Beomaster 4000 (2406): Installation of Custom Designed Toroid Transformer - Final Impressions

I finished up the toroid transformer implantation in the Beomaster 4000 that I am currently restoring. I added a bolt to fix it in its 3D printed cradle and used the mounting plate and rubber shock absorber that came with the toroid to secure it in place. Here is an impression:

Lovely! A very happy look in my opinion! 

Then I put on the bottom plate:

and with plate installed:

This shows a the detail around the 'gills' of the Beomaster:

I like how the red shines through a bit. Since the toroid represents a performance upgrade of the Beomaster and brings its power supply into the current millennium, I like that it can be seen a bit if closely scrutinized. Putting in a toroid into a Beomaster 4000 is a bit like installing Brembo calipers on a vintage BMW 5-series, if you catch my drift...;-):

Toroids have a much improved EMI performance than conventional "EI" style transformers due to their geometry. That is the main reason why most modern low noise analog power supplies employ toroid designs.

While I am writing this post I ran the unit together with the Norwegian Beogram 4000 that I am testing right now, and I can report that it sounds absolutely awesome! No humming on any input and everything stays absolutely cool including the toroid and I am cranking it up quite a bit right now. Appropriate for Jethro Tull's Aqualung!

This pretty much concludes the restoration of this Beomaster (if nothing comes during the testing period). Here is a picture of the units with exchanged parts:

Beomaster 4000 (2406): Installation of Custom Designed Toroid Transformer

This is a follow up to my recent posts about a strongly humming transformer in a Beomaster 4000 (2406). This Beomaster is in great condition otherwise, so it made sense to install a new transformer to get this unit ready for the next 40 years of service. Most modern audio designs with analog power supplies use toroid transformers due to their much better EM characteristics. After characterizing the original transformer current loads in the secondaries, I designed a replacement and had it custom made by the Toroid Corporation in Salisbury, MD. It costs a pretty penny to get this done, but the result is pretty, too (at least from a geeky engineery point of view...;-). 

While the installation of a new transformer is fairly straight forward, the details can be pretty daunting. The first order of business was to remove the old 'rectangular' transformer. Here is an impression of the original state of affairs:

This shows the 'cavity' after removing it:

The challenge here was essentially to fit (and hold securely) a donut into a square space. Furthermore, The Laws of Beolove require to not drill additional holes into any B&O equipment, ever! My technical upgrades are always 'reversible', i.e. the units can be returned into their original state if so desired at a later point in time. So I set out to design a 3D printed mounting bracket that would fit exactly to the mounting holes of the original transformer case.

Since this bracket represents a fairly large part from a 3D printing perspective (the final design took about 26 hrs to print), I began the design process with an initial study that 'put the mounting holes into the right positions in space', but that needed minimal printing effort so the design could be iterated in a  reasonable amount of time (it is difficult to do precise measurements within a crowded enclosure). Here is an impression of one of these study parts implanted into the cavity:

Once the holes were close to the proper positions, I completed the design and created a final version of the bracket that was able to hold the toroid and the main rectifier in place:

This shows the final design after four prints (and about 1.5 kg of plastic filament...;-) that did not fit precisely and had other fit-related issues. It turned out that the 'study' prints were slightly flexible, which allowed me to bend the holes a bit to the right positions, while this final design is very rigid and so there was no more room for mismatch. This shows the toroid and the rectifier added:

Before I was able to install the toroid assembly, I needed to replace the reservoir and speaker capacitors to make a bit more room for the toroid. The original reservoir capacitor is a bit thicker than its modern replacement, and that gave me five more millimeters that were required to fit the toroid:

This shows the installed bracket with rectifier in place and the new capacitors::

Add toroid:

The next step was connecting the toroid to the input voltage selector. This shows the arrangement at the input selector:

The it was time to verify that the connections were made correctly for proper voltage transformation at all four input voltage settings. This was the moment where I would get proof that my transformer design was correct. All this can be a bit confusing since the input selector switches the various primary coils of the transformer in series and parallel depending on the tuns ratio that is needed for a particular input voltage. So I hooked up my 110-to-250V variac to the line plug of the Beomaster (turned 'on') and then slowly ramped up the transformer voltage for each of the input villages while monitoring the secondary voltages. This shows the setup:

