How much frequency is good for a Phaser LFO? from 1Hz to like 30 Hz?

Started by savethewhales, October 01, 2020, 10:05:01 AM

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savethewhales

I have no idea how it would sound on 1 Hz or even 30Hz, so that's why I'm asking to find someone who knows!

Mark Hammer

1hz is actually pretty fast.  Many phasers have sweep cycles that take 10sec to complete (=0.1hz).  Thirty hz puts you into audio range and will yield "rubber band" ring-modulator-like tones.

Since, by your post count, you appear to be a fairly recent member (and if not, my bad), you likely haven't seen the myriad of past threads in which the topic of LFO range for modulation pedals was discussed.  There will generally be a single capacitor in the LFO, whose value sets the general range of weep speeds/rates.  Not a new trick, either.  I have one of those old blue rackmount MXR digital delays sitting in the basement.  Look closely at the Frequency (Speed) control and you can see that pulling the knob out would multiply the sweep rate x100, which would put the LFO into the audio range and get wonderful ring-modulator type tones.

If you have a schematic for a phaser, and the LFO shows a 100nf capacitor in the feedback loop of an op-amp, that cap is setting the overall range of sweep rates.  Make it larger and the range shifts downward.  Make it smaller and the range shifts upward.  So, if 100nf got you .1-10hz in that circuit (and a 100:1 ratio is pretty damn good), using 50nf would get you 0.2hz-20hz, and 10nf would get you 1hz-100z.  This was used to useful effect in JOhn Hollis' "Frobnicator" tremolo ( http://www.geofex.com/PCB_layouts/Layouts/frobn.pdf ), where a toggle is used to add a 2nd cap in parallel, reducing the total feedback capacitance, and lowering the modulation range from well into the audio, down to sub-audio rates.


ElectricDruid

Quote from: savethewhales on October 01, 2020, 10:05:01 AM
I have no idea how it would sound on 1 Hz or even 30Hz, so that's why I'm asking to find someone who knows!

+1 What Mark said. Somewhere from 0.1Hz to 10Hz covers most typical modulation pedals. Above 10Hz is getting into "almost-audio" territory, and below 0.1Hz is getting vvvvvveeeeeerrrrrrryyyyy ssssslllllooooowwwww indeed. For specific cases, you can probably limit it a bit more.

Danich_ivanov

I like it somewhere around 1/4 of a second to 8 Hz, 6 octaves, which should work well with a single pot. If you want faster AND slower than that, it will likely require a switch.

savethewhales

Guys thank you very much for the answers. Now that I have my computer, I can answer accordingly:

The general idea that I got was to use aproximately 0.3 Hz to more or less 8-10 Hz, and not reach audio frequencies.

The reason for my computer to be brought to this case is:



This is the circuit that I will use on my phaser (yes, simple as that), and I wonder if R4 being a potentiometer of 470k and C1 as 15u will make it.. I guess so, just have to do math when I get my hands on my calculator.

Best regards, Fred

Vivek

On one of my Digital MFX, the Phaser LFO goes from 0.1 to 10 Hz

savethewhales

Quote from: Vivek on October 02, 2020, 07:22:16 AM
On one of my Digital MFX, the Phaser LFO goes from 0.1 to 10 Hz

Do you like that? I mean would you rather use more or less frequency or it's good for you?

amptramp

One way to get extended frequency range on a phase-shift oscillator is to use a stereo dual pot.  Where you controlled one pot before, now you have two on the same shaft.

savethewhales

Quote from: amptramp on October 03, 2020, 12:20:21 PM
One way to get extended frequency range on a phase-shift oscillator is to use a stereo dual pot.  Where you controlled one pot before, now you have two on the same shaft.

Why would I want a dual? I didn't get the point I guess..

11-90-an

Quote from: savethewhales on October 04, 2020, 05:47:18 AM
Quote from: amptramp on October 03, 2020, 12:20:21 PM
One way to get extended frequency range on a phase-shift oscillator is to use a stereo dual pot.  Where you controlled one pot before, now you have two on the same shaft.
Why would I want a dual? I didn't get the point I guess..

maybe wiring the 2 pots in series?  8)
you would be able to get 2x the range, in a single turn... would be a bit finicky, though, as the resistance will increase 2x faster than before... maybe an A taper?
flip flop flip flop flip


11-90-an

pots with larger value than 1M are *rarer*...

also, there are some LFOs which also require dual-gang pots... the Univibe, for example.
flip flop flip flop flip

amptramp

Check out the tremolo oscillator in the lower left corner of the Univox U65 schematic below:



If you replaces the 330K resistor on the left with a pot, you could extend the frequency range of the oscillator.  You could use a dual 500K pot to get much lower frequencies but still top out at a higher frequency with another 22K fixed resistor in series with the new pot.  Stereo 500K pots are common and would permit a frequency range beyond anything you would actually need.

savethewhales

Cool! I'm gonna think about that. I don't think that for now I need a very big frequency range (maybe just 0.3 to 8 Hz or smth) and I was thinkin with only one pot it would be possible...

PRR

It's not the resistor(pot) size, but the ratio of range.

A common linear pot, say 100k, can be turned to 10k, maybe 5k, but not much lower or it gets twitchy.

(Try this at home. Ohm-meter and pot. You can easily dial-in 10%, maybe 5%. But you will struggle to dial-in 1% (1k on a 100k pot), and again every time you try to find 1%.)

The C-R-C-R-C-R phase-shift oscillator, _IF_ you vary all three Rs together, is linear in F with R. So you can conveniently dial-in 5%, which is same as a 20:1 frequency ratio, or for 20:1 of R you get 0.5Hz-10Hz.

But 3-gang pots are rare as 3-wing chickens.

If you vary ONE resistor, the F varies as *third-root* of R, or for 20:1 of R you get 2.7:1. This is how many old amp-trems did it: it covers an "octave" of rate so you can find a sub/multiple of your BPM.

If you vary Two resistors, the F varies as *two-third-root* of R, or for 20:1 of R you get 7.3:1. I have not seen that in production.

Reverse(!)-Audio-taper pots improve the spread per turn but can have trouble with matching and finding enough loop-gain to stay in oscillation. (Forward-Audio taper works if you can turn your hand backward.)

However if you are not content with ~~2.7:1 of rate there are better paths. Generally a function generator making a triangle wave clipped and filtered to semi-sine. With exponential control the old ARPs would cover 0.01Hz-16kHz in two bands, over 1000:1 range in one go.
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ElectricDruid

Quote from: PRR on October 04, 2020, 05:25:08 PM
However if you are not content with ~~2.7:1 of rate there are better paths. Generally a function generator making a triangle wave clipped and filtered to semi-sine. With exponential control the old ARPs would cover 0.01Hz-16kHz in two bands, over 1000:1 range in one go.

+1 this.

The 1978 Boss BF-1 flanger LFO covers 0.05Hz-10Hz in one go, a 200:1 range. That's a simple Schmitt trigger/integrator dual op-amp style LFO. You could waveshape the triangle output with a diff pair or clipping diodes to get something more sine if that's vital.

More complicated VCO designs could cover a much wider range. Or you could use a '3340 as an LFO (the Sequential Pro-One did this) if you don't want to build your own! These days that's cheap enough to be feasible.

For 0.3 to 8Hz or so (4.7 octaves) there's no need to go beyond basic stuff.

PRR

IIRC, Bob Pease's VtoF converters approached 100,000:1 range with low error and all for 19 cents.

But agree, "there's no need to go beyond basic stuff."
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savethewhales

Yeah, I guess I'll stick with the basic stuff for now. But I'll keep the information.
Thanks