RogerK · 2 years ago
Nice clean and powerful amplifier - Update
As I had planned, I replaced the little 25 volt transformer with an Avel Y236803 500VA 35V+35V Toroidal Transformer (part # 122-665). Now I get 49.4 volts DC on the filter caps. So how does it work now? Well, I ran the amplifier into a pair of 8 ohm non-inductive dummy loads, connected the amp inputs and outputs to the oscilloscope in summation mode (to view any DIFFERENCES between input and output) and started cranking. The amplifier was clean right up until the output voltage was around 35 volts RMS with a 1000 Hz sine wave input. At that point, the amp started to clip. Into an 8 ohm load, that corresponds to around 150 watts RMS. This matches the specs almost exactly (remember, I was driving an 8 ohm load). While doing this, the dummy load resistors got hot - the amplifier got mildly warm. I only have 2 dummy loads, so I paralleled them and tested one channel at 4 ohms. I got the same result - the amp was clean up to 35 volts RMS where it started to clip. This corresponds to around 306 watts RMS (again, right on spec). Driving one channel at 300 watts made the amplifier only slightly warm (about the same as the 2 channel 8 ohm test). I couldn't run it long enough to see how warm the amplifier would ultimately get (because the dummy loads were getting hot!), but it ran long enough that I'm sure the amp probably would be comfortable to touch at full power. I am completely happy with this little board. It works like a charm - and meets the published specs too!
A final note: If you ever plan to test out this board or any other board like it with external test equipment (oscilloscope, meter, etc) be sure you use an ISOLATION TRANSFORMER because the speaker outputs are BOTH are above ground. You cannot safely connect a test instrument to the board if it's directly grounded to the power mains. Failure to do this can give you a shock, burn out the board, blow fuses, start a fire or all other kinds of bad things. Be careful!
