Beogram 4004: Characterizing a MMC20EN Cartridge with an Analogue Productions Test LP

I recently bought 'The Ultimate Analogue Test LP' from Analogue Productions. I felt I needed to be able to characterize the output of my cartridges and I read about this LP. It is a heavy duty 180g high-quality vinyl that has all the relevant test tracks for basic measurements.

I connected the oscilloscope to the DIN5 connector via a DIN-to-RCA breakout and two RCA-BNC adapters. This allowed me to form a shielded signal path from the output of the Beogram 4004 to the oscilloscope. However, the measurement was still pretty noisy, so I resorted to using a low pass coupling for the trigger and averaging. This gave me usable traces. Here is a shot of the oscilloscope screen on the 1 kHz test track:

It is obvious that the MMC20EN cartridge that came with my Beogram 4002 puts out a well balanced signal on both channels. The peak to peak amplitude of the 1 kHz signal is about 15 mV. This also seems to be a pretty good way to calibrate the RPM of the platter...

Here is a shot of the 100 Hz track (timebase is 10 ms/sq):

As expected the amplitude is considerably lower (maybe 4 mV without the noise) than for the 1 kHz signal due to the RIAA pre-emphasis. This is necessary at low frequencies to keep the groove width reasonable (Faraday's law prescribes that the induced EMF is proportional to the change of the magnetic field, which in turn is proportional to the velocity of the moving magnet, iron or coil in a cartridge). I found an excellent summary that explains the background behind the RIAA curve and the historic path towards it. The article is by Gary A. Galo and can be found here.

In short, the RIAA curve lowers the amplitude progressively towards lower frequencies to reduce the groove width (otherwise long playing records would not be possible). At high frequencies the amplitude is increased to stay ahead of the noise, which, due to its high frequency content is an effective EMF generator. Hence the signal amplitude needs to be a few magnitudes larger than the noise amplitude, that in the amplifier signal + noise can be reduced together, thereby reducing the noise relative to the original signal level. This is a similar approach as is used in Dolby noise reduction systems. 

The 10 kHz track on the Analogue Productions LP produces a standardized -20 dB amplitude (i.e. 10x smaller than the other two tracks, which are at 0 dB. 0 dB apparently corresponds to a 7 cm/s stylus velocity, as suggested on the LP sleeve. Here is the (pretty noisy, disregard the Freq measurement indicator) measurement (timebase is 100 us/sq):

Without the noise the amplitude might be about 10 mV or so, i.e. it were at about 100 mV at 0 dB. That would give us about a factor 7 relative to the 1 kHz signal. 20*log(7)=16.9 dB, which, considering the noisy measurement, is reasonably close to the RIAA curve (from stereophile.com, the blue curve is the curve used on records, the red one is an idealized 6 dB/octave curve, which would be best, but cannot be implemented due to practical reasons), which suggests a pre-emphasis of about 13 dB at 10 kHz:

The 100 Hz measured amplitude above is also reasonably close to the curve: 20*log(4/15)= -11.5 dB (compared to about -13 on the curve....

All in all another interesting evening at Beolover's lab...;-)

Beogram 4002: Signal Path between DIN5 Plug and Cartridge, Plug Replacement

I finally wanted to try out the Beogram 4002 that I am getting ready for the 21st century. While everything seems to work now, the DIN 5 plug looked really bad. Rusty and bent...so I decided to replace it with a modern quality plug (Neutrik AG Rean NYS 322G) with gold plated pins to ensure lasting contact at my Beomaster 6000 Aux input. Here is a pic of the old plug...pretty ugly! Not sure what the previous owner was up to...:

Before I soldered the new plug on, I noted the signal path down to ensure that I would make the right connections after cutting off the old plug. Here is a schematic of my measurements (note that the cartridge connectors are shown on a mirror that I held below the arm):

It is interesting to note that the cartridge is essentially directly connected to the amplifier with the recommended 'star arrangement' ground (all ground connections come together at one central point and loops are avoided). The Beogram has its ground connected to the chassis, which could cause a ground loop if it were connected to the phono preamp ground and there were a conductive connection between the Beogram chassis and earth (usually the Beogram chassis floats due to the isolating feet and the absence of a earth connector at the standard two-prong power entry). The shield of the cable, however, is connected to the chassis, and will make a direct connection to the amplifier chassis, i.e. in the end the Beogram ground is connected to the amplifier ground. But this should be o.k due to the star-arrangement, even if there is a potential difference between the two units. And indeed, I was not able to discern any hum from the speakers after hooking the Beogram up.

Another interesting detail is that the back channel connections are fed to the plug, even in a 2-channel Beogram 4002 or 4004. I guess, when these decks were new, the CD4 upgrade was available for an easy plug-in at the Bang & Olufsen dealerships. Sometimes, I wish I had a time machine!...;-)

Here are a few pics from the plug replacement procedure. This was actually not trivial due to the heavy duty double shielded cable, which required a few sweat producing manipulations of the 22 gauge leads that carry the signal, while soldering on the heavy duty shield and signal GND connection:

And here the final result: 

Just hooked the Beogram to my Beomaster 6000, and it sounds beautiful! Andrew Hill: Dance with Death on an original 1970s Blue Note pressing. Fantastic!