[Project] Phase 90 w/ ramp LFO. Need some help

[mdaudet]

Hi everyone.

My name is Matt and this is my first post, but I'm a legendary reader at this forum

I have some experience building guitar effects, but now I have a major issue that I can't solve by myself.

In addition to have more freaky & weird sounds, i was planning to add an extra LFO to the phase 90 phaser. The chosen one was the "Ramp up/down" designed by Mr. R.G Keen.

http://www.geofex.com/FX_images/p90ramp.pdf

I've designed the PCB (wich i would share to the community if it works) and mount all the components, exactly as RG's schematic. I reproduce the whole LFO section, so I can choose between the stock LFO and the Ramp one (next i was planning to combine them  ) So, my layout consist in two opamps: LM358 and a TL072, with all the parts involved.

To the point: This LFO doesn't oscillate (sweep, phase, juiiiiuuu, etc) at all. For some reason the schmitt trigger opamp does not turn the transistor off to get the oscillation. Moving the trimmer I can hear some sweeps, but only when I move it, like a manual LFO. The stock LFO works fine by the way.

I've read all the topics related to the phase 90 ramp mod (there are only a few) without getting any idea to make this thing work.

Have anyone tried this mod? Any suggestions? Any idea?

I'll be very, very happy if you help me to make this thing work.

thanks a lot.

regards...

Matt

[dschwartz]

hola matias!!!
have u tested the speed pot? maybe is sweeping too fast, and that´s why you hear a change in sweep when turning the trim

[mdaudet]

You say that i should change it for a higher value, like 100K? The 10K isn't broke at all, so I'll perform the mod you suggest. Thanks friend.



The trimmer I referred on my previous post is the one that is part of the P90 circuitry.

One curious thing that happens is when i move the Hi (or low) speed pot very quickly I can feel an almost imperceptible "sweep" (like it changes somehow the notches, but very subtle). But still no oscillation at all.

There are not many enthusiast builders who performed the ramp mod, in deed, the only two post about it said that it doesn't work. The closest build report to mine is this one:

http://www.diystompboxes.com/smfforum/index.php?topic=61702.0

Hope someone could help me.

Thanks.

[R.G.]

I'll dig through it.

[dschwartz]

What if you try a darlington instead of the transistor?

[R.G.]

I got out the old sim file. It works. I breadboarded just the LFO. It works.

Are you using anLM358 or LM2904 dual opamp for the LFO as specified? It **will not work** if you use other opamps for these. The inputs of these two types can go to ground and operate correctly. A TL072, for instance, will not operate correctly with one or both of its inputs within one volt of ground. The LM324 quad will work there too; the 2904 is a dual version of the LM324.

A 2N3904 works for the switching transistor too (that's the "2n5088" at the bottom, with a 10K base resistor and a 51K collector resistor to the - input of the opamp). But you simply must be able to drive that transistor on and off.

So - go take the voltages and let's dig it out and make it work.

One caution - the lowest speed on that LFO is something like ten *seconds* in the circuit as shown. If you have the speed all the way down for some reason, it could be working, but too slowly to notice. It will go even lower if the input to the voltage control is truly 0V. Maybe days.

[dschwartz]

i think he is using lm358..

[R.G.]

Sorry, yes, I see that; presumably the LM358 is in the LFO.

What if you try a darlington instead of the transistor?

Won't work. A single transistor will saturate its collector to within tens of millivolts of ground under these conditions. A darlington will always have the second transistor active, not saturated, and will only get down to about one diode drop. That will severely limit the oscillator's range, and might make it not work at all.

The way this works is that the first opamp is an active integrator. It integrates the *difference* of the voltages on its two inputs. The + input voltage is always 1/2 of the input voltage, divided by those two 51K resistors. The - input is either the 100K feeding directly into the - input when the transistor is off, or 51k/151K of the input voltage through 51K||100K when the transistor is on. These conditions are such that the integrator integrates up and down at the same rate, with only the direction depending on the transistor being on or off. This whole section is a substitute for the 10uF cap to ground in the original circuit, but it is a *voltage controlled* integrator, replacing the 10uF cap that was faking an integrator. For slow speeds the input voltage may be very near ground, and so the inputs to the opamp simply have to work at ground voltage.

The second opamp is a Schmitt trigger, and is identical to the single opamp in the original, to give the same trip points. The output drives the 10K to the base of the transistor instead of a 500K speed pot.

