[sdiy] Square to Sine

[Magnus Danielson]

Elaine,

On 04/05/2017 11:03 AM, Elaine Klopke wrote:
>
>> You where talking about a 2 pole filter, which is different from the original 3-pole filter, or at least that is what I saw from your contribution.
>
> I think I can clear this one up... I did a search on google for the thing and there's a guitar delay pedal that uses basically the same filter layout, but it ends up being a third passive pole tacked onto the front end of a two pole Sallen-Key bjt filter.

There is plenty of such examples.

>> Now, a smarter approach is to use a resonant lowpass filter setup, as the resonance located on the fundamental will provide an additional fundamental gain offset for all overtones, and then the roll-off takes care of the rest. This is fairly feasable to do for a fixed frequency thing like the organ/tone-wheel thing.
>>
>
> Can you describe this a little more? Is it as simple and space-effective as the transistor one?

The Sallen Key low-pass filter is in its core very simple, but by 
selecting the components differently you can get a resonance. You might 
actually need to introduce an initial damping for the resonance not to 
clip, but that comes at an additional resstor loading the first 
resistor, the parallel value will be that of the normal Sallen Key 
dimensioning.

Some care in the precision of value is needed for the resonance to occur 
at the right frequency.

https://en.wikipedia.org/wiki/Sallen%E2%80%93Key_topology
http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm

Cheers,
Magnus