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Friday, April 21, 2017

Beogram 6000 (5512): Restoration and Characterization of CD-4 RIAA Preamplifier

The most interesting part of the restoration of the Beogram 6000 that is on my bench right now was  working on the 4-channel CD-4 preamplifier board. This board renders the 6000 capable of reproducing CD-4 quadraphonic vinyls from the 70s. While there are not so many CD-4 vinyls that were cut for this short-lived format, this preamplifier board is also a veritable stereo preamp, which is very convenient if the Beogram is to be used with a modern amplifier (who often do not have Phono input stages anymore). The CD-4 board allows connecting the Beogram to any standard high level input like a DVD or AUX input.

The first step was to replace all the electrolytic capacitors on this board. This shows the board in the turntable after taking the keypad out:
I removed the board
and then replaced the capacitors and the indicator light bulb:
The light bulb can be replaced with a standard red LED and a 1k resistor:
This is how the LED peeks out through the cutout in the keypad that permits its light into the CD-4 indicator:
This shows the CD-4 preamp in action playing my The Fisher CD-4 test record (my only CD-4 record):

The more interesting part here is however: How well does this preamp perform for listening to stereo records. The most important items here are how much noise is added and how faithfully does it deemphasize the RIAA curve. I connected the Beogram DIN5 to my QA400 audio analyzer and measured a noise spectrum:
These curves were measured for the left and right channels. They look quite identical. They were measured with a cartridge installed and the turntable running with the arm lowered next to the platter. 

This made sure that the measurement was performed with the motor running to see if there is any crosstalk through the power supply of the Beogram from the motor (there is not) and that the preamp was actually amplifying the signal from the cartridge. When the arm is up the signal in the preamp is grounded, i.e. one only sees the noise from the amplifier itself. This is a boring measurement, which yielded for this CD-4 preamp a flat line at about -130dBV. This number corresponds to the noise numbers that are often given for external phono preamps that are sold for considerable amounts of money. -130dBV is a big negative number and impressive. However, it is meaningless for any practical use of a phono preamp. The measurement shown above is much more crucial, since it gives the noise floor defined by the cartridge, which is what you hear when a record is played.

What we see from the graph is that the 1kHz noise is about -110dBV. Considering that a 0VU level  is at about -20dBV, we can say that with this amplifier and a cartridge we have about 90dBV maximum signal-to-noise. What does this mean in practical terms? 20dBV correspond to a 10x difference in the amplitude of the signal, i.e. 90dB means that the noise contributed by the cartridge is less than 1/1000th of the audio signal at the highest level. 

You may wonder why I stopped writing about the amplifier noise and just mention the cartridge. Well, the amplifier noise is -130dB, i.e. it is another factor 10 smaller than the cartridge noise, i.e. pretty irrelevant at this point. 

An interesting question is: Why does the cartridge make most of the noise? (actually not-the vinyl surface is even more noisy than anything 'electronic' I am discussing here...see my discussion of the BeoloverRIAA internal amplifier for the Beogram 4002
After all it is a passive component that has no active (powered) electronic components, except one coil per channel that picks up the signal from the moving magnet connected to the needle. Here is where physics comes in: All conductors generate Johnson-Nyquist noise, which is generated by thermal movement of electrons in the conductor. This movement is random in direction, i.e. the electrons move forth and back at high speed through the wires of the cartridge coils and the connecting leads, which generates a small fluctuating current that is permanently fed into the amplifier input, hence one can hear a bit of hiss (white noise) even if the needle does not touch the platter.

One more interesting question: Why is the noise in the above spectrum higher at low frequencies than at higher frequencies? The answer is: This is a direct consequence of the RIAA deemphasis of the amplifier. Records are recorded in a way that low frequencies are engraved at a lower amplitude than higher frequencies. The reason is that lower frequencies need larger 'wiggles' in the groove to generate the same acceleration of the magnets (=mV output signal from the cartridge) like higher frequencies at the same audio volume. This trick allows to squeeze the grooves closer together and more music can be put on a side of a record. This shows the theoretical RIAA curve (from wikipedia):
The red curve is the playback curve. So if we have a flat noise spectrum (white noise) coming from the cartridge, the spectrum should drop by about -40dBV across the 20Hz-to-20kHz range. And that is what we see in the above measurement. The drop is about -36dBV, i.e. the RIAA deemphasis of the CD-4 board is slightly off from the theoretical curve from wikipedia. This difference, however, is pretty irrelevant in practical terms since a) -4 dBV it is hardly discernible when listening to music, and b) the RIAA emphasis (blue curve) of records from different labels are all somewhat different, too, i.e. they use different RIAA curves to begin with.
One last point: I just assumed that the noise coming from the cartridge is 'white', i.e. flat. Is this really the case? Yes, it appears so. Thermal noise is constant per frequency up to the GHz range:
This graph was taken from this interesting webpage, where thermal noise is discussed in some detail. This means that, in absence of a white noise generator, using a cartridge connected to the pre-amp seems to be a pretty decent way for measuring the quality of the RIAA deemphasis of an amplifier. 

Another interesting point here is that the thermal noise level depends on the resistance of the conductor that produces it. Hence, shorting the input of an amplifier connects essentially 0 Ohms, which kills most of the noise at the input, and one then measures only the noise generated within the amplifier.

Allright...after this little excursion into the land of physics, it is time to put this Beogram back together, do some final adjustments and then finally enjoy some lovely vinyl! This time through the line-level Phono4 input of my Beomaster 6000. Exciting prospects!








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