can you amplify an LFO?

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I have an LFO I built from a circuit that has been floating around (it is the LFO used in the uglyface mods). It works, but it is not very strong. It does not light up an LED very brightly, nor provide a very strong modulation when I tried it directly into the VCF from the circuit snippets site.

Could I make it stronger with a simple booster or buffer?

[R.G.]

You can pretty much amplify anything.

However, LFOs take a little care. They're tricky in several ways. What determines what your LFO needs to do is what is being driven - as you have found out. Just any LFO won't work.

An LFO has to provide a waveform that has the right size, matching the minimum to maximum range that whatever it drives needs, as well as the right DC level for whatever needs driven. Generally, you set the DC level to match whatever is being driven at one point, usually the midpoint of whatever it does, then change the AC size of the LFO around that DC point. This produces a variable sweep on the LFO. Some LFOs have a zero volts DC level, and all of the variation is UPwards (or downwards) from that zero point. Some have a non-zero "middle".

Every modulated circuit has different LFO requirements.

Some other complications: Some LFOs need to be a current, not a voltage. Notably  the OTA based circuits need current type LFO inputs. And you can't always drive an LED (as you might guess by reading the above) because LEDs have their own quirks of what they need. They want a DC level just about the LED on voltage, and primarily a current variation, not a voltage variation. You can fake a current drive from a voltage LFO by simply putting in a resistor, but you have to size the resistor correctly to get a good current range. And if you do this, you have to remember that driving an LED is NOT what you wanted the LFO for in the first place; modulating something else is. Unless the LFO is a low impedance voltage-source-like voltage and happens to be about an LED voltage in the middle, this may not work well without other things to drive the LED. If you have a voltage-needing circuit the LFO is driving, then hanging an LED on the LFO may let the LED clamp the LFO voltage so there's much less effect change caused by the LFO.

All that being said, yes, you might get lucky. I like to use buffers on any LED drive by an LFO for the loading reasons. And you can always use an opamp to both change the AC voltage size of the LFO and to change its DC voltage. That ...might... happen to drive an LED OK. Or you could use multiple buffers, one to match the LFO size to drive the effect better, another to drive the LED.

Am I giving you the idea that just picking an LFO from a circuit snippets site might need some more work to get running? 

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OK.... well...

Thank you for that pile of info 

So let's say I wanted it to light an LED. (I am looking to use this LFO, which I have already built, to modulate the delay time pot on a delay that I plucked from a karaoke boombox).

Where would I start? Would a regular buffer, such as those used at the beginning of many FX circuits, work? Do I need a special one for low frequency stuff?

This is the LFO circuit, BTW... I don't think it is Tim E's design, but something that someone added as a mod:



I notice that the "easy LFO" from the PWM uses a transistor at the end, would it be worth trying something like that?

[aron]

I think a synthesizer LFO circuit might be the way to start.

[PRR]

> let's say I wanted it to light an LED.

OK, we'll say you want to ligth an LED.

Wait! That's easy! Battery, resistor, LED..... light!

So what's with this LFO?

Oh, you want the light to vary?

Figure out how many volts and amps the LED needs to go off and on the way you want.

You may find that 1.5V is too dim and 1.6V is too bright; we often add a resistor to spread-out the voltage range.

Now you have a possible plan (with electrical specs) for the LED.

Go back to your LFO. How much power can it put out? Voltage and current?

It may be possible to change the voltage swing of an LFO. However with 9V supply and real chips, it may be hard to swing to extremes. This may particularly hurt if you need to swing OFF.

The raw chip output may have 10mA available. But some is used to make the LFO action happen. And sometimes "output" is tapped from a sensitive timing network, hardly any power available.

It would help to spend a decade working with analog computers. ANYTHING can be computed (simple amplification is a multiply computation). But that's a Lost Art.

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I am sorry guys, but I figured out a really simple answer to this question.

See that resistor at the end of the schematic? The 1K one? R6?

It is now 100ohms

The LFO works beautifully.



*edit* 470ohms works even better.

[R.G.]

I am sorry guys, but I figured out a really simple answer to this question.
See that resistor at the end of the schematic? The 1K one? R6?
It is now 100ohms
The LFO works beautifully.

*edit* 470ohms works even better.

Kewl. Whatever works! 

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Kewl. Whatever works! 

I do wonder if it would be the same situation though if I were to try to use it to sweep something with an actual CV in rather than lighting an LED.

For this purpose though, this works perfectly, it seems it just had too much resistance between the LFO and the LED.