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!

Beogram 4000: Installation of New Dust Cover and Aluminum Trim

The customer of a Beogram 4000 that I restored in January-February 2025 recently let me know that the new DKsoundparts hood that I installed during the restoration broke off one of the hinges. He included this sad picture:

I decided to replace the cover under warranty since it cannot be ruled out that this was caused by mounting screws that were tightened a bit too hard.

He sent me the hinge. And I installed a new hood and aluminum trim. See here if you are interested in this service. This shows the parts before installation:

First I improved the design of the plexiglass cover by installing small 3D printed plastic bits into the top ends of the 'channels' they designed into the side areas where the aluminum trim is supposed to adhere: 

These channels make it difficult to achieve a perfectly sharp bend with the trim, since it is not supported when it is bent, causing a more rounded corner.

The next step was to check whether there is any interference between the metal back and the plexiglass part when the hinges bend. The new hoods seem generally a bit tighter than the original ones and bent metal hinges can cause interference between metal and plexiglass. The resulting stress can crack the hinge areas of the plexiglass cover. I bolted the metal back onto the hood and checked if the metal would touch the plastic in any way during bending the hinge. On both sides I saw a minimum 1 mm spacing between the parts, i.e. this was a pass:

I removed the metal back again and centered the aluminum trim using my alignment pieces:

I usually use a strip of blue tape to make a pencil mark between strip and the hood:

Then it was time to bolt the metal back in. I put some white wood glue on the threads of the holes in the hinges:

This holds the bolts in place even if they are not tightened very hard.

The final step was applying the aluminum trim

and bending it down on both sides:

Nice precise corners due to my little plastic inserts! Beolovely! Here an impression of the entire dust cover assembly:

Time to send it back to Australia!

Beogram 4000: Installing a New Carriage Motor and a New Hood

Yesterday, I finally received the first batch of my new custom manufactured Beolover Carriage Motors for Beogram 4000, 4002, and 4004. They are a direct drop-in replacement for the original motors, which also suffer from dry oilite bearings, similar to the DC platter motors of the later Beogram 4002 and 4004.

Unfortunately, these motors cannot be opened up very easily for oil infusion of the bearings and therefore I had a suitable replacement motor designed. Since they use modern coreless technology they draw less current than the original motors at the same mechanical performance. This is great news for the often failing power transistors in the H-bridge of the carriage drive. They also vibrate less. Read about the motors here, where I describe my tests of these motors in comparison with an original motor.

This was good news for the Beogram 4000 from Australia that I recently restored. My customer had opted for upgrading his carriage motor and so this project was on hold until I would receive the motors. This shows the original motor still installed:

I extracted the motor:

This shows the upper half of the housing removed and the new motor for comparison:

The new motor is slightly shorter and so the leads can conveniently be fed through the bottom cutout of the housing:

This shows the new Beolover motor installed:

I positioned the motor inside the housing making sure the carriage belt sat straight between the two pulleys. This is easily done since the motor is held in place by two vibration insulating EPDM rubber rings, i.e. the motor can be laterally shifted within the housing if necessary to achieve perfect fit.

This Beogram also still needed a new hood installed. This shows the new hood from the dksoundparts store in Denmark together with a reproduction aluminum trim:

I used the my alignment aids to center the strip

and marked the position with a pencil:

After installing the re-painted hood hinge and glueing in the aluminum trim I installed the hood on the Beogram:

Beolovely! I will play this deck a bit more, and then it will be time to send it back to Australia!

 

Beogram 4000/4002/4004: Test of New Custom Manufactured Beolover Carriage Motor

Sometimes carriage motors in Beogram 4000, 4002 and 4004 can be pretty noisy. Most likely their bearings are also in need of an oil infusion under vacuum, like the DC platter motors. Unfortunately, both the original Faulhaber and Maxon motors cannot be opened easily. So the remedy until now was a drop of oil on the shaft and hoping that this would do the trick for some time. After all these motors run much less than the platter motors.

But I always thought this was a less-than-Beolovely solution. Therefore, during the last couple years I tried out a variety of small motors with similar footprint but had to find out that all standard motors were way too noisy and vibration generating. Only after I sent one of the original motors to the supplier of the motors that I use in the Beolover SyncDrive for evaluation it became clear that the carriage motors used in the Beogram 400x are something special:

They are so called "coreless DC motors". Such motors have a rotor constructed as a hollow, self-supporting coil, often in a basket or honeycomb pattern, which eliminates the need for an iron core. This results in reduced mass, reduced eddy currents, lower inertia, and smoother torque/lower vibrations. The result is a motor that runs much more quietly and draws less current compared to normal motors with iron cores for the same performance (but that also has a significantly higher price - you get what you pay for!...;-). The quietness is crucial for driving the carriage since the slightest vibrations on the floating chassis can translate into sound audible in the speakers since they directly couple into the pickup. During my trials of standard motors the carriage drive was quite audible while listening to music at moderate volumes.

