Direct digital feedback amplifier benefits
The common metrics for amplifier performance show excellent performance, especially if a detailed analysis is conducted. Figure 6 shows THD over a range of output powers. Of significant interest is the performance at very low powers, where competitive systems would show a raised value, due to the noise dominating.
Figure 6: THD vs. output power for DDFA
In this case, the noise contribution at low power is minimal. To illustrate this, Figure 7 shows the output spectrum in a test with a -120dB input signal; a noise floor of better than 150dB ref 30V gives less than 60V audio band noise on the power output.
Figure 7: Output spectrum of DDFA, with -120dB FS signal
Other advantages of direct digital feedback
Through the processing of error information in the digital domain, there is a new opportunity to act upon the information in an intelligent fashion. For example, error levels correlated with output signal amplitude can be judged for operational acceptability. The digital amplifier knows the output voltage, so can make a decision when the error is disproportionate. This would typically occur if the output is subject to a short circuit.
On a more subtle level, the information can be used to infer the load impedance and real-time output power. This gives the opportunity to build in intelligence to the amplifier, which allows it to tolerate difficult applications. For example, consider the situation where the amplifier is loaded with 4 loudspeakers. The load impedance will be reduced to 1/4th and the system will attempt to deliver 4 times the normal power and current. This could well be too much for the output stage or power supply. Using the knowledge of the load impedance, the system could alter the gain or impose a ceiling on the output power.
Put simply, there is the opportunity to create a fully intelligent and manageable solution to this final and critical stage to any audio pathway.
The future for direct digital feedback amplifiers
Digital processing and control of the amplifier provides benefits in terms of functionality, low cost complexity, and intelligence. The DDFA allows very high theoretical performance levels to be approached in practice, without the need to specify expensive or specially selected electronic components. This translates into excellent aural results, allowing users to enjoy previously unheard detail and clarity in an involving listening experience.
About the author:
David E.L.Jones, Audio Systems Engineer at Zetex Semiconductors for 3 years, has been involved in audio and signal processing since 1983 when he worked on the first professional CD players, building on his power electronics experience and lifelong passion to design superior audio amplifiers. He holds a BSc in Mathematics and is a member of the AES.