A Schematic for the Microsynth Filter using LM13X00 OTAs:

[liquids]

I spent some free time this weekend working on adapting and breadboarding(!) the filter portion of the Microsynth to the available LM13X00 chips, and comparing it to my actual Microsynth.  It works.

I drew up a schematic of what I have, which probably has errors and is not the whole microsynth, but it's a starting point for the initiated.  The common schematic drawn by Fabian Hartley is more or less what the below is meant to be, but with some of his alleged resistor value errors corrected and a potential pinout arrangement for LM13X00 chips, plus some other interesting options you may or may not find useful.  Here's what I have to offer:



Yes, the rest of the microsynth OTAs can be built to use LM13X00 from the ground up, but the filter is all I care about so that's all I have here...

I'm sure someone else can take it from here!

Mr. Giles?  



Have fun!

[liquids]

I've corrected and tweaked the schematic image in the gallery, which is linked/shown to above...but it doesn't seem to update in the thread when I do.

Here's the link: Schematic?

[Morocotopo]

This is like an envelope filter, right? I´m not familiar witht the original. Does this one have any special characteristics that set it apart from other env filters? Could be nice to make a pedal...
Great work on adapting it. Should be easier to build.

[liquids]

This is like an envelope filter, right? I´m not familiar witht the original. Does this one have any special characteristics that set it apart from other env filters? Could be nice to make a pedal...
Great work on adapting it. Should be easier to build.

It's not an envelope filter, it's a 'triggered' filter. That is, the filter doesn't 'follow' or attempt to follow the envelope of your playing dynamics; moreso, it is 'triggered' by the attack of a note and then it's sweep is determined by the controls, mostly independent of your playing dynamic, until it is 're-triggered' or reset again by a strong enough signal.

I say mostly, because, contrary to what Stephen Giles says/wants (a triggered filter with envelope controled start frequency), I find that there is SOME small range where the 'start frequency' is affected by how hard I pick. Just as picking too softly causes the filter not to trigger at all, somewhere between there and full playing at force, the filter will trigger and sweep, but not as 'dramatically,' depending upon how the controls are set by comparison.

[Figuurzaagje]

Sorry to interrupt.

I need to repair a vintage Micro Synth. It squeals random, like a constant trigger.

A. I can't figure out what schematic to use; there are many topics about "correction" but I can't find the definitive and corrected schematics: Who can help me?

B. What are the two adjustable pots for? I guess one is trigger sensitivity bias...

C. Are there any weird obsolete components in this thing? I'd like to build one myself, to get back into shape before diving into the electro world again.

D. Some say an entire VCA section should not be build but just replaced by pots. How can that be okay?

[StephenGiles]

This is like an envelope filter, right? I´m not familiar witht the original. Does this one have any special characteristics that set it apart from other env filters? Could be nice to make a pedal...
Great work on adapting it. Should be easier to build.

It's not an envelope filter, it's a 'triggered' filter. That is, the filter doesn't 'follow' or attempt to follow the envelope of your playing dynamics; moreso, it is 'triggered' by the attack of a note and then it's sweep is determined by the controls, mostly independent of your playing dynamic, until it is 're-triggered' or reset again by a strong enough signal.

I say mostly, because, contrary to what Stephen Giles says/wants (a triggered filter with envelope controled start frequency), I find that there is SOME small range where the 'start frequency' is affected by how hard I pick. Just as picking too softly causes the filter not to trigger at all, somewhere between there and full playing at force, the filter will trigger and sweep, but not as 'dramatically,' depending upon how the controls are set by comparison.

That's very interesting, I don't have a unit to play with now but I was never aware of this small range when I did have one. Perhaps what I want is more like an "adaptive peak peak following start frequency"

We are in Spain for a week soon so whilst my wife is roasting herself in the sun, I can have a good think about this!!

[oskar]

Sorry to interrupt.

Hi! Please start a new thread.  Chances are slim that this is going to be a trivial fix so this thread would be
effectively hi-jacked.   

I need to repair a vintage Micro Synth. It squeals random, like a constant trigger.

Start here -> http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

A. I can't figure out what schematic to use; there are many topics about "correction" but I can't find the definitive and corrected schematics: Who can help me?

In your new thread. Link to the schematic you do find.

[StephenGiles]

There are only 2 versions as far as I know using CA3094, the original  which used an obscure voltage regulator and ran very hot, and the other one which didn't and used fewer CA3094.

[YouAre]

I might be blind, but I absolutely cannot find where "B" goes to from the stop/start section...

[oskar]

I might be blind, but I absolutely cannot find where "B" goes to from the stop/start section...

Inverting input IC2a to rightmost transistor ("under" the Sweep Rate pot)

[YouAre]

Inverting input IC2a to rightmost transistor ("under" the Sweep Rate pot)

I meant where does that point connect to? I'm assuming there's two connections to B, so I can't find the other one.

[oskar]

If you're on the 12/12/78 revision schematic I think it's equivalent to the feedback net pin 3/8 on IC A13.

[YouAre]

If you're on the 12/12/78 revision schematic I think it's equivalent to the feedback net pin 3/8 on IC A13.

I was just looking on the schematic posted above. I'm really interested in building this triggered filter and/or experimenting with other uses and filters.

[PRR]

> looking at the start/stop frequency portion of the circuit, do you have any insights into simplification

I'm not real sure how it works. There isn't much there. I doubt you want me to rack my remaining brains for hours to save you a dime or two of parts.