My supplier was able to design a modern coreless replacement that has a torque and voltage that is close to the original motors. I had a few samples manufactured and recently received them.

Here is a picture of the new Beolover carriage motor fitted with a custom pulley::

These motors are now available at the Beolover store.

The motor is easily mounted in the original motor enclosure using two EPDM rubber O-rings that help dampen any vibrations the motor may emit and also align it with the enclosure:

Since the new motor is a bit shorter than the original Maxon and Faulhaber motors, the leads can be fed though the cutout in the bottom of the enclosure:

This makes for a nice and clean installation:

It is a good idea to replace the original usually wobbly plastic pulley on the carriage spindle with a new precision machined Beolover aluminum pulley when exchanging the motor.

I performed some measurements that compare an original Maxon motor in one of my Beogram 4002s with the new motors. These oscilloscope traces show the voltage at the motor terminals while playing a record. The first curve corresponds to the original motor:

And the second one to the new Beolover motor:

The oscilloscope settings were the same for both measurements. We see that the new motor responds at a lower voltage, which corresponds to a smaller deflection of the tonearm necessary to get the motor to move. This is most likely a result of the lower friction inside the motor due to its fresh bearings that provide optimal lubrication. The consequence is that after implantation of the new motor the tracking feedback may need to be adjusted slightly to get the specified sensitivity. If a Beolover Tracking Sensor LED Light Source has been already installed in the tracking sensor, this can be achieved by simply reducing the light intensity of the LED slightly by adjusting the built in trimmer.

An important factor in motor replacements is the current draw under identical operational conditions. The H-bridge on the main board already struggles a bit with the current consumption of the original carriage motors, which is evident in the occasional need to replace the power transistors, especially those providing the current for the travel direction towards the home position (>>). So I definitely did not want replacements that would draw more current than the originals.

These are my measurement results for the two motors:

The only difference between the two columns is that the original motor was replaced with a new one. All other parameters like the <, > speed trimmers were kept the same.

It is clear that the new motors draw considerably less current in the same setting. Their performance however is very similar to the original motors as can be seen from the time it takes to complete a full platter sweep. This was done with an empty platter and measuring the time it takes between pressing Start and the moment the deck shuts off after the carriage came home again. The original motor did it in 18.0 seconds while the new motor was slightly faster at 17.5 sec. It should be noted that these absolute numbers will be somewhat different in every Beogram due to friction variations caused by adjustments and lubricants etc...But it is clear that the new motors will result in much less heat load in the H-bridge.

Another happy aspect of the new motors is that during these tests it became apparent that the new motors run noticeably quieter than the original ones. Which may have to do with their more modern design, but also with their fresh bearings and better lubrication.

In summary, it appears that the Beolover carriage motors are an easily installed cure for noisy original motors.

Beogram 4000: An Interesting Tracking Issue and Installation of a Beolover Efficient 24V Power Supply and Main Capacitor Array

I recently received a Beogram 4000 back that I restored in February 2024 for a customer in Massachusetts due to an issue with the tracking system.

The phenomenon described was that the carriage occasionally would not track after the needle was lowered into the run in groove. I tried for some time unsuccessfully to reproduce this issue until it finally happened after I put a new record on the deck. It became clear to me at this point that it depended on the particular record played since the problem was fully reproducible with this particular record, i.e. happened every time I started the deck.

After a bit of head scratching I figured out that when facing a 'fast' run in groove, the arm would move so fast to the left, that the carriage motor was not able to keep up moving the carriage along before the tracking sensor aperture had already completely passed past the photo resistor opening in the sensor, in effect turning off the motor. The motor voltage is directly controlled by the resistance of the photo resistor and if it is dark the motor stops running.

The big question at this point was why this would happen with this particular Beogram 4000 and not with all the other ones I restored so far. 

I set out to measure the motor voltage with my oscilloscope. This trace shows the evolution of the motor voltage during regular play of a track on this Beogram 4000 as I received it:

What we see on this graph is that the motor voltage 'oscillates' and that there are larger jumps every few oscillations. It turns out that the oscillations correspond to the rotation of the platter: Note the 4s per unit time constant and that there are two oscillations per scale unit. It takes about 2s for a rotation at 33 RPM...

