ECE4450 L22: Moog Ladder Filters Analyzed (Analog Circuits for Music Synthesis, Georgia Tech course)
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- Опубліковано 22 лип 2024
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NOTE THE TITLE OF SPRING 2019 ON ONE OF THE SLIDES IS AN ERROR
This was going to be the last in-person lecture of the Spring 2020 offering of my ECE4450: Analog Circuits for Music Synthesis class at Georgia Tech, but the Coronavirus derailed that plan. I recorded the lecture for my students to watch remotely, and thought I'd post it here on youtube in case anyone found it interesting.
The schematic at 3:31 is missing a cap to ground at the emitter connected to the current source. I was wondering how it would work, but that cap showed up in the small signal circuit equivalent at 11:47 .
Also, the the double ladder, that cap isn't needed because it is replaced by the differential emitter coupling to the other side of the ladder.
Thanks for your comment -- I "pinned" it.
@@Lantertronics Are temperature controlled peltier cooling mechanisms used to stabilize some filters?
So is it kind of similar to a monophonic analog synth keyboard the way the audible frequencies are split to a series of transistors through voltage dividers in series and then each transistor is basically a bandpass amp? (thus making basically a kind of amplified EQ) If so, why is it possible to get a smooth sweep across all audible frequencies from a single knob with a relatively small number of transistors (mini "bandpass amps").
By all means please post plenty for us non-students. Your lectures are awesome.
Hi Aaron, Thank you for the mention and you're correct about the use of the OTA in the Monowave. It was based on the Taurus ladder VCF, however the CA3080 was going obsolete, so swapping in an LM13700 and tweaking the bias gave me a "nice" sound. just a touch of distortion, no-one likes a super clean VCF, VCFs need character :)
There's a whole bunch of other tricks you can do with the ladder filter that make it such a fabulous building block.
Hope you and your loved ones are safe.
Paula
@gerhardumlandt3873 Or you think that you like it clean. Look at it using good oscilloscope and distortion analyser ;) There is no such thing as clean vcf. Even digital ones to some degree.
I'm a EE major in the bay area and this is great! Building some synth DIY stuff and would love to see more videos posted for non-students. Thank you so much!
This is pure gold! Very good job on talking to an empty room!
This is my first UA-cam comment ever. I'm posting because I couldn't watch this without saying thank you. BSEET student in Oregon currently taking a transistor amplifier design class. This was exactly what I needed to connect my studies, my work, and my passion for synthesis! Thanks and here's hoping you share more!
I have the full playlist for the course up now.
Thank you for sharing. I hold a B. Sc. in electronics and have always been fascinated by this beautiful circuit and all its implications. Although I have seen analyses of the ladder filter before, I still learned something from your presentation, especially regarding the different implementations by Moog and others. Thank you, and stay safe.
Not in your class. I am hobbyist who taught himself electronics (I got a math degree from an engineering school so engineer speak is clear to me). I am very happy to see this filter broken down so anybody with a decent understanding of AC electronics can understand it. Very clear explanation! I can now not only see how it works, but I also now can can see how change it for different results.
I'm a hobbyist interested in homebuilt synthesizers, and my university doesnt offer much beyond extremely basic circuit classes. These videos are invaluable -- not just for understanding the particular circuits in question, but also for gaining a greater intuition for the operation of nonlinear circuits as a whole. Thanks so much!
Awesome lecture! It's hard to find great content like this. Thanks!!
As a software engineer getting into electronics by building a synthesizer, this was exactly the level of detail that makes things understandable.
It would be amazing to hear the rest of the class! I can email cute pictures of quokkas if that helps. ;)
Hey Professor Lanterman, thank you so much for doing this, I'm in the last quarter of an EET Associates program and just finished a term on active filters and I was a little disappointed that we didn't get into this in the scope of my studies. I've been trying to understand this topology for years and you did a great job of explaining it! Now I'm off to see what other treats you may have in your post history...
I really enjoyed the way you did this lecture. Really wish my professors in college had a similar delivery taking care to just discuss the way YOU are conceptualizing everything, really makes it more understandable and fun.
my understanding of why in a ladder only the top and bottom of the ladder needs to be a tightly matched, thermally bonded, pair, is that the top and bottom pairs lock the bias current in both arms of the filter to be the same. Given the transistors were made in the same batch and are physically located in the same place, if the current through them is the same the temperature rise in them will also be about the same which keeps them "close enough"
with modern transistors "weakly controlled" parameters are actually pretty consistent if the parts are from the same batch. I once sat and measured 2 dozen BC548's and their Hfe's all came out at exactly 220.
Many thanks for sharing this video. The lecture is really interesting and very well done. This is a mix of "Control/System Theory" and Electronics and you explained all the concepts exactly as I would expect, even with the same notation I would have used! Again, many thanks for this lesson!
