Characterization of a MMC 6000 Cartridge Restored by Tonabnehmerservice.de in Comparison to Two Good Sounding Original Cartridges

I recently sold a fully restored Beogram 4002 (Type 5503 with AC platter motor) to a customer in California. The customer also wanted a newly rebuilt cartridge. He settled on getting a restored MMC 6000 from tonabnehmerservice.de. Franz Josef at tonabnehmerservice has provided a number of cartridges to some of my customers over the years, and they all seem pretty happy with his work. This particular cartridge is one of his top-of-the-line restorations with a beryllium cantilever and a nude Shibata diamond. 

In this case, my customer asked Franz Josef to send me the cartridge directly so I could check it out before sending it along with the restored Beogram. I received the cartridge last week. It arrived in a little box fitted with a proper plexiglass protector:

I carefully extracted it:

Looks very nice! This shows the beauty playing on the restored Beogram:

The test setup for cartridge characterization is simple. All I had to do was plug the Beogram DIN5 output plug into proper adapters to interface it with the BNC inputs of my new QuantAsylum QA403 audio analyzer. All measurements were done at 1.2g tracking weight (calibrated with a digital gauge).

A few years back I wrote a couple blog posts about this type of measurement. This one discusses the frequency response measurement and this one the 'trackability' measurement.

Let's focus on the frequency response first:

There are several ways to do it. In the old days, they used test tracks that actually played a frequency ramp from 20 to 20000 Hz. The cartridge was hooked up to an AC voltmeter, and a voltage vs. time plotter recorded the signal as the test track played. Add a proper scale to the time axis that translates the time it took to play the track into the proper frequency scale, and the frequency response becomes visible. 

These days we can play a noise track and use an audio analyzer to record a Fast Fourier Transform Spectrum (FFT). A FFT spectrum shows the volume of all individual frequency components of the measured audio signal on a frequency axis. A clean 1kHz test tone would, for example, show up as an isolated peak at 1kHz. A noise signal would look more like a line parallel to the frequency axis since noise is a composition of many frequencies.

I used Tracks 2 and 3 on Side 2 of the Analogue Productions "Ultimate Analogue Test LP" as noise tracks and played them directly into the QA403 analyzer. 

Tracks 2 and 3 are "Pink Noise" tracks: Unlike white noise, pink noise distributes its energy equally per octave rather than per individual frequency. Because of this, its Fourier transform (FFT) is not ideally a horizontal line along the frequency axis, but rather a downward-sloping line that drops at exactly -3 dB per octave (which translates to a -10 dB drop per decade). In plain English: pink noise emphasizes the lower frequencies and gradually reduces the volume towards the higher frequencies contained in the noise. White noise features all frequencies at the same level, i.e., would be a parallel line to the frequency axis.

Now let’s go from the theoretical construct of pink noise to its real embodiment on a test record. Records are usually cut with RIAA pre-emphasis, which superimposes an upward tilt of roughly +6 dB per octave (or about +20 dB per decade) to the signal. If you go to the link, you will see that the RIAA curve is not a line, but has some wiggles in it. That is a consequence of the fact that in real life electronic filters are used to produce the pre-emphasis. But for simplicity's sake, let's assume a +20dB/decade line.

Therefore, if a pink noise test track is played back from a record without using RIAA de-emphasis, it has the RIAA pre-emphasis superimposed on the original pink noise signal. In practical terms, this means that the -10 dB drop of the pink noise signal is overcompensated by the ~ +20 dB RIAA signal increase per decade. This results in a measured FFT spectrum that should roughly slope upward at about +10 dB/decade.

At this point we did not consider the influence of the electromechanical characteristics of the cutting and playback processes on the signal. The good news is that in a first-order approximation we do not have to! To understand why, consider the ‘constant velocity characteristic’ of the cutting and reproduction processes by the cutting machine and the cartridge. Both have cantilevers that end in iron pieces inside coils. On the cutting side, signal voltage changes fed into the coils translate into cantilever movements cutting the groove, while the cartridge does the exact opposite: It translates the 'wiggles’ in the groove back into a voltage signal at the coil leads. So in a first-order approximation, we can say that the influences on the signal by the cutting machine and the cartridge basically cancel out! Therefore, for understanding and discussing the measured FFT curves on the test track, we only need to consider the pink noise characteristic and the RIAA pre-emphasis (since we plug the cartridge signal directly into the QA403 without RIAA de-emphasis).

