LM3886-based gainclone amplifier

[WonderNomad]

I recently completed building a LM3886-based gainclone amplifier. The build seemed to go smoothly, all connections checked multiple times, and I'm getting audio output.

However, I'm plagued by a significant and persistent 50/60Hz hum (sounds like mains frequency) that's present even with the input shorted and the volume at minimum. It increases slightly with volume but is always clearly audible. However, I haven't figured out what's wrong yet.

 

[solderdude]

Could be wire routing (power, input or output loop), could be a grounding issue, could be power supply, could be an oscillation.

 

[antcollinet]

I recently completed building a LM3886-based gainclone amplifier. The build seemed to go smoothly, all connections checked multiple times, and I'm getting audio output.

However, I'm plagued by a significant and persistent 50/60Hz hum (sounds like mains frequency) that's present even with the input shorted and the volume at minimum. It increases slightly with volume but is always clearly audible. However, I haven't figured out what's wrong yet.

As @solderdude said - many possibilities.

Other than playing with cable routing, you'll probably need to use an oscilloscope to diagnose.


EDIT - if you take a picture of the circuits and wiring, and provide your circuit diagram, someone might be able to offer advice on what to try.

 

[audio_tony]

Check your ground paths. Ideally you should be using a 'star' ground technique. The OV (center tap) of the transformer(s) should also be brought to the star grounding point. Not using a star ground scheme is a common cause of hum.

Here's an example.

And this is how I did one of my projects - note how all the black ground wires are connected to a central point.

 

[antcollinet]

You might also want to move this to the DIY section.

 

[somebodyelse]

https://neurochrome.com/pages/taming-the-lm3886-chip-amplifier might be worth a read.

 

[DVDdoug]

What are you using for a power supply?

You may get hum into the electronics from a "linear" power supply, especially if it's unregulated. Voltage regulators are great at filtering-out that last of the ripple.

It's less likely with a switching supply which operates above 50/60Hz and usually above the audio range. And higher frequencies noise is easier to filter-out.

Or of course, a battery is "pure DC".

 

[antcollinet]

Or of course, a battery is "pure DC".

Except it isn't. The internal battery resistance will cause the output voltage to vary as the output current varies.

Of course - when the amp is not doing anything, this won't apply.

 

[bmc0]

You may get hum into the electronics from a "linear" power supply, especially if it's unregulated.

The typical PSRR of the LM3886 is 120dB for the positive rail and 105dB for the negative rail. You won't get "clearly audible" hum due to power rail ripple unless something is seriously wrong.

 

[wwenze]

What are you using for a power supply?

You may get hum into the electronics from a "linear" power supply, especially if it's unregulated. Voltage regulators are great at filtering-out that last of the ripple.

It's less likely with a switching supply which operates above 50/60Hz and usually above the audio range. And higher frequencies noise is easier to filter-out.

Or of course, a battery is "pure DC".

Unfortunately the general idea of the meta is that 50Hz power supplies for speaker amplifier output stage will not use a linear regulator. (But don't take my word for it, meta analysis can be done by anyone for free). Primarily because a linear regulator of that current and voltage is expensive and unavailable, plus it overlaps the same job as the amp chip.

It's definitely within considering and many lower power devices e.g. headphone amps do spam linear regulators e.g. a pair of LM7815/7915. Just not when you need ±50V. You can do your own circuit with pass transistors and chonky heatsinks, but, yea...

 

[wwenze]

I recently completed building a LM3886-based gainclone amplifier. The build seemed to go smoothly, all connections checked multiple times, and I'm getting audio output.

However, I'm plagued by a significant and persistent 50/60Hz hum (sounds like mains frequency) that's present even with the input shorted and the volume at minimum. It increases slightly with volume but is always clearly audible. However, I haven't figured out what's wrong yet.

Scope definitely helps. 50Hz noise and 100Hz noise have different root cause but 100Hz just sounds an octave higher. The shape of the waveform e.g. is it half a sine wave also tells us what different root cause.

