Retrofitting an LFO

[LP Hovercraft]

I have posted previously about my 24-stage Ropez build.  I have it on a switch to select every even stage between stage 4 and 24.  There is a problem that I am having with the LFO's sweep when set higher than 10 stages.  At the highest points in the sweep, the sweep becomes rather erratic like there's ripple on the oscillation shaped like a bunch of little upward ramps, kind of sounding like a sparkly feedback pitch shifter noise.  At the bottom of the sweep I get a low pitched oscillation out of my speaker.  To remedy this, I am thinking about building a triangular LFO to use instead.  The 9V Mistress' LFO on Tonepad looks like a good one to use.  In order to accomplish this, would including the 3rd LM324 (after the filter matrix switch) stage with the range network give me good results, or be completely unnecessary as being an intended part of the 4013's clock network?   

[gez]

What LFO are you using (schematic)?  Perhaps it could be tweaked.

[jimbeaux]

Possibly the LFO doesn't have enough drive to handle more than 10 stages.

Cascading transistors to the output of the LFO could possibly be the solution - each driving a "bank" of 8 stages.

I don't have the technical know-how to come up with an actual schematic of the solution - but I'm sure someone in this forum does.

In looking through several OTA based Phase Shifters - the maximum number of stages I see is four.


(BTW - what does 10 stages of phase shifting sound like?)

[LP Hovercraft]

Gez-The LFO I'm looking at dropping in there would be the one for the 9V Mistress on Tonepad.  I did a lot of Google searching for an appropriate one with regards to compatible voltage requirements, waveform output, and simplicity and came back to the Mistress'  I'm really interested if I were to include everything up to the input of the LM311 chip including the range control if that acted to move the phase shifting to different parts of the audio spectrum. 

Jimbeaux- That's interesting food for thought about a darlington pair configuration at the output of the LFO.  However, I am concerned that too much output will damage the OTA's at the phase shift stages.  Granted, I don't know enough about this myself.  Maybe a unity gain buffer in series with the output of the LFO might be enough to correct it. 

I wish I could provide a concrete example of what a ten-stager sounds like.  It subjectively sounds like the bridge of the Ween song "Can't Put My Finger On It."  Still very much a phase shifter sound but richer than a four stager due to 5 notches massaging the signal instead of 2.  I really like the sound of a six-stager, too.

Thanks for the replies!!

[jimbeaux]

What kind of power supply are you using?

Have you ckecked to see how much current you're drawing?

[LP Hovercraft]

I'm using a 0-12V (variable with a pot) 1A DC supply that I got from Fistell's (the local electronics store).  Yes, Maybe I should check if I go over that amount.  Will report back soon!

[jimbeaux]

1A is more than adequate.

[gez]

I missed your thread, I have no idea what you've built.  Without a schematic, or outline of what you've done, it's impossible to advise. 

Personally, it makes more sense to me to get working what you have in there, but you didn't say what it was.

[LP Hovercraft]

The Ropez is the name of the Ross Phaser layout on Tonepad.com   I have modified it for 24 stages. 

[gez]

I don't have the schematic handy (always helps if you link to one) but if my memory serves me well the LFO's darlington buffer connects to the Iabc pins of all the allpass stages as well as connecting to its own Iabc pin in order to 'stretch' the triangle into a hyperbolic waveform.  With that many stages, as has been pointed out, you might well be loading the darlington and, therefore, distorting the LFO's waveform.

Easy fix would be to connect an op amp follower or two (use a dual, and preferably JFET/MOSFET input) to the output of the darlington, divide your OTA stages into banks and run each one from the output of one of the followers.  The followers are simply wired up with + input connecting to the emitter (output) of the darlington and the output connecting to the - pin.  Although the LFO output won't swing any further south than 1.2V (approx), it would be advisable to use an op-amp with common mode input swing down to earth potential.

The wave form the Ross kicks out is well suited to phasers.  Keep the LFO.

PS  With that many stages you might need more than two op-amp buffers.

[LP Hovercraft]

  I'll giver a whirl and report back!  Thanks man!!!!