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Need help with amp hummmm

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Quercus

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Hello all,

I've been working on an audio amp for a while now based on the TDA7294 amp module; it has three inputs, a mixer, variable low-pass subwoofer output w/ volume control, and a headphone amp with independent volume control. The schematic, as well as images of the completed boards are attached.

The amp and mixing are all working as expected, except that I can't get a persistent hum to go away. I've read many a thread on this seemingly persistent problem, have tried many of the solutions proposed. The hum at this point is small and is not a problem while music is playing, but is sufficiently audible when no sound is coming from the amp that it would be an annoyance when silence is expected.

To isolate the issue, I have bypassed all components of the circuit and tie my source directly into the amp circuit (see label "direct input to amp" on the images). This is normally where the signal from the master volume control would enter the final amp circuit. I have a commercial amp-on-a-pcb that I purchased which is similar to the amp circuit I made, and when I connect my amp's power supply and use similar signal input and output connections to the commercial amp-on-a-pcb, the hum is negligible. So I know the problem is somewhere in my amp circuit and is not coming from the power supply or my signal source. I should also note that when I had this very circuit bread-boarded, the hum was sufficiently negligible.

Assuming that the hum is a grounding issue, I have made ground connections to the amp circuit in a much of a star pattern as is practical. All ground connections come together on a board "ground bus" that consists of a 12 AWG bare solid wire heavily soldered to the board. Each amp circuit, the snubber caps, input + and input - ground connections all have a direct path to the ground bus.

So I am out of ideas now and looking for suggestions. Any thoughts on what I can do to reduce the hum is appreciated. As a reminder, I have bypassed all elements of the circuit except for the amp, so (for now), please ignore the mixing circuit, headphone and subwoofer elements.

Many thanks!
 

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ronsimpson

Well-Known Member
Most Helpful Member
You get hum in the power amps.
Do you get hum in the headphone amp?
....The small amp only pulls up. It can not pull below ground. You do not need two caps on each output. \
....10K pot?? I think this should be 1k or lower. Are you using 8 ohm head phones? I might have put the posts on the input to the amp not the output.
upload_2017-4-8_10-46-20.png
 

chemelec

Well-Known Member
Is your common point ground at the Output or Input.
It should be at the Input for least Noise or hum.
 

Quercus

Member
Do you get hum in the headphone amp?
I'm not even using the headphone amp at the moment, just the main amp. I'm sending my input directly into the amp, bypassing the mixer, headphone amp and subwoofer output. (The back-to-back caps is just a work-around. I really want a 500 uF there, but didn't have one - but that's a problem for another day.)

Is your common point ground at the Output or Input.
It's both, really. The input and output are located next to each other on the ground bus. See image below.

Thanks for the comments!

Ground Connections.jpg
 

chemelec

Well-Known Member
BOTH?
That will Cause GROUND LOOPS!

All Grounds should connect to ONE POINT at the Input.
 

ronsimpson

Well-Known Member
Most Helpful Member
I'm not even using the headphone amp at the moment
I was trying to see if the inputs are quiet and the noise is in the power amp.

Because I have many power supplies, I can replace each supply with clean power and see what happens.
Because I have many capacitors, I can add more power supply caps and see which supply the noise if coming from.
 

chemelec

Well-Known Member
looking AT THE SIZE OF YOUR FILTER CAPACITORS, I Doubt the Hum is from your power supply.

Especially at Minimum Volume.

Is this a Line Type of Hum, (60 or 120 hz)
Or an Oscillation of sorts?
 
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Quercus

Member
All Grounds should connect to ONE POINT at the Input.
That's what I'm trying to do. Think of it this way... the input - the one point - is the bottom of the ground bus (see "Ground from line input enters here" in image above). The speaker output is connected right next to it, and the other ground connections are made on this same ground bus which consists of a relatively large and low resistance conductor. All ground connections are made on this one ground bus - it's pretty darn close to a single point. So I don't see the ground loop.
 

Quercus

Member
I was trying to see if the inputs are quiet and the noise is in the power amp.
The inputs are quiet. I substituted a commercial amp-on-a-pcb in place of my amp circuit, still using my power supplies, speakers and line inputs, and it was all quiet. So I know the problem is in my amp circuit.
 

Quercus

Member
Thanks for the diagram. Your interpretation of how I've wired this is correct.

I have not cleaned any of the flux off of the board - I've read conflicting posts on whether residual flux will or will not be a problem for electrical circuits. How much of the hum might be from that?
 

chemelec

Well-Known Member
It Always a good Idea to Remove Solder Flux, But VERY UNLIKELY that would Cause HUM Problems.

Do you have any Resistors Across the Inputs to your Power Amps?
Possibly 100K to Lower the Impedance and help prevent Stray Pickup?

And your STAR POINT GROUND is Not really what is needed.
It is Not a Star point ground for Each Section.
More so, Just for the Total number of Sections
 
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