Some other things seem dubious.

Seems like the "2N5087" can never turn on, no way to forward-bias the base. Anybody know what am I missing here?

Top center, the "1.5K 15K" resistors should be some ratio but I don't think 10:1 is correct. Either 1:1 or 2:1... I forget but a little searching on precision full-wave rectifiers should give a clue.

The +9V 330K:470 divider top-right seems awful steep. 13mV threshold, but the log-amp before it has output up to 2V. I'm wondering if should be 33K:470... easy to try. The increased threshold will need more Trigger Gain, but also raise levels through the top path, perhaps reducing the "how hard I pick" issue.

[liquids]

Paul, thank's for sharing your insights...

Seems like the "2N5087" can never turn on, no way to forward-bias the base. Anybody know what am I missing here?

No idea...though it is right out of the schematics I have.  Note that the resistor I have at the base as 12k is the 'Bass' version value, 'guitar' version is 1k.

Top center, the "1.5K 15K" resistors should be some ratio but I don't think 10:1 is correct. Either 1:1 or 2:1... I forget but a little searching on precision full-wave rectifiers should give a clue.

Good catch - that's a typo, what is labeled 1.5k should be 7.5k

The +9V 330K:470 divider top-right seems awful steep. 13mV threshold, but the log-amp before it has output up to 2V. I'm wondering if should be 33K:470... easy to try. The increased threshold will need more Trigger Gain, but also raise levels through the top path, perhaps reducing the "how hard I pick" issue.

This area is actually also per schem AFAIK. 
[Edit: was looking at the wrong part of the original schematic at first]

There isn't much there. I doubt you want me to rack my remaining brains for hours to save you a dime or two of parts.

It's not so much dimes as it is a combination of ,my complete lack of understanding this part of the circuit with a dose of curiosity.  The 'equivalent' replacement for the unique part of the CA3094 with the stacked BJTs and two resistors seems so clunky.  But, I guess it works...

[StephenGiles]

You may be able to substitute a start/stop circuit based around the one in the EH Mini Synth, which used a transistor + a couple of pots and resistors - but why not leave as it is - it works.....albeit in a fixed sort of way!!

[liquids]

I wanted to note that using the trigger circuit into an OTA wired as a VCA (see something like the final OTA/output of the microsynth schematic) creates a fairly nice attack/decay. 

Start frequency pot becomes start volume, end frequency pot becomes end volume, sweep time is sweep time.  This means you can not only do volume swells (zero output volume to maximum output volume), but staccato-like decay.

A heavily compressed signal of some sort is probably preferable to get maximum usage from this effect...likewise, I imagine the full-fledged EHX attack decay allows for much more as I believe that one can do both a rising and a falling amplitude, not just one or the other, but it works.  Using the trigger portion to control a full-fledged ADSR envelope generator --> op-amp VCA might still be be simpler than an attack decay clone.

[liquids]

I've been trying to update the schematic with the little corrections but alas, the old image still shows and post can no longer be edited.  Anyone interested in working off of the schematic should go to the gallery, but they're mostly just component value tyops as compared to the common schematics out there....

I keep forgetting to try the following out for myself (I have the whole trigger and start/stop sections breadboarded currently and driving a similar but different OTA filter)...I am curious about that transistor pair that follow the start/stop OTA - the pair that drives the filter OTA controls and simultanously feeds back to the start/stop OTA.  

My question is, could the start/stop OTA feed the base of a single transistor, with said transistor's emitter driving the filter OTA's control inputs, and the transistor's collector only feeding back to the start/stop OTA's inverting input?   To clarify, that would eliminate the 2.2k resistor to V+, the 47k resistor connected to the filter OTA's controls inputs, and the one transistor.  I do intend to try it because I have no understanding of what that pair effectively does.  Also trying to scan over the current XO version which uses 13700 otas and much have a work around, but alas my brain hurts everytime I try to follow the traces and SMD components.

Can someone explain why that transistor pair/resistor pair is critical here?  The discrete replacement (which again, does work properly as-is) came from the intersil datasheet, should be able to view the snippet here:  

[liquids]

[liquids]

I doubt many people have built this off my work, but I found a few connection errors in my original schematic that are critical errors as well...it wouldn't work as originally pictured.  So please refer to the gallery where I try to keep the most updated schematic.

Also, I did indeed find that I can eliminate one of the transistors and make use of the unused 13x000 buffer.  I simmed it and it seemed that the sink output and 'source drive output' are more or less outputting the same waveform at identical phase (which surprised me).  My best guess is that the network functions to buffer the signal, and then feed the second/Q13 transistor, all as a Q&D way to merely have two isolated outputs (collector and emitter) for the signal at the base.

So, I found that if you substitute what is shown as 'Q12' in the CA3094 datasheet (or the first transistor in my work-alike transistor), along with it's 2k collector resistor to V+ (noise rediction?) and 47k resistor to V- from emitter (emitter follow resistor?) with the simple darlington in the 13x00 chip as a buffer, it works fine... then from the output of the darlington (with it's standard resistor to V- of course) you'd do the same as show in the schematics: feed the output it to a discrete transistor's base, and connect that transistor's collector and emitter connect as follows on the drawings.  Works just fine; that in effect makes use of the otherwise unused buffer, eliminates a discrete transistor and two resistors.  Why not.