I did play with the thing a bit. I find that I would mod it a bit now. I would either make the emitter follower at the speed control section be a darlington or an opamp to get higher impedance on the run up/down 100uF cap and the ramp up/down pot be 100K so the speed up/slow down times are longer.

Hmmm... I thought of a possible reason for not working. If the Schmitt trigger opamp output does not go fully to ground, but stops maybe a diode up, it would not turn the transistor off. You can check this by putting one or two diodes in series with the transistor base, so they conduct current into the base, but block it coming out of the base. We don't need the blocking, but the diode(s) add one or two diode drops in series with the transistor, so if the output of the opamp stops as much as a volt above ground, the transistor will still go off. Some LM358 outputs may not go fully to ground. This will let it stay above ground and still work the transistor.

[mdaudet]

Hi. I measured the whole thing.

Voltage readings: One circuit with 2N3904 and the other with 2N5088

Notes:

LM358: Using the 1,2,3 pin section (course with 4 to ground and 8 to V+) Other pins are not connected (is this right?)
TL072: Same as above
Power Supply = 9,34V

USING 2N3904 on transistors:

LM358 (pin):

1. 8,08V
2. 0,60V
3. 0,89V
4. 0V
5. 2,03V (Not Connected)
6. 1,42V (NC)
7. 8,08V (NC)
8. 9,34V

TL072:

1. 1,38V
2. 7,96V
3. 3,62V
4. 0V
5. 2,24V (NC)
6. 2,22V (NC)
7. 1,48V (NC)
8. 9,34V

Q1 - 2N3904 (the one with Emitter to ground):

E: 0V
B: 0,63V
C: 0V

Q2 - 2N3904 (The one with Collector to V+):

E: 2,07V
B: 2,65V
C: 9,34V

USING 2N5088 on transistors:

LM358:

1. 8.1V
2. 0,71V
3. 1,05V
4. 0V
5. 1,3V (NC)
6. 1,28V (NC)
7. 8,1V (NC)
8. 9,34V

TL072:

1. 1,37V
2. 7,97V
3. 3,62V
4. 0V
5. 1,13V (NC)
6. 1,36V (NC)
7. 1,50V (NC)
8. 9,34V

Q1: 2N5088 (the one with emitter to ground)

E: 0V
B: 0,6V
C: 0V

Q2: 2N5088 (the one with Collector to V+)

E: 2,11V
B: 2,64V
C: 9,34V


I tried the LFO with the speed in all positions. No luck.
The only change I perform to the circuit is the 3M3 Resistor which was swapped to 2M2 in order to get more depth* in the oscillation. Hope this small change do not affect the performance of your design.

Thanks Daniel and RG for your help.

regards,

Matt

[R.G.]

OK, that's one problem.

You should use both LM358 sections for the LFO, not one of the LM358 and one of the TL072. The TL072 cannot drive the direction-change transistor properly. I updated the schematic with pin numbers for example.

What's happening is that the LM358 is fully high on pin 1, and that is driving the pin 1 of the TL072 as low as it can go - but that's only 1.38V, not enough to turn off the transistor. That transistor base/10K needs to be driven by a voltage under 0.5V to turn the transistor off. You could ...try... sticking three diodes in series with the 10K to the transistor base, and that might work. Much better to use the other LM358 section.

[mdaudet]

Thanks a LOT RG. 

I would try the new circuit as soon as I can. The update of your schematic is now much clearer.
I thought that one part of the original LFO circuit must be intact (TL072) and the extra parts are using the LM358/2N5088 for the integrator.
You've been such a great help to make this project come alive. Thanks for your time

I let you know the results (hope now works) 

Cheers.

Matt

(Thanks again)

[R.G.]

I let you know the results (hope now works) 

I hope so too. But even more important to contact me if it doesn't work. We can make it work.

[mdaudet]

YES!!

Its ALIVE!!! Works perfect... I was wrong with using two opamps, I rewired the thing in order to use just the LM358, as the new ramp mod scheme figures.

Many many many thanks RG and Daniel for your help.

The good thing is that the ramp up/down switch along with the HI/lo speed pots (5K) are so interactive that you can manage a lot of phase, detune, envelope sounding filters.

