[sdiy] Pink?

Rutger Vlek rutgervlek at gmail.com
Thu Sep 6 11:02:12 CEST 2018


Hi Ian,

Thank you for the compliments! The OTA design was indeed hard work, and I made quite a few failing prototypes before I had something I feel I could share with others. Spice was a big help, though reality is always a little different. I started by doing complex designs (e.g. discrete CA3080 attempts, borrowing from LM13700 a bit), and then started doing the maths on circuit cost and tolerances. Soon I realized I could never achieve consistent performance and a sensible price point doing these complex designs discretely, so I explored where I could simplify. In the end I came up with a design that uses a 4-transistor (super-?) Wilson current mirror on one side, and a more mundane current mirror on the other side. I use closely matched transistor pairs in SMD for it, and tweaked the signal and CV levels until I found a good balance between noise, bleed, headroom, etc. It will never be as good as a CA3280 under all circumstances, but I think I found a balance that works musically well for a wide range of applications. It's fairly compact, moderately affordable, and doesn't require trimming for most applications. The other good thing: it's constructed from components that have a low chance of becoming obsolete, ever! Unless we say goodbye to transistors and resistors. 

Regards,

Rutger

ps. While your email was private, Ian, I hope you don't mind I share my reply with the list. Perhaps it's of value to others thinking/working on discrete OTAs.

On 18 aug 2018, at 14:51, Ian Fritz wrote:

> Rutger --
> 
> Nice work!  I remember reading in a book on mastering that a famous studio's magic sauce was final filtering at 1-2 dB/oct. It's really great to see this available at the modular level.
> 
> I once worked at trying to make a discrete OTA that worked as well as a CA3280. I got it eventually, but it was very expensive -- MAT transistors, fully symmetric current mirrors, etc., so I appreciate the hard work that must have gone into your design.
> 
> Best,
> 
> Ian 
> 
> On Aug 18, 2018, at 3:12 AM, Rutger Vlek <rutgervlek at gmail.com> wrote:
> 
>> Oh here's the product page of this spectral tilt module with a demo: http://riversynths.com/product/marble/
>> 
>> Rutger 
>> 
>> Op vr 17 aug. 2018 16:13 schreef Mattias Rickardsson <mr at analogue.org>:
>> Hi all,
>> 
>> I often experience VCOs "too bright", having too much high end in the sawtooth & pulse waves.
>> I often experience VCF resonance peaks "too bright", being too strong at higher cutoffs.
>> I often experience white noise "too bright", having too much high end...
>> 
>> ...while pink noise feels more balanced. Equal power per octave, not per Hz.
>> Also, good sounding loudspeakers/studios tend to have a bit pink-ish spectrum roll-off from the sound source to the listener position, suggesting that some sort of pinkification of clean sounds could be desirable.
>> 
>> So, why aren't VCOs and VCFs more spectrally pink?
>> Have there been any attempts historically to alter their characteristics a bit in synths? :-)
>> 
>> /mr
>> 
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