ECE4450 L26: Sallen-Key Filters & the Korg MS-20 VCF (Analog Circuits for Music Synthesis)
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- เผยแพร่เมื่อ 25 เม.ย. 2021
- MAJOR CORRECTIONS: 1) In the schematic shown at 13:45 and again at 28:37, the inputs of the first OTA need to be switched. The positive input should be grounded and the other connections should be going to the negative input. 2) Around the 12:15 mark, I switch feedback paths so that they're taken from the output of the buffers instead of the input. This only works for the SECOND OTA output, since the current from the OTA is going through Cg. I should have left the FIRST OTA alone, since the current from the OTA needs to go through Cf. The error persists through the 14:35 mark, and appears again at 28:37.
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I recorded this during the Spring 2021 offering of ECE4450: Analog Circuits for Music Synthesis, but this material will likely be appropriate for future offerings as well.
Be sure to check out Alex Ball's amazing Daft Punk synth tutorial, featuring the Korg MS-20: • Daft Punk Synth Tutorial
I just re-watched your video after a long time, and I took the schematics of my Befaco BF-22 dual filter eurorack module to compare it with your lecture. I stopped your video a few times and checked the Befaco filter. It seemed a bit strange, apparently using two bipolar transistors as variable resistors. And then, near the end you answered all my questions! The Befaco module is an almost straight reissue of the 'MS10 version of the Korg35-based filter', as analyzed by Tim Stinchcombe, including the Q2SC1623 transistors. The only main difference is the addition of a LM13700 to enable voltage control over resonance. So, as it turns out, the Befaco filter is actually an interpretation of the MS10 filters, and maybe its name should be BF-12.
Thanks a lot for your videos, and for mentioning Tim Stinchcombe's paper. I've got some reading to do :)
So much lol on Bach! LOVED IT!
It's three o'clock in the night and I have to get up early, but this is just too interesting. I've come to the point in my EE bachelors where I can both fathom what you're presenting and as a bonus, it's relevant for my upcoming exams in both control theory and circuit analysis! I really love the MS-20 filter so I'm totally going to build and experiment with one. Thanks for making these great lectures public.
i just love the style of your lectures. Thanks!!
Thank you again for making these, very inspiring!
Another great lecture! Thank you so much!
So well explained, I could have used such a great video when I made my Dual Filter module :) I started from the same circuit and added an OTA for resonance CV control.
Wish I could have taken this course, enjoy all these videos!
I wish I had the math background to understand these lectures more. Makes me want to go back to school. I have made several of these filters but as for designing my own I am at a loss.
Thanks Aaron, I had watched it twice and I didn't catch the inputs on the OTA either. Very happy that you covered the MS20, I've been baffled by the filters used in various korg synths(MS,delta,poly's), this clears up alot. Also, would you mind explaining what those 2 transistor pairs 2sc945 on the original korg design schematic are doing? Korg also uses them in other synths in that fdbk/resonance ctlr loop in front of those op-amps. thx
Looking forward to the next lecture where we are going to look at J.S. Bach's sick acid filters:)
Very nice!
I would say though that the sum of a band pass filter and a high pass filter is still a high pass filter, it's just that the (Korg) high pass filter in question has a 6 db/octave roll off instead of 12 db/octave. But it's still got the resonance, so I don't think the second pole is wasted. Maybe 6 db/octave actually sounds nicer for a high pass filter in some cases, to avoid making the sound too thin? I haven't made any audio comparisons though.
Just rewatched the video. Another SK-cell bandpass filter is the Steiner Parker, although it uses diodes and the bandpass output is achieved in a different way compared to the usual ones
That S-K one is really interesting and weird -- the overall topology changes depending on what you plug into it.
Would you check Colin's Fraser's "mini Korg filter - Keio TS201012" page? or much Better VCF mini korg by j. Bergfors
explain the HP & LP Filter and the resonance in the Korg 770, it's some sort of diode arrangement.
Hi, thanks for sharing the knowledge. Whats the formula at min 21 for getting the constant to multiply by the current to give us the natural frequency? Couldnt understand where that came from.
