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

[cheater00 .]

On Sat, Dec 14, 2013 at 1:56 AM, cheater00 . <cheater00 at gmail.com> wrote:
> Neil,
>
> 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?
>
> Cheers,
> D.

Hooking up D23 in series with a ~100 ohm resistor, the LEDs shine nice
and bright, even if four patterns are selected at once in play mode
(for chaining). Minimal dimming happens when the blinking LEDs are on.
No hum is present. I'll keep it that way. I might revisit this if I
get some nice inductors, like the ones from J.W. Miller / Burns that
look like upright electrolytic capacitors. They have low DC R.

As a side note, I tried removing the bypass which I did years ago. It
goes from the volume pot ground pin to the headphone jack ground.
Robin instructed me to add it to remove the buzz. Removing it
generated an insane amount of buzz again, so apparently it's still
necessary.

Cheers,
D.