Moog VCF resonance control -  Any analog synth guru here? - Page 1
Forum section: electronics / projects

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: January 27, 2013, 12:10:23 pm
content: I suppose it's not the most discussed topic here and by some considered ancient.Be that as it may, I'm playing around with the Minimoog VCF (voltage controllable filter), like in this here version of the schematic:http://dropmix.xs4all.nl/rick/Emusic/Moog/moogvcf_schematic.gifClassically, the resonance (yeah, those filters have it on purpose, even if some of you might cringe now *g*), called "emphasis" by moog, is not made voltage controllable, and I've not found a single schematic on the net that did this for this particular filter.Well, I'd like it to be, either voltage controlled or just diretly digitally controlled, since I intend to connect to a MCU board for control stuff anyway.The resonance amount is controlled here by the feedback pot P11 50k log "Emphasis", to the upper right.How would one replace that pot, with what, to get resonance voltage / digitally controlable?I was thinking about digi pots, but from some data sheets I've seen, those usually like current in one direction only, whereas voltage on P11 swings in both directions. (have the circuit here on perfboard and measured, with 2 scope probes and A-B mode - correct way to do it?).I have a bunch of LDR + bright LEDs for experimenting, but would rather not like to use this "deprecated" for good reason stuff in my regular devices I build.Now I'd appreciate not getting comments about "deprecated" for the next idea, using an OTA based (b/c low parts count) VCA as replacement for the pot - at least some of them, like LM13700, are still being made for reasons only the FSM knows, which is a pretty neat status for parts in analog synth DIY landBut an OTA (operational transconductance amp) has differential inputs - I wouldn't know what difference I'd be amplifying in that circuit, nor do I know whether voltage or current is the inportant thing there.Can anyone help me with that, or do you have alternative ideas to what I just listed?It can be a very low update rate solution (hence I shortly considered LDR), it's only for storing & retrieving a setting.Btw., my electronics knowledge is rather humble, I'm a programmer dabbling a bit in the realm of electrons on weekends :-DSo forgive me stupid questions or not stumbling upon seemingly obvious stuff myself :-D

author: nctnico
author_rank: Super Contributor
timestamp: January 27, 2013, 04:25:58 pm
content: You should be able to use a digital potmeter but you'll need one that likes bipolar voltages. I've used this one in the past and I think it may suit your needs:http://www.analog.com/en/digital-to-analog-converters/digital-potentiometers/ad7376/products/product.html

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: January 27, 2013, 05:19:31 pm
content: Quote from: nctnico on January 27, 2013, 04:25:58 pmThat thing looks awesome from the specs! Thanks a lot!Seems a bit pricey where I found it, but looks like a simple, no-worries solution. Cool.

author: CarlG
author_rank: Regular Contributor
timestamp: January 27, 2013, 05:34:29 pm
content: Great tip! Looked around a bit;AD5290could be a cheaper upgrade, orAD5293with even higher resolution for about the same price.

author: Hideki
author_rank: Frequent Contributor
timestamp: January 28, 2013, 01:03:08 am
content: Moog Source uses an OTA to control resonance (emphasis), as seen inhttp://www.retrosynth.com/~analoguediehard/studio/keyboards/moog_source/moog_source-schematic-analog-4.jpg

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: January 28, 2013, 08:09:55 pm
content: Oh that's cool, the "Source" wasn't on my radar at all :-D

author: bradfordtoney
author_rank: Newbie
timestamp: January 30, 2013, 04:14:39 am
content: So i'm not a synth expert either but i'm building something similar and i noticed that the transistors for the exponential converter are similar to a design i'm working on. Originally i was very confused about how it worked but i found this PDF about how they work in great detail. This may assist you some:http://electronotes.netfirms.com/s019.pdf

