Why IC Power Amplifiers?
There was a time when the use of ICs (Integrated Circuits) in high-end audio was almost universally regarded as undesirable. The first ICs designed in the 1960's and 70's had poor slew rates and a host of other problems, making them inferior to discrete designs. However, times have changed. Over 40 years of collective design experiences have transformed the scene. We now understand more about various mechanisms of distortion and have the advantage of more advanced fabrication technologies and faster transistors. We at Anedio believe that IC power amplifiers built using National Semiconductor's LM3886/LM4780 offer truly exceptional performance, on a par with or even superior to discrete designs.
Its sonic potential was first noted by 47 Laboratories, which introduced an IC-based amplifier called GainCard. It was a paradigmatic shift in the philosophy of high-end amplifier design, and it started a small revolution in the DIY world, with countless hobbyists confirming the quality of its sonic performance.
The beauty of the IC amplifier is its simplicity. It is remarkably easy to build, and even novice hobbyists can build a good sounding amplifier out of it. At the same time, it requires just as much attention to details as any other amplifier to obtain the best possible performance. With careful design and tuning, its sonic performance can truly blossom.
"Of all our human resources, the most precious is the desire to improve" -- Anonymous
We started our own investigation by building several versions of IC power amplifiers based on National Semiconductor's Overture series. Through our investigations, we discovered that its potential had not yet been exhausted and that it could be pushed even more to a higher level by optimizing the circuitry around it and the PCB layout. The result of this optimization has demonstrated that the performance of an IC power amplifier can equal or exceed that of expensive discrete designs. The only limitation is its moderate power capability, which still is adequate for most home listening environment.
Key Advantages of IC Power Amplifiers
What are the key advantages of IC power amplifiers over conventional discrete designs? Three factors stand out:
Small Current Loops: With power amplifier ICs, a fully-functional amplifier can be built using merely a handful of capacitors and resistors. This austerity lends itself to a compact layout, with tiny current loop areas compared to discrete designs. Why is the loop area important? The smaller the current loop, the less noise picked up by the circuit. The inside of the amplifier enclosure is actually a noisy place, with lots of high-order harmonics from the power supply (for Class AB amplifiers) and multiples of 60Hz from the transformer floating around. Given such a noisy environment, the small loop area is a great antidote to unwanted noise. The compactness of the IC power amplifier results in a very clean noise floor, well below the realms of audibility.
Small Parasitic Capacitances and Inductances: The integrated-circuit technology reduces drastically the capacitances and inductances associated with the layout. Within the IC, wiring distances are orders of magnitude shorter compared to discrete designs. Also, outside the IC, the layout can be compact, as already mentioned. All of this, along with the sound circuit design of LM3886/4870, contributes to superior speed without resorting to expensive technologies. Even with a 2uF capacitor load, just about the worst capacitive load it will drive, the amplifier remains stable and well-behaved. Its excellent transient response is well suited to driving all types of loudspeakers.
Close and Fast Thermal Tracking: Another key advantage of IC amplifiers is the close thermal tracking of all the transistors. Since all the transistors are on the same chip, any excessive heating in the output transistors can be detected fast and reliably, and actions can be taken quickly to protect the amplifier and the loudspeaker (See National Semiconductors Application Note AN-898 on SPiKe protection circuit). Moreover, the bias devices for the output stage can track the output transistors closely in real time over a wide range of temperatures. The result is consistent performance whether the amplifier is cold or hot and whether it plays softly or loudly. The amplifiers settles into its optimum operating condition within a few minutes of turn-on, and its performance remains solid over a large range of temperatures and power levels.
These three factors, we believe, are the most significant ones that contribute to the excellent performance of LM3886/4870 amplifiers.
Are there then any limitations to IC power amplifiers? The power handling capacity of a single IC is moderate. But this is not an inherent limitation. The power output can be increased significantly through the use of bridge and parallel techniques (See National Semiconductor's Application Note AN-1192).
Understandably, some are concerned about thermal distortion, which arises from the heat generated in the output transistors flowing into the input transistors. The first generation IC op amps indeed suffered from this effect, but again, a significant progress has been made. By carefully controlling thermal gradients and by using the common-centroid layout for the input devices, the thermal distortion can be made negligible. If it is present, it would manifest itself as an increase in harmonic distortion at low frequencies. In Anedio's implementation, no increase is observed, and the total harmonic distortion at 20Hz remains below the threshold of measurement, about 0.0002%.
In conclusion, IC power amplifiers, such as the LM3886/4870, can be pushed to state-of-the-art performance by carefully optimizing the circuitry around it and the PCB layout. And such a level of performance can be achieved at a fraction of the cost of typical high-end amplifiers, making them accessible to all audiophiles. Most important of all, it offers exceptionally musical sound that engages the listener, powerful transients that communicate the emotional impact, and extraordinary definition that draws the listener closer to live performance.