The secondaries wires are held securely apart from each other by some carpenter clamps in concert with thick cardboard strips. This shows the measurements for the various input selector settings vs the 50V secondary. 110V input:

130V input:

220V input (and 22V secondary):

and 240V:

After this I put the input switch back into its box together with the fuse holder (secured in place with some double sided tape):

After this I connected the secondary windings to the two rectifiers:

And then it was time for the magic moment! I slowly ramped up the variac and the 'ON' light came on. Then I connected speakers and an antenna, and set it to FM. And everything seems to work! And of course no transformer hum at all! This is Beolove! I still need to procure a nice bolt to hold the toroid in place in its bracket, but I think it is pretty safe to say that this Beomaster 4000 is back in business. I will give it a full recap and then give it some play before sending it back to Italy! 

Beomaster 4000 (2406): AC Current Measurements in Transformer Secondaries

This post is a first update on the transformer issue that I identified in a Beomaster 4000 that is in an outstanding cosmetic condition otherwise (i.e. worth fixing!).

I recently entered discussions with a custom transformer manufacturer, and naturally the question of current in the windings came up. The Beomaster 4000 has two secondary windings, one delivering 47V AC RMS and the other 22V AC RMS. The service manual specifies a 275VA transformer, but makes no reference to the distribution of power among the two windings. This shows the pertinent part of the circuit diagram:

The 47V winding directly powers the output stages, while the 22V winding takes care of the rest of the unit such as the preamplifiers and the FM section.

In order to measure the current in each of the windings I added 0.18 Ohm shunt resistors in series into each of the circuits. This shows the measurement on the 22V section. The black and white AC lines connect to the far right back corner of the Beomaster where the rectifier sits:

I unsoldered the white lead and put the resistor in series:

Then I hooked up a voltmeter across the resistor and turned the receiver on with speakers connected. The voltages that I measured were a constant 30 mV for all settings except phono, where I measured 36 mV AC RMS. These results were independent of the volume setting. Even with white noise on the FM section or a 1000 Hz test tone on the Tape input and volume set to 10 (that is pretty loud...;-) this reading did not change. I pretty much expected this since all output current is provided by the 47V winding. 

Doing the math yielded therefore a constant 200mA RMS current indicating that the VA requirement of this winding is a mere 22Vx0.2A=4.4VA.

On to the 47V winding:

For this one I connected a 5Ohm heatsinked resistor to the output and then fed in a 'worst case' (i.e. the scenario if someone hooks up an iPad with he output fully cranked up and loud music playing) modern consumer nominal line signal of 2.8Vpp into the Tape 1 input. Then I measured the voltage across the resistor as shown in the above picture (the resistor is again in series with the white lead which was disconnected right at the rectifier mounted to the transformer).

Interesting aside: It was pretty impressive how the speaker capacitors start 'singing' under this condition. The test tone became clearly audible, with no speaker connected when going to volume 10.

I did the measurement for 100Hz, 1kHz and 10kHz connecting the resistor to L and R channels. The measurements did not vary much depending on the frequency (to be expected with a resistor, a speaker will yield different readings due to the frequency dependent impedance curve). 

With the volume fully up I measured 566 mV AC RMS across the resistor for each channel. This corresponds to a 3A current into 5 Ohm, i.e. we can extrapolate that a 4 Ohm load would drain about 3.7A RMS. This means the bottom line is that both channels on full duty would cause a 7.2 A RMS current, resulting in a total power of 347 VA. This is a bit higher than the rated 275 of the original transformer. I think that is a result of my 'brutal' input signal, which probably caused a lot of distortion in the outputs. 

I repeated the experiment with a DIN spec input signal of 250 mV (RMS), which corresponds to 707 mVpp. With fully ramped up volume the measurement yielded a more modest 300mV with 5 Ohm on one channel corresponding to a 1.6A current. Extrapolated 4 Ohm on both channels, this corresponds to a current of 4.2 A RMS and 197VA, which is conservatively below the 275VA rating of the original transformer. 

So I think the bottom line is, for maximum safety it is probably best to put a 350VA rated transformer in there, but a 275VA rated one can probably be used if properly fitted with a thermal resettable fuse (as the original unit that is in there right now).