Current draw fluctuation with op amp + LME49600 amp

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BuckarooBanzai

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Joined 2015
I've recently finished putting together a basic headphone amplifier consisting of an op amp (presently a NE5532) followed by a LME49600. Schematic is the one from The Wire SE-SE (SE-SE-Schematic.pdf - Google Drive)

I was in the process of power up testing using a dummy load. With inputs shorted the current draw is stable at 20mA for one audio channel. Once I applied a signal (1kHz at 1Vrms) I noticed my bench power supply output current was fluctuating a little. I then put multimeters on the positive and negative supply lines. The current draw peaks at 30mA, but drops to 20mA and then back up to 30mA about twice a second. The same happens at 1Vp-p but the fluctuation is smaller.

Any idea what is causing this and is it something I should be worried about?
 
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BuckarooBanzai

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PRR said:
Current should rise with signal power to load, in any class AB power amp.

Should be steady. Twice a second is odd. Unless this is a DMM with rounding error and an update rate like that?
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One of the meters is a Keithley 2015 with VFD display. On the slow update setting the change isn't visible. On the medium and fast settings it's there. The other one is a regular handheld DMM which just barely shows it.

I tried another power supply and got the same thing.


Michael Bean said:
Where are you breaking into the circuit to read the current draw? Sometimes the shunt in the meter is too high of a resistance and can cause problems with the DUT.
Click to expand...

Between the power supply and the amp. The power supply shows the fluctuations without there being a meter in the path, just with less detail as its update is slowish.


:boggled:
 
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jan.didden

jan.didden

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Joined 2002
linearaudio.net
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BuckarooBanzai said:
No, the voltage remains completely stable to within 1mV. Measured at both the board input and at the IC pins.
Click to expand...

Well, if that is the case, and if the 'fluctuation' you see is also there with a completely different amp, that is evidence that you are not observing the right thing. So most likely nothing wrong.
Is the fluctuation on the power supply current meter? That would mean the meter or its circuit is flaky.

Jan
 
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BuckarooBanzai

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Joined 2015
I figured it out! I borrowed an oscilloscope from a friend and had a look at the waveform. Turns out the software I'm using isn't providing a constant output, but rather in pulses.
Switching to an online tone generator gets rid of the current fluctuations. Thank you everyone for your input! :cheers:
 
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