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

Tom Wiltshire tom at electricdruid.net
Sat Dec 14 00:40:27 CET 2013


On 13 Dec 2013, at 22:42, 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).
> 
>>> 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.

Or replace the LEDs with something modern and high efficiency, thereby allowing you to increase the LED resistors massively and reduce the current drawn by a similar amount. That ought to reduce the noise, as well as reduce the load on the power supply. I admit it's a fiddle to do, but I'd have thought a 303 was worth it.

T.




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