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: March 10, 2013, 12:46:31 pm
content: Quote from: Hideki on January 28, 2013, 01:03:08 amThat old dual FET transistor (Q23, just left of the VCA tag), I can't find anything about, like a replacement type.I looked for a dual jfet (dual because of thermo coupling I guess) at RS, the only one I found is some real *tiny* SMD one and I don't know whether it would even work, to be worth the touble getting an adapter pcb.Can this be done differently? Those two form a differential amp, right? Can an op amp replace this? Or just 2 bipolar transistors, like the rest of the circuit is using?Does anyone here understand the circuit well enough to be able to tell why Dr. Moog chose those FETs there?

author: CarlG
author_rank: Regular Contributor
timestamp: March 10, 2013, 06:46:58 pm
content: Quote from: TinkeringSteve on March 10, 2013, 12:46:31 pmNot easy case, but I foundNTE461...not for freeAlternatively, depending on your demands and how stubborn you are, you could buy a bunch of single TO-92 JFETs, try to match and glue them together. If you just want to experiment it would be feasible. If you don't want the matching trouble you trythis link at diyaudio.Otherwise, Linear Integrated Systems (like LSK389) may be an option. Can't find it at any internet store, though.

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: March 16, 2013, 12:32:10 pm
content: Thanks, I'll ask that guy.I might hack together a matching device with some MCU devboard, but it would mean that I'd have to buy a lot of transistors, doesn't it :-D

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: March 21, 2013, 05:24:36 pm
content: The guy didn't reply yet.Can anyone suggest what single JFET (to match two myself) I should try in this circuit?I have no experience with JFETs whatsoever, and wouldn't know which one would be good for this application.

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: April 18, 2013, 07:41:46 pm
content: For anyone who cares, the part involving the JFETs I got working using two J113, which is also not the most easily found component, but obtainable´. If I am not mislead by the numbers, NXP makes an SMD version of its "bigger brother" J111, the NXP PMBFJ111,215, which seems more readily available.

author: icon
author_rank: Regular Contributor
timestamp: April 18, 2013, 09:25:51 pm
content: Hey! I care! Sorry, I missed this thread 1st time around, but I just noticed the dual fet question - I started this threadhttps://www.eevblog.com/forum/projects/dual-monolithic-n-channel-jfet-in-8-pin-dip/a while ago - is it relevant?. I have a spare monolithic dual N channel JFET that I got at the time in a little can package with 6 legs. I easily got it to fit in an 8 pin dip socket.If it's any use to you still, you can have it.John

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 13, 2013, 01:38:40 pm
content: icon:Thanks for the offer! I haven't gotten the OTA feedback thingy working well behaved, though.I had found another later moog circuit with OTA for feedback and without the JFETs, which behaved just as badly and I have given up on that for now.I have gotten it to work with a similar digi pot as mentioned by nctnico, although there's something strange going on, which I will soon make another thread about.

author: Odysseus
author_rank: Regular Contributor
timestamp: July 14, 2013, 10:04:16 am
content: I built a VCF based on the Moog transistor-ladder filter a few years back. And guess what the one thing I wish it had? Voltage controlled resonance!Well, that and maybe a temperature controlled oven for the exponential current source. Breathe on it and it drifts a semi-tone or two, which is pretty annoying when you use the filter as a sine-wave oscillator by turning the resonance all the way up.I can't beat my self up too much though. At the time it was a school project with a deadline, and I decided that it wasn't worth the added effort , despite knowing exactly how to implement it. Currently, the resonance is controlled by increasing the negative feedback signal with a pot and some opamps, but I could've replaced it with an analog multiplier circuit, of the Gilbert Cell variety. It's similar in vain to an OTA, I suppose, although the multiplier lets you make the resonancenegative. It becomes an overdamped instead of an underdampd 2nd-order system.If I had the motivation, however, I would probably go ahead and add two separate multipliers driven by the same CV. One to gain the negative feedback resonance path, and one to gain the input to compensate for the loss of overall gain in the filter caused by the negative feedback. This way, turning up the resonance would not increase the output noise.

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 14, 2013, 11:48:10 am
content: Yeah the temperature control in the schematic of my opening post is quite neat.I wonder if abusing a transistor as a heating has bearing on the lifetime of the chip, though...