[sdiy] SMD LED intensity and lenses

[rsdio at audiobanshee.com]

On May 25, 2017, at 11:06 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
> On 25 May 2017, at 17:48, John Speth <john.speth at andrews-cooper.com> wrote:
>> 	• PWM is noooisy.....
>>  
>> So true.  I recently abandoned a design that tried to PWM a vactrol with the intent of "linearizing" response.  PWM noise was everywhere.  It wasn't worth the expense to try to eliminate it.  Another lesson learned.
>>  
>> JJS
> 
> I've had different experiences. Perhaps you have to be careful with PWM, but noise "everywhere"? Nope, not at all. Perhaps it depends how the PWM is generated?

Couldn't this be related to the infamous TB-303 headphone noise that tracks LED states? In the case of the TB-303, the LED ground is shared with the audio ground. As the current to the LED switches on and off, the ground reference for the audio changes, creating noise in the headphone output. There are mods to fix this, but apparently even Roland can make a mistake with ground loops.

I have a hunch that most PWN noise could be fixed by proper isolated grounds. However, since any given modular rack shares its power supply across many modules, it still seems possible that modules further from the supply might see PWM noise on the ground if the LED current is high enough.

Brian Willoughby
Sound Consulting