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 Forum index » DIY Hardware and Software » Thomas Henry designs
TH LFO controller range
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prgdeltablues



Joined: Sep 25, 2006
Posts: 186
Location: UK
Audio files: 11

PostPosted: Thu Nov 13, 2014 2:41 am    Post subject: TH LFO controller range Reply with quote  Mark this post and the followings unread

Hi, in the process of building a TH LFO controller. Made various mods to run on +/- 12V, but I also planned to incorporate a range switch, and to reduce the range of adjustment of the rate pot. So instead of C3 being 4n7, I've used a switch to switch between a 1n and a 10n cap. That works fine.

I've tried to reduce the range of frequencies on R57 by reducing it from a 100k pot to a 10k pot, and putting a 100k resistor in between the top end of the pot and pin 7 of IC1b ( in other words, to keep roughly the same overall resistance between pin 7 and ground via the feedback loop of R57/R1). I've found I've had to increase R1 to about 500R to prevent stalling. The reduced range pot works but with a smaller range than I had expected from an LTspice simulation. With 1n it varies from about 20Hz (just about visibly flickering) down to about 2 Hz, and with 10n, from about 1Hz to 0.2Hz, whereas I was expecting (and had simulated) a greater and overlapping range.

I must admit I don't really understand how the various resistors function to determine the frequency of operation. I gather that they determine the rate of charging of the timing capacitor, which ramps up the voltage on pin 1 and thus pin 5, eventually causing the comparator to flip, thereby reversing the current flow, but not how R57, R1 and R64 interact.

Any thoughts appreciated.

Peter
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elmegil



Joined: Mar 20, 2012
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Location: Chicago
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PostPosted: Thu Nov 13, 2014 8:24 am    Post subject: Reply with quote  Mark this post and the followings unread

I can't really speak to LTSpice simulation, but if you have a 100K between the pot and IC1b, you basically have set the fastest rate to the maximum of the original circuit. I would do something more like a 50K resistor there and a 50K pot, or an 80K resistor and a 20K pot....
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