Response Checker RC-1 Frequency response measurement

Measurements and Performance

Frequency response measurements

For frequency response measurement i have used very special device: Response Checker RC-1 (with calibrated microphone) from Nippon Audio.

Special thanks to my friend Mike Ostapenko, who gave me this device for testing.

The frequency plots below show that has excellent characteristics up to at least 15–16 KHz:

This is not surprise, because the test was conducted under a dummy load. The next plot diagram indicates frequency range response into real situation — with Klipsch RF-15 and calibrated microphone:

Look ugly, but this test is not representative, because it shows the loudspeaker distortion and the living room influence.

THD measurements

For THD (total harmonic distortion) measurements I have used Digital USB Oscilloscope. It's very easy to use and is not picky about the computer resources, so as not to depend on the parameters of the audio card. As advantage can also note the presence of galvanic decoupling.

Its software include a spectral analyzer. Four Fast Fourier Transform (FFT) windows (Rectangular, Hamming, Hanning, and Blackman-Harris) are available to match an analyzed signal here. I have choosed the Blackman-Harris window, because it is better for periodic signals.

Test Conditions:
Input = CD (2V RMS), adjusted for 1 W output.
Test frequency = 1 kHz.

As a result I got:
1.8% with 8 Ohm dummy load
2.5% with with Klipsch RF-15 speaker

Most significant term is 2rd harmonic (1.73% with dummy load and 2.43% with speaker), but 2nd harmonic is almost inaudible, since it is a full octave higher than the original musical spectra and is musically consonant. Odd harmonics (3rd, 5th, 6th, 7th and 9th) are very dissonant and ugly sounding, but fortunately they are small.

The second test I conducted with the following conditions:

Input = CD, adjusted for 2 W output (exactly before clipping).
Test frequency = 1 kHz.

I received following THD values:
2.8% with 8 Ohm dummy load
4.5% with with Klipsch RF-15 speaker

But it should be noted that the level of ordinary plays equals of 1 watt output.


Level Five 2A3 Schematic