The voltage fluctuations are essentially a result of the eccentricity that most vinyls have to varying degree. The graph means that the carriage only weakly moves during ~6 or 7 rotations and then in one large burst after the voltage gets big enough for the motor to overcome stiction. Then the carriage drives forward until the arms are almost parallel and then the process starts again.

This suggested that the amount of light falling on the photoresistor was not enough to generate a high enough motor voltage through the H-bridge.

I decided to modify one of my standard Beolover Tracking Sensor LEDs to give the LED a bit more current to make it light up more:

After I installed the modified light source I re-calibrated the tracking feedback gain to start moving the carriage after about 3-4 rotations and measured again:

This time the curve looked like this and the carriage reacted speedily after hitting the run in groove of the record that caused the above issue. During play of the same track as above it gave me this trace on the oscilloscope. This shows that the carriage is now basically moving every rotation of the platter.

There are still a few variations between the peaks, which probably indicates a variation of the friction the motor encounters as the spindle rotates. Most spindles are not completely straight and so there is some change in friction during a rotation. It appears that each rotation of the spindle corresponds to maybe 4-5 turns of the platter. Of course this depends on the particular record since the groove density varies from record to record.

It seems this issue is fixed, but of course I am wondering why this particular Beogram 4000 needs a higher light intensity. It may be that the carriage motor itself is a bit different and turns less easily than others at a given voltage. Another possibility is that the photo resistors in the sensor are different, or from a different batch. The Beogram 4000 has two photo resistors in the tracking sensor that allow the carriage to actually track both forward and backward (this was discontinued in the later 4002 and 4004 models which only have one resistor for forward movement).

This unit seemed to track about the same in both directions when the arm was moved to the right or left manually, so I think it is not an issue with one of the resistors. If it were, I would expect that only one direction be affected. B&O has a history of making small circuit changes over the manufacturing run of a design. Maybe they simply used somewhat different resistors or there was variation between batches of them. Possibly the potentiometer for adjusting the voltage into the bulb that is normally found on the original configuration of the tracking sensor of the 4000 is an indicator that they already had this issue when they produced them, and someone calibrated the bulb intensity before Beograms were shipped out.

We may never know!...;-). Anyway, let's hope this tracking mechanism is fit for duty again!

While the unit was on my bench my customer decided to let me install one of my new efficient 24V power supply and main capacitors boards for Beogram 4000:

These boards elegantly replace the big reservoir and motor capacitor mess that is normally found in Beogram 4000s. It also updates the old-fashioned linear regulator based 24V power supply that causes a lot of power loss and heat emission in Beogram 4000s. This board will make any 4000 run more efficiently and the unit will get much less warm.

This Beogram of course already had my previous capacitor replacement kit implemented:

Out with the old

and in with the new! This shows the board bolted in. The solder pads are clearly labeled and in the right spots for easy connections:

This shows all the leads soldered to the board and the motor re-installed:

All good now with this Beogram 4000!

I will play it a bit more and then it will be time to send it back to my customer!

Beogram 4000: New Main Capacitor Array with Integrated Efficient 24V Power Supply

If you ever experienced a Beogram 4000 you probably know that it gets pretty warm after a while! The main reason for this is the 1970s style voltage regulator based power supply, which is a bit of an energy hog.

So when I decided to replace my main capacitor replacement kit with a more modern PCB based design, I thought, why not integrate a modern buck converter based 24V supply that would smoothly replace the original 24V regulator?

This is the new design that resulted (it is available for purchase at the Beolover Store, there is also a version of this board for the AC motor Beogram 4002 Type 550x):

The many round capacitors are high-quality 105C rated Panasonic electrolytic capacitors that are connected as an array to match the original reservoir capacitor values. The row of small 'boxes' on the left is an array of Samsung X7R type ceramic capacitors that add up to the 150uF of the non-polar original electrolytic motor phase capacitor. Ceramic capacitors are much better for this application since they are inherently non-polar and they can take AC current much more easily than electrolytic capacitors. The circuit on the far end of the board is the buck converter based 24V power supply.

And here an impression of an installed board:

It bolts directly into the mounting holes of the capacitor clamps of the original setup. The solder pads for the wires are in approximately the same locations as the original connections to the big capacitor cans. This makes it straight-forward to replace the old capacitors. Simply unsolder them and then tack the wires to the pads according to position and color labeling.

This is an impression of the original setup that is being replaced by the board:

This schematic tries to make sense of the wiring around the capacitors and the voltage regulator:

For the installation of the new board this diagram does not need to be fully internalized. Simply match the colors of the wires with the labels next to the solder pads. This shows the board with all the connections in place:

Note that the 'thin blue wire' from the collector of the original voltage regulator (see below) needs to be moved over to the PCB. Solder it to the pad labeled "lgt. blue" up front where also the orange wire is connected. The blue wire connects the solenoid to the 45V coming from the rectifier.