EE for many years; very well-explained. Thanks!
Thank you so much for sharing these fantastic videos!
“Analog circuits for music synthesis “ looking at Moog! Wow, I had no idea this could be a course. I wish past me would know that this would be something I would be interested in in the future. I was a music major, and am a musician very interested in analog synthesis! Of course my music path did not send me in your direction. Later in life, I have found joy in learning electronics, and also playing analog synths like Moog synths. This is so awesome. I have zero idea of what you are talking about, but I can say that I truly appreciate that you have left this video here. I am sure that kids are learning this, and don’t appreciate it! I hope they appreciate how freaking cool this is. My guess is a lot of them are just rolling their eyes, and are just memorizing the exam answers. What an awesome course, and you are killing teaching this…. My degrees is music education, so I had a lot of education classes, and taught for 5 years. Great kid dude!
Thanks!
Great lecture and very interesting! Hard to believe that was the first one you did in this format.
Excellent lecture on the Moog ladder! Thanks Aaron!
Thanks for the explanation, really apreciate your work.
Thank you for publicly posting your lecture. It is exciting to watch and a pleasure to see such a friendly explanation for this great topic.
You are welcome! :)
Good to see you again Professor. I'm rooting for the powers that be to permit you to share as much as you'd like. :)
I have the whole playlist for the course up now, and am also posting materials now for my "Guitar Amplification and Effects" class. :)
Hi! I study electronicl engineering at a South African university (Stellenbosch). It has been amazing coming across these lectures, since my university does not offer music synthesis courses. This is truly inspiring thank you!
So cool to analyze the legendary ladder filter in a EE course! Awesome material Aaron and Georgia Tech!
Thank you for your kind words!
this is an awesome class, I wish you could release the entire series so we all can learn the great and fun EE class! It is fun to connect the theory learned from book with the real application. Thank you for the offering!
I have the full playlist up here: ua-cam.com/play/PLOunECWxELQS5bMdWo9VhmZtsCjhjYNcV.html
incredibly well explained and easy to understand. Thanks
Thanks for this, The moog 24 db low pass filter sound is what drove me to building my own synth circuits. Still haven't tackled this one, but I'm slowly gaining confidence. I have so much to learn, but how you explained the transistor ladder was very helpful.
Thanks! I hope to post more videos like this in the future.
Thanks! Found the channel today. Great!
Interesting analysis, had to refresh my frequency domain studies from my college classes. The Moog Voyager uses a trick from the modular to realize a highpass filter response. The Voyager doesn't implement the highpass architecture in the patent or the 904B module. They just use an opamp to subtract the unfiltered audio from the output of the ladder filter (with zero resonance). The passband of the filter output is out of phase with respect to the unfiltered audio; so the pass band frequencies cancel, but the frequencies above the passband remain... and you have a highpass filter. Yes I have schematics.
This was even more understandable than in your awesome lecture series 👍🙂 thanks for uploading
The difference between current controlling and voltage controlling just continues to mess with my brain. 🤷♂️
This is great. Thanks for posting it here
Thanks!
I'm currently running a class called "Guitar Amplification and Effects" -- see my latest videos if you're into that kind of thing.
That's awesome and I will check it out
This is amazing!
Thank you very much for sharing.
Great explanation; thank you for sharing! I would love to have seen the rest of the course
Here you go: ua-cam.com/play/PLOunECWxELQS5bMdWo9VhmZtsCjhjYNcV.html
@@Lantertronics Thank you!
Invaluable. Thank you.
You are welcome! :)
10000% lovely, thank you very much
LOL, I will suppress the urge to email you a capybara.
wooo! upload more ???
ua-cam.com/play/PLOunECWxELQS5bMdWo9VhmZtsCjhjYNcV.html
Awesome lecture, thank you :)
Thanks!
I immediately jumped into the full course playlist :D
Very cool, wish this was offered back in my day :-)
This is brilliant and explains a lot! Not sure whether it would be relevant to your course materials, but I would love to find a video tutorial on J/P Fets & N/P channel MOSFETS in circuits.
vOUT = VOUT + vout. Thanks for the lecture!!! It was awesome!!!
wow thanks a lot!
Would love to attend the course run by Aaron 😍
This is fantastic! I'd love to have the full course. :D
Stay tuned... :)
I'm coming back to this video reading comments after a couple of years... anyway, you've probably seen that I have the full playlist for this course up now, and am currently posting materials for my "Guitar Amplification and Effects" class.
Thanks.
Its fantastic! I want more :)
Here you go: ua-cam.com/play/PLOunECWxELQS5bMdWo9VhmZtsCjhjYNcV.html
Great!
Thanks!
