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Oliver Acoustic Research AR-94 specifications
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This amplifier contains 4 Maplin 150w MOSFET amplifier modules but I have used higher supply voltage than + - 55 Volts as I have used + - 65 volts supply. This have increased the power output up to 200 watts RMS into 4 ohms. But I wanted more power as it was kind of a challenge so I put 2 modules as a bridge amplifier and the power jumped up to something very close to 500 watts RMS into 4 ohms.


The Project

The way I have driven the modules to supply an inverted and non-inverted signal was by LM324 quad op-amp. Every 2 op-amps are driving one channel. A very simple circuit with X1 amplification. I also added 2 loudness switches ( attenuators ) since I have used a center tapped gang volume. So that was a good option to set. Finally I have also used speaker delay switch to avoid the DC current flowing into the speaker until the amplifier stabilizes its bias.

You can see the schematic diagram of it. It's almost identical to this one I used from http://homepages.keme.com/~daatkins/proj_subamp.htm he has used the same module I used. So I used simply an RC circuit that triggers a Darlington transistor with a relay on its collector. The web link above has used a better circuit but the effect is the same. You can use a 555 timer too. Anything that delays a switch on for few seconds 3 - 5 seconds just to allow the amplifier to stabilize its output. Sure the relay connects the speakers to the amplifier output.

board schematic diagram (click to enlarge)

speaker delay circuit (click to enlarge)

Funny addition I made but vital is you have kids around !! ! my little brother once played with the volume and then switched the amplifier ON!! That time I had it connected to a speaker switch to use headphones and the amplifier burned the headphone converter box and burned my amplifier MOSFETS!! So ! I had to put a code switch to avoid such a disaster. Looks funny but it works.

Regarding the ventilation especially that it's built in a wooden box I have installed 2 PC fans one that sucks air from the bottom and one blows the air on the heat sink I have made a simple duct to make the air revolve around the heat sink to the air exit. The pictures can show this more clearly.

The amplifier is split into 2 chambers ( Left & right) the chamber is simply the power supply, the right chamber is the amplifier modules chamber. I have used a carton paper to split the 2 chambers, not electronically, but to force the air flowing from the bottom fan directly to the heat sink blowing fan. Otherwise the air pressure will be lost in the whole amplifier box and I will not make use of the ventilation.

The ventilation is necessarily needed since I'm using the amplifier in extreme conditions, and the bipolar drivers usually heat up from the quiescent current of the amplifier module. The ventilation makes them warm and steady other wise you need to change the bipolar drivers to accept more current. Also when the output MOSFETS delivers the power to output the heat sink gets really warm and you will feel the hot air from the back vents, I really recommend this option or you will need a huge heat sink.


top view of the amplifier chamber

the 2 fans .. one that sucks in and the other that blows on the heat sink

plastic cover to drive the air to the outlet. For driving the air from the front of the heat sink to the back of it leading the hot air to the exit holes in the back of the amplifier box.

where the air exits the amplifier ( hot air )

blower fan in the heat sink duct

the LM324 board

the amplifier module