60 Cylce Hum Driving Me Batty

[K1ngR0y]

I have a circuit which takes a 15VDC supply from a wall adapter.
Most connections are made by shielded cables and I have employed the manufacturer's (and others) recommendations for hum reduction.
Is it possible to insert a 60-cycle notch filter or other high-pass that'll attenuate the hum?

 

[D.A.(Tony)Stewart]

Loop inductance can be reduced by area of the loop.

The amplifier has 50 dB PSRR but an unknown CMRR to conducted or radiated ground noise with 50 k input spec on Amp.

Measuring the ripple on DC output should be << 10 mV.
The hum was eliminated by cutting both conducted PS paths. Reset by cutting only 1 path ( DC +) but remain 0V to ground connected to batteries gnd.

This will test if the noise is injected by radiation or conduction CM noise.

There is some imbalance in emitter impedance effect on IN +/- gains but reduced by negative feedback thus unknown CMRR.

 

[betwixt]

I think we also need to consider the load as a source/sink of ground currents. This is why I queried whether the hum was present with a loudspeaker connected directly as a load or whether the higher impedance output was feeding a later stage of amplification. There is a possible ground loop between wall supply and a post-amplification system.

We really need to see how it is constructed and how it fits in the bigger picture.

Brian.

 

[D.A.(Tony)Stewart]

It looks like a low gain ~(50) final stage to speakers but supply noise is still TBD for both CM and DM.

 

[K1ngR0y]

I think now it does not make any difference for the Caps whether in or out.

As I suspected 50% pot position might be the loudest as that is the highest impedance for stray current to be amplified.

Compare all the grounds for L/R as there must be something missing or different.

When Vcm hum is conducted, the highest unbalanced impedance creates the largest voltage and usually gets partially rectified to double the frequency by the input stage protection diodes. grounding the supply to PE gnd at the source may help using a spade or ring lug.

--- Updated Jul 16, 2024 ---

The LM386 input impedance is only 50k and the source impedance of the pots varies around the 0 to 100k. This makes for very poor CMRR while a balanced high input impedance has good CMRR. But there has to be something different on the L vs R signal to ground path coupling to the signal.

Since batteries work as expected, raising the CM impedance is usually done by a Balun/ CM choke. But at 60 Hz this might look like a 100mH 1:1 transformer which shouldn't be necessary. There seems to be a ground loop or open shield.

You may have to re-route your passive input paths.

Thank you. I am going over everything with fine tooth comb. (And I'm looking into a proper power supply.)

 

[D.A.(Tony)Stewart]

Consider 12V from any PC PSU, also it is PE grounded and just tap into any Molex plug.

 

[Analog_Joe]

This sounds like a classic ground loop problem. So I'll just throw it out there and probably take the heat for it, but if you ever get tired of trying to trace the culprit with no success, get a couple of audio input transformers and they will, most likely, solve the issue. Transformers do add some distortion and artifacts, particularly at low frequencies, which also increases as signal drive increases, but good transformers (aka Jensen Transformers) reduce distortion to a minimum. EDCOR manufactures good and relatively cheap transformers, they might not be as high-end as other brands but they are fairly good and get the job done.
I received a call (around 12 years ago) from a friend working at a record label asking me to help them solve an issue with their meeting room. In it they had a large speaker system, several video screens and a very expensive-looking remote conferencing system, kind of like Zoom but before Zoom ever existed. Every time someone connected their laptop to the video screens and to the the PA at the same time a massive hum/buzz blasted through the speakers. It didn't look good for them whenever they were trying to show off the new music video from artist X with a massive hum & buzz attached to it. Anyway, to make a long story short, I used a healthy dose of audio transformers and the problem was gone for good.