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Old 16th February 2017, 10:35 PM   #1
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Default Sap15 biasing in power amp

Hi,

I am trying to set the bias on two musical fidelity xas100 power amps (4 mono amps).

Chips used are SAP15.

Datasheet has an explanation, as do sites/forums online but I'm struggling to interpret the information and apply it to the amp. I'd say my knowledge is begginer.

Data sheet:

"1. Recommended Operating Conditions
➀Add a variable resistor (VR) between diode terminals to adjust the idling current. The
resistor having 0 to 200Ω is to be used.
➁Adjust the forward current flowing over the diodes at 2.5mA.
➂Adjust the idling current at 40mA with the external variable resistor.
Both the temperature coefficients for the transistor and the diodes are matched"

200 Ohm VR fitted between (D-D)
Assuming the 2.5mA will be set by design (across D -D)
All I need to do is adjust the idling current to 40mA Question is, where do I take readings from to determine the current?

Any advice/assistance is much appreciated.

Thanks,

Kevin
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Old 16th February 2017, 11:11 PM   #2
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For those who may want to offer assistance, the datasheet is attached to this post. The circuit diagram of the bias network is shown below.
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Old 17th February 2017, 02:29 AM   #3
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Take the reading from pins 4 and 5 on the sap15n's, 8.8mV for 40mA.
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Old 18th February 2017, 06:29 PM   #4
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Thanks for the info, much appreciated.

I have set the (first after the VR) SAP15 NPN side across pins 4+5 (S+E) to 8.8mv on all 4 amps.

Measuring the DC across the speaker terminals shows varying amounts on each amp. Also, now that they have been on for 30mins and settled each heatsink is the same temperature at 40deg C. All emitter R now measure 9.6R.

Amp 1: 30mV
Amp 2: 3mV
Amp 3: 21mV
Amp 4: 52mV

Should I be concerned about the differing DC voltage at the output?

Thanks
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Old 18th February 2017, 09:42 PM   #5
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Nothing wrong here all amps, without servos at least, have a bit of offset. under 100mV is not a danger to speakers.
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Old 18th February 2017, 10:14 PM   #6
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Splendid.

Thanks again.
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