LDA162, a low distortion power amplifier

The LDA162 power amplifier is capable of delivering 200 Watts into a 4 ohm load with very low distortion (the 1kHz THD of the first and second prototype measured less than 0.0008% @ 75 Watts into 6 ohms, and degraded to only 0.0013% @ 200 Watts into 3.5 ohms). Moreover the amplifier has very low THD at all power levels below 200 Watts and up to the highest audible frequencies. This amp should be suitable for the most demanding home audio applications. My intention was to design it to be very reliable.

If you want lower maximum power, that is viable without component changes, just by lowering the supply voltage. At 120 Watts power and below you may use only 1 output transistor pair instead of 2 pairs.

The circuitry includes protection circuits and an ultra low distortion solid state relay, which is faster than a mechanical relay for protection purposes.

Prototypes and measurements

In November 2018, I built a mono, then later

a stereo prototype.

 

The distortion measurements revealed an extremely low THD at 1kHz.

 

Other, very challenging kind of distortions, like 19+20kHz Intermodulation (IMD) or Transient Intermodulation (DIM) are also very low.

 

 

This is the protection circuitry.

 

The power supply includes an ultra low distortion solid state relay, which has several advantages over a mechanical relay.

 

Lately I have designed the second version of the amplifier PCB, and built another stereo prototype with this new PCB layout. As you can see, this is a medium power version, with only one output transistor pair.

 

Measurement results are somewhat worse at heavy loads, but I suspect it would be about the same if I had used 2 output transistor pairs. I have also designed a new power supply board (PSMR101S) with mechanical relays for those who would like to go cheaper, because the ultra low distortion solid state relay is much more expensive than a mechanical relay.

Listening test

In February 2019 I conducted an ABX style listening test with three young people, two of them were musicians. None of them could distinguish the LDA162 from a commercial amplifier (namely a Sony STR-DE598). The Sony has about 10x higher harmonic distortion in the output voltage range at which we listened to music. The test music was (1) the first 30 second of Mike Oldfield's Five Miles Out, and (2) the first minute of 2CELLOS' I Will Wait. The official recordings were played through an EMU 1212M sound card. The loudspeaker was a pair of PM200's that I had built back in 2003.

Non-profit LDA kits available

Right now, I'd like to sell a few low distortion amplifier kits without any profit. More information here.

 

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