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 Forum index » DIY Hardware and Software » Developers' Corner
RA Penfold LFO, where's 0V rail?
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dcjim



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PostPosted: Sat Nov 03, 2012 12:26 am    Post subject: RA Penfold LFO, where's 0V rail? Reply with quote  Mark this post and the followings unread

Hello

I've been building some of RA Penfold's synth modules. The LFO's not working and the schematic is a little ambiguous.

Pin 4 of the LF353 goes to -12V as do pins 3 and 6. I'm thinking these last two should actually go to 0V and that the bottom rail is mis-labeled.

Anyone?


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analog_backlash



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PostPosted: Sat Nov 03, 2012 2:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi,

I've just looked at the schematic and it can't work the way it's been drawn. I suspect that you're right that the rail at he bottom has been mis-labelled as -12V instead of 0V. The only connection to -12V should be pin 4 of the IC. I would also be inclined to connect the "loose end" of RV102 to 0V, so that you can go from zero modulation, to maximum modulation as you turn the pot.

This is in one of the many Babani books on electronics. They were pretty good (I have quite a few of them) but they did make some mistakes. Usually, you got an errata with them, but I can't find any for this one.

Incidentally, the output of pin 7 is a square wave at the same frequency, so you may also want to use that. You'd need to replicate the RV102/R104/C102 setup to control its level (which will have a greater amplitude than the triangle wave).

Gary

Last edited by analog_backlash on Sat Nov 03, 2012 3:13 am; edited 1 time in total
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analog_backlash



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PostPosted: Sat Nov 03, 2012 3:08 am    Post subject: Reply with quote  Mark this post and the followings unread

P.S. I've just noticed that you've only just joined, so I feel that I should do the WELCOME thing! welcome

I'm pretty new to the site myself.
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Danno Gee Ray



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PostPosted: Sat Nov 03, 2012 9:51 am    Post subject: Reply with quote  Mark this post and the followings unread

like this?


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analog_backlash



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PostPosted: Sat Nov 03, 2012 4:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

Sorry about the delay - I've been out. Yes, that looks exactly right. Pins 3 & 6 need to be connected to 0V and as you've also shown, I would connect the previously unused end of the pot to 0V. That should work. Use a non-polarised capacitor for C101 (i.e. not an electrolytic or tantalum - it says polyester in the text). Also, I mentioned using the square wave from pin 7 with a similar set-up as RV102/R104/C102. If you do this, forget about C102 or else it won't be a square wave anymore! Looking at the text of the book, I think that the value of C102 has been chosen for the specific purpose of modulating the frequency of the VCO. Normally, a general purpose LFO output wouldn't have a capacitor as it will change the shape of the waveform.

I hope that this helps,

Gary
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Blue Hell
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PostPosted: Sat Nov 03, 2012 4:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

Also that 2M2 value for the volume pot is a bit much. Probably fit for the circuit it was lifted from, but for general purpose I'd use like 10 k, or even add a buffer opamp on the output (after the pot).
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analog_backlash



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PostPosted: Sat Nov 03, 2012 5:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

The pot value did seem rather large to me as well, but I thought that Mr Penfold may have had some reason for using it... I did also think about mentioning buffer amps and also the possibility of mixing the square and triangle waveforms together (as I've done on LFOs I've built previously), but I didn't want to over-complicate matters!

Gary
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Blue Hell
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PostPosted: Sat Nov 03, 2012 5:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

Fair enough :-)
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JovianPyx



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PostPosted: Wed Nov 07, 2012 6:54 am    Post subject: Reply with quote  Mark this post and the followings unread

One thing I've seen quite regulary about Penfold on the Synth-DIY email forum is that Penfold's books contain many errors of many different kinds. Before using any of his circuits, I would strongly suggest finding others who have already successfully corrected the designs and use their updated schematics.
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dcjim



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PostPosted: Wed Nov 07, 2012 10:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks everyone

To confirm I built the LFO with the correction and it's working. I finished the rest of it too. It's all working! The VCO is a bit nasal but the VCF is pretty sweet. It was a project for study anyway more than anything else

Jim


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Danno Gee Ray



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PostPosted: Wed Nov 07, 2012 8:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well done.
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analog_backlash



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PostPosted: Thu Nov 08, 2012 6:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Glad that it's all working now Jim.

Yes, I tend to agree with JovianPyx about the Penfold circuits (or circuits from the Babani books in general - you'll recognise them by their distinctive printing style). I think that they have some reasonably good and generally easy circuits in them though, so I wouldn't dismiss them altogether.

Just my opinion.

Gary
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