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DB120A Power Amplifier
I built two of these old amplifiers back in 1997 and I have been using them for many years. They have worked really well and probably run almost every day for 20 years without any fails. It was intended that each amplifier should be made with four channels. On the PCB, which is symmetrically constructed, there are two amplifiers, each with 5 pairs of output transistors. It runs at +/- 63Vdc so there is plenty of power in it. The idea was that the four channels in each enclosure should be bridged two by two so that more than 600 watts in 8 ohm stereo could be pulled out of each amp.
On the PCB there are two mono amplifiers of 200 watts into 8 ohms with 10 output transistors each so this board can be used for stereo mode or a more powerfull bridged (full balanced) mono amplifier. These amplifiers are DC coupled with a DC servo so they are always 100% in balance with a DC offset of less than 10mV. The frequency range is 2Hz-150KHz -3dB. There is also an output relay for each channel with delay and DC protection. I have measured the output power of it in bridge mode to be 760 watts in 8 ohms and 1,440 in 4 ohm RMS.
I also made a 4 channel vu meter to it with different status LED's so you could see if the speaker relay was connected, if there was too high a temperature, if the amplifier clipped and much more fancy stuff. Unfortunately the project was never completed.
The sound is perfect and there is really good control over the speakers. It has very low distortion (below 0.01%) and is very powerful with 10 output transistors per channel (or 20 when bridged). The power supply is built with a big 1,500VA toroid transformer and two giant computer grade electrolyte capacitors of 65,000uF each. These capacitors are rated at 55 Vdc and 65V surge. I have been running them hard at 63 volts for almost 20 years now and they are still completely fresh. This amplifier was built for hobby only so not much was done for safety.
The biggest mistake I made with it was to assemble it and test it without finishing it all. Doing so can end up being a really really long test. In my case, it was more than 20 years before I gave it to my son who has now separated it to reuse the power supply for a new project, see here, Jonas Amp. When I assembled it, many things were missing. There should be two amplifier PCB's in each box (4 channels) as well as VU meter, status lamps, soft starter, auto power up/down, over temperature shut down and much more. Do never assemble your project before you have finished all the PCB's, because otherwise there is a risk that you will never get it done once it plays.
Here is the PCB layout. It is single-sided and the size is 180x200mm. It is made in Protel For Windows V2.5. The drawing date: 19960717.
Here are a few different pictures of the amplifier.
