I managed to pickup two LA300M Mosfet amplifier modules at a really good price and could not resist the temptation to do an amp build. Then came the shock of what a 1kVA 50-0-50 transformer costs. Fortunately I got one on ebay along with a PSU board and a speaker protection board. The power supply board has 4 x 10 000 uF Capacitors. I was also lucky enough to get an old Parasound HCA-1500A case from an amp that had failed and the owner was throwing it out. The capacitors had leaked and there was some corrosion on the bottom plate of the case.
The modules have fins and I thought it would a bit of a gamble to rely on natural airflow so I opted to add four 80mm x 80mm 12VDC fans, one over each of the vents in the bottom of the case. I then had an aluminium bracket made up to lift the modules above the fans.
I still have to setup the fan control and I have been prototyping a fan control which uses a LM35 on each module with a x 3.2 gain then onto two diodes which means highest of the two temperatures drives the fans. I then use the two inputs to go into a 11X gain DC amp to produce 4.3 V at 31 degrees C and this is proportional up to to 12V at about 55 degrees C. I setup a Schmidt trigger comparitor to switch on the fans at 41 degrees C at which point the Fan voltage is just under 8 volts. If the temperature starts to decrease the voltage drops proportionally until the comparitor switches off the fans at 31 degrees. If the temperature increases the fans speed up until they reach 12 V at about 55 degrees C. With four fans running at 12 Volts it sounds like a bee hive but below 5 Volts that are extreemly quite. I am not sure if these temperatures are too conservative or if I should do nothing until the amp reaches say 65 degrees C and switch then off at say 50 degrees. Anyone got any experience/ advice in this area.
I must also still do the clipping circuitry. I found the following on the internet which is what I was thinking of using. The front panel of the amp has a pair of current overload LED
http://circuit-diagram.hqew.net/Power-Amplifier_4876.html
The front of the Amp looks as shown below
The modules have fins and I thought it would a bit of a gamble to rely on natural airflow so I opted to add four 80mm x 80mm 12VDC fans, one over each of the vents in the bottom of the case. I then had an aluminium bracket made up to lift the modules above the fans.
I still have to setup the fan control and I have been prototyping a fan control which uses a LM35 on each module with a x 3.2 gain then onto two diodes which means highest of the two temperatures drives the fans. I then use the two inputs to go into a 11X gain DC amp to produce 4.3 V at 31 degrees C and this is proportional up to to 12V at about 55 degrees C. I setup a Schmidt trigger comparitor to switch on the fans at 41 degrees C at which point the Fan voltage is just under 8 volts. If the temperature starts to decrease the voltage drops proportionally until the comparitor switches off the fans at 31 degrees. If the temperature increases the fans speed up until they reach 12 V at about 55 degrees C. With four fans running at 12 Volts it sounds like a bee hive but below 5 Volts that are extreemly quite. I am not sure if these temperatures are too conservative or if I should do nothing until the amp reaches say 65 degrees C and switch then off at say 50 degrees. Anyone got any experience/ advice in this area.
I must also still do the clipping circuitry. I found the following on the internet which is what I was thinking of using. The front panel of the amp has a pair of current overload LED
http://circuit-diagram.hqew.net/Power-Amplifier_4876.html
The front of the Amp looks as shown below