Let's have a look at what I measured:

This graph shows the frequency response of the restored MMC6000 cartridge measured on Track 2 (lateral pink noise):

I tried to 'graphically evaluate' the slope of the spectra and found maybe +9dB between 100 Hz and 10000 Hz, less than the above estimate of +10 dB per decade. This result probably has a bit of an error bar on it due to the superimposed 'RIAA pre-emphasis wiggle'. I think this probably means that the pink noise tracks do not precisely follow the theoretical -10 dB/decade energy distribution across the bandwidth. This is supported by the fact that I see the same slopes for the other cartridges I measured.

Nice is that both channels match very well. The two peaks at the bottom end of the spectrum at 60 Hz and 120 Hz are interference from the power grid. Not surprising considering the very small signals coming from the cartridge. The sharp drop-off after 20,000 Hz is probably caused by the pink noise signal itself. It probably only contains frequencies up to 20,000 Hz. Sadly, there is no detailed information about the Analogue test record available, and my emails have not been answered so far. I guess analog audio is only analog if there is a bit of woodoo in the signal path!...;-)

The next measurement I did with this cartridge was on Track 3 (horizontal pink noise):

This spectrum looks pretty similar, but there is an approximately 5 dB difference between the channels between 15,000 and 20,000 Hz. This difference means that the left channel will reproduce very high frequencies at a slightly lower volume. In the bigger scheme of things, 5 dB is not a very big difference. I certainly did not notice it when I listened to records with this cartridge. 

This is not surprising for several reasons. The most important one is probably that the 62-year-old Beolover likely has a significant roll-off at around 15,000 Hz and does not hear much of the frequencies under consideration here in the first place.

Furthermore, music has only very little information in this frequency range; only the sizzle of cymbals and similar noises are there. We also need to realize that the vertical signal contains the stereo localization (L-R), while the lateral (horizontal) signal contains the mono information (L+R). So a bit of difference in the vertical signal only affects the localization of instruments on the sound stage. Since the human brain does the locating mainly with mid-range frequencies, a small difference at high frequencies is not very significant for this process.

On to the next cartridge: A Beogram 4002 (5523) that I recently restored for a customer in Louisiana came with its original MMC4000 cartridge. I played it a bit with the restored unit, and I thought it still sounded very good. Here is Track 2 played by this cartridge:

and Track 3:

Very similar spectra compared to the MMC 6000. Left and right are in good agreement in both directions.

Next, I measured my favorite MMC20 EN that I use for daily listening in my workshop. These are the measurements:

First Track 2:

and Track 3:

These curves also look very similar to the previous curves, but here both directions have a small difference between the channels. Here, the left channel is a tad stronger than the left in both directions.

In my opinion, this cartridge also sounds very good.

On to the trackability measurements:

I use a Clearaudio "Trackability Test Record". This record has tracks of a 333 Hz test tone at different volumes. They give the 'volume' directly as lateral deflection, i.e. the physical dimension of the 'wiggles' superimposed on the groove. The tracks start at 50 um lateral (i.e. mono) deflection, and then they go to 100 um in 10 um steps. To put this into perspective: Loud dance music tracks have maximum amplitudes around 70 um. So if a cartridge can track 70-80 um without too much distortion, it is pretty good and can play most music ever pressed on vinyl.

So I played all the tracks with each of the three cartridges and noted the total harmonic distortion (THD). THD is essentially the ratio between the fundamental peak and its harmonics. As an example, these L/R spectra were measured on the MMC6000 playing the 50 um track:   

You can see that the THD estimated from the ratio of the second harmonic to the fundamental peak is less than 3%. After measuring six sets of spectra like the above for the three cartridges for each of the test tracks, I ended up with this plot of THD vs. amplitude:

All three cartridges track fairly well up to 70 um. At larger amplitudes, the original cartridges begin to distort fairly strongly. The restored MMC 6000, on the other hand, is considerably better. It has the lowest distortion across the entire range and could even reproduce 100 um signals with tolerable distortion.

So the conclusion of these measurements is maybe that 'good-sounding' original cartridges are still good enough for everyday listening, but that for audiophile listening experiences a restored cartridge should be procured. My money is on tonabnehmer.de. I think Franz Josef does a very solid job, and you can send him your original cartridges for restoration without hesitation!

MMC Cartridge Box: Now Available on Shapeways

What better way to celebrate my 400th beolover post with making the Beolover MMC Cartridge Box available on Shapeways?? It is time for adequate protection for our valuable cartridges!

