What does it sound like?
It sounds similar to a good example of the original. It has the same frequency response, LDR modulation and a sine wave LFO. I ignore two differences:
What is the value of the capacitor right after the input?
- Possible non-linearity in the transistors, as high-gain darlingtons in low gain phase splitters are pretty damn clean.
- Thermal lag in the optical coupler, which I originally simulated and threw out when I realised one could tweak the coupling instead. The onboard preset covers the useful range of variations.
The input cap is 1nF.
What light dependent resistor (LDR) should I use?
Pretty well anything should work as the trimmer can adjust for differences. All four must be the same type.
As it happens I use Farnell Electronics part number 179611.
Does it matter which LED is connected to which LDR?
No, it makes no difference which of the flashing LEDs go to which LDR.
What are the two LEDs in the upper left of the schematic used for?
Those two LEDs are used to ensure the flashing LEDs get the right bias regardless of battery voltage or make/size/colour of LED. The LEDs should all be the same. I suggest low current 3mm red ones. Avoid yellow ones as the brightness of them can vary with production.
Is the 1k drive pot one of those
small trim pots and what does
this pot control/adjust?
The 1K drive pot is a small trimpot used to tweak the LED drive. Turn the depth most of the way up and adjust the trimpot for an even sweep. The depth control is designed to slightly over modulate at the maximum setting which helps to get the nice swaying effect that univibes are known for.
Any recommendations for a good
inexpensive LDR to use in this
Use the miniature ones that are about 5mm across and go down to 10-20K. Avoid the huge ORP12 as it makes construction difficult. I get mine from Farnell Components in the UK.
Why the 10uF cap across the comparator output?
It's a fudge, but it does prevent faint clicking noises at high gain settings
and it was more effective than tricky wiring arrangements that are hard for folk to duplicate.
I am allergic to TL064s, can I use different op-amps?
Almost anything should work. The TL064s were choosen to minimise current consumption.
If you use something with better drive capabilities that trick with the 10uF across the comparator output
may not work so you may get faint clicks. Try removing the 10uF or even increase it's value.
What modifications could I make?
It's not working, what do I check?
- Add a pair of break jacks to the LFO rate control so you can use a volume pedal to control the speed.
- Add a four pole two way rotary switch that substitutes 0.01uF capacitors in the phase shift network to make a normal phaser.
- Add some feedback, a 1nF in series with 470K from the last stage to the input should do the trick.
- If you do both the above mods you should get a passable substitute for the EH Small Stone.
- Make sure you have at least 7 volts on the supply rail.
- The bias LEDs should be glowing.
- The bias should be at 3.8 volts.
- The op-amp outputs should be centered on 3.8 volts.
- With speed at minimum you should see the LFO op-amps working even using just a meter.
- With depth turned up the coupler LEDs should be pulsating.
Last updated: 26th May 2002