Beomaster 4400 - Australia Project - Wrapping Things Up

This Beomaster 4400 from Australia passed its final bench testing today.

I can also report that musically the Beomaster 4400 sounds great.

Here are a couple of pictures of my first listening tests on the bench.

I will install the Beogram 4400 cabinet cover next.  Then move the receiver to a listening room where I can spend more time checking out all of its features.


Prior to this listening test, I ran my usual harmonic distortion and frequency response tests on the Beomaster 4400.
I use a dummy speaker load of 8 ohms and use a 1 KHz, 0.316 VRMS test input sine wave into the Tape 1 inputs (L & R).

I run the harmonic distortion (THD) and frequency response tests with the Beomaster 4400 at its maximum rated output.  For an 8 ohm speaker that is 50 Watts.  I actually tested it at 55 Watts in this case.

Here are the measurement results.

The signal drop off above 22 KHz on the frequency response measurement is due to the measurement device (QuantAsylum QA401 Audio Analyzer) ending its test run.

The THD measurements at 55 Watts are nicely below the Beomaster 4400 rated value of 0.1%.

Those measurement results are what I have been getting from my Beomaster 4400 restorations so they are good measurements to make and compare to.

To get to this final bit of the Beomaster 4400 restoration testing I spent some extra time installing some power protection features to the receiver.

On all of my Beomaster 4400 restorations I have been installing an arc suppression device to protect the Beomaster 4400 power switch.  The Beomaster 4400 On/Off switch switches full power across its contacts and I have seen one Beomaster 4400 where those contacts were pitted so bad the switch had to be replaced.

My previous Beomaster 4400 restorations were all Type 2419 versions though.  They only required a single arc suppression device.  The following schematic shows the Type 2417 and 2419 power switch differences.

The On/Off switch of the Type 2417 routes both the hot and neutral AC wires to the Beomaster 4400 transformer while on the Type 2419, only one of the AC wires runs through the switch.

On the Type 2419 version I only have to put one arc suppression device across the switch contacts because the two sets of contacts are connected together for the one AC wire they handle.

The Type 2417 uses a set of contacts for both AC wires (hot and neutral) so I added arc suppression on each contact.

Here is my installation of the arc suppression for the Type 2417.  I had to build a 3D printed support to mount them inside the Beomaster 4400 cabinet.  The black mounting screws for the assembly are plastic.

Another power protection install I did was to install a bleeder resistor on each of the two ±35 V rail reservoir capacitors.  The bleeder resistors insure that both 10,000uF capacitors discharge together and relatively evenly when power is turned off.  This also provides a bit of safety when the Beomaster 4400 is opened up for service.  The two 10,000uF capacitors will be at zero volts in that case.

For the bleeder resistors across 10,000uF capacitors 0C8 and 0C9 I chose 5100Ω.

The final power protection installation I did was to build and install a soft start circuit for the ±35 VDC rails.  A lot of amplifiers do this as the soft start circuit slows down the in-rush of current to the reservoir capacitors when power is turned on.  Limiting the in-rush current will put less stress on the transformer, fuses, diode bridge and reservoir capacitors.

The way it works is when the Beomaster 4400 power switch engages, the ±35 V rail voltages go through a pair of 22Ω resistors instead of directly to the 10,000uF capacitors.  That slows down the charging up of those two capacitors.  After about 1.7 seconds, a relay switches out the two 22Ω resistors so the remaining 5 to 10 VDC of rail voltage directly charges up the reservoir capacitors.

Here is a picture that shows the 10,000uF reservoir capacitors charging up with and without the soft start circuit.

A nice side effect of the soft start circuit is that it greatly reduces any "thump" sound through the speakers when the Beomaster 4400 turns on.  However, the reason I added the soft start circuit was for long term protection of the power components.

Here is the soft start circuit installation that I implemented.  I made a small 3D printed mount for the circuit board so I could easily attach it to the inside of the Beomaster 4400 cabinet.

The soft start circuit is powered by the Beomaster 4400 22V power supply.
The inputs to the soft start are the ±35 VDC from the Beomaster 4400 bridge rectifier.
The outputs from the soft start go to the the two 10,000uF capacitors (0C8 and 0C9).

Note: For a Type 2419 Beomaster 4400 I remove the two auxiliary AC plugs that the Type 2417 does not have.  Removing the two AC plugs provides room for the soft start circuit and I created a 3D printed plate to cover the rectangular cabinet cutouts for those plugs.

Now for some more listening to this Beomaster 4400 before packing it for shipment back to Australia.