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Sunday, November 12, 2017

Bang&Olufsen MMC20EN Vintage Cartridge Test

The Beogram 4004 parts that I recently restored came with a MMC20EN cartridge, which was to be tested.

There are two preceding posts about my (still evolving - Beolove never ends..;-) approach to cartridge testing. One outlines the test of the frequency range of a cartridge and the other discusses testing the 'trackability', i.e. how well can the cantilever track deflections in the groves of the record. This essentially tests the mechanical aspects of the cartridge, most notably the state of the cantilever suspension.

I currently use three test records:
1) The Analogue Audio Ultimate Test LP. This record has a pink noise track that is perfect for checking the frequency response of the cartridge in the audible range. Unfortunately, this track tops out at 20kHz, and so we need to use a different record to see if the cartridge can also reproduce higher frequencies.
2)  The Fisher Test & Performance Standard Quadradisc: This record has some CD-4 quadraphonic tracks which feature the 30 kHz carrier frequency that is needed to encode the two back channels. The test tracks have this carrier without any encoded sound, which is a great way to see if the cartridge can reproduce 30kHz.
3) The Clearaudio Trackability Test Record: This record is used to determine the condition of the cantilever suspension. It has 333 Hz tracks with different groove amplitudes from 50 um to 100 um.
This allows a measurement of how well the cantilever can deflect and at what deflections significant distortions occur that would negatively affect the sound quality.

Aside from these test records an audio analyzer with the capability to calculate a Fourier transform of the voltage signal coming from the cartridge is required. I use the QuantAsylum QA400 (now replaced by the QA401), which (aside from a somewhat lackluster user interface) is a very capable device that has a high sensitivity enabling plugging the turntable directly into its input without needing amplification. This allows analyzing the signal from the cartridge directly. I simply use a DIN to RCA adapter and connect the DIN5 of a Beogram directly to the analyzer input.

These are the results measured on the MMC20EN cartridge (at a tracking weight of 1.2g):

This is the 20 Hz to 20 kHz frequency response of the cartridge as measured with the pink noise track:

And this is the CD-4 carrier signal (sharp peak at 30 kHz):
The amplitude of this peak is supposed to be 1/10 (or -20 dBV) of the 0 VU 1 kHz signal, which is usually at about -47 to -50 dBV. The peak measured in the graph is at about -70 dBV, which is about where it should be. So far so good!

These are the results of the trackability test:

I used the 50, 60 and 70 um 333Hz test signals on the Trackability record. The 50 um track corresponds approximately to a 0VU signal, which is normally the maximum amplitude encountered on a standard record. The 60 um amplitude level approximately corresponds to a +6 dBV signal level. I measured these THD percentages:

50um left:     1.8%
50um right:   3.5%
60um left:     2.8%
60um right:   10%
70um left:      4.9%
70um right:    20%

Since THD levels up to 5% go unnoticed by most human listeners, this cartridge can still be used for records that adhere to standard amplitude levels, but at +6dB amplitudes distortions will be audible on the right channel...

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