Phono Frequency Response

I had a nagging to play vinyl on Saturday Night.  On top of some pile I can't remember buying I found Queen's Greatest Hits.  Having heard these same songs on digital, in some cases high resolution digital, this was different, having more presence, transparency, liquidity, and magic (and some noises).  But also, seemingly, less bass.

I think the less bass part is primarily a problem with this "Greatest Hits" pressing, which may not have the extended bass of the originals.  But also I suspected it could be my turntable system.  Often when there is less bass, we hear it as greater presence and transparency.

So I got out Hifi News and Record Review Analog Test LP, and played Track 7 Side 2 which is a sweep tone.  One of the very annoying things about this record is that each band is walled off from the others.  In order to play Track 7, you can't just cue to the space in between 6 and 7, because that just reverts back to 6.  You have to pass the wall at the beginning of Track 7, and cue precisely there.  It's hard to hit that point precisely, and when you do, the tonearm is still stabilizing at the beginning of the track.  That might, partly expaln the bass below 30 Hz in this recording shown here in Audacity.  There's also a matter of intermodulating with the tonearm and antiskate system resonances..

 The bump at the bottom where it more or less stabilizes is 30 Hz.  Up from there to the midrange level represents about a 1dB increase.  So it appears the bass below 100 Hz is gradually rolled off by 1dB at 30 Hz, then looks funky.  (The scale above is linear so it shows changes that look larger than they would on a logarithmic dB scale.)

I wouldn't think that 1dB loss would be that big a deal, I routinely make changes of that amount or more in the bass level, and in fact I tried adding 5dB (by changing the DAC output level from -15dB to -10dB) and the Greatest Hits recording still sounded like no bass, until the last track.

The instability below 30 Hz might be a problem, but possibly worse there is a peaky resonance just below 20khz which shouldn't be there.

Another measurement I made indirectly shows the instability below 30 Hz is not a matter of initial tonearm/table stabilization.  I happened to drop the needle at 30Hz instead of 20 Hz, and the reproduction is pretty clean, only showing the barest overlay of the needle drop stabilization, which disappears after 13 cycles of 30 Hz (just over 1/3 second).  That initial instability could also relate to the slight peak (small fraction of a dB) at 30Hz shown in the previous measurement.  This means that the instability shown above is caused by something else, presumably interaction with tonearm vertical and horizontal resonances that needs fixing for good bass below 30 Hz.  I'm showing higher magnification in the selected portion just after the needle drop (also shown) for clarity.

Digging through my files, I found a measurement with my Mitsubishi LT-3 linear tracking turntable, with damping added to tune the bass just as with the Linn (though it was an earlier job I didn't do as much or as well).  They both have a Dynavector 17D3 playing through an Emotive XSP-1.  I'm showing this somewhat aligned with the Linn result above.

The Mitsubishi is on top here.  I think it has slightly flatter bass below 120 Hz (perhaps 0.5dB loss down to 30 Hz vs 1.0dB for the Linn), it lacks a weird resonance in the Linn around 120 Hz, and the bass extends possibly a tad farther before it becomes unstable below 30 Hz--but they are so little different in that regard it's hard to tell, in both cases the bass becomes unstable just below 30 Hz.  They both seem to have a resonance just below 20kHz, I'm wondering if that's coming from the cartridge body or the screws.

I wonder if the resonance around 120 Hz in the Linn is the infamous flexible subchassis resonance and "flabby midbass."  From the day the LP12 was introduced people wondered why they were getting such a thin and flimsy subchassis with an expensive turntable, when even much cheaper turntables like the AR Turntable had very heavy gauge metal.  HP of Absolute Sound denounced both the subchassis resonance and the flabby midbass in his first review of the LP12.

Famously Linn doesn't have a subchassis upgrade except for the very expensive Cirkus bearing.  I figure this is because the original bearing has a noise that is absorbed by the flimsy subchassis.  The expensive Cirkus bearing eliminates that resonance, so you can even have specially cast and machined subchassis with it.  Such were never offered for the original bearing (except by outside vendors...and they were not universally loved).

The flimsy subchassis could have eliminated a bearing noise.  But the price may be that funny ripple in the frequency response around 120 Hz.  Remember the cartridges are the same model.

Although the Mitsubishi turntable yields better frequency response, I don't think it sounds as good.  It has larger and more undamped arm/cartridge resonances and/or motor and bearing noises.  The movement of the linear tracking tonearm subtly affects tonality.  I could possibly fix the larger arm/cartridge resonances, with the application of more damping material, and the electronic motor is probably due for a tune-up.  My Linn Valhalla board has been replaced at least twice, the Mitsubishi has it's original electronics and it's a wonder they're even still working.