[sdiy] An Improved Sine Shaper Circuit

Wed Apr 22 20:51:00 CEST 2020

Hi all, and apologies to David,

This one came through fine.

It’s not the size. As I said before, up to 1 MB messages go through unfiltered. Bigger ones are held and I can manually moderate them. The problem was that it was a .gif. That was filtered out <duh>. I don’t know why this was, but I’ve changed the settings so shouldn’t be a problem in the future. Probably some anti-spam feature from way back when.

Ben

> On 22 Apr 2020, at 20:17, David G Dixon <dixon at mail.ubc.ca> wrote:
> 
> Hello All,
>  
> The total size of both images is 59 kB.  If this is too big, then whatกฏs the point.  I suspect that this กฐembed imagesกฑ thing only works for certain software.  Iกฏm using Outlook 2003 (yeah, donกฏt give me shit about it จC MS Office 2003 does things that later Office users can only dream about).
>  
> As a test, hereกฏs the 5 kB image I tried to embed before:
>  
> <image001.gif>
>  
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org <mailto:synth-diy-bounces at synth-diy.org>] On Behalf Of Pete Hartman
> Sent: Tuesday, April 21, 2020 2:29 PM
> To: Jimmy Moore
> Cc: synth-diy mailing list
> Subject: Re: [sdiy] An Improved Sine Shaper Circuit
>  
> Same here Jimmy, I contacted Dave off list about it.  I suspect that perhaps they exceed the size limits that Ben described earlier this week.
> 
> Pete
>  
> On Tue, Apr 21, 2020 at 3:58 PM Jimmy Moore <jamoore84 at gmail.com <mailto:jamoore84 at gmail.com>> wrote:
>> Not sure if the issue is on my end, but the picture attachments aren't rendering.  Anyone else?
>>  
>> On Tue, Apr 21, 2020 at 2:29 PM David G Dixon <dixon at mail.ubc.ca <mailto:dixon at mail.ubc.ca>> wrote:
>>> I just simulated the Thomas Henry transistor-pair sine shaper, and compared the output to a 90-degree phase shifted pure sine wave of equal amplitude.  It is virtually impossible to tell the two apart จC THD = 0.57% -- light blue is the pure one, orange is the shaped one.
>>>  
>>> 
>>>  
>>> Hereกฏs the sine shaper circuit Iกฏm simulating จC this is exactly the circuit I build into all my VCOs (transistors are 2N3904, opamp is TL07X จC the 11k resistors get me closer to 10Vpp than 10k):
>>>  
>>> 
>>>  
>>> In what way does the output of the diff pair not look like a sine wave?
>>>  
>>> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org <mailto:synth-diy-bounces at synth-diy.org>] On Behalf Of Donald Tillman
>>> Sent: Tuesday, April 21, 2020 12:45 PM
>>> To: Renจฆ Schmitz
>>> Cc: synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>
>>> Subject: Re: [sdiy] An Improved Sine Shaper Circuit
>>>  
>>>  
>>>> On Apr 17, 2020, at 8:53 AM, Donald Tillman <don at till.com <mailto:don at till.com>> wrote:
>>>>  
>>>> 
>>>> On Apr 17, 2020, at 1:56 AM, Renจฆ Schmitz <synth at schmitzbits.de <mailto:synth at schmitzbits.de>> wrote:
>>>> 
>>>> Interesting circuit, and a great article.
>>>> I'm pretty sure I have seen a similar technique before, because I have used it. (cusp canceling)
>>>> 
>>>> I am very familiar with cusp cancellation.  I've used it also.  And it's mentioned in the article.
>>>> This is not cusp cancellation.
>>>  
>>>  
>>> I'd like to expand on this for a moment...
>>>  
>>> I think the phrase "cusp cancellation" has, accidentally, been misused a lot.  And that's caused confusion.
>>>  
>>> "Cusp cancellation" should mean that we've already got a pretty good approximation going, but the cusps of the triangle are still coming through a little bit.  And we can cancel those by subtracting a small amount of the original triangle wave.  Sweet!
>>>  
>>> This would be because the transfer curve of the diff amp pair isn't completely flat at the top and bottom.   The tanh() curve is asymptotic, so there will always be a little slope on the peaks.
>>>  
>>> The most common next step is to apply negative feedback around the diff amp pair.  This could be in the form of a feedback resistor, or by adding small emitter resistors.  The negative feedback plumps up the curve and flattens the slope at the peaks for a better overall fit.  Nice!
>>>  
>>> But here, with the Colin/Henry/Guest/Tillman (Have I got everybody?  In order?) approach, the output of the diff amp pair isn't remotely close to a sine wave.  Not even trying.  And none of us are using negative feedback to plump out the curve.  We're not in the cusp cancelling business, we're doing something else.
>>>  
>>> I got here by applying actual cusp cancellation to an actual diff amp pair with negative feedback and a pretty good sine approximation.  Then I refined it with thousands of simulations, which lead me away from cusp cancelling, and toward considering a compound curve of tanh(x) - ฆยx, expressly for the bumps and the sine shape in between.  And the rest as I described.
>>>  
>>> So I guess Dennis Colin (ARP, Aries) got to the circuit first.
>>>  
>>> So, I'll claim that if a small amount of the original triangle wave is subtracted from a wave that's roughly sinusoidal, then it's actual cusp cancellation.
>>>  
>>> But if the diff amp pair contribution doesn't look like a sine wave, and there's no negative feedback, and the transfer function can be put into the form tanh(x) - ฆยx, then it's this other approach that Dennis Colin pioneered.
>>>  
>>>   -- Don
>>> --
>>> Donald Tillman, Palo Alto, California
>>> http://www.till.com <http://www.till.com/>
>>>  
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