Class AA audio circuit

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Class AA audio circuit [Copy link]

There are two types of tone controllers that we commonly use now, namely attenuation tone and negative feedback tone. Negative feedback tone is widely used in various amplifiers with lower harmonic distortion and higher signal-to-noise ratio. However, as people's appreciation level improves, they find that due to the existence of negative feedback network, signal transmission is affected, and its tone is always a bit dry and lacks brilliance. Especially when the treble is increased, the sound is even more harsh. The second type is attenuation tone. Due to the attenuation method, the impedance of the entire circuit will change during the adjustment process of this tone. If the components are not selected properly, the noise of the entire circuit will be greater. However, its transient characteristics, distortion and subjective listening experience are significantly better than negative feedback tone. Panasonic has designed a class AA amplifier, the most significant advantage of which is that it still works in the class A state even when connected to a heavy load, even if the voltage and current waveforms are different from the composite dynamic impedance. Therefore, the attenuation tone resistor-capacitor network is a perfect match as the front of the class AA amplifier.

The circuit of the AA-type attenuated tone board is shown in Figure 1. The op amps are preferably AD827 and EL2604. The op amp A1 is used for voltage amplification, and A2 is used for current amplification. The working current for driving the load is provided by the current amplifier A2. The resistors are all 1/4W five-color ring high-precision low-noise metal film resistors. The capacitor has a great influence on the timbre of the entire circuit. The red WIM A variety is selected here. When the machine is connected, it is measured once with a digital meter to ensure accuracy. The potentiometer is an exponential type (Z), and the power supply can be powered by an active servo, as shown in Figure 2. The transformer uses a 25W ring cow. Please note that the voltage regulator blocks 7815 and 7915 of the active servo power supply are added with a first-level RC filter circuit before and after. All electrolytic capacitors have been replaced with EC film capacitors, and four voltage-dropping resistors have been added. The 10Ω resistor added after the rectifier tube and the 2000μF EC capacitor behind it form the first-stage RC low-pass filter, which greatly filters and suppresses various clutter and pulse interference in the power grid and the pulse interference generated by the power rectifier tube during the opening and closing period. The filtering and suppression effect is much better than the original use of 4700μF large electrolytic capacitor alone. The 8Ω resistor added after the 7815 and 7915 voltage regulator blocks is to make it form the second-stage RC low-pass filter with the 160μF EC filter capacitor behind it. They play two roles here: first, suppress and filter the current ripple output by the voltage regulator block when working, and also suppress and remove the power supply pollution leaked into the first-stage RC network, so that the current change obtained by the operational amplifier AD827 when working is very pure; second, isolate the audio signal so that the signal source no longer passes through the voltage regulator block with a complex internal structure to enter the ground, thereby greatly reducing the nonlinear distortion of the audio signal.

After the completion of the tone board, I inserted it between the TDA7294×2 amplifier and the CD and listened to it. I felt that the resolution was extremely high and the tone was relatively balanced.