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Old 31st October 2006, 03:32 AM   #1
JesseG is offline JesseG  Canada
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Default Hum hunting - LM4562 Phono stage

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.
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Old 31st October 2006, 03:34 AM   #2
JesseG is offline JesseG  Canada
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Default And here is the PCB Layout

The PCB is single sided and looks like this
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Old 31st October 2006, 03:40 AM   #3
JesseG is offline JesseG  Canada
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Default And a photo of the amp

Heres a photo so you can see what it looks like.
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Old 31st October 2006, 04:15 AM   #4
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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...
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Old 31st October 2006, 06:11 AM   #5
JesseG is offline JesseG  Canada
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Quote:
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?
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Old 31st October 2006, 07:29 AM   #6
Eva is offline Eva  Spain
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- 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.
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Old 31st October 2006, 08:10 AM   #7
kubeek is offline kubeek  Czech Republic
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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.
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Old 31st October 2006, 04:42 PM   #8
JesseG is offline JesseG  Canada
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Quote:
- 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
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Old 5th November 2006, 06:01 PM   #9
JesseG is offline JesseG  Canada
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Default Fixed the hum

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
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Old 31st December 2012, 09:14 PM   #10
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Default making boards

On my first board - I sent out to have my board done - by Professionals. Requesting the 'work-up' for the project, my result was a Walk-Thru of the procedures for my board developement, the chips' placements - passive & active - and guidelines to make my OWN BOARDS in the Future.
All-In I spent $212 for Tutalage & Labor. Battery Power the Preamp boards and a simple B+ monitor. Shield all boards - regardless.
Ground ALL NEG - TO REAL GROUND. Drive a rebar into the ground if you have to.
Whatever You Preamplify - will REALLY be amp'ed - through your $500 250W amp !
Grampa Pre-Amp's in the basement - and cables that into his living room - so when he Blasts a song, the Preamp is not 'getting his feedback' !

-P-
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