Graphical Data of the BPA768-400:

The Measuring Technique

The diagrams shown here have been taken using an Audio Precision SystemTwoCascade (S2C).

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Generation of Audio Data

This served both the generation of analogue or digital audio data as well as for measuring of the analogue results.

Operating Condition

The results values given in brackets are operating points at which they were measured e.g.: (+32.4dBV/4Ω).

Ampl.-Sweep 10Hz-200kHz

The diagram shows with the red curve the amplitude frequency response of the transimpedance amplifier with final stage analogue measured. It is remarkable that the deviation up to 50kHz is smaller than 0,050dB, an exceptionally good value.(+32.4dBV/4Ω)

Damping Factor

The blue trace shows the slope of the damping factor versus frequency. Shown is the suppression which is caused by a signal fed from outside. The feeding was carried out via a 8Ω resistance.

Ampl.-Sweep 10Hz-200kHz with Filter

The amplitude frequency response of the transimpedance amplifier with final stage and antialias filter of the D/A-Converter is shown with the red trace measured analogue. Due to the optimal adjusted filter a deviation of less than 0,020dB to 100kHz results, a value not to be surpassed any more.(+32.4dBV/4Ω)

Phase-Sweep 10Hz-200kHz with Filter

The blue trace shows the phase for this arrangement including the delay in time through the complete amplifier. More than remarkable is the fact that at 100kHz a phase shift of only -28° is reached. Unfortunately, the Audio Precision S2C does not show any group delay, otherwise it could be shown that this phase turn is caused only by the delay time and therefore is not a real phase fault of the amplifier.

Ampl.-Sweep 10Hz-55kHz with DAC

The diagram shows the amplitude frequency response via the complete final stage of with D/A-Converter. As can be easily recognized, the drop in the frequency response starts already relative early due to the low sampling frequency of 108kHz with -0.3dB @ 20kHz. (+32.4dBV/4)

High Sampling Frequency

By that it gets once more clear how important a high sampling frequency is. When using 192kHz (SACD) this drop will appear at not just the double frequency. Unfortunately, the Audio Precision S2C cannot produce such a sweep digitally.

Harmonic and Hum at 1 kHz

Well to be recognized are the low value harmonics of better than -105dBc at 1 dB under full modulation. (+32.4dBV/4Ω). At the same time the 50Hz Mains Hum is at -113dBc or better.

Harmonic and Hum at 1 kHz

The same like above at only 10dB less in modulation. (+22.8dBV/4Ω) The amplifier still yields -100dBc.

THD+N versus Frequency

A further important statement about the quality of the BPA768 is the measuring shown here. It shows values better than -100dBc for the complete construction: DAC, filter, amplifier + final stage. (+32.4dBV/4O)

THD+N Residual

This measuring shows another THD+ N representation than the one above. The carrier is suppressed here and the interferers can be judged one by one. (+32.4dBV/4Ω)

Intermodulation at 10kHz

This is one of the most meaningful measurements at all. Almost everything can be seen here: the behaviour at a two-tone rejection. it is moreover very important, that the IMD products fade away as fast as possible, what by far is not the case at many other final stages. (2x +26.4dBV/4Ω)

Measurement is a RF-measurement

The two tones are chosen as (10kHz+/-60Hz) to make the IMD-products of the 50Hz from the power supply also visible. With -117dBc "wonderful" results were reached here. This measuring results from the knowledge in the RF measuring technique and has already been adopted by some other audio experts too.