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!

Beomaster 8000: Evaluating the Audio Performance

This Beomaster 8000 receiver has been playing music in my lab since I finished the recap and output amplifier adjustments. The sound is great as it always is with these receivers but human ears are subjective and we like to measure some key values with test equipment to have some actual numbers to compare with. For amplifiers we typically use a sine wave test input voltage and measure the output of a preamplifier or power amplifier to check the total harmonic distortion (THD) and sound to noise ratio (SNR).

For this Beomaster I set my test up to check some input voltages at some key frequencies and measure the output amplifier at the speaker outputs.

For the speaker output measurement I connected a pair of fixed 8Ω loads. Obviously the fixed resistive load isn't exactly like a real speaker system load but by using the fixed 8Ω load I should always have a good measurement value I can compare other amplifier results to.

Here are my dummy speaker loads. They are power resistors connected to a very large heat sink. When I drive the output amplifier up to its rated power output (100 Watts into and 8Ω load in the case of the Beomaster 8000) the load resistors will get quite warm.

I am going to measure the THD and SNR values with my QuantAsylum QA400 audio analyzer. Since the high speaker output voltages would damage the analyzer inputs I use a low noise differential probe to lower the signal to a level the analyzer will accept.

This picture shows the Beomaster 8000 left and right speaker outputs connect to the dummy 8Ω resistor loads where the differential probe is attached and the signal connected to the QA400 inputs.

For the test inputs I want to use a constant sine wave of 1Vrms at frequencies from 20Hz to 20KHz.
I use the signal generator of an HP8903 audio analyzer for the test input signal and check it with an oscilloscope as I route the test signal to the Beomaster 8000 Tape 1 (TP1) inputs.

I started with a 1KHz signal as that is a common frequency for a lot of the audio specs manufacturers publish. It is also in the middle of the frequency range on the log scale.

It should be noted that before starting the measurements I adjusted the Beomaster 8000 source input levels to set the left and right channel levels as close as possible to each other. Those input level adjustments are on the left side of the Beomaster cabinet.

Using the Beomaster volume control I increased the volume of TP1 while monitoring the output with the QA400 analyzer and the Beomaster clipping lamp.

With the 1Vrms input signal I got to the clipping point when the Beomaster reached 5.9 on the volume indicator. I backed off that volume to 5.8 and measured between 28Vrms and 29Vrms at the speaker load. That corresponds to about 100 Watts of output.

Here is what the QA400 analyzer measured.

The THD levels are very good. Both channels are comfortably less that 0.05%.
For the SNR value I notice that B&O specifies their Beomaster 8000 SNR value as "A Weighted" and should be >77dB for the Tape input. The frequency and output level isn't specified but I am doing my measurements at the maximum rated output level so I turned the A-Weighting on in the QA400 and got these measurements (at 1KHz).

The SNR values are below the expected 77dB for the Beomaster and the left channel THD went up some. It is still below 0.05% but I thought it was odd that it moved and the right channel didn't move much.

Checking other frequencies (400Hz, 10KHz and 15KHz) resulted in the left channel being within THD specs but not as good as the right channel.

These results are not bad but I would like to see closer values for the left and right channels during this testing as I am using the same input signal and the load resistors are identical. A common culprit in the amplifier performance for the Beomaster 8000 are the OpAmps used in the preamplifier board and tone control board. Those are the LF353N, TL072CP and uAF772TC OpAmps. The audio source signals go through these OpAmps so their performance directly affects the Beomaster amplifier performance.

In the case of changing out the OpAmps it is best to change them all (left and right channel). That way all of the signal paths are using new OpAmps that should be pretty much identical.

When replacing the OpAmps I am also going to install 8-pin sockets for the integrated circuits. Here is the preamplifier board with its original OpAmps (seven total)

 Here are the sockets for the new OpAmps.

Here are the new TI LF353N OpAmps installed.

Next is the Tone Control & Filter board. This Beomaster 8000 unit has five OpAmps to be replaced on this board. Note that some Beomaster 8000 units have a six.  This picture shows the five OpAmps replaced.

The OpAmp identified as 4IC6 is for the Beomaster volume control circuit. Whenever that OpAmp is changed it is very likely that the volume control circuit offset adjustment will be necessary to be performed.

Here is the circuit schematic.

The section outlined in red shows the left channel trimmer for the volume control offset adjustment.
You immediately know if the adjustment is necessary because you will hear audible clicks in the related channel's speaker as you turn the volume up or down. Adjusting out the offset removes those click sounds.

Now that the OpAmps are changed out and the volume control offset is readjusted I remeasured the amplifier outputs.

The THD and SNR (A-Weighted) both improved plus the left and right channels are close to the same.

I have done a couple Beomaster 8000 restorations where I replaced the OpAmps with a higher performance TI OPA2134A (SoundPlus) device. However, in measuring the performance with that OpAmp versus a new TI LF343N OpAmp I could not see any measurable differences with the analyzer. Since the OPA2134A costs four times that of a new LF343N I recommend using the latter.

As a final check I used the QA400 impulse stimulus and measurement system to make a frequency response measurement of the Beomaster. This test also uses the same test setup. The difference is the Tape 1 stimulus is from the QA400. I have to admit I am not 100% comfortable with this test yet but it is worth looking at and recording for future reference with other Beomaster unts. The results look pretty good from 20Hz to 15KHz. However, I was hoping the 20KHz level would be a little higher. As it is it measures around -0.8 dB instead of the -0.5dB.

I will finish closing the Beomaster 8000 cabinet back up then do some actual audio component tests using the Beogram 8000 and a Beocord 9000. That will make sure all of the playback and recording features work as well as the Beomaster 8000 remote control.