DIY LM3886 Chip Amplifier (Gainclone) Kit
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.
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
Construction - LM3886 Chip Amplifier (Gainclone) Kit
For my gainclone build I purchased the circuit boards for a Non-Inverted LM3886 amplifier and power supply from chipamp.com. I decided against the partial kit that includes the LM3886 IC, rectifiers, capacitors and resistors because I already all had pretty much all the required components on hand. For the resistors I opted to use carbon film resistors which I had on hand. The carbon resistors are standard 0.5W jobs and you have to stand them up as shown in photograph 4 in order to fit the PCB which is sized for compact size resistors. For my subsequent DIY gainclone amplifier builds I have been using 0.5W 1% compact metal film resistors which have worked out well. Note that the Zobel and bleeder resistors needs to be rated at 2W or greater. For the 10000µF filtering capacitors shown in the power supply, I used 15000µF caps which I also had on hand and fit the power supply PCB. The capacitors do have an impact on sound and using good quality capacitors will result in improved sonics. I like the results obtained using Nichicon Muse, Panasonic, Black Gate and WIMA capacitors. The semiconductors are National LM3886 and for the diodes are ultra-fast MUR860 which I had in my collection as samples. The toroidal transformer used for the gainclone amplifier build is a 160VA Plitron with 22VAC dual secondaries and recovered from a previous amp build. The heat sinks that I used are larger than required are also from a previous DIY amplifier build.
Photograph 4: LM3886 IC Amplifier Assembled PCB with Heasink
For myself, I always find that the most difficult portion of a DIY amplifier project is often the enclosure. I had built a few vacuum tube amplifiers using plain Hammond aluminium chassis, so I decided to use a Hammond enclosure since I already had the heatsinks to use for the amplifier. The 1444-32 Hammond chassis was available locally in Winnipeg from Tip Top Electronic Supply for about $33 (May 2006). Aluminium is a nice material to work with for project enclosures as it is easy to drill, file and sand.
Photograph 5: Hammond 1444-32 Aluminum Chassis
For the mains power connection, I used an IEC receptacle rated at 6 Amps and with a built-in EMI filter. Consistent with a lot of the other parts, the IEC socket was recycled from a dead CRT computer monitor. The fuse holder (also from my used parts bin) holds a 2 Ampere slow blow fuse. For your wiring connections consult and follow the governing electrical codes for your area.
Photograph 6 below shows the underside of the LM3886 chip amplifier build. I twisted all the AC wires together and for obvious reasons, I kept the RCA inputs and amplifier PCB close together and far away from the AC. In the Photograph, you can also see the star grounding. The chassis turned out to be quite a bit larger than required so one could easily fit this amp into a smaller enclosure. The chassis and heatsinks are both larger than required so I did not bother to add ventilation the chassis as it would not be necessary. However, if you use smaller heatsinks, be sure to provide airflow around them for cooling. On the front of the gainclone amplifier build is a small power switch with a built-in LED that is run off of the power supply board.
Photograph 6: LM3886 Chip Amplifier (Gainclone) Wiring
From the rear view of the LM3886 gainclone amplifier shown in Photograph 7, you can see the plain nickle plated RCA jacks and speaker binding posts were used. The enclosure handles are old cupboard handles. There you have it, my first gainclone amplifier with less than $100 invested. If you have to purchase all the parts for yourself, including a nice looking enclosure you should plan to spend about $300 US. With the kits and guides from chipamp.com the entire process is made quite simple for you.
Photograph 7: Rear View of LM3886 Chip Amplifier (Gainclone)
Before you use the gainclone amplifier for the first time you need to check and ensure that there is no significant DC offset at the output terminals of the amplifier. To check, use a multimeter to check across the amplifier outputs for DC voltage. For this test no source is required. There is no need to worry about DC offset less than about 50 mV. Also, please note that there are no DC blocking capacitors in the signal path so you must also ensure that your source has DC blocking capacitors on the output and is not a source of DC.
Sound - LM3886 Chip Amplifier (Gainclone) Kit
When I fired up the DIY LM3886 gainclone amplifier build for the first time, I was immediately struck with how much power was available, the level of detail and the nice bass response. The LM3886 gainclone amplifier build sounded far better than I was expecting and much better than it should considering the simplicity and the fact that I used inexpensive components and a 160VA supply.
Photograph 8: Front and Top View of LM3886 Chip Amplifier (Gainclone)
For a component shootout I compared the sound of this LM3886 gainclone amplifier with the DIY K-12M tube amplifier kit I had recently built. The comparison between the LM3886 chip amplifier and the K-12M tube amp is a fair one in that they are both DIY amplifiers and similar in price (about $200 - $300). However, that is where the similarities end. The main different between the two DIY amplifiers is output power - into 8 ohm the K-12 tube amp is about 8W, while the LM3886 is 35W+. From a practical point of view, the LM3886 chip amplifier is far more flexible as it has much more available power and can easily drive a wide variety of speaker loads. For the listening comparison I used speakers with a rated sensitivity of 91dB/1W/1m and low to moderate listening levels. Between the two different amplifiers I found that I prefer the bass response of the LM3886 chip amp over that of the tube amp. The K-12M tube amp has a superior mid-range performance. The high frequency response of the LM3886 gainclone is further extended and crisper. For the cost, both these DIY amplifiers sound very good. I'll give the K-12M only a slight edge due to it's mid-range performance. However, at times and with some recordings the LM3886 gainclone amplifier does sound more detailed to me.
More Chip Amp / Gainclone Amplifier Projects
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- DIY TDA2050 IC Hi-Fi Chip Amplifier
- Synergy - a LM3875 Gainclone Amplifier
- DIY LM3875 Chip Amplifier (Gainclone) Kit
- Nanoo - DIY LM3875 Chip Amplifier (Gainclone)