Old Ross Phaser LFO amplitude Q? and idea

[frank_p]

I am messing around with the Phozer (particularly the LFO that is the same as the early Ross Phaser) and there is something bugging me about the output voltage amplitude of this LFO.

http://www.home-wrecker.com/ross_phaser_orange1.png
http://www.runoffgroove.com/phozer.html

When the lower power supply pin of the op amp is connected to 0V (ground), the output voltage swings from 3V to 6V. Lets just say that if I connect the LED (+resistor) for a vactrol, it is not blinking a lot.

I just want to be sure: Is it normal that I get such a DC with the triangular waveform, and such a *small* triangular swing ? Anybody had such weak results ?

I've tried LM358 and RC4558 op amps and also tried to switch 150k resistors to other values, ex:
http://www.diystompboxes.com/smfforum/index.php?topic=81072.0

Note that if I connect -9V to the to the lower power supply of the op amp, I get a 0V to 6V signal at the output of the LFO and the output LED does now have good luminosity and amplitude.  (But this Ross Phaser does not have bipolar supply).

So, did I made an error on my breadboard... or this LFO is subtle  


Now, I had a charge pump on the bench and decided to try it with the LFO to make a bipolar power supply for the op amp ( 0V for the Vin+ voltage divider lower reference and the pool capacitor; and +9v -9v to the op amp).  It works well (about the same as a normal bipolar power supply).  And if you do so, you can also change the Vin+ lower voltage divider resistor to -9V instead of 0V and the output of the oscillator will go 6Vpp centered on 0V.

If I made no errors on the breadboard, is this LFO is the good choice for the Phozer. Would putting a charge pump there would be a stupid choice, or the Phozer would benefit from an other oscillator ?

[frequencycentral]

A swing between 3v and 6v is normal AFAIK.

I've tried LM358 and RC4558 op amps and also tried to switch 150k resistors to other values, ex:
http://www.diystompboxes.com/smfforum/index.php?topic=81072.0

Look at Figure 4. You can get a wider swing by reducing the value of R7 (the amplification is set by the ratio of R6/R7).

[frank_p]

A swing between 3v and 6v is normal AFAIK.

I've tried LM358 and RC4558 op amps and also tried to switch 150k resistors to other values, ex:
http://www.diystompboxes.com/smfforum/index.php?topic=81072.0

Look at Figure 4. You can get a wider swing by reducing the value of R7 (the amplification is set by the ratio of R6/R7).

Ok, thanks Rick. Going to experiment a bit with that...
Hmm... And since there is a Schmitt Trigger in there, I am wondering also if it's possible to bring the bottom of the wave closer to 0V by messing with (R1, R2 and R4(R3))...
See here, and scroll down.
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/schmitt.html

[PRR]

The tri-wave's thresholds are set by the 150K:150K:150K network to be 1/3rd and 2/3rd of the supply voltage. On zero and 9V, 3V and 6V.

Tinkering the PFB 150K to 220K slightly increases output (and lowers frequency), but also gets you close to not-oscillating (if the threshold reaches the limit of either input or output swing, it stalls).

You could stand the bottom of the LED up on a 1.5V-2V point, so the 3V tip doesn't leave enough voltage on the LED to light-up well. However photoresistors will sense very dim glow, and making a solid 1.5V supply is not trivial here. (You could stick in an AA battery, and it won't run-down, just rust-out in 2-3 years.)

Since you have an idle buffer, do what Rick says: wire some gain there. That won't mess-up the oscillator. You can never swing a TL072's output TO the negative rail, but I think it can get close enough to darken the LED.

Also try green and yellow LEDs. Photoresistors love those colors, and the higher LED voltage may be easier to darken.

[frank_p]

> The tri-wave's thresholds are set by the 150K:150K:150K network to be 1/3rd and 2/3rd of the supply voltage. On zero and 9V, 3V and 6V.

Yes, theoretically those are the threshold values given by the formulas of the Schmitt Trigger equations with those given resistor values.  But I have 3V and 5V on Vin- thresholds on the breadboard (even with all the parts disconnected in the neg. feed. loop and the variable power supply connected...) ... Experimental errors  
.....................

> Tinkering the PFB 150K to 220K slightly increases output (and lowers frequency), but also gets you close to not-oscillating (if the threshold reaches the limit of either input or output swing, it stalls).

Yeah... played with some values but still experimenting.
......................

> You could stand the bottom of the LED up on a 1.5V-2V point, so the 3V tip doesn't leave enough voltage on the LED to light-up well. However photoresistors will sense very dim glow, and making a solid 1.5V supply is not trivial here. (You could stick in an AA battery, and it won't run-down, just rust-out in 2-3 years.)

Three LEDs in series (with lower limiting resistor value) will make them go on-off. But since The LEDs won't lit up before some amount of current: two LEDs in series (two volatge drops) with the resistor makes a perfect compromise. The LED now get pretty close to NO light at the minimum of the wave.
.....................

> Since you have an idle buffer, do what Rick says: wire some gain there. That won't mess-up the oscillator. You can never swing a TL072's output TO the negative rail, but I think it can get close enough to darken the LED.

Playing with the value of the limiting resistor of the diode will make it fine (and keeping the output buffer), no need for more gain.  ( There was not a trimpot for nothing on the Phozer schematic   ). Just lowering the resistor value got me a good maximum light.
...................

> Also try green and yellow LEDs. Photoresistors love those colors, and the higher LED voltage may be easier to darken.
Yes, they are more sensible to this part of color spectrum.

Thanks a lot Paul and Rick, I am continuing to play on this oscillator...

But, en résumé, two LEDs in series and sizing the value of the limiting resistor makes it great (= much more lighting amplitude than before and very homogeneous dimming no matter the speed of the LFO).  The best combination is to have a red LED to ground and a yellow LED to the resistor and op amp: this way you can get as close to no light at minimum without staying not lighten (at the bottom of the wave) and have the yellow LED used for the LDR.

[PRR]

 > I have 3V and 5V on Vin- thresholds on the breadboard

The theoretical numbers are for an amp which swings to the rail.

5V top-swing at input suggests 7.5V at output and 9.0V-7.5V= 1.5V drop is not a bad guess for some opamps under some load conditions. (Or if your assumed 9V isn't really 9.0V.) (Or if you used a bipolar amp instead of the specified FET amp.)

[frank_p]

> I have 3V and 5V on Vin- thresholds on the breadboard

The theoretical numbers are for an amp which swings to the rail.

5V top-swing at input suggests 7.5V at output and 9.0V-7.5V= 1.5V drop is not a bad guess for some opamps under some load conditions. (Or if your assumed 9V isn't really 9.0V.) (Or if you used a bipolar amp instead of the specified FET amp.)

After op amp swappings, more precise measurements, what seemed to be bad contacts on the breadboard and calculating more realistic values...

Vtrig = (1/3) * 9V + (1/3) * 8V = 5.66V

I now have the same values on the scope.

Thanks Paul