Beomaster 2400 Type 2902 - Canada Project Bench Testing To Collect Some Reference Data

Listening to this restored Beomaster 2400 has been very enjoyable. The amplifier sounds as good as any Beomaster 1900 and 2400 receiver I have tested in the past.  But that is a little subjective isn't it?

In my ongoing effort to collect some quantitative data so I can compare my restoration results I returned to using my QuantAsylum QA401 Audio Analyzer and some fixed resistive, dummy speaker loads.
Since I last used it QuantAsylum has released a couple of software upgrades that will help me with my amplifier testing.

If you are not familiar with the QA401 Audio Analyzer, it can output stimulus signals into an audio amplifier and measure the outputs either from preamplifier outputs (which a Beomaster 1900/2400 does not provide) or output amplifier speaker outputs.

A nice feature of the QA401 analyzer is that it can do single-ended measurements or differential measurements. I have experimented with both in the past. For this test setup I went with differential measurements. 

Choosing the measurement points based on the expected output is important.  For that reason I constructed my fixed resistor, dummy speaker loads like this -

...and here are the connections to the QA401 Audio Analyzer.

I have a dummy speaker load assembly for two amplifier output channels.
I decided on 8Ω fixed loads.  If I had to it would be quite easy to desolder and resolder my configuration for 4Ω fixed loads. 

An important reason for having multiple series load resistors in the fixed load assembly is because the QA401 inputs have limits to how much voltage they can handle. I have tested amplifiers on my bench that have 20 Watt outputs all the way to amplifiers that have 250 Watt outputs. With the Beomaster 2400 I am only looking at 20W into 8Ω.

My dummy speaker load assembly is constructed like this (from the positive lead to the negative lead):
4Ω to 0.08Ω to 2Ω to 2Ω

That gives me a total 8.08Ω for the fixed resistance load across the amplifier speaker outputs.

The 0.08Ω resistor is in there for the large 100 plus watt amplifiers. I need a small sensing resistor to connect my QA401 differential measurement probes without exceeding their input level limits.

Determining what the sensing load I am going to use is important in the configuration of the test measurement system of the QA401. It has a popup context menu for setting the Input Gain level of the measurement. If I was going to measure across the full 8.08Ω loads then I would use an Input Gain value of 0.00 dB. However, as I explained above, the QA401 has input voltage limits so it is common to be measuring across one of my individual (smaller) resistor segments. When doing that you must calculate the Input Gain value to put in.

For example if I am concerned about the amount of voltage going into my QA401 measurement then I could begin by measuring the amplifier output using the small 0.08Ω sensing resistor.
In using that I calculate my Input Gain value using the formula 20*log10(0.08/8.08)) which gives me -40dB.

Also note there is the option to display the output as Vrms or Watts.  The actual load impedance value field here is for the QA401 to calculate the values it displays. This is the impedance the amplifier sees, not the individual sensing resistor value.

Why not just use the 0.08Ω sensing resistor for all amplifier measurements?
The reason to not do that and the reason for having the other measurement point options are for sensitivity of the measurement. Basically the resolution for accuracy.  According to the QA401 documentation and forum discussions you get the most accurate measurements when your test signal is not too close to the maximum input the QA401 can handle and not too small either.

For the Beomaster 2400 and its 20 Watt power output rating across an 8Ω load I am going to make my measurements across one of the 2Ω resistors in the assembly.  That means a QA401 Input Gain setting of -12dB.

Continuing on with some actual measurements...
I wanted to first check the QA401 measurement setting configuration and my earlier service manual adjustment of the Beomaster 2400 volume level (for the Medium Volume Preset switch position).
Remember that test setup asked for a 200mV, 1KHz test sine wave into the Tape inputs.
Turning the Beomaster 2400 on using the Tape source selection should result in 100mVrms at the speaker outputs at the medium volume level setting on the Beomaster.

Here is that check.

Note that I set the QA401 measurement units to volts so I could see what it measures for the speaker outputs. The voltage output levels are in the 100mVrms range. I was hoping they would be closer but this is the first time I have recorded that setting so perhaps that is typical. I will start noting that on all of my Beomaster 1900 and 2400 restorations from now on. 

The bench tests I really want to collect data on is the total harmonic distortion (THD) at 1KHz and the frequency response. For all of these measurements I will need to use the Beomaster Tape source as the point of entry for the test signal. I decided for these tests to keep my Tape test signal at 200mV.
On the signal generator of the QA401 that calculated out to be -13.8dB for the generator configuration.

That value was determined using a generator test option on the QA401.
I set the QA401 generator amplitude value while measuring the signal the QA401 was putting out using a DMM.

The bench testing generator is now configured to go along with the measurement configuration.

I began the bench testing with the THD test option on the QA401. The generator is sending a 200mV, 1KHz test signal into the Beomaster 2400 Tape inputs. I increased the volume level on the Beomaster 2400 while watching the QA401 left and right measurements coming off the speaker load assembly.
I set the measurement units to watts for these tests and here are my results with the Beomaster producing 20 Watts of power.

Less than 0.02% THD.  That is a very good number and that is at the maximum output level for this amplifier.

I left the Beomaster 2400 at the volume level that produced the 20W output (at 1KHz) and removed the test signal.

Next, I setup the QA401 for a frequency response test using that same volume level.
I let the QA401 do a single shot sweep frequency response test from 20Hz to 20KHz and saved the result. 

Another nice measurement. The swing of the signal from 20Hz to 20KHz for each channel stays within 1.5dB from the 1KHz point. 

That is all I will collect for now. I believe I have a repeatable test configuration I can use on future Beomaster 1900 and 2400 receivers.