It was a lengthy and difficult process to get this part designed and I had to go through many design iterations to get the 'feel' right. I desired to have a solid press-fit between the cartridge and the 'base' of the box to afford maximum protection to the cartridge in case someone accidentally drops the base after inserting a cartridge. The top on the other hand was supposed to fit with some, but not too much friction to enable easy access. Some manufacturing tolerances at Shapeways did not help this process either, and so it took me many design iterations and test prints to get to a point where the print outcomes between different print-runs were satisfactory.

So here we go: This shows the bases with cartridges inserted and the matching tops:

And with closed tops:

And why not get base and top in a duo-tone combination??:

There are 49 color combinations possible as they are available in seven colors...check out the parts at my Shapeways store!

Or use these direct links below for ordering. You can select the color during checkout. It usually takes about two weeks to get parts from Shapeways in most parts of the world.

MMC20 Cartridge Mount Replacement: Second Design Iteration

This is a follow-up to my initial post about the design of a MMC20 cartridge mount replacement. In the meantime, I acquired a Beogram 4000 with a broken off mount, so now this is 'personal'...;-). Ouch!:

I just received the redesigned Version 2 prints from Shapeways. The main issue with the initial version was that the back part would not fit on the lower part of the original mount that will be reused, and that the contact tab would make intermittent contact. I realized that the issue was the 'bump' that I designed into the tab to press the contacts into the cartridge contacts. One I looked at the broken off tab of my Beogram 4000, I saw that a simple straight shape should work. So version two of my design aspires to closely resemble the 4000 version of it.

Here are a few impressions. Along with the changed shape, I also needed to redesign the tool bodies that are used for pressing the circuit board into the mount while the glue dries:

Hers is a picture of the curing process:

And here the result in comparison with the original mount (this one is from a later Beogram and was supplied by Sonavor (Beoworld)):

Then I tested it on a MMC20S cartridge. It made good Ohmic contact and I measured the usual 770 Ohms per coil:

This iteration now fits very well on the lower (original) part of the assembly:

Consequently, the part also fits into the arm tube:

However, the mounting angle is a bit too steep. From the gap, using a protractor, I deduced that I need to reduce the angle by about 4 degrees. Oh well, back to Autodesk Inventor for Version 3.  While I will be waiting for the Version 3 prints I also need to figure out how to plate the PCB traces with gold to ensure long term stability. We do not want to have oxide layers getting in-between our precious vinyls and our ears...;-)

Designing a new part: Quite a laborious process - A resuscitated Beogram 4000: Priceless! This is Beolove!

MMC Cartridge Box: Another Iteration

This is a follow up to my recent post about the design of MMC20 and MMC4000/6000 cartridge storage boxes. I just received the next iteration from Shapeways. I changed the lettering on the covers and also designed a cover with a view port to directly see what type cartridge is inside. Here are a couple of impressions:

I also made the covers a tad thicker in the hope to get a tighter fit, but now they are a bit too tight for my taste. So, I guess the next iteration may be the final one...;-)

MMC Cartridge Box: A First Impression

I accumulated a few MMC20 and MMC4000/6000 cartridges over the years, and I only have a couple protective covers. I always wanted a better solution for keeping them safe. Protecting them definitely makes sense considering the prices they fetch these days in good condition. So I set out to design a 3D printable small box. First I took exact measurements of a MMC20 cartridge and built a model in Inventor. Then I created the lower part of a box moulded to the metal housing of the cartridge and a cover.

Yesterday I received the third version of the design from Shapeways, which finally seems to fit the cartridge perfectly, and also has enough friction between the cover to give a nice open-close feel. The issue with 3D printing really is that every technology and material behaves differently, while having specific tolerances and 'quirks'. So it takes a bit of trial and error to get a working product from a CAD design. Well, here are a few impressions:

This shows the empty container. The bottom part shows the characteristic slant of the bottom part of the cartridge. The front part is cut out deeply to make sure that the cantilever and tip never touch anything:

Pretty amazing the resolution that Shapeways can achieve. The slightly grainy surface is a result of their Selective Laser Sintering (SLS) process that blasts the plastic granules in the printing bed to fuse them into a solid body. 

And here with inserted MMC20CL cartridge. The cartridge body makes a nice smooth press-fit with the part to ensure protection even if turned upside down:

And closed. The cover shows the cartridge type on top:

I think I will redesign that a bit with bigger lettering. I will make a specific cover for each MMC20 and the MMC4000 and MMC6000 models that one can easily find the desired one. Shapeways also offers a bunch of nice colors, i.e. color coding will be possible, too.

Stay tuned for the final version...They will be offered at the Beolover Shapeway store.