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Old 8th February 2013, 08:37 PM   #1
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Default Quiescent current way high on ESP P3a

I recently built two ESP project 3A amps from the official circuit board. One works excellently but the other will get hot fairly quickly because it draws over 600mA of quiescent current! The trimmer pot VR1 measures 2.02 kOhms and even when I short the other 1K fixed bias resistor (R16) the current only goes to around 480mA.

I have replaced Q9 (bias servo) and Q6 (driver) and neither fix helped. I no longer have a working oscilloscope either so I use a multimeter to measure drop on the 5 watt output resistors.

Has anybody fixed this issue themselves? Does anyone know what the potential errors with my amp could be? Thank you all.
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Old 8th February 2013, 09:29 PM   #2
jaycee is offline jaycee  United Kingdom
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Does R15 get hot ? If so, the amplifier is oscillating.
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Old 8th February 2013, 09:55 PM   #3
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R15 does not get hot. I just did a 2 minute finger test. I might add that this amp works perfectly except for the quiescent current issue. A speaker is hooked up as well as an input source but the source is muted. The reason I had these hooked up is because I like to do minimal signal testing (with the 22 ohm testing resistors in place.) Strangely, with them in place the voltage drop on them was only 1 volt and they remained cool.

Also, the current rises as the amp heats up... shouldn't it stay the same or even decrease? (From about 200mV to 300mV across op resistors = from 600 to 900mA :O )

I always make sure to turn the amp off when it is no longer "touchable" on the top side of the TO 3P transistor cases. That, in my experience, is the hottest part at any given time.
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Old 8th February 2013, 10:29 PM   #4
jaycee is offline jaycee  United Kingdom
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There must be something different in the circuit. Try measuring voltages at the collector of Q3, collector of Q4, and the base on Q6 for both channels (referenced to ground).
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Old 8th February 2013, 11:08 PM   #5
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Never mind, it was oscillating. I got my amp working again. Apparently the frequency was so high that R15 only got warm, and only with the input disconnected. I had left a small ( less than 1/2 inch) length of the input cable unshielded/ungrounded between the board and t he cable, which was common grounded at the other amp board... so basically I fixed it by adding... a ground loop?!

Now I want to know why the thing would register a "DC" current on my multimeter when there were HF AC oscillations...
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Old 8th February 2013, 11:48 PM   #6
jaycee is offline jaycee  United Kingdom
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the reason is because its oscillating, which is effectively a signal on the output stage. That causes more current to flow. Also the outputs wont be switching off quick enough, so youre getting cross conduction.

I've found the P3A is a bit twitchy with only 100nF decoupling capacitors on the main board. Adding 220-330uF decoupling often helps.
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