A cpl questions about 27xx LFO
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Author:  patrickvf [ Fri Feb 14, 2014 12:37 pm ]
Post subject:  A cpl questions about 27xx LFO

1. What kind of waveform does this LFO use? Sine or triangle? And if it's a triangle LFO, would I be able to say route the symmetry trimmer to the front and use that to change the triangle to rising/falling ramp?

2. Which components affect the depth and rate of the LFO and what would you suggest to replace those with to get a much faster/slower speed and more depth? And any suggestions on replacing the rotary speed switch with a continuous pot for setting the coarse speed without jumping to predefined values?

3. I have another LFO with some issues - the oscillation often ceases when I switch between speeds, or it barely oscillates, or the oscillation slowly picks up. Am I correct to assume that some capacitors that set the speed are bad and need to be replaced or is there more behind it?

Thanks ahead for your help :D

Author:  PAiA-Scott [ Fri Feb 14, 2014 5:29 pm ]
Post subject:  Re: A cpl questions about 27xx LFO

The 2720-5 (4750) Control Oscillator / Noise Source kit assembly manual is posted in this area:


Notice the front cover describes the output as sinusoidal. It is such a simple circuit that to add and change for the features you're after, you might as well start anew.

The power supply voltage being the specified 18V is important (within a volt or so) and check to see if the transistors Q1 and Q2 have been replaced with substitutes. If so, the biasing might not be optimized for them -- it might take some resistor changes to get things balanced out right.

Author:  Billb3 [ Fri Feb 14, 2014 7:30 pm ]
Post subject:  Re: A cpl questions about 27xx LFO

I am working on a simple but usefull LFO. I hoipe to have it ready in a week or so (subject to getting my oscillators in tune)


Author:  patrickvf [ Fri Feb 14, 2014 9:36 pm ]
Post subject:  Re: A cpl questions about 27xx LFO

There is a simple LFO schematic flying around the web somewhere. If I remember correctly, it puts out a triangle and a squarewave, and it utilizes a quad op-amp and just a handful of rc components. I'll have to look around and find it again. It's very simple and low parts count. I'll post a link to it if I find it again.

Author:  patrickvf [ Fri Feb 14, 2014 9:49 pm ]
Post subject:  Re: A cpl questions about 27xx LFO

Ok, this is not the same circuit I was talking about but this one is even simpler and uses one dual op-amp, 10 parts total if you count in the biasing circuit. I haven't tried this though, so don't ask me about it's performance. ... %20LFO.gif

PS: If I'm not mistaken, you could ditch the biasing circuit and use the Paia's -9V rail directly which would probably increase the depth/power of the LFO, but also decreases the parts count by 3 (unless you need to use some capacitors for conditioning)

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