Meet Handmade by Dandy – the DR700w mini room amplifier, where I am going to demonstrate how to test parametrically.
The Need to Test
I would like to know the characteristic of my project electrically so I have to subject it to a test by using a signal generator as the source of signal ( music) and the load will be resisitive (speaker), I will be viewing data using oscilloscope and HP 8903B AC signal and distortion. This kind of test will show those electrical parameters that normally cannot be heard by my (our) naked ears.
- Offset Voltage – the doc voltage appearing at the speaker, the ideal is zero volt dc, the normal is -50mvdc to +50mVdc (-0.05Vdc to +0.05Vdc). The reading for the Left (L) = +10mVdc, Right(R) = +14mVdc both channle loaded with 8 ohms resistive load (its not speaker).
- Residual Noise – in class D, there will be a signal appearing at the output even though there is no input signal. This output signal is the class- D clock, about 352 KHz.
if I am going to expand the time division to see clearly the frequency, it will be like this, its pretty ugly but this is the real-world characteristic of class-D amplifier. Teh 352 KHz is way above the audio spectrum of 20Hz ~ 20KHz and it looks like that 352KHz may not be reproduce from the Tweeter speaker and will not be heard by human, even a superhuman would not listen to this frequencies.
3. Distortion @ 1Watt 1 KHz – I measure the distortion at 1Watt, when the voltage swing at the resistive load is about 2.8V @ 8.2 Ohms. The distortion at this power level is about 0.0276% as read on the HP-8903B.
4. preCondition before testing the Maximum Power Output it can deliver to the load. Input a music source from PC or smartphone, operate at 1/4 output power, if it is 200W, then operate it at about 50W per channel. I am using a resisitve load and not a speaker, as at this power, it will be very-very noisy in my room. the setup is as follows :
5. Maximum Ouput Power @ 10% distortion @ 1KHz – after item #4 above, then take the maximum ouptut power it can deliver to the load, both channel – left and right should be simultaneously operated. Here is the output of the left channel, the voltage at less than 10% is 39.94Vrms. The output power will be Pave = 39.94 x 39.94 / 8.2 = 194.5 Watts (sometimes it is written un-acceptably as P = 194.5 Wrms), which simply means, the power computed from a rms voltage).
6. Input Sensitivity – I am now interested to know the signal input to caused a maximum power output at the load @ 1KHz. I found it to be 982mVrms (let say 980mVrms or 0.98Vrms). This input is good enough that many Tone Control and other music player can drive it without having any problem.
7. Efficiency – the AC power consumed at maximum power when both channel driven is 450W, and the combined power output of the DR700W mini is 388W, my wild guess for the efficiency = 388/450 x 100 = ~86%.
Note: I should have measure the Power Output and efficiency when the line voltage is 180Vac and 240Vac.
Now, I have the basic understand of what my DR1700W mini class-D amplifier can do. I can do more parametric test like the (a) Frequency Response test (b) Stereo Separation, but not (c) Rise Time or Slew Rate, which may not applicable in class-D.