Let's discuss the Beogram 4000 24V power supply a bit. The 24V rail is the main supply and energy provider of the Beogram. The motors, light bulbs as well as the circuitry that translates the commands from the 6V powered control logic beneath the keypad into actual behaviors of the Beogram are powered by it. The 6V supply that provides power to the control logic is only a marginal energy consumer since it only drives the logic chips producing control signals.

This is a clipping from the circuit diagram that shows the 24V setup (the red dotted rectangle indicates the parts replaced by the new Beolover board):

The rectifier 0D1 is fed about 46-49V RMS from the transformer secondary. The rectified voltage is smoothened by capacitor 0C3 into an unstabilized DC voltage of about 45V. This 'rough' DC voltage is fed into the collector of 0TR1, which is set up as an emitter follower controlled by a Zener stabilized ~24V voltage at its base. At the emitter of this voltage regulator a second big capacitor 0C4 removes most of the remaining DC ripple and a stabilized ~24V rail results.

This picture shows how this circuit is implemented in the Beogram 4000:

Since this regulator circuit produces a large amount of waste heat in the transistor, the transistor itself is bolted directly to the metal enclosure in its own metal compartment. This makes for an effective heat sink. The collector of the transistor is connected to the metal bar that clamps the transistor down (the screws go in from the bottom of the enclosure). Left and right of the collector bar the base and emitter leads poke out through insulating sleeves. The big maroon colored resistor on the left is 0R1, which pulls the Zener up to the unregulated 45V from the rectifier, which is connected to the collector bar via the red wire seen on top of the picture. The Zener itself is the metal can whose cathode is soldered to the base. The anode end of the Zener is anchored to GND, which is achieved by soldering the lead to the connection point of the two negative terminals of the biggest (3000uF) capacitor cans of the setup, which are also connected to GND via the black and green wires (see schematic above).

The blue wire that goes to the same solder spot like the red wire connects directly to the top of the arm lowering solenoid. In other words the solenoid directly gets the unregulated 45V and bypasses the 0TR1 regulator (this wire needs to be moved to the Beolover PCB during installation).

I think this explains why the B&O designers chose to regulate 45V down to 24V. The solenoid needs such a high voltage to actuate reliably, and apparently they did not see fit using a transformer with a 3rd dedicated secondary winding for the 24V rail. I guess in the 1970s it did not really matter that much if a device wasted almost 50% of its energy intake...;-), because this is about what this approach 'achieved'. 

Let's have a look at different ways that can be used to convert a voltage down:

This figure shows the three main ways to go from a higher voltage to a lower voltage:

The most straight-forward way is a simple voltage divider. In this figure the 'load' represents the entire 24V connected circuitry of the Beogram. If we want to go from 45V to 24V, we simply calculate R1 according to R1=((45V-24V)*R(load))/45V and as long as the load resistance remains stable we have 24V on the load. The problem with this setup is that the current is the same in R1 and through the load. In the case 45V->24V this means that R1 dissipates almost the same heat as the load, so close to 50% of the energy is directly wasted into heat.

The next step up is the voltage regulator like it is employed in the Beogram 4000. Here we replace R1 with an emitter follower and a zener/resistor divider that puts 24V on the base of the transistor. The great improvement with this approach is that it can keep the voltage pegged to 24V regardless of variations of the load resistance. In the Beogram example, the load resistance would for instance go down as soon as the platter motor turns on, which would draw more current. Whenever the carriage is moved the load resistance goes down a little, too. And so on, bulbs on/off etc....

Since loads are rarely constant, the voltage regulator is a popular and simple way to achieve a regulated stabilized voltage rail.

But since the emitter follower basically only replaces R1 and essentially still acts as a resistor, albeit a variable one, we get a similar heat dissipation like in R1 in the simple divider. In fact, it gets even worse, since we have to feed the Zener divider, which constantly carries a current to peg the base to 24V. This current is actually fairly substantial in the Beogram setup since the regulator is a simple bipolar power transistor with a low gain, maybe in the 25-50 range. This means that the base needs to constantly carry a current that is a few percent of the collector-emitter current through the transistor into the 24V rail, proportionally increasing the heat load. Hence the big 5W power resistor!

While this approach was maybe the best approach in the 1970s, it is not used much anymore in modern designs since we have now MOSFET transistors, which can switch at high speeds with a very low heat loss due to their very low ON resistance. This enabled the development of the so called buck converter, which is able to step down voltages with high efficiency. Modern designs can achieve 90-95%, minimizing heat loss dramatically.

