56W/8Ω amplifier based on LM3875

Rohit Balkishan Dubla

Figure 1 shows the amplifier schematic (one channel only - two ICs are required for stereo). It is based on the 56W power IC LM3875 from National Semiconductor. The circuit is based on the typical application given in the datasheet but with a few changes as follows:

• 1. Voltage gain is slightly increased from that used by the typical application.

  2. The input pot has been removed and replaced with a high-pass filter comprised of R1 & C1.

  3. There is a zobel network at the output.

The power supply is ±35V DC from a 25-0-25V transformer. The supply must be properly decoupled using 100nF decoupling capacitors close to pins 1 and 4. With this, the amplifier will deliver about 56W into an 8Ω load. Also, note that the schematic shown has a different component labelling than that on the datasheet schematic - bear this in mind while comparing the two. For stereo operation, the transformer may be rated between 150VA to 225VA. I have used a 225VA transformer** since this amplifier is going to be used mainly for bass (in a 3-way active set-up) with low-frequency equalisation which gives a 6dB boost to the signal at around 30Hz. However, a 150VA transformer can be used without any problems if no low-frequency boost/eq. is applied, at the cost of a small reduction in maximum power. The rectification and filtering are done by a 25A bridge and 6800µF/50V capacitors per rail, respectively.

The rather big (6800µF) filter capacitors and the 225VA transformer may appear to break the gainclone "norm" of using 1000µF filter caps (and perhaps a smallish transformer) - with bigger caps and transformer the low frequency performance improves by a large margin. With small capacitors and/or transformer, the bass response will suffer rather badly as the rail voltages will simply droop with bass transients - the result will be either clipping or "suck-outs". With the transformer and capacitors that I have used, the LM3875 sounds as good as any other "standard" or discrete power amplifier. In fact, I doubt if anyone could ever tell the difference simply by hearing it. I would recommend 120VA as the absolute minimum transformer rating for stereo operation, with 150VA being a good compromise between cost and performance.

The IC must be mounted on a heat-sink whether or not a signal is applied (when powered up, of course). Although the IC's SPiKe protection scheme will prevent any damage due to over-heating, unnecessary heating will shorten the device's life and reliability. To maximise thermal transfer between the IC and the heat-sink, the insulating mica washer may be omitted. However this will cause the heat-sink to be at the -ve supply - not a problem for my current set-up since the amplifier is housed in a wooden enclosure.

This amplifier is part of an active 3-way setup. Click here for photos.

** - the 225VA figure is based on an assumed efficiency of 68% each for the IC's and the rectifier/filter section (yeilding a final VA of 216 - 225VA is the closest standard). These are conservative figures and in practice even a 150-180VA transformer will be found to be quite sufficient.