Disconnect everything including internal wiring (so you don't have to worry about "star ground" or whatever), leaving only the power supply plus the output. Measure the output. Actually, measure the power supplies first to make sure your +V -V and ground are connected and working first.

Add: For clarity, is this a kit from aliexpress or did you design the thing from the ground up?

 

[DVDdoug]

Unfortunately the general idea of the meta is that 50Hz power supplies for speaker amplifier output stage will not use a linear regulator.

Aa far as I know, almost every good amplifier has a regulated power supply. I've got an amp from the 1970's and it has a regulated power supply. (A discrete circuit, not a LM78xx which wouldn't handle the voltage or current.)

Except it isn't. The internal battery resistance will cause the output voltage to vary as the output current varies.

I didn't say it's an ideal voltage supply but there's no 50/60Hz (or higher) power supply noise.

 

[audio_tony]

Aa far as I know, almost every good amplifier has a regulated power supply. I've got an amp from the 1970's and it has a regulated power supply. (A discrete circuit, not a LM78xx which wouldn't handle the voltage or current.)

If you said preamplifier you would be correct.

Solid state (power) amplifiers with regulated power supplies are the exception rather than the norm (although the proliferation of Class D amplifiers is rapidly changing that).

I don't know a great deal about valve amplifiers, although I think some have choke regulated power supplies, but I'm open to correction on that.

 

[bmc0]

I don't know a great deal about valve amplifiers, although I think some have choke regulated power supplies, but I'm open to correction on that.

Choke is just another name for inductor. An LC (inductor-capacitor) filter has a 2nd-order low pass response and thus filters noise much more effectively than just a capacitor, assuming component values are chosen appropriately. However, it doesn't make for a regulated supply in the sense that most people use that term (i.e. active voltage regulation).

Tube/valve amps do indeed commonly have chokes. High PSRR is difficult to achieve with tubes, so chokes help reduce hum to acceptable levels.

 

[somebodyelse]

I don't know a great deal about valve amplifiers, although I think some have choke regulated power supplies, but I'm open to correction on that.

Musical Fidelity used chokes in some of their solid state power amps too, but very much the exception rather than the rule. Sorry for taking it further off topic.

 

[wwenze]

A choke is an inductor when it is used to block AC but pass DC i.e. it's a functional name

Some people call it a current regulator because it keeps current constant because it resists changes. Come to think of it, regulators output voltage also changes with load, like 12V @ 0% load and 11.6V @ 100%. The main difference is that (active) regulators are very good with DC regulation, while chokes do jack all at DC.

 

[MaxwellsEq]

Aa far as I know, almost every good amplifier has a regulated power supply. I've got an amp from the 1970's and it has a regulated power supply. (A discrete circuit, not a LM78xx which wouldn't handle the voltage or current.)

Almost all Class A/B amplifiers made in the last few decades do not actively regulate the power into the final output current delivery stage.

In contrast a high percentage of Class D amplifiers with SMPS power supplies do regulate the power into the final stage.

 

[wwenze]

With any power supply, your choice with dealing with 50Hz ripple is

a) A lot of capacitance
b) Regulation

In terms of impedance, adding a lot of capacitance reduces output impedance of lower frequencies, while regulation also reduces output impedance. Thus both serve the same purpose.

With SMPS, adding regulation is the clearly cheaper method. If I remember correctly I have only seen one speaker amp that uses unregulated SMPS, and it uses the "lots of capacitance" method.

 

[antcollinet]

All this talk of regulation is fine - but probably has little to do with the OP's noise issue - which is far more likely to do with incorrect grounding arrangements of the various circuits.

 

[wwenze]

This is why I asked if it was a kit or a self-design. A kit would have the ground figured out at least on the PCB, and my recommended troubleshooting steps is based on that.

And it would be funny if the hum came from mathematically insufficient 100Hz filtering.