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Old 23rd May 2008, 03:27 PM   #71
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We check the voltage between V+ and PG+
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Old 23rd May 2008, 03:32 PM   #72
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And between V- and PG- (black probe on PG)

If we measure approx. +/- 20V (positive will be lower because of LED) we are ready to connect amp boards.

Please note that voltages are much lower than expected beacuse of lack of smoothing caps.

If you have 10uF caps installed on rectifiers board both voltages would exceed 30V DC
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Old 23rd May 2008, 05:19 PM   #73
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The important aspect about sharing a single transformer and common rectifiers per two amp channels is power star ground.

While you could simply run PG+ and PG- (ground) wires from rectifier board to each amp board, in some setups hum problems were reported and presently I always use star (power) ground in all my amps.

I simply achieve it by connecting output grounds (OG, on the opposite side of speaker wire connection) with a 14ga solid core copper wire.

The center of that wire is my power star ground and both PG+ and PG- grounds from rectifier board connect here. In the picture below the connections are so short that I used single runs of wire, normally I use two wires for both PG+ and PG-.

Please note that power ground is also directly connected to the chassis.

I specifically refer to it as power star ground, as signal wires from RCAs are not connect directly here. They connect through separate traces on PCB to OG (output ground) pads.
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Old 23rd May 2008, 05:32 PM   #74
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More info on star and safety grounding was posted here: http://www.diyaudio.com/forums/showt...92#post1509392

If you need to implement ground break circuit between amp's power ground and chassis, you may consider the use of thermisor (CL60) as per F series of amps from Pass Labs.

I get my high gauge copper wires from electrical cables available at Home Depot.
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Old 23rd May 2008, 06:06 PM   #75
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We finish off by connecting power supply wires V+ (red) and V- (blue) from rectifier board to amp boards and this concludes the assembly.

Before we connect the amp to the speakers we need to test it.
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Old 23rd May 2008, 06:09 PM   #76
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Quote:
I specifically refer to it as power star ground, as signal wires from RCAs are not connect directly here. They connect through separate traces on PCB to OG (output ground) pads.
Should that read SG (signal ground) pads, Peter?
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Old 23rd May 2008, 06:18 PM   #77
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You are correct, wires from RCAs connect to SG (signal ground) pads. Signal star ground (on the board) connects with separate traces to OG pads, which are directly connected to the ground plane: http://www.audiosector.com/images/lm3875_se_pcb.gif
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Old 27th May 2008, 04:16 AM   #78
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The first thing we do before connecting any speakers to the amp is measuring the DC offset. This can be done directly at the binding posts with no load connected. If the potentiometer is installed, the offset will vary depending on volume setting. This particular amp (right channel) measures 32mV offset when volume is completely down (input shunted to ground).
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Old 27th May 2008, 04:22 AM   #79
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and 70mV when volume is at the maximum (15K combined input impedance of 50k pot and 22k input shunt resistor)

This is rather average chip when comes to offset, and for selected ones the offset is usually at 50mV or less at max setting.

The offset will be also lower when low impedance output preamp is connected.

When we switch the amp off and no speakers are connected, we can observe that for a moment or two the offset jumps pretty high to 4V or so. This is normal behaviour, and when load is connected (you may try it with 10R resistor across binding posts) the offset will stay low when turning the amp off.
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Old 27th May 2008, 05:14 AM   #80
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We'll also measure the power into 8ohm using dummy load connected to the amp's output (4 x 30R, 50w resistors) and feeding the amp with 100Hz sine wave. The measurement is taken before amp starts clipping.

At +/- 30.6V rails, the output voltage is approx 18V RMS with the input signal of 550mV. This translates into 40W output.

During such test, the heatsinks are way too small and the temperature rises substantially. At normal listening material and average levels though, the heatsinks are still fine.
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