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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Adding a skew control to the LFO on Synth Experimenter board
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jukingeo



Joined: Oct 24, 2007
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PostPosted: Sat Dec 07, 2013 1:22 pm    Post subject: Adding a skew control to the LFO on Synth Experimenter board Reply with quote  Mark this post and the followings unread

Hello all,

After looking around here on typical LFO designs, one feature that struck a chord with me is having an adjustable 'skew' control on the LFO.

This feature I first saw in an older MFOS modular LFO here:

http://www.musicfromouterspace.com/analogsynth_new/OLDIESBUTGOODIES/LFO/images/coolnewlfo_sch.gif

Then it also came up in this discussion here:

http://electro-music.com/forum/viewtopic.php?highlight=lfo+skew&t=32372
(2nd post down).

Since I already have the DIY Synth Experimenter's board, I am curious if I could add this feature to it. As it is the LFO lacks a ramp wave output. Another side effect of the skew control is having control over the duty cycle of a square wave output. I find this to be a MUST HAVE feature on my Synth Experimenter.

Looking at the schematic below:

http://www.musicfromouterspace.com/analogsynth_new/EXPERIMENTERBOARD/images/schempg4.gif

It would seem that the 'wave shaping' or 'skew' control would be placed (along with a couple of diodes) right before the inverting input of IC U6 a (or U6 d for the 2nd LFO). Would I be correct in this assumption?

I came up with that hypothesis after looking at the LFO for the Sound Lab Mini Synth here:

http://www.musicfromouterspace.com/analogsynth_new/SOUNDLABMINISYNTH/images/synthkit_lfo_noise_power_001.gif

In this case there is a switch to select Ramp-Tri-Sawtooth. But I could just as easily put a pot here for a variable skew, right? So I am thinking if I add this circuit to the Synth Experimenter, I should get the same feature, correct?

Thank You,

Geo
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SineHacker



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PostPosted: Fri Dec 20, 2013 2:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

I can't help you with skew, but I might be able to get you a ramp wave, I posted this earlier without seeing your thread - my schematic might help you and I would like to get some dialog going on modding the experimenter:

http://electro-music.com/forum/post-394011.html#394011

I haven't tried the mod with the LFO yet as I was more interested in getting it working with the VCO. Looking at the MFOS schematic, the LFO is based on an integrator/comparator pair - I think if you follow my eagle schematic shown in the linked thread, you should definitely be able to get ramp or saw - the LFO is almost identical to what I had on the breadboard whereas there are some differences with the VCO circuit.

My SDIYE project PCBs are pretty much complete, its just not wired up or in a box yet, I'm at the stage where I want to poke around and see if I can squeeze any more out of it - I'm not at home to test it till after Christmas though santa

looking at your first link, it would be cool to get a waveshaper style thing going on either the VCO or the LFO! even if it is crude Smile

Let me know how you get on if you get there first!!

Edit: I read the bottom of your post properly and yes it looks like the Soundlab uses the same concept - I guess we just need to try it Razz

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jukingeo



Joined: Oct 24, 2007
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PostPosted: Sun Dec 22, 2013 12:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

SineHacker wrote:
I can't help you with skew, but I might be able to get you a ramp wave, I posted this earlier without seeing your thread - my schematic might help you and I would like to get some dialog going on modding the experimenter:

http://electro-music.com/forum/post-394011.html#394011


I took a look at your schematics and compared it to Ray's on the MFOS site. It looks like the diode arrangement & skew control are within a feedback loop on his schematics, whereas on yours you have it outside of the loop and set to ground out. Since it is outside of the 'active' loop, I didn't think it would be effective.

I was thinking that I would build the experimenter as is first and then attempt mods this way if something goes wrong, I know where to look.

Quote:

I haven't tried the mod with the LFO yet as I was more interested in getting it working with the VCO. Looking at the MFOS schematic, the LFO is based on an integrator/comparator pair - I think if you follow my eagle schematic shown in the linked thread, you should definitely be able to get ramp or saw - the LFO is almost identical to what I had on the breadboard whereas there are some differences with the VCO circuit.

