Hum hunting - LM4562 Phono stage

[JesseG]

Hi all:

I recently read about National's new audio opamp - the LM4562.

I got a few chips and decided that my first project would be a phono stage. I decided to build the example single gain block design provided in the datasheet.

So, a little pcb layout work with PCB Express, some etching and drilling and I had a board. Some soldering and I had a circut. Power up and, well, Ill be d^&*ed - it works!

BUT (theres always one - at least) it hums.

It has a pronounced 60Hz hum that I cant identify the source of. Looking with the scope, the PS rails are clean, the input is clean, but there is a distinct AC hum on the output - about 15% of the total volume.

Can anyone suggest how I can find the source of this hum?
I would appreciate any suggestions.

[JesseG]

The PCB is single sided and looks like this

[JesseG]

Heres a photo so you can see what it looks like.

[EchoWars]

Since you have a full bridge, I'd expect the hum to be 120Hz + harmonics.

I don't see any bypass caps on that chip...

[JesseG]

Since you have a full bridge, I'd expect the hum to be 120Hz + harmonics.
I don't see any bypass caps on that chip...

Could you please explain = bypass caps on which chip, where??

The datasheet indicates that the 4562 has a PSSR of >110db. Would this not eliminate any ripple from the rails getting into the output? And, there is no mention of bypass caps in the applications section.

What am I missing?

[Eva]

- Your PCB layout contains several ground loops and it's overall quite poor. As a result, PSU capacitor ripple current from the unregulated side is being directly fed as voltage into the inverting input of the op-amps and amplfiied with the full RIAA gain.

- A phono preamplifier involves sub-milivolt signals and high impedance nodes, thus it requires careful electrostatic shielding, yet your PCB is attached to a bare wooden box. That's absolutely unsuitable.

- The 7912 regulator is not properly wired in the PCB mask shown.

I suggest starting a new PCB layout from scratch (conceptually the circuit is fine), but you have a lot to learn about PCB routing. You should have posted your layout here and asked for improvements before etching the PCB.

Tip: Read about star grounding techniques.

[kubeek]

Eva, could you please post some links about pcb design? I am interested in ways of designing more complex circuits than this one, but some basics wont´t hurt too.

[JesseG]

- Your PCB layout contains several ground loops and it's overall quite poor. As a result, PSU capacitor ripple current from the unregulated side is being directly fed as voltage into the inverting input of the op-amps and amplfiied with the full RIAA gain.
- A phono preamplifier involves sub-milivolt signals and high impedance nodes, thus it requires careful electrostatic shielding, yet your PCB is attached to a bare wooden box. That's absolutely unsuitable.

- The 7912 regulator is not properly wired in the PCB mask shown.

I suggest starting a new PCB layout from scratch (conceptually the circuit is fine), but you have a lot to learn about PCB routing. You should have posted your layout here and asked for improvements before etching the PCB.

Thanks, Eva - as you say, I have a lot to learn about pcb layout. This is my third attempt, the first two being non-critical ccts.

I am very happy to start over. Any suggestions on where to begin?

PS: I caught the error on the 7915 reg during the build and managed a work-around.

Jess

[JesseG]

Hi all:

For those of you who might be interested how this turned out, I got the hum fixed.

A freind offered a suggestion for a very simple mod (see PCB image. This killed the ground loop and the hum.

Initial listening impressions are: superb bass, very balanced and natural overall. Highs seem a bit reduced (but just a bit). This is probably due to my choice of eq caps.

Next one I will get first-rate caps and, in the meantime I will be working on my PCB skills. If anyone has any suggestions where to get beginner's info on PCB layout, I would appreciate it.

Onward and upward (hopefully).

Jess