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Old 28th February 2004, 01:55 AM   #1
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Join Date: Feb 2004
Location: Midwest
Default Strange Humming issue...could it be?

Ok, I built an amplifier with an LM4766T (stereo IC) and LM837 functioning as a unity-gain 2nd order high pass filter. The amp works great. The filter works great. I have a hum of 6mV at the speaker leads.

This is what I have from beginning to end:

Signal Input to RCA input. RCA Grounds to plane where LM837/filter is referenced. RCA Grounds are isolated from chassis. Signal passes through 20k volume control wired as voltage divider. Unity Gain NI Buffer of LM837 passes the divided signal through to a L-R 2nd order HP filter also configured for NI unity gain. From here, the signal goes straight to the amp with a 50mV offset that is negated by a DC blocking capacitor in the amplifier gain loop (typical National app note Ci). Power supply consists of a 40VCT EI core and 2 x 3300uF along with decoupling caps right at the LM4766T. The OpAmp gets its supply from a pair of Zener regulators consisting of a 420 ohm resistor feeding 12V Zeners.

The amp is built on a 2 sided FR-4 board. It is star grounded. The RCAs are isolated from chassis ground, but are connected to the same plane area as the LM837. disconnect the input of the LM4766T and there is ZERO hum. Adjusting the gain does not change the amplitude of the hum.

Looking at the 'scope I see a little 6mV divot occuring at 120Hz intervals. This would tell me that I do not have enough power supply filtering - so, is it the Zener regulator that does not provide enough filtering? Should I use a 3 terminal regulator, or will this not help the situation?

Now, here's the kicker. I built another amplifier without the filter as a prototype. It is dead silent. It uses ALL the same parts - the only differences are the Zener regulators and the filter components.

So, why on earth would this be humming?? This is not the first amplifier I've built, but it is the first with these components. Please help!!
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Old 1st March 2004, 08:13 AM   #2
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Join Date: Mar 2007
Looking at the 'scope I see a little 6mV divot occuring at 120Hz intervals.


This probably is AC poorly converted into DC, check your rectifiers and capacitors, that the only guess I could make.
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Old 3rd March 2004, 03:55 AM   #3
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Join Date: Feb 2004
Location: Midwest
Default Persistence pays off!

Fixed! Everything was right except the feedback resistor ground connection was right in the path of capacitor charging currents. I set it aside for the weekend and found the problem right away today.
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