My SDIYE project PCBs are pretty much complete, its just not wired up or in a box yet, I'm at the stage where I want to poke around and see if I can squeeze any more out of it - I'm not at home to test it till after Christmas though santa


Well, another fellow here recommended that I first get the PCB working as per the original instructions first and then move on from there.

Quote:

looking at your first link, it would be cool to get a waveshaper style thing going on either the VCO or the LFO! even if it is crude Smile


I also thought of just building the LFO Variable Skew on the MFOS site on a separate strip board this way I would have a working reference to work off of prior to making mods on the Experimenter board. Once working and after I gain a full understanding of how it works, then I would be in a better position to mod the LFO's on the Experimenter PCB. Furthermore if I am successful, I would have an extra LFO or two. As it is, you can't have enough LFO's and I can see already that by playing around with one capacitor, you could change the range and get the LFO to run pretty high in frequency...probably would be good for modulation sources for VCO's as well as ring modulators, and so forth.

Quote:

Let me know how you get on if you get there first!!


Sure, I will post my findings here. But as of now I tried a new company, Futurlec, for my first parts order and they seem to be pretty SLOOWWW at getting my order out. While I like their prices, the slow shipping is turning me off. They are also out of 13700 IC's and it wasn't until I asked about the shipping status that they told me that. While one is needed for the LFO Variable Skew, it is only used for the sine wave output. I am mostly interested in the variable skew part and how it affects a triangle and square wave right now. Since the sine wave circuit encompasses nearly half of the LFO Variable Sku, I figured I could leave it out on the first strip board attempt. Then I only need ONE TL084, so I am good.

Quote:

Edit: I read the bottom of your post properly and yes it looks like the Soundlab uses the same concept - I guess we just need to try it Razz


Yeah, I noticed that the LFO on both Sound lab versions (original and MKII) AND the Ultimate (which is identical to the SLMKII), use a center off switch to set the output to Saw, Triangle, and Ramp. The diodes in this part of the circuit must act as some kind of short circuit and depending on the polarity of that diode determine whether you get a saw or ramp output. Leaving the switch at center off removes the short and you would get a symmetrical triangle wave. Now if you remove that switch put in a pot in that location, boom! You would have your variable skew. Comparing the schematics to all of these LFO's to the variable skew one's schematic does verify this. In all cases the skew control (or skew switch) is within the feedback loop. This is why your circuit didn't work. I think you have to put the diodes and control within the feedback loop AFTER the LFO rate control.

Once I get the parts and complete the Experimenter, I will try out some tricks on a breadboard and see what I can come up with.'

EDIT: I did look at the schematics again and the LFO on the experimenter is very similar to that of the original Sound Lab. It appears that the extra components needed are R88 1k, D9 & D10 and a 50k pot. The circuit takes place on one side of the LFO frequency control. This control usually has pin 3 and 2 tied together, but in this case it has to be separated with the #2 pin going to the inverting input of the 'a' op amp. The #2 pin needs a 1meg resistor. The #3 pin of the control goes to the two diodes together, the diodes are set up in opposite polarity across the 50k pot. The wiper of this 50k pot goes via a 1k resistor to the inverting input of the 'a' op amp. Now the #3 pin of the frequency control goes also to the output of the 'b' op amp. That would complete the circuit for the LFO on the Experimenter. BUT the VCO is a different ball game. There are about two places where I can see this same circuit going, but I don't have a frequency control to reference from. It could be that you would put the circuit I outlined above on the LM3900 'a' op amp, since that is the op amp that has the capacitor in the feedback loop (and that is also the triangle output), but I am not sure.

Thanx,

Geo

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SineHacker



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PostPosted: Sun Dec 22, 2013 3:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

Just to warn you, I have been to the pub with my parents and this was after a boozy social, so language, meaning and intent may break down... santa santa santa

If this helps:

The square side uses a comparator - I probably won't do to well trying to explain how this works, but here goes (I am still learning myself Embarassed ) but it basically will flip between positive and negative voltage depending on its input, which in our case is the triangle waveform - when the triangle reaches is positive or negative peak, it causes the comparator to flip its output state. The triangle side uses an integrator, **I think this is how it works** the output tries to match the input, but this happens over the period the capacitor takes to discharge, our input is a square wave so the op-amp is constantly tracking the changing voltage state of the square and as the output takes time to match the input, we get the ramps of the triangle wave. So the whole thing is a feedback loop, before and after the frequency resistor.