At 20:30 or so I realized I'm still dropping '-' signs in math, which I had a problem in EE school... LOL Thank you for sharing the maths exposition.
i'm not part of your class, but thanks a lot, that was interesting :)
This is fascinating, I would seriously consider moving to Georgia state to take courses in analog synthesizer design.
I've got the full playlist for my Analog Circuits for Music Synthesis course up here: ua-cam.com/play/PLOunECWxELQS5bMdWo9VhmZtsCjhjYNcV.html
Very well explained👍could you please explain how that pnp with its base grounded converts a vairiabe voltage to a variable current?
At RPI I saw the same capital/lowercase bias/small signal notation used in ECSE courses.
I'm using that notation now in my Guitar Amplification and Effects class.
Hi Mr Lanterman, thank you so much for sharing this with us, this is exactly the content that helps me understand more about these circuits. I'd love to take part in one of your classes - have you ever considered doing some sort of online class? I've got an EE degree but I found that many of the essential stuff for music synthesis is not tought anymore in todays classes, sadly! I'd totally help fund you via patreon or direct donations if that would enable you to put more time towards recording videos like this one. Once again, thank you so much for this valuable content!
I have the full playlist up here: ua-cam.com/play/PLOunECWxELQS5bMdWo9VhmZtsCjhjYNcV.html
It took a while, but I finally worked out a deal with the Powers at Be at Georgia Tech by which people can support my work on this channel via a targeted donation to the Georgia Tech donation earmarked for my work. Any amount is welcome, and it really helps in showing my colleagues that there is a hunger for this kind of material. Here's a video with instructions: ua-cam.com/video/VBu-LST1p9c/v-deo.html
thx for sharing ;)
The non-linearity of the ladder is supposed to be cancelled out because it's a differential set up. But why they chose to use a TCA instead of an instrumentation amplifier is an interesting question!
That's a myth. These cascades are highly non-linear for all but small AC voltages. A couple hundred mV, at most, if we care about distortion, tens of mV if we wanted it to be "high fidelity". The thing is... in an analog synthesizer we don't care about it! The filter comes after the oscillator and it does not need much dynamic range because the VCO output voltage range is always the same. More importantly, the VCO spectrum is one fundamental frequency and its harmonics, hence all the VCF distortion will do is to change the overtone spectrum by a few percent, which is barely perceptible. If we were to use this filter on a polyphonic signal or a mix of multiple instruments, then the intermodulation distortion would sound horrible and the noise floor would be pretty bad for signals with low volume. Thankfully none of that happens in a synthesizer and this cheap and easy circuit works just fine.
The Moog filter pretty nonlinear in practice. There's a great paper on digitally modeling it that includes the hyperbolic tangent nonlinearities.
Behringer has an implementation of the 904B high-pass out now. I assume that they have lifted the design straight off.
The diode-connected transistors in the TB-303 make me wonder if it would be possible to rig up a ladder filter with ganged potentiometers such that one extreme is fully diode-connected and the other is like the classic Moog design, a kind of mooginess control.
Robert Moog was brilliant
That´s a masterclass! Thank you so much.
How would you go about implementing a FET (or MOSFET) transistor ladder?
Would it be even possible?
fantastic! now I can finally start to understand how the ladder filter works! I've always wanted what the circuitry is actually doing so I can start making some of my own and-
*wait what the hell is this guy talking about*
Hah! ;)
Hey Aaron, I really appreciate your video. If you could explain, im curious why when you do the small signal model of the half circuit, you choose to call the emitter of the input BJT an AC GND. Wouldn’t that pin just be floating because the current source would be an open circuit?
Thanks a lot, professor. You made it really clear to me, but still, looking at the equations, i don't get why we can't just replace transistors with resistors? And why won't that be Ibias dependent? In fact, increasing current through simple RC filter can also change it's cut-off frequency to some extent, am I right?
Sorry, I'm not really following your question. If you replaced the transistors with resistors, you'd get a fixed cutoff; modifying an extra DC current through the ladder wouldn't change anything. A basic RC filter (input to one side of a resistor, other side of resistor to output, output also has a capacitor to ground) will have cutoff defined by 1/(2*pi*R*C). The only way to get variable behavior by using resistors here would be to use potentiometers as variable resistors -- in which case, you'd have a physical control but not voltage control -- or to use a light dependent resistor, like in a vactrol, and change the amount of light shining on it.
@@Lantertronics I see how that could work if we changed base voltages, for example. Then CE resistance would change and a transistor would act as a potentiometer. But you say that BE current is negligible and base voltage is constant, but then there is no difference between transistor and resistor. It manifests itslelf only as CE resistance, isn't it? In this case I don't see how that system can be controlled by Ibias. In the final equation for cutoff frequency Ibias appears because of gm. It's stated that gm = Ibias/Vt, but it also equals 1/r. What if we say that this r is simply a resistor staying there? Why doesn't it work that way? Thank you.
