[sdiy] [synth-diy] Success - hum in 303 output gone

cheater00 . cheater00 at gmail.com
Sat Dec 14 01:56:17 CET 2013


thanks for your reply.

On Fri, Dec 13, 2013 at 11:42 PM, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> Hi,
>>> I went for the largest inductor in the ballpark:
>>> http://de.mouser.com/ProductDetail/Bourns/70F501AF-RC/?qs=sGAEpiMZZMsg%252by3WlYCkU6J1Kh%252bkI7ZW63PlG49mKtY%3d
>>> Bourns 70F501AF-R
> Firstly, that choke is pretty much at the limit of its DC current
> handling (30mA per datasheet).  Anything above 30mA and it stops
> behaving like a choke, just a piece of wire (the core saturates and
> there's no more magnetic magic happening).

That's what I thought too, at first, but notice its spec for DC
resistance. It's going to limit the current much lower than 30 mA.

>>> wasn't any buzz. The LEDs were bright enough to see in a normally-lit
>>> room. When the blinking LED turns on, the other two go a little dim.
>>> That's all really.
> Secondly, if the source of the noise is the LEDs switching, have you
> considered routing the LED power circuit (supply and ground) directly
> to the PSU, perhaps their own dedicated smoothed supply rail.  That
> would reduce the amount of LED switching noise from reaching the audio
> circuits.
> Neil
> --
> http://www.njohnson.co.uk

Those exist already. If you look at the layout you'll notice D23 gets
its own trace coming directly from the power supply, and GND is even
connected with a wire directly to where the battery negative terminal
comes in via its wire. What you're saying could be a problem if the
analog portion of the synth and the LED matrix shared a single trace
for either the voltage rail or gnd, and the LED matrix were using
loads of current for that, making the trace resistive for the tiny
currents going to the other parts. However that is not the case. What
I believe is happening is that the power supply has a high output
impedance, which makes it sag with current. The symptoms are exactly
the same as trace congestion which you hinted at.

Would you suggest that I try hooking the LED driver voltage rail up
even more directly? Thicker wire, or even replace the trace going to
D23 with a wire as well?


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