Amp Testing & Bias Setting

The amplifier is now being tested and the bias set from the driver stage.  I am using an LM4702B to drive the 2SK1058 / 2SJ162 MOSFET output stage.  These devices will have a bias of 115mA to take them just into Class AB and make sure their ratings are within the correct SOA (Safe Operating Area) and that crossover distortion is kept to as low as possible without helping global warming.  So 573mV quiescent volts on the gate of the 2SK1058.  The 2SJ162 needs a slightly different bias to the 2SK1058 but this value is good for both.

Although the LM4702 is a dual device with provision for 2 channels in each (for stereo); in order to keep excellent stereo separation (lowest cross-channel crossover) I am using one per channel.  At some point I will start looking into and testing using the spare channel in each to give an extra line-drive to provide a bass speaker or treble unit for those that want that.  Or they could be used to drive two extra loudspeakers in other rooms.  This would be quite a flexible arrangement so worth the extra effort and cost.

In the photo, you can see the voltmeter showing 573mV, you can see I am using +/-20V and the quiescent current being used.  You can also see that the positive, negative and earth connections all have star junctions.  Also shown is the temperature monitor with the sensor silicon pasted to one output transistor.  The Pure Jongo A2 is being used as a signal source with my iPad Mini providing a continuous 1KHz signal.

Output stage bias setting for LM4702, 2SK1058 and 2SJ162.

Output stage bias setting for LM4702, 2SK1058 and 2SJ162.


About Matal

Hand made HiFi, photos, a friends poetry.
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