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Sunday, April 2, 2017

Testing B&O MMC Cartridges with a QuantAsylum QA400 (Pt.2): Trackability

After making my first post about testing vintage MMC cartridges with a QA400 audio analyzer and some test records, I was advised by another B&O enthusiast that measuring the frequency range of cartridges is only half of the story (at best). The second important test is 'trackability', which tests how well a cartridge can reproduce signals at different amplitudes (='audible volume'), and at what volume it starts significantly distorting the signal. The crucial parameter in such measurements is the "Total Harmonic Distortion (THD)" that is superimposed on a principal test signal depending on the total deflection amplitude of the cantilever. The deflection amplitude becomes larger at higher audio volumes: The grooves 'wiggle' more strongly and so the cantilever has to make larger movements to convert the higher volume into electrical signals. This stresses the suspension of the cantilever more and the elasticity of the suspension becomes crucial for faithful conversion of the mechanical motion into the electrical signal. A hardened or otherwise damaged suspension will counteract the deflection and this causes distortion.

Yet another test record is needed for such trackability measurements: I used the "Trackability Test Record" from clearaudio for the measurements discussed here.

The test setup was a recently restored and properly adjusted Beogram 4002 (5513). The DIN5 cable of the turntable was directly fed into the RCA inputs of the QA400 analyzer using a DIN-to-RCA adapter, i.e. there was no RIAA amplifier in between. The sensitivity of the QA400 is vastly better than what is needed to measure the MMC signal, which is typically in the <10mV amplitude range.

The first experiment I did was with the MMC20CL cartridge that was also used in the initial post about the frequency range. This graph shows the spectra measured while playing the 60um and 80um trackability tracks of the test record:

These spectra show a Fourier transform of the signal that comes out of the cartridge. In essence the signal is split into its individual frequency components. The intensity of the peak is proportional to the amplitude of the particular component frequency. The left-most peak is the principal test frequency of 333 Hz. The peaks emerging to the right of this main peak are the harmonic distortion  (HD) peaks that are superimposed over non-harmonic noise. The blue spectra are from the 60um track and the red ones from the 80um track. One can see that the harmonic distortions become more pronounced at 80um compared to 60um. 

If you think that this (pretty nice sounding!) cartridge has a lot of distortion, even at 60 um, consider that the dBV scale is logarithmic. This means that a 6 dB amplitude difference on this scale represent a factor 2x in volume. So if you compare the principal peak with the first HD peak the scale indicates that there is about a difference of -30 dBV. This corresponds to a volume difference of 2^5=32, i.e. the first HD peak is 32x weaker than the main signal. This corresponds to a THD value of about 3%. The subsequent HD peaks are even weaker, adding only insignificantly to the THD value. Considering that the human ear can detect harmonic distortion at about 5% this measurement indicates that a 60um signal could still be enjoyed with this cartridge.

An interesting question is: What is a 60 um signal in the context of actual volumes occurring on normal records? An answer to this question is (like with most things analog) a bit complicated and does not have a 100% definite answer. An indication of the amplitude of a so called "0 VU" signal (i.e. the highest signal amplitude that is found on standard vinyl records) is given on the Trackability Test Record itself: Track 3 on side 1 is supposedly a 1kHz "standard maximum" mono signal. My measurements on this track yielded a 7 mV signal amplitude (~ -43 dBV) for the above MMC20CL cartridge. Note that "The Ultimate Analogue Test LP" that was used in the preceding post has a slightly different opinion about what a 0 VU signal constitutes: My earlier measurements with this MMC20CL cartridge on its 0 VU track yielded a signal of only 4 mV. So you see that this is not an exact science. And of course any avid vinyl listener will know that there is quite a bit of variance in how loud a particular vinyl is, i.e. different labels may always have used different 0 VU specs. 

Anyway, let's go with the 7 mV level as 0 VU signal for now:

In comparison, my measurements on the 333 Hz 60 um and 80 um tracks yielded signal amplitudes of 7.5 mV and 10 mV, respectively. Here we need to consider that the frequency spectrum is encoded on records with RIAA emphasis, which means that a 333 Hz signal that had the same volume than a 1 kHz signal when it was recorded should yield an approximately 50% smaller signal amplitude when played back without RIAA de-emphasis. Hence, since the amplitude measured on the 60 um track was about the same as the 1 kHz 0VU amplitude, we can hypothesize that the 60 um track corresponds approximately to a +6 dBV signal (i.e. a signal that is about 2x of the usual standard maximum signal found on records after RIAA deemphasis), so this track is a bit of a stress test already. In comparison, the 80 um track yielded a 10 mV (maybe a +9 dBV) signal.  I also measured the 'loudest' 100 um track, which yielded 13 mV, i.e. would corresponds to a +12 dBV signal.

The liner notes of the Trackability Test Record state that cartridges that can reproduce the 80 um track without too much distortion can be considered very good. So the bottom line for my vintage MMC20CL is that it is still o.k. and that it can reproduce 0VU levels without much noticeable distortion, but that it is probably not that fresh either. 

Of course we do not know how they measured these values back then, i.e. a direct comparison may not be appropriate, but we can glean from the report that "trackability 300Hz lateral +9 dB" came in with a 0.38% distortion when they did their measurements. If the above estimations are correct, this measurement might approximately compare to my measurement on the 80 um track. That we are at least in the right ball park with comparing these dBV levels is supported by the stated 1 kHz sensitivity number of 0.85mV per cm/s lateral deflection velocity. The Trackability Test Record gives a 8 cm/s velocity for its 1 kHz track, i.e. the output of the cartridge should be 6.8mV, which is close to the 7 mV value I saw in my measurements. 
I guess it is time to get a rebuilt MMC20CL cartridge from tonabnehmerservice.de and do this measurement again to see what a new suspension can do for the cantilever's ability to track....more Beoloving excitement in my future.

After measuring my MMC20CL, I also measured the MMC4000, MMC6000 and MMC20EN cartridges that were featured in my first post about the frequency range measurements. These are the values I measured for these three cartridges in comparison with the MMC20CL:

The x-axis starts out with the 1 kHz measurement on the left. The points to the right are the THD values measured on the Trackability Test Record 50 um to 100 um tracks. All cartridges show a significant THD uptick after the 60 um track. This is to be expected.

The single green trace at the top of the graph is from the MMC4000 cartridge, which apparently has a right channel that is in trouble. Even at 1 kHz and the 50 um track it has more than 25% THD. It definitely needs a rebuild. The other cartridges seem to be in a 'usable' condition since they are below 5% THD even on the 60 um track. It is remarkable that this cartridge did not show any misbehavior during the frequency range test. This is a clear indication that trackability is probably the more important measurement for determining if vintage cartridges are still performing adequately.

A remarkable feature is that for all cartridges the THD increases more strongly on the right channels than on the left channels. I immediately thought there is an issue with the equipment, and so I tried the measurements on two other Beograms that were in fully restored condition. And the result did not change. One hypothesis for explaining this phenomenon may be that the tracks are curved, i.e. they have an outer and an inner edge which contain the information for one or the other channel...but that is a topic for another post. The good news is that at or below 0 VU this difference is not significant and it only plays a role at very high amplitudes.
Allright, time to put the working cartridges to some exciting listening use instead of boring them with 333 Hz and 1 kHz test tones....;-)

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