@@user-yz2jw2ue7z The base voltages are just setting DC bias points to make sure all the transistors are on. I'm not quite understanding you when you say "but then there is no difference between transistor and resistor." To get a sense of where gm=Ibias/Vt coms from, read this by my colleague Marshall Leach, particularly pay attention to equation 13:
leachlegacy.ece.gatech.edu/ece3050/notes/bjt/BJTBasicsSu10.pdf
The gm=1/r is just notation. My presentation was in terms of gm, but the patent and most discussions of it are written in terms of r. But that "r" is a *dynamic* resistance in the small signal model. When you say "r" is simply a resistor staying there," again, I'm not sure what you mean. If you put a fixed resistor in there you're back to a fixed filter. The whole point is to change "r" (equivalently, gm, since they are reciprocal) by changing Ibias.
@@Lantertronics Thank you so much, professor, especially for the link. Now I see, we're talking about dynamic resistance, that's the point. I could not be sure enough about that before.
And one last question then: why won't they use transistors as potentiometers, controlling their resistance via BE voltage? That seems to be an obvious idea, but maybe I'm wrong.
I'm now studing analog synthesis and Moog modules, and I would really appreciate if you gave me some advice on what to read about that, like that BJT article, or maybe some book. And again, thank you so much for spending your time.
@@user-yz2jw2ue7z There's an Electro-Harmonix pedal called the Bassballs that uses a BJT as a voltage controlled resistor by changing the BE voltage. I think there's a limiter design build into some old cassette tape recorders that works similarly.
Stumped - is there a video series of the full course?
There is now: ua-cam.com/play/PLOunECWxELQS5bMdWo9VhmZtsCjhjYNcV.html
Also check out the latest videos I'm posting for my "Guitar Amplification and Effects" class.
I'm not your student but I wish to send you a cute kitty for sure. thank you
To manufacture analog equipment , the Design process can NOT use the approximations
assumed in 11:10/35:06 r]pi , r]o = infinite.
1st stage design uses r]pi-type , r]o-type at I]bias -type
- - -testing of the circuit becomes CRUCIAL to eliminate the many units that are faulty.
A more refined design uses hfe]LO ,hfe]HI values AND
- - - most importan transistors hfe must be measured REPEAT ONLY hfe]measured !
Thank you for your comment! I am curious, did you (or do you) work for a company that makes Moog ladder filters? I would like to know more of your insights. Many many years ago we could assume you were working for Moog, but there are many companies putting Moog ladders into products nowadays, especially with the proliferation of modular synth manufacturers.
Also, are you saying that the hfe values just need to be matched, or are you saying that they must also be chosen for specific values? I've heard that people match at the pair at the top and the pair at the bottom, but I don't think I've heard of people selecting for specific hfe values.
I doubt that you will end up with a lot of faulty units using modern transistors. These filters don't need to be calibrated much, anyway. People are tuning the instrument by sound, not by potentiometer value, so if the gains are off, the user feedback loop will calibrate that out.
awesome job.
however, i did improve the Buchla wave folder / timbre ... not being a professor.
my profile has a video of it.
this drawing from Robert looks a bit different.
www.synthfool.com/docs/Moog/modular/moog_904b.gif
I am not sure how, but your presentations appear to be unclogging a forty-year 'wtf! Transistors do what? & how? hole flow wtf? you're kiddin' me.." that sort of deal seems to fade each view. It is actually since 1969 when Wendy Carlos' "Switched-on Bach" blew my brain apart to want to learn how this works...... Now, a 8 stereo/2x8 mono inputs by same outputs VCA cross-point Matrix Mixer forces me to understand so I can build it and be able to route anything to anything at will, something I missed on the Buchla racks available to me for 10ish years back then.
Thanks!
Ah! You have laid hands on a real Buchla? The sacred Buchla? (I am obsessed with Buchla)
course, quokkas produce the least cute sound on the planet
sounds like a cackling witch from hell, even more unnerving from 20 feet up a tree.
You have to be slower and more detailed on math expressions. And explain every expression's member. That is most interesting and complex part.For me it looks greek.
For a bit of context, this was recorded for my Analog Circuits for Music synthesis class at Georgia Tech, and in particular was the last lecture in the Spring 2020 semester; I recorded it when we went to online learning because of Covid. So the students had spent the semester following my style and notation.
Definitely, it's helpful on UA-cam that you can stop and rewind. :)
@@Lantertronics youtube has pause and rewind but narration does not explain the expression members. It takes time to deduct it, which is not easy with phone. Anyway, I love what you did -- great material.
I think I saw The Roots Of Unity open up for Living Colour in the 90's