Adding a skew control to the LFO on Synth Experimenter board

jukingeo

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

SineHacker · Joined: Mar 09, 2010 Posts: 99 Location: United Kingdom

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

jukingeo · Posted: Sun Dec 22, 2013 12:59 pm Post subject:

SineHacker · Joined: Mar 09, 2010 Posts: 99 Location: United Kingdom

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

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).

SineHacker · Joined: Mar 09, 2010 Posts: 99 Location: United Kingdom

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.

jukingeo · Posted: Sat Dec 28, 2013 2:06 pm Post subject:

analog_backlash · Posted: Sat Dec 28, 2013 3:06 pm Post subject: Re: Futurlec

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

Link removed as it didn't work!

SineHacker · Joined: Mar 09, 2010 Posts: 99 Location: United Kingdom

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.

analog_backlash · Posted: Sat Dec 28, 2013 6:48 pm Post subject:

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

jukingeo · Posted: Sat Jan 04, 2014 7:55 pm Post subject: Re: Futurlec

jukingeo · Posted: Thu Jan 09, 2014 5:27 pm Post subject:

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

jukingeo · Posted: Mon Jan 13, 2014 6:48 pm Post subject: Futurelec order update

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

jukingeo · Posted: Sun Jan 26, 2014 8:13 am Post subject:

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