A very simple schematic model of such a converter is shown in the above figure. At its heart the R1 resistor is replaced with an appropriately dimensioned inductor. This inductor acts like a resistor plus energy storage due to the fact that a coil upon turn-on has a high resistance due to the EMF that is acting agains the inrush current. As the magnetic field builds up in the coil the resistance drops asymptotically towards zero. So in the end a coil is basically like a straight wire with some residual Ohmic resistance.

So how do we go from 45V to 24 with this process? This is achieved by chopping the 45V input into a high-frequency 'pulse width modulated' (PWM) signal. This results in a continuous partial ramping up of the current in the coil during the ON 'duty' cycle, and a release of the stored energy in the coil into the load during the OFF (-duty) period when the voltage is cut.

As a consequence, most of the energy in the 45-24=21V drop that is wholly dissipated in the divider and regulator circuits is instead fed into the load!

Pretty cool (in the literal sense of the word!...;-)! Of course, nothing is free in nature, and the disadvantage of this setup is that there is a bit of noise on the output voltage caused by the incessant chopping of the input voltage. This is dealt with by connecting a reservoir capacitor across the load, which stabilizes the voltage. 

The above basic circuit also omits the necessity of a feedback based control circuit that adjusts the PWM duty cycle depending on the load resistance to keep the voltage on the load constant. So this setup is in reality quite a bit more complicated than the simple regulator circuit of the Beogram. Luckily, one does not really have to deal with this 'complication'. Nowadays integrated circuits take care of all of this and one simply has to select the right external components (i.e. resistors, capacitors and the inductor) for the desired output voltage and current needs.

The two traces at the bottom of the above figure show the result of the simulation: The chopped input voltage (green) and the resulting ripple on the output voltage (blue). With a fairly modest 100u cap the ripple comes out to about 27mV, or ~0.1% of the output voltage. The Beogram has 3000uF at the output (0C4), and so it is much smaller, probably easily in the range of the original regulator output.

Let's have a look at the actual 24V supply circuit on the new Beolover board:

The big 'box' in the back is the inductor and the small 8-legged integrated circuit in front is the buck converter. The passive components around it take care of the feedback to keep the voltage constant.

Alright! On to some measurements! I measured the current draw into the 24V rail and the temperatures of the transformer and at one of the screws that bolt the motor down before and after the implantation of my new board.

It is important to state the motor voltage at which these measurements were made since this voltage directly influences the current draw of the 24V rail. This is a oscilloscope trace measured at the purple lead (the nice sine wave also demonstrates that my ceramic capacitor array works happily as motor phase capacitance):

The 13.3V amplitude I chose balances enough motor torque for sweeping dust off the record with a still reasonable current draw.

The temperature measurement location at the motor made sense since the voltage regulator is directly next to it and so I would measure the heat in the front left corner, coming from the motor and the regulator. This shows my measurement setup (Beolover PCB already installed):

The temperature meter above shows the two temperatures and the multimeter is hooked into the 45V line as current meter. The picture shows the current draw in ON condition with running platter. About 190mA go into the 24 power rail. This table shows the measurement results for the original setup vs. the new Beolover 24V supply:

The current difference in OFF condition is 70mA vs. 15mA (original vs. new) and 360mA vs. 190mA. We see that the power consumption of the 24V rail goes down by almost 50% in ON condition and about 80% in OFF (standby) condition. The better performance in standby comes from the absence of the Zener diode voltage divider, which is absent in the buck converter circuit on the Beolover board. In OFF condition the Zener divider represents the main power draw of the 24V rail.

The standby power draw of the original setup of 3.15W is pretty impressive. Considering the hours in a year (8760), this means that over a year about 27kWh are dissipated by a Beogram 4000 just by sitting on the shelf being plugged in. This is reduced to about 6kWh with the new Beolover circuit. Just to put it in perspective, 1kWh allows driving a typical electrical vehicle for about 2-3 miles. So this is a pretty substantial power waste for a standby audio device. 

This more frugal power consumption of the Beolover circuit became immediately evident in my temperature measurements. The original setup reached 47C at the motor and 41C at the transformer in ON condition (after about 1 hr runtime). In contrast, with the Beolover board installed much lower temperatures of 37C and 34C were measured, respectively. 

So in conclusion one can say that the Beolover board reduces power intake of the 24V rail by almost 50% while playing the deck, and by almost 80% while it sits in standby.

This Beogram 4000 has arrived in the modern age in power supply terms!