That said, I think the schematic I posted is wrong (and it does contradict what I typed in that post). I think I drew that schematic as I remembered it from Ray's webcast a short while ago - but I found on the breadboard that the diodes had to be at the output of the comparator in order to take the effect we want, which is what you said I think Razz (I will have to build it again to be sure now).

Quote:
I took a look at your schematics and compared it to Ray's on the MFOS site. It looks like the diode arrangement & skew control are within a feedback loop on his schematics, whereas on yours you have it outside of the loop and set to ground out. Since it is outside of the 'active' loop, I didn't think it would be effective.


It is interesting what you have pointed out here, it has made me think - In my circuit I think that the triangle and square waves oscillate between a +V and -V. Whereas in the SDIYE VCO the LM3900 doesn't use the negative pole as a reference, it uses +V and ground, so I think that coupling the LM3900 comparator output to ground though a diode is creating a fundamentally different circuit to when I do it with my circuit. I will check the circuit again you mentioned as this might be an approach that is more compatible with the SDIYE VCO.

PS, the frequency control with the SDIYE VCO works by taking a reference voltage from the pot, this goes through the inverting buffer TL074 and effects the transistor in the feedback loop which acts as a voltage controlled resistor - it is a cool concept and very stable! Sorry I can't explain it much better than that, particularly in my current state drunken

I will probably take another look over your reply tomorrow and throw around some more ideas, its good to be able to actually talk to someone over this!!

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SineHacker



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PostPosted: Sun Dec 22, 2013 3:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

Also, as I said, my SDIY EB is near completion so I can test out anything you want me to throw at it - I am pretty sure that my schematic does show an example that will work with the LFO. I will try connecting to the inverting input of the integrator (U6 as you mentioned) but using a pot to wipe between the two diodes connected to ground - if science is having a good day then we will get a morph between tri saw and ramp, but that is probably me being incredibly optimistic Laughing If that doesn't work I will also see if I can find a way to include the diodes in the actual loop of the feedback path, though I imagine this would definitely involve some changes to the actual board which might be fiddly.

I built a bench 12V~ supply recently so I could quickly breadboard the LM3900 based VCO as well to see if I can change up something there. I am very hopeful I can get the saw ramp and hard sync working.

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jukingeo



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PostPosted: Sat Dec 28, 2013 2:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

SineHacker wrote:
Also, as I said, my SDIY EB is near completion so I can test out anything you want me to throw at it... .


Sorry I have not responded as of yet, I have been busy with Christmas. We had festivities both on Christmas Even and Christmas Day. All went well. I hope your holiday's events also turned out well.

In terms of my synth project, I don't have my parts as of yet! So far my initial dealing with Futurlec has been mediocre. I put my order in on the 11th and I didn't hear anything until the 20th when they told me the 13700 was on back order. So I asked if they have 13600's and if they did to sub them in the order. They got back to me on the 23rd saying they did have those chips and they will get my order out and I should see it in 10 to 14 days.

THAT is a LONG time! As is was, my wife's Christmas present to me were the PCB's for the MFOS 16 channel sequencer. She placed that order on the 18th and it came before Christmas AND before the Futurlec order.

So I am having second thoughts about using Futurlec again. I am curious if others have had issues with their slow turn around time. If so, then the next place I would like to try out is Small Bear Electronics in NY. Given that they are so close I would assume the shipping will be better.

So that is what is cooking on my end. I am just waiting for the order to arrive. I am hoping everything will be there and there will not be any more delays.

Have a good one!
Geo

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analog_backlash



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PostPosted: Sat Dec 28, 2013 3:06 pm    Post subject: Re: Futurlec Reply with quote  Mark this post and the followings unread

Hi jukingeo.

I haven't ordered from Futurlec, but I have seen people complaining in other fora about shipping times and changes to shipping costs. However, some here seem to like what they get (even if it does take a long time). It's on my list of possible suppliers, so I guess I'll watch this space.

