do the suspension elastomeric 'donuts' most MM/MI cartridges (at least) have. IME, the latter tend to harden, in some cases, with troublesome rapidity (maybe related to local air quality or maybe exposure to bright and/or UV light?).

Second guess: Check all tonearm figures of merit (so to speak): VTF, VTA, alignment, antiskate.

EDIT: Bronze-medal guess One or more iffy connections in the interconnect cables (e.g., solder connections to the plug(s)) causing a change in capacitance.

I'm grasping now.

 

[teched58]

Measure the resistance of each of the four tonearm wires, with one probe of your multimeter at the cartridge end of the tonearm wires and the other at the end of the cable coming out of the turntable.

Moving the TT could've caused any cold or iffy solder joints in the signal path to start acting up. (This type of intermittent problem I file under the "give the unit a big whack on the side to get it back into normal operation.")

 

[Thomas_A]

How are the spectrogram curves aligned? Are the songs exactly the same length from start to finish?

 

[board]

Speed change between moves?

If one recording is faster than another, it will show as small ripples throughout the orange line, not as a massive dip (unless, I suppose it's a massive speed difference, such as e.g. 33 to 45 rpm).

 

[board]

Occam's Razor guess: Time for a new stylus. They do wear out (especially if misaligned or at non-optimal VTF), as do the suspension elastomeric 'donuts' most MM/MI cartridges (at least) have. IME, the latter tend to harden, in some cases, with troublesome rapidity (maybe related to local air quality or maybe exposure to bright and/or UV light?).

Second guess: Check all tonearm figures of merit (so to speak): VTF, VTA, alignment, antiskate.

EDIT: Bronze-medal guess One or more iffy connections in the interconnect cables (e.g., solder connections to the plug(s)) causing a change in capacitance.

I'm grasping now.

As mentioned in my original post, I mounted an identical almost new cartridge on the turntable and had the same results.
As for capacitance, since this is a moving coil cartridge, as far as I know, it will essentially be immune to capacity changes.

 

[board]

Measure the resistance of each of the four tonearm wires, with one probe of your multimeter at the cartridge end of the tonearm wires and the other at the end of the cable coming out of the turntable.

Moving the TT could've caused any cold or iffy solder joints in the signal path to start acting up. (This type of intermittent problem I file under the "give the unit a big whack on the side to get it back into normal operation.")

As mentioned in my original post I don't have any measurement equipment.

 

[board]

So, I think I figured out what the issue was: temperature.
I tried turning up the heat quite a lot, and then play a few records, and now recording the same song as in my first chart and comparing it to the same, older recording done with the same cartridge, the difference now looks like this:

I also recorded another record, where it was the same result - a dip in the recording done in a cold room after moving compared to the the recording done before moving, but now the recording from a warm room is essentially identical to the recording done before moving.
I did, however, also unscrew the caps on the interconnects from the turntable, just to check, and everything looked fine, but it's possible that there was something wrong with them, although I doubt it.
I can try to make a recording another day in a cold room and see if the frequency response dips again.
So, I hope this means that the issue is resolved now.
But I must say that I was shocked that it was the temperature! I had never seen this issue before after several years of recording records.

 

[mhardy6647]

As mentioned in my original post, I mounted an identical almost new cartridge on the turntable and had the same results.
As for capacitance, since this is a moving coil cartridge, as far as I know, it will essentially be immune to capacity changes.

That is correct, sorry.

 

[Balle Clorin]

What kind of temperature levels / difference is an play here

 

[board]

What kind of temperature levels / difference is an play here

I can't say precisely, but it could be 7-8 degrees difference celsius.