ACCE3 CFA - current feedback power amplifier (Accuphase topology)

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ACCE3 CFA - current feedback power amplifier

This project is dated of 2016. I started it for the reason that I was interested in a current feedback topology behavior. The current feedback concept is well described in Renesas company application notes. For those who are interested, I recommend to download the application notes that I have linked below. I could hardly say anything more.

https://www.renesas.cn/cn/zh/document/apn/an9420-current-feedback-amplifier-theory-and-applications

https://www.renesas.cn/cn/zh/document/apn/an1993-voltage-feedback-versus-current-feedback-operational-amplifiers

Amplifier schematics
is as follows. Not shown is the DC servo that keeps amplifier output DC voltage close to zero. The circuit was inspired by Accuphase topology used in E-306 and E-406 amplifiers.

ACCE3 CFA

Accuphase topology:

The goal was to design a simple and robust amplifier that would be able to drive difficult loads, would be stable and would have balanced time and frequency domain characteristics. The lowest nonlinear distortion was not the design goal, just to keep it inaudible.

The amplifier has now been reliably operated for 9 years, without a single problem.

Output devices are robust OnSemi MJL21194/93. Drivers are MJE15030/03. Similarly as in case of my PA4 amplifier, the rugged and reliable amplifier was my goal.

The amplifier is strictly dual-mono, channels do not share any common ground point, so no ground loops are created.

Photos of the amplifier

Front view

Rear view

Teardown

Basic specifications

• Output power 2 x 150W/4ohm at THD+N = 1%
• Frequency roll off (-3dB) 225kHz
• Harmonic distortion <0.02%/1kHz/100W/4ohm
• CCIF 19kHz+20kHz 0.01%/30W/4ohm
• Rise time of step response 1.4us
• Gain 27dB
• Output noise voltage 68.58uV/BW20kHz, 35.50uV/A-weighted
• S/N ratio 92.3dB/2.83V/BW20kHz/linear, 111.1dB at full swing
• S/N ratio 98dB/2.83V/BW20kHz/A-weighted, 116.8dB at full swing
• Dimensions 450 x 440 x 190 mm
• Weight 27kg approx.

Distortion measurements

THD+N vs. output power into 4ohm at various frequencies, @BW=20kHz

The amplifier distortion is almost independent of signal frequency and the maximum power does not change with signal frequency.

THD vs. frequency into 4ohm load at 3.5W – 55W

THD slowly rises above 4kHz. The profile remains almost unchanged with output power.

The amplifier is fast, with nice aperiodic step response and short rise time, which are the benefits of the CFA topology.

 

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@KSTR has suggested to post distortion residual instead of distortion spectrum. So, I am posting both. Distortion residual is obtained when fundamental test frequency (1kHz here) is notched (filtered out) from the amplifier output. The method has been usual since old days of audio and still sometimes used in Stereophile. I am posting both methods below.

1. Distortion residual shape in time domain

2. Distortion residual in frequency domain

3. Complete spectrum with 1kHz test signal

 

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Test with capacitive load

Another test I always perform is a THD vs. power and THD vs. frequency with capacitive load 2.2uF added in parallel to the load resistor. I am showing results with 4.7ohm load compared to 4.7ohm//2.2uF load below.

1. THD vs. output voltage at 1kHz

The plots are identical, there is no difference if the capacitor is added or not.


2. THD vs. frequency at 7.41Vrms output voltage

We can see some small rise of distortion above 5kHz when the capacitor is added.

These results are excellent and confirm stability with complex load and overall robustness of this topology.

This is a very different story when we compare it to results obtained with class D top performers Hypex and Purifi. These are the results measured previously with Hypex NC252MP and Purifi 1ET400A. These amplifiers have big problems with capacitive load, as I have shown many times. This applies to most of class D amplifiers and to me it is an important reason to keep preferences of class AB amplifiers. Side by side with continuous output power and long term reliability.

FYI, Accuphase E-306V datasheet:

https://www.accuphase.com/cat/e-306ven.pdf