120hz spikes at amp output, crappy layout or something else? - diyAudio
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Old 6th February 2014, 01:31 AM   #1
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Default 120hz spikes at amp output, crappy layout or something else?

At the amp output, I'm getting 120hz spikes as shown below:

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In the layout I have star grounded as much as possible, maybe the layout is crappy elsewhere?

PS Layout (using 15000uf caps):

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Amp sch and layout:

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Old 6th February 2014, 02:04 AM   #2
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Hi,
Does spike look like when the rectifiers are switching at zero crossing. The spikes are every 8.2 ms. Can you post your PS schematic.
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Old 6th February 2014, 02:30 AM   #3
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Quote:
Originally Posted by tauro0221 View Post
Hi,
Does spike look like when the rectifiers are switching at zero crossing. The spikes are every 8.2 ms. Can you post your PS schematic.
It looks like it, but unfortunely I only have 1 working scope, so I can't 100% confirm.

The sch for the power supply is simply... just the rectifier caps and crimp connectors. The rectifier is on another board.

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Old 6th February 2014, 03:05 AM   #4
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Hi,
I was looking for the rectifiers. What are you using diodes in a bridge or a bridge?
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Old 6th February 2014, 03:07 AM   #5
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Well, the layout could be a lot better, TBH. There are some very obvious problems:
- The biasing diodes are close to the Q10 rail on the supply side.
- The feedback path wanders about through a maze of power rail traces
- There is a general maze where low level signal is way too close to the rail currents
- no local decoupling caps for the rails near the front end.
- the circuit design itself doesn't have a good PSRR.

Designing a good amplifier PCB is no simple task. A couple of things it doesn't have to do is look pretty or be scrunched up into a tiny board that is inherently noise prone. A couple of things it does have to do is separate noisy class AB power rails from the signal input, feedback and any low level paths. Apart from layout, there is no cheaper solution than physical space or distance between noise source and signal path - use a roughly square format for a starting design or take a look at P3a design beginner threads - JLester's comes to mind as a good basic result. There are several over the last 2 years here, for indications how well you can do this without a lot of effort.

Another useful pointer is, where necessary to cross low level traces over noisy ones, do so at 90 degrees, using a component, link or top layer trace. Avoid any parallel runs with these.

Generally, you have a very busy PCB that for such a simple circuit, doesn't need to be and doesn't even need to be double sided. Double sided boards solve some layout problems neatly but are just a trap unless you are aware of EMR and what it does to your precious audio.

Every inch of rail current trace is more radiated noise and distortion. Eliminate or prune it right back and keep its tracks and wire connections, feed and return current, in close parallel to cancel some radiated noise at least.
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Old 6th February 2014, 03:19 AM   #6
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Hi,
You can try solder a .01uf cap across the diodes to minimizes the diode switching and a .01 cap across the filters caps.
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Old 6th February 2014, 03:57 AM   #7
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The layout is very tight because I have limited space to work with inside of case.

But is a general summary that I need to separate the low current and high current wires and not snake things so much. I'll try to do that, but as you can see the space is extremely tight. I'll also try to add the coupling caps.
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Old 6th February 2014, 03:58 AM   #8
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Quote:
Originally Posted by tauro0221 View Post
Hi,
You can try solder a .01uf cap across the diodes to minimizes the diode switching and a .01 cap across the filters caps.
Which diodes? Do you mean the diode bridge for my filter caps?
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Old 6th February 2014, 03:59 AM   #9
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Also, besides the snaking and general power rail issues, how does my grounding look? Do I need to change anything?
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Old 6th February 2014, 07:30 AM   #10
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Can't offer the wisest or most recent views on grounding, sorry.

However, you might also look at isolating the voltage reference shared for CCS of the input and VAS stages. See Douglas Self's blameless design and chapters explaining the much improved distortion and noise reduction possible with a few cheap parts there. If you can't afford to read books, there are always earlier editions for free download.
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