Minimoog voltage controlled filter - Page 1
Forum section: electronics / projects

author: Dajgoro
author_rank: Frequent Contributor
timestamp: March 03, 2016, 07:54:57 pm
content: Hi.I've been doing some simple synth gizmos for fun, and I have been looking into voltage controlled firers.Googling I discovered that old school synths used specialized IC whic are not available widely anymore, and the only IC alternative would be the LM13700. Bit later I stumbled across the Minimoog filter schematic which uses only discrete components. I have been trying to understand how does it exactly work, but its still a mystery for me.Can anyone explain how does it work?

author: German_EE
author_rank: Super Contributor
timestamp: March 03, 2016, 09:43:08 pm
content: Some guesswork here:Audio input on pin 4Audio output on pin 7 (Maybe) or on that line labelled 'to VCA'Control line on pin 5It's an RC filter of some sort and as the control voltage increases more and more of those dual transistor pairs turn on, switching in additional 0.068 uF capacitors. The filter output is balanced and fed to a buffer stage where Q6/Q8 convert it to an unbalanced signal.

author: Andy Watson
author_rank: Super Contributor
timestamp: March 04, 2016, 01:53:40 am
content: Q8, Q7, Q5 and Q5 form a differential-input amplifier to buffer the audio signal - an output goes to the VCA and also to the "emphasis" on pin 7. Presumably the emphasis signal can be switched back to pin 6 to alter the response of the filter (i.e. change it from low-pass to band pass etc..)The filter-voltage control signals are scaled by Q26 to give various control laws and also temperature compensation such that Q28 generates a current that is exponentially proportional to the control voltage.The audio signal is mixed with the emphasis signal in the differential long-tailed pair of Q29 and Q30. This pair have as their collector loads the "ladder" of Q2, Q3, Q10, Q11, Q19 ,Q20, Q23 and Q24. The pairs of transistors present a low-impedance toward the driving side, i.e. the lower side of the ladder and a high impedance toward the next stage, i.e the upper part of the ladder. The key to understanding the operation of the filter is to realise that the impedance presented at each rung is due the capacitors (C3, C7, C11, C16) and thesmall-signalemitter resistance of the transistor pairs. The latter parameter being determined by the current through the transistor - which is the same current though the entire ladder and is controlled by Q28.Quote from: Dajgoro on March 03, 2016, 07:54:57 pmPresumably it worked adequately given the available technology of that era!One of the difficulties is preventing feed-through of the control signals into the audio path. Another problem lies in the assumption of "small-signal" properties - when the audio signal becomes large there will be some interesting distortion happening!Edit: The patent gives a better description:http://www.moogmusic.com/news/patent-no-3475623-moog-ladder-filter

author: calexanian
author_rank: Super Contributor
timestamp: March 04, 2016, 03:23:20 am
content: I have never built one but i have been told by others who have that transistor matching is very very critical in those circuits and it is why they are not in common use anymore.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: March 04, 2016, 04:51:01 am
content: Thanks for the explanation!I did not even realize that was a patent.