Simple LFO with problem (help needed plz)

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After modifying an old module, In the same case I had my first simple LFO which has always been a little strange . It works well with VCO pitches but not with other stuff like VCAs.
It was an opportunity to test it.

And I actually measured around -2V and + 2V max at the output !

I rechecked everything several times, the schem, the component values, the solder … everything looks fine. I don’t understand.

Here is the schematic I used (sorry i don’t know the source), I just changed a few resistance values ​​next to the Range pot to adjust the speed like i want it, but I don’t think that’s the problem.

Could someone tell me what resistance values ​​I could change to increase the output voltage please.

Thank you in advance

simple LFO ......
simple LFO ......1085×733 68.2 KB

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This appears to be identical to version 1.2 of David Haillant’s Simple LFO (he’s now on version 1.4, which I think only changes the LED driver). As noted R10 and R11 are changed to 22k and 100k from Haillant’s 2.2k and 10k. That shouldn’t affect the gain. Haillant says the output should be ±10 V and I’ve done a simulation that more or less reproduces that. The square wave is produced by a comparator that should give ±11 V or so at U1 pin 7, and then U1C is a gain stage with gain 1.22 so the only thing that should limit its amplitude is the pot R9. So there must be some error in your build. As you say, changing the resistances on the left side shouldn’t affect the amplitude.

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Wait, was it the triangle or the square or both that was ±2 V? What were the resistor changes?

Also, I just noticed another significant difference: In Haillant’s version the LFO Square tag is on U1 pin 7. In your version it’s on the line at the top, which connects to pin 7 via a voltage divider which will reduce the amplitude to about ±8 V. Of course that doesn’t explain your ±2 V.

Ah — now I see your version corresponds exactly (? — except the noted resistor changes) to Version 1.1.

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Thanks, yes on all versions (except mine) the Square output is on pin 7 !
but it’s low on both Square and Triangle.

For the pot Range : i change the 100K on wipper by a 22K (trimpot) and also add a 2 or 3K on pin 3.

Tomorrow i will check everything again.

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I found !!!
I had put a 47K instead of a 4.7K for R7, i have around 8V ouput now.
it is very often a stupid mistake like that :upside_down_face:

Thank you @analogoutput for taking the time to watch it :slight_smile:

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Sorry for bumping this thread but I’m having an issue with the David Haillant Simple LFO module as well, however, a different one.

I have two of this module in my system. They’re working well but I noticed they affect other modules as well, especially noticable in the pitch of the VCOs in the system: the pitch is modulated according to the LFO frequency, i.e., a pitch drift. A very annoying one.

Worth mentioning that this effect is less noticable if the LFO is not connected anywhere in the system. Also, the Amplitude knob of the LFO defines the level of the pitch drift. If at the minimum, the pitch drift goes away. So I guess the output somehow introduces a noise? But I still don’t know who to proceed.

I though this was an issue with the tiny power supply I was using but recently I swapped my PSU for one based on Rich’s MFOS-ish schematic (Thanks for that by the way!).

All my modules have 10 uF decoupling caps for the power rails and 100 nF caps for the ICs. I installed additional caps for the LFO but that didn’t solve the issue.

Do you have any ideas what shall I do? Do any of you who has this module have a similar experience? Should I install additional caps for the VCOs?

UPDATE: This was a power supply issue. The LFO works great.

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Presumably it’s one of two things: The power rails are picking up the VCO signal (your PSU swap suggests not, but it’s not definitive), or the VCO CV is.

Does it happen whether or not a CV is plugged into the VCO? Does it happen regardless of the CV attenuator (if any) on the VCO, and the initial frequency knob on the VCO? Answers to those might help narrow down.

Have you tried powering the LFO and VCO with different power supplies? Since you evidently have two. Does it still happen then?

Is there a good connection between the LFO and VCO grounds? Should be if they’re connected to the same power distribution board but it can’t hurt to check.

Do you have a scope? If so you could look at the VCO power rail voltage (AC coupled) and see if there is a visible variation correlated with the LFO. And likewise with the CV.

The bypass caps aren’t likely culprits, I think. They exist to keep chip voltages constant when there are transients in the chips’ power draw. Not to filter out low frequencies, in fact if you use 10 Ω resistors they (the 10 µF ones) form low pass filters with a cutoff in the audio range.

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Thank you! I’ll check these.

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I checked.

Does it happen whether or not a CV is plugged into the VCO?

Yes.

Does it happen regardless of the CV attenuator (if any) on the VCO, and the initial frequency knob on the VCO?

Sadly, I don’t have CV attenuators on any of the VCOs (I built them early on my path and didn’t know better). And yes, regardless of the initial frequency knob.

Have you tried powering the LFO and VCO with different power supplies? Since you evidently have two. Does it still happen then?

I tested a VCO and an LFO using an isolated PSU. With these two, no issue is heard or visible on the scope.

Is there a good connection between the LFO and VCO grounds? Should be if they’re connected to the same power distribution board but it can’t hurt to check.

I checked continuity using my multimeter. It beeps, even with the PSU powered down (and without jack connections).

Do you have a scope? If so you could look at the VCO power rail voltage (AC coupled) and see if there is a visible variation correlated with the LFO. And likewise with the CV.

No visible signs of the LFO influencing the rail voltage or the CV on the isolated PSU.

Conclusion: the root cause is not the LFO. So this may be a wrong thread for my issue.

Also, I noticed that if I remove a VCO from the system, all others perform better, and the noise disappears (completely? I’m not sure). Could it be that I reached the PSU’s maximum performance?

And one more thing: the clock divider module may have something to do with the situation. I noticed that if I disconnect its clock input, the noise is also reduced, almost unnoticeable. I usually use one of the LFOs as a master clock, this is why I thought the LFO is the issue. Turns out that connecting other modules to the clock divider lead to the same issue.

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Yep, this was a power supply issue. Too many modules were connected to it, draining too much current, I suppose. I should measure power consumption of my modules once again (I did that some time ago, and of course lost my notes). I reinstalled the tiny PSU to help its big brother. This solved the problem, no noise whatsoever.

Sorry for the false alarm. The LFO is great and had nothing to do with my issue. Besides working as designed, obviously.

Thanks for the quick help, Rich! Your questions helped me easily find the root cause instead of trying to figure out what’s wrong with the LFO.

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