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

cheater00 . cheater00 at gmail.com
Fri Dec 13 19:13:15 CET 2013

On Fri, Dec 13, 2013 at 7:09 PM, cheater00 . <cheater00 at gmail.com> wrote:
> Hi guys,
>
> first of all, I tried moving my huge capacitor bank from bypassing
> C204 on the switch board, up to being across C65. This was a bad move.
> The hum was back. So I went back to checking out my idea with a series
> inductor:
>
> On Thu, Dec 12, 2013 at 7:39 PM, cheater00 . <cheater00 at gmail.com> wrote:
>> I have thought of putting an inductor in series with D23. Is that a good
>> idea? It seems that it would need to reject above ~50Hz. How does one
>> calculate the inductor? Googling came up with nothing.
>
> I've read up some more on inductors to freshen up my memory of how
> they work. I used this great page:
>
> http://www.animations.physics.unsw.edu.au//jw/AC.html
>
> I calculated that I would want the inductor to be about 1000 Ohms at
> 125 Hz. My reasoning was that the capacitors I'm using are likely
> around 1 Ohm Rs, and that the LED driver would be getting its voltage
> from a voltage divider formed by the SMPS + L on one side and
> capacitor + its Rs on the other side. I wanted this to be 1:1000,
> since that's 60 dB down from the default. I figured this out by the
> fact that 0 dBV is 1V, and 60 dBV is 1000V.
>
> (Connecting a 1 ohm resistor in series with D23, I scoped that I'll
> need at most 30 mA.)
>
> The RMS voltage across the inductor is V = I \omega L, so the
> impedance magnitude is V/I = \omega L = f * tau * L = 125 * 6.28 * L.
> That's roughly equal to 800 * L.
>
> I went for the largest inductor in the ballpark:
>
> http://de.mouser.com/ProductDetail/Bourns/70F501AF-RC/?qs=sGAEpiMZZMsg%252by3WlYCkU6J1Kh%252bkI7ZW63PlG49mKtY%3d
>
> Bourns 70F501AF-RC
>
> That would only give me 400 Ohms. Still, worth a try I thought. The
> datasheet said that its maximum DC resistance is 700 Ohms. This isn't
> really great - I might have to search around some more.
>
> Still, I decided to try and see if the circuit would work at all with
> the DC resistance in place. Surprisingly, after adding a ~500 Ohm
> resistor in series, the circuit not only works. Removing all the
> bypass capacitors I added to the switch board (15000 uF!), there still
> 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. I think this might be a pretty good hack unless I
> can find a really good inductor to use instead, which would give me
> low resistance at DC and high resistance at 125 Hz (the frequency of
> the LED switching). However, I'm not sure if there's a mode where more
> LEDs are on, in which case something might go awry.
>
> BTW, of course having the 15000 uF capacitors across C204 meant it
> took a few seconds for the LEDs to come on after the 303 was turned
> on. It actually looked kind of cool. Still, with just the stock
> capacitor, the LEDs come on right away.
>
> Cheers,
> D.

BTW, I'd be grateful if someone could look over the reasoning I posted
above and let me know if it's correct, or just plain wrong, and what