[sdiy] Square to Sine

[Peter Pearson]

http://hackaday.com/2017/03/08/dont-fear-the-filter-lowpass-edition/

http://hackaday.com/2017/03/29/dont-fear-the-filter-cascading-sallen-keys/

On Wed, Apr 5, 2017 at 5:31 AM, Magnus Danielson <magnus at rubidium.dyndns.org
> wrote:

> 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
>
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