Gary

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SineHacker



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PostPosted: Sat Dec 28, 2013 6:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

I haven't got around to prodding the Sdiye yet either - but I can say this, next time you order parts check out Tayda Electronics.
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analog_backlash



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PostPosted: Sat Dec 28, 2013 6:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ditto Tayda, although they usually take 2-3 weeks to arrive as well (but very cheap and good quality so far).
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jukingeo



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PostPosted: Sat Jan 04, 2014 7:55 pm    Post subject: Re: Futurlec Reply with quote  Mark this post and the followings unread

analog_backlash wrote:
Hi jukingeo.

I haven't ordered from Futurlec, but I have seen people complaining in other fora about shipping times and changes to shipping costs. However, some here seem to like what they get (even if it does take a long time). It's on my list of possible suppliers, so I guess I'll watch this space.

Gary


I have not heard about shipping problems with Futurlec...but then again I really wasn't looking either. But here it is Jan 4th and NO parts. I was hoping to do some PCB populating this week.

Actually since you mentioned hearing about issues in other forums, I think the next time around I am going to try Small Bear Electronics. Since they are fairly close to me, their shipping should be pretty fast. Also I like their selection of controls more than Futurlec. Futurlec only has PCB mount controls, whereas Small Bear has the solder lug type controls. Granted the price break isn't as good as Futurlec, but I would be getting more of what I would like rather than 'settle' and fudge something else.

So Tayda is another good company? I guess I will check them out. I have to see where they are located. Hope they are pretty close too.

(checking)

Oiyah! OUCH! They are in Bangkok, Thailand! Nope, no dice. I definitely want something closer than that.

Well, since I was going to get my controls from Small Bear anyway, I will report back with my findings with them. Hopefully I will have good dealings with them.

Geo

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jukingeo



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PostPosted: Thu Jan 09, 2014 5:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hello All,

Futurlec gave me the tracking information in regards to my order. As of the 7th, it arrived in NYC. I am HOPING that I will see it by tomorrow or even Saturday. We will see.

I really didn't pay attention that Futurlec was overseas as well as Tayda. Well, now I know. I just was hoping that I would have had the parts by now as I am a week behind on my winter projects. So much so that I am actually going to work on an antique radio project first.

Geo

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PostPosted: Mon Jan 13, 2014 6:48 pm    Post subject: Futurelec order update Reply with quote  Mark this post and the followings unread

Hello All,

Well, it has been a month and 2 days and my order from Futurlec is STILL not here.

The only good thing is that I do have the tracking number and I was able to follow the package from Germany on. But I am somewhat infuriated that the package has been sitting in Kearny, NJ for THREE DAYS! Granted we just had a weekend, but doesn't the post office work on Saturdays? I know mine does.

Anyway, it said it left Kearny on the 10th. I am hoping it finally arrives this week.

I don't know if a month lead time is normal with these Asian companies, but if you have a project plan or want something quick, definitely not the way to go.

EDIT: Ok, so the day AFTER I write this the package FINALLY arrives. I have not taken an inventory of the parts yet, but it looks like it was packaged well with the parts put into a bubble bag and this was wrapped on itself twice. Then this was put in a shipping bubble bag. So I give them props for the shipping.

Now I have a question: Is a month shipping the norm for all Asian companies? Or did I get a bum steer here and my wait time was excessive?

Thanx,

Geo

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jukingeo



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PostPosted: Sun Jan 26, 2014 8:13 am    Post subject: Reply with quote  Mark this post and the followings unread

Hello All,

Just touching base. I have begun to construct the DIY Synth Experimenter board along with a couple other projects I am doing.

Yesterday and today, I spent quite a bit of time on the Tayda Electronics site. They have WAY more product than Futurelec, as just about all of the items I couldn't get through Futurelec, they have.

I placed an email to them and they assured me shipping is within 3 weeks, which beat out Futurelec's one month+. Granted on Futurelec's behalf, I did order during the holiday season, so it is possible that I might have received the order sooner.

But all in all, I believe my next order will be through Tayda. I will follow up here when I do place an order with them, which will be a while since I am still working on the Synth Experimenter. For Christmas my wife got me the MFOS 16 step sequencer boards, so I will be placing an order with them within a couple months.

Special thanx to Sinehacker and Analog_Backlash for pointing me in the right direction.

Geo

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