One curious thing is the 10K speed pot. It seems like it doesn't affect at all at any setting. Other thing is I can't get fast (or really slow) sweeps by moving the 10K pot, only I can handle some speed variations with the other pots. The pot is to short? What should this particularly pot do?

Now I'll perform some mods, like adding the diode to see what happens, increase the 10K speed Pot, etc. Suggestions are welcome.

I just mix the two LFOs (the stock and the ramp) and it WORKS freakin' great. I almost reach a pseudo "random" LFO. I did it simple: I put both LFO outputs together.

Wave outs are together
The 3M3 resistors that come from the circuit are together.

I lost some range, because I can hear only high freq sweeps.

I'll tell you more when I finish test the circuit

Thanks A LOT.

Me, now, very very happy. 

Cheers

[DougH]

Any chance you could do a sound clip to let us know what it sounds like? I've got the phase 90 almost finished on the breadboard right now and am getting ready to experiment with it.

Thanks!

[mdaudet]

Any chance you could do a sound clip to let us know what it sounds like? I've got the phase 90 almost finished on the breadboard right now and am getting ready to experiment with it.

Thanks!

Hi Doug.

I will, just let me finish the experimentation process in order to obtain the best of the circuit. There are still some things that I guess we can improve. Once I build the whole effect, i will post a sounclip about it. Stay tune.

regards,

Matt

[R.G.]

YES!! Its ALIVE!!! Works perfect... I was wrong with using two opamps, I rewired the thing in order to use just the LM358, as the new ramp mod scheme figures.

Great! I was hoping that was all the problem was.

One curious thing is the 10K speed pot. It seems like it doesn't affect at all at any setting. Other thing is I can't get fast (or really slow) sweeps by moving the 10K pot, only I can handle some speed variations with the other pots. The pot is to short? What should this particularly pot do?

The 10K "speed" pot is "how fast it moves from the high/low to the low/high speed" when you flip the high/low switch. At low settings of that pot, it should be almost instant. With a 100K instead of a 10k, and set to max resistance, it should ramp up or down over a few seconds like a motor speeding up/slowing down in a leslie. It's "how fast does the speed change?".

I just mix the two LFOs (the stock and the ramp) and it WORKS freakin' great. I almost reach a pseudo "random" LFO. I did it simple: I put both LFO outputs together.
Wave outs are together
The 3M3 resistors that come from the circuit are together.

That's a cool idea. Yeah, should be almost random.

[DougH]

I will, just let me finish the experimentation process in order to obtain the best of the circuit. There are still some things that I guess we can improve. Once I build the whole effect, i will post a sounclip about it. Stay tune.

regards,

Matt

Believe me, I completely understand. Thanks!

[dschwartz]

You should see the beautiful pedals that matt makes, check his blog.

we´re good friends and learn a lot from each other, and we are thinking on making our own fx page ala ROG..

i cant wait to compare this phaser with my "nazca pets"

[mdaudet]

Hi again.

I did make some test to the circuit and finally I discovered what the 100K (10K listed on the previous schem) ramp speed pot does. It controls how spend to the speed to change between HI and Low when ONLY you alternate the speed via the switch. It does NOTHING with a particular setting on the other speed pots (5K), so is more like a "How fast the speed changes when you press the up/down switch". This pot will be useful only if you planned to use a Footswitch to change the speed, so its more interactive and "on the fly". In my case, I wouldn't use a footswitch, just a toggle switch for the "UP/DOWN" Oscillation.

Complicated for me, because in my pedal I was planning to use only two footswitches: 1. bypass, 2. Sine LFO/Ramp LFO. That particular 100K pot makes me to add another Footswitch to worth the function of the Ramp Speed. I guess I would eliminate the 100K speed and place a Jumper to make the transition faster between the UP/DOWN switch modes.

was the above right? or I'm fooling around? 


I Know this mod to the LFO section on the P90 was made to replace the stock LFO, not to change between two LFOs.

I tried a Darington for the emitter follower section (MPSA13) and it works fine, just like the BJT 2N5088. Maybe lowering the value of the 100uF cap would make any difference?

I put a diode (1N4148) in series with the 10K R and it doesn't seems to affect the Oscillation. I know is optional in case the LFO doesn't sweeps, but i try it in order to "try" 

Mix: Still working in a proper way to mix both LFO and don't loose depth.

Next target: Shape MOD.

Suggestions are welcome.

regards,

Matt