Hi all.  i have a PT2399 chip on the way from Steve Daniels, and i'm planning on building the rebote2 delay from tonepad.com.

it has a 1k resistor in series with a 50k pot, and i'd like to make the 1k resistor CV variable between 100-500ohms and 5k or so.  the exact numbers aren't important.  LDRs only seem to have minimum values of about 5k tho so i was owndering if it was possible to use a FET like this?  (it's to use the delay as a chorus - drive the CV with an LFO.)

i'm not too sure how FETs actually work as variable resistors, maybe someone can point me at some theory to pore over?     or does anyone have a better idea?

thanks!

FETs work fine as variable resistors.

Not knowing anything about this circuit I don’t know if this is a wise idea.  However, I assume you know what you’re doing so here’s one way you could do it with a MOSFET.

Replace the 1k with a 5k1 (or larger) but swap its position with the 50k pot so that it’s wired to ground.  Connect one end of a 560 ohm (this acts as a stop resistor and you might have to tweak the value) resistor to the drain of a n-channel MOSFET and the other end of this resistor to the junction of the 5k1 and the 50k pot so that this ‘resistance chain’  is parallel with the 5k1 (the MOSFET's source connects to ground).

Wire up the outer lugs of a 100k trim from 5V to earth (keeps bias consistent if the supply varies) and connect one of the outer lugs of a 1M lin pot to its wiper.  The other lug connects to a 4u7 NP cap and the wiper connects to the gate of the MOSFET.  The CV is fed via the 4u7.  The 1M acts as a depth control and the trim adds DC bias to the FET to get the range right.  You might need to add a resistor in series between the input cap and the 1M pot - all depends on the amplitude of your control voltage - in which case you could use a smaller value depth pot.

PS I can send you a schematic of this if you're confused - mail me with your details.

thanks heaps gez.  

yeup, i've got the idea of what you were saying.  i guess the cap coupled input would be ok for LFOs but maybe not for pedal input.  the cap would discharge through the 1M pot + part of the 100k trim to ground, so i guess if it kills a pedal CV too much i could just up the capacitor value.  it looks like it would work without the capacitor anyway though.

i'll give it a go!

thanks again

Quote from: idlefaction.  i guess the cap coupled input would be ok for LFOs but maybe not for pedal input.  the cap would discharge through the 1M pot + part of the 100k trim to ground, so i guess if it kills a pedal CV too much i could just up the capacitor value.  it looks like it would work without the capacitor anyway though.

The cap is necessary to isolate the LFO's DC voltage from the MOSFET's gate, if you don't the LFOs output level will contribute to the bias of the MOSFET which may bugger things up - all depends on the amplitude of the LFOs waveform and the threshold of the MOSFET.  

What type of LFO are you using?  An op-amp triangle job won't be loaded by the 1M (plus some) input resistance of the MOSFET.  Same goes for many phase-shift oscillators - the EA trem's LFO is coupled by a cap.

4u7 is usally fine with 1M input resistance for a LFO, but as you rightly said it can be made bigger.

I usually aim for .75-1V peak-to-peak across the 1M pot (you may need to divide down to get this).

P.S. I can send you a schematic of a LFO I use all the time which has the above setup.  Only thing is, it requires a ICL7621 op-amp which may be hard to find.  However, it's worth it as everything I've used it in so far is click free.  It has a pretty good approximation of a sine wave as an output, though it still has the option of a triangle output.

i'd be interested in your LFO, for sure!  i need a click-free oscillator for the phaser project planned for after the delay/chorus.  the hyper-triangle converter on mark hammer's site has my attention for that one.

darren(at)wadda.org

I've sent you a couple of schematics, one has a sine output the other a rectified sine (though it uses a different method to the one at Mark's site).

thanks gez!  that looks like a really useful circuit.  

I'd like to see that oscillator too, if you dont mind.

In my delay to chorus experiment I used an opamp triangle oscillator and it was click free when outside the box. When i mounted in, the small board with the osc was too near the delay board and aparently it emited some kind of interference that made the clicks apear. I had to isolate it with an "experimental" shield of aluminum paper connected to gnd and the clicks were gone. It was strange to me, as I first thought the clicks were transmitted across the power lines, but that was not the case.

Also, i used a 4u7 cap to connect the LFO to the fet so the fet could be biased right. I just replaced a resistance to GND that was connected to the delay time pot with the source of the fet to GND and voila!

Good luck

Miguel

Quote from: bioroidsI'd like to see that oscillator too, if you dont mind./quote]

Anyone who wants the schematic just email me with your details.  I cant GUARANTEE that it'll be click free as you still have to obey all the rules of good layout; as you've already experienced you can couple noise from LFOs through to the audio chain via capacitance of wires etc.

The circuit is just a triangle wave oscillator but the chip has CMOS output (allows for a wide output swing without the possibility of 'snagging') and can be run as low as 2V.  I run it a 3V with a diode shaping network to get a pretty good approximation of a sine wave.  Run at this voltage any current spikes produced by the Schmitt bit of the circuit are miniscule so it's click free (well, so far it has been for me!).

You can do the same trick with inverters from a 4049UB chip - just use two in series for the Schmitt - but the triangle output isn't as accurate.

Sounds great to me, as I'm always looking for a simple/better oscillator to test. My email is bioroids@yahoo.com.ar

Feel free to ask questions (if you think I might have some idea :shock: ) or other schematics.

Thanks

Miguel

Miguel, I've sent them so check your inbox.

If anyone sent me a email and I didn't get back to them, it's probably my junk mail filter (it tends to delete some stuff that comes via the forum).  If that's the case leave me a private message with your details.

Hi, I just sent you a mail thanking you for the oscillator schematic.

Luck

Miguel