Ah, the 19.2, that's 1/(2 * VT) where VT = 26 mV is the so-called "thermal voltage" -- 26 mV is a typical value for "room temperature."
See my lecture "Voltage Controlled Amplifiers: Operational Transconductance Amps": th-cam.com/video/96j2tNKFCPI/w-d-xo.html
@@Lantertronics will do it. Thanks.
@@Lantertronics that was not what i was in doubt but thats good to know anyway. My problem is that i thought the formula was wn=19.2*iabc√(1/C*C), i must have missed something as I cant seem to understand how that translates to the one you have used. I'll go watch the other videos hopping to get my answer. Thanks.
@@wizegeek Ah, in that case check out my lecture titled "Single-Pole OTA-C Filter" th-cam.com/video/pTHHzFsa4Ss/w-d-xo.html
Hi could you please tell me what is controlling eg1trig in? In schematic
Google on Korg MS-20 schematics and you can find the service manual that shows the whole thing.
22:02, think you got that wrong there, the first pole in inverting but the second is not.
probably an inverting opamp in front of the first.
At 14:00… the first OTA dumps its current output into a voltage buffer. Is that intentional?
Most of the time an OTA is used there is a voltage buffer at the output, a lot of OTA chips like the LM13xxx have the buffer included in the chip. The reason for this is that unlike op amps, OTAs have a high output impedance, meaning that it could heat up/fail very easily if the load demands more current than your average op amp could handle.
@@tired.mp3 I think what you're missing is that there generally is a path for the output current to flow, e.g. a resistor, an inverting opamp buffer or a capacitor. In this case, I think that path was accidentally left out. The OTA is a current output device and that current can't be dumped into the high impedance input of an opamp. I guess Prof. Lanterman was using the opamp symbol in place of an abstract buffer circuit.
@@TheSlowGrowth Ah you're right! The first OTA is missing a load resistor.
@@tired.mp3 I think the loads for both OTAs in these circuits are capacitors. One of them is the feedback capacitor.
@@possible-realities Theres a buffer between the feedback capacitor and the first OTA, so that one needs some load resistor
So I have built the Rene version with both the ca3080 and the lm13700 otherwise every other component is the same but they sound different there is a big personality difference 🤷♂️ any idea why 🤔 But at least I have a better understanding of what I built because of this video 👍
You might find this of interest: synth.stromeko.net/diy/OTA.pdf
If the "simplified schematics" on the datasheets are to be believed, the upper current mirrors (the ones near the positive voltage rail) are slightly more complex on one chip than the other, but I'm not sure how much of a difference that would make.
Ah, it does look like the current mirror at the bias current input on one is different than the other -- the input of one chip looks like it's two diode drops above the negative voltage rail, and the inputs of the other one looks like it's one diode drop above the negative voltage rail, but that should only matter if you're trying to directly create the control current with a resistor and voltage source and ohm's law.
I've never compared them myself but I have heard people say that the CA3080 distorts before the LM13700. Does the 3080 version sound grungier in general? Do the differences between the version seem less prominent when you put in quiet signals?
Short answer is: I could guess at bunch of theories, but I actually have no idea. ;)
aa Preciso de um CIRCUITO CROSSOVER ATIVO TRI WAY e ou TREE BAND AMPLIFIER, baixa potência, três circuitos independentes cada um SÓ com seus 2 ou mais amplificadores operacionais, em Classe D, com fonte SIMÉTRI-CA comum, circuitos quase iguais e SÓ sintonizados na entrada de áudio. Veja que não é SÓ Pré ou SÓ Equalizador. É bem simples, mas é AMPLIFICADOR e sem potenciômetros nas tonalidades. Por favor. Muito obrigado.
Today on Electroboom, Mehdi attempts to put 1kA through a small microchip.
E então? Obrigado.
Have any of your videos ended in the middle of a sentence?
BTW a running meme with my friends is how quickly i end phone calls.
The TH-cam experts tell me I should say things like "thanks for watching, and if you liked the video please like and subscribe and ring the bell to be notified" -- and they're right, I just hate taking up people's time. ;)
Math is hard 😭
Hang in there! :)