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).