얼마 전부터 시작한 프로젝트인데 그 동안 중단했다가 다시 손을 대 볼가 합니다.    아래 일본의 47 Laboratory란 회사에서  나온 제품을 300B 암프와 비교해서 손색이 없다고 주장하는 글을 읽어본 적이 있어 호기심이 생겼습니다.  여기서는 LM3875보다 더 출력이 큰 LM3886을 사용해 보려고 합니다.

 

4706 Gaincard Amplifier and Gainclone Amplifiers

So what the heck is a Gainclone amplifier? Originally, the term Gainclone amplifier was generally understood to be a clone of the 47 Laboratory Gaincard amplifier. The simple Gaincard amplifier is based around the National Semiconductor (now Texas Instruments) LM3875 integrated circuit (IC). The 47 Laboratory Model 4706 Gaincard amplifier has these features:

• 9 parts per channel based around the LM3875 IC
• A very short signal pass and NFB loop
• PSU filter with 1000µF to 2200µF capacitors
• Voltage regulation with a 170 VA cut-core transformer
• 50 Watts output power (25 Watts per channel)

However as time has gone on, to many Gainclone amplifier is now generally understood to mean: any low component amplifier based around high power IC and intended for high performance audio. Photograph 1 below shows a 47 Laboratory Model 4706 Gaincard which retails for about $3300 US. Despite the high retail cost of the Gaincard, you should easily be able to build a great sounding gainclone amplifier yourself for about $200 to $300 US.

Photograph 1: 47 Laboratory Model 4706 Gaincard and Power Humpty

The Model 4706 Gaincard amplifier was introduced in 1999 by 47 Laboratory. The Gaincard amplifier with its unconventional design that had far fewer parts, less capacitance and simpler construction than similar power Hi-Fi amplifiers. The Gaincard amplifier raised a number of eyebrows in the audiophile community when it was discovered that the Gaincard amplifier cost less than $100 US in parts, but sold for $3300 US with its small power supply. The controversy ensued after a number of positive magazine reviews of the Gaincard amplifier. Fast forward a number of years and now the Gainclone is probably the most commonly built and well-known DIY amplifier project amongst audio hobbyists. In this DIY Audio Project I run through a build of a LM3886 gainclone amplifier for myself to show how simple it can be to build these amplifiers and to hear the gainclone sound for myself.

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LM3886 PCB from chipamp.com

OK, so now you are interested in building a Gainclone chip amplifier for yourself and you are wondering where the heck should you start? You can start at chipamp.com where you can purchase printed circuit boards (PCB) and part kits for LM1875 (20W) and LM3886 (68W) amplifiers. This leaves you with sourcing the remaining parts, power transformer and enclosure. Photograph 2 shows the LM3886 amplifier PCB and power supply PCB from chipamp.com. These are very nice quality boards with short and thick traces. I have used these PCB for a number of gainclone amplifier builds now.

Photograph 2: LM3886 Amplifier and Power Supply PCB from chipamp.com

For detailed information about this non-inverting LM3886 amplifier circuit, the PCB and implementation, see the Users Guide for Non-Inverted LM3886 Kit - (PDF 280KB). For reference, the amplifier circuit from the above users guide is shown below in Figure 1.

Figure 1: LM3886 Chip Amplifier (Gainclone) Schematic

The amplifier circuit is built around the LM3886 IC (Photograph 3). The LM3886 is a high-performance audio power amplifier capable of delivering 68W of continuous average power to a 4 ohm load and 38W into 8 ohm with 0.1% THD+N from 20Hz - 20kHz. For the full details and operating parameters of the LM3886 IC refer to the LM3886 Datasheet - (PDF 1.2MB). The gain of the LM3886 amplifier circuit is set about 33. To adjust the gain to suit your Hi-Fi configuration refer to the users guide or LM3886 datasheet above.

Photograph 3: LM3886TF Itegrated Circuit (IC)

For reference, the power supply schematic, also from the users guide is shown below in Figure 2. Refer to the charts on the datasheet to choose the power supply voltage rails for the amplifier that are best suited for the speaker load you intend to drive. Using a power transformer rated between 160 and 300VA will be suitable for a stereo gainclone amplifier build. For my more recent builds I have been using 250-300VA and dual mono power supply boards. For further guidance on the selection of a suitable power transformer, refer to section 2.1 Choosing a transformer, in the Users Guide for Non-Inverted LM3886 Kit.

Figure 2: LM3886 Amplifier Power Supply Schematic