Interesting LM13700 Phase Shifter Vibrato article - thoughts?

[tubegeek]

Phase modulator vibrato for musical instruments by Craig Sellen

http://archive.siliconchip.com.au/cms/A_111525/P_2/article.html

The schematic is below. An LFO feeding an opamp driving two sets of phase shift networks via the two halves of an LM13700. I found this googling "LM13700 tremolo" I think. The text is as follows:

Code Select Expand

"Phase modulator vibrato for musical instruments
Click for larger image
The term "vibrato" is often used mistakenly instead of "tremolo" which describes a regular variation in signal or loudness. Vibrato, on the other hand, is modulation of pitch or frequency

In this circuit, vibrato is achieved by using two cascaded phase shift networks. The two different phase shift networks are connected in parallel and are fed the same audio signal. The forward gains of the two networks are varied by two transconductance amplifiers which are controlled by a single low-frequency oscillator.

Following the transconductance amplifiers, the two separately phase-shifted signals are resistively mixed to obtain a vibrato output.

In essence, each phase shift stage is a low-pass filter with a different corner frequency, as marked. When signal has passed through the two paralleled phase shift networks, the two outputs are 90° out of phase. Dual op amps IC1, IC2 & IC3 provide the six phase shift stages. Op amp IC1a acts as a unity gain buffer. Its non-inverting input is connected to 0V via a 1MΩ resistor to provide a high impedance – important if an electric guitar is the signal source.

The two phase-shifted signals are applied to LM13700 transconductance amplifiers IC4a & IC4b. Their individual gains are controlled by the DC voltage fed to IC4's pins 1 & 16.

The slowly varying control voltage is generated by op amp IC1b which is configured as a low frequency sinewave oscillator. Its output at pin 7 is fed to a phase splitter stage comprising op amps IC2c & IC2d. IC2c is a unity gain inverter while IC2d is a unity gain non-inverting buffer. Hence the gain control signals applied to pins 1 & 16 of IC4 will be in anti-phase. Hence, IC4a will be at maximum gain when IC4b is at minimum and vice versa.

The net result of the common signal at the junction of the two 10kΩ resistors is that its amplitude is essentially constant while its phase is continually shifted back and forth over a 90° range. The overall gain of the entire circuit is unity.

The six phase-shift stages need to be calibrated one at a time and the easiest way to do this is with an oscilloscope connected in XY mode. For example, for IC2a, first connect your scope to the input and output of the phase shift stage and set your audio oscillator to 206Hz. You then adjust trimpot VR1 until a circle pattern appears on the scope screen.

You then do the same thing for IC3a at 1657Hz and so on until all stages have been adjusted.

Craig Sellen,
Carbondale,
Philadelphia, USA
"

Any thoughts? I'm curious about the calibration step of the phase shift network for one thing - is this likely to make any audible difference from just using design center/nearest standard value performance? Is there any close-cousin circuit that's more familiar in the pedal world? This looks like an interesting starting point for either the vibrato effect as drawn, or for trying to build a harmonic tremolo by using simpler filters on each branch and flipping one of the LFO op-amps 180º.

[StephenGiles]

Could this be a starter building block for a thru zero phaser of sorts?

[Scruffie]

The drawing reminds me of an old Moog schematic which handily flicked a switch, I think this is an implementation of dome filters.

[StephenGiles]

The drawing reminds me of an old Moog schematic which handily flicked a switch, I think this is an implementation of dome filters.

Aha, where is Puretube and his dome filters?

[tubegeek]

Jurgen Haible has info that seems relevant here:
http://www.jhaible.info/fs1a/fs1a.html

Thanks for the mention of Dome filters!

[Eb7+9]

Could this be a starter building block for a thru zero phaser of sorts?

You won’t be catching non-linear rainbows with this circuit I’m afraid ...

I spent some time converting the original tube based 3-phase version of this idea which does a semi-constant 120 degree shift of audio in three bands and mixed them in a continuous, or semi-continuous rotary manner

The output stage mixer is where the rotation takes place

interesting to see only two 90degree shifters being used here instead ...
in principle you would need four such paths ... then requiring a 4-phase oscillator and mixed together to sweep thru 360 degrees of shifting

but then again, this is very probably done to keep things “simple” ... or merely “doable” in a typical diy context

only providing part of the full effect

[ElectricDruid]

There's more explanation of what that is actually doing (or trying to) on Jurgen's first Frequency Shifter page too:

http://www.jhaible.info/tonline_stuff/hj_fs.html

[Scruffie]

There's more explanation of what that is actually doing (or trying to) on Jurgen's first Frequency Shifter page too:

http://www.jhaible.info/tonline_stuff/hj_fs.html

Speaking of phase shifting and quadratures, I've been wondering when you're going to recreate a Solina style LFO on a digital chip

[ElectricDruid]

Speaking of phase shifting and quadratures, I've been wondering when you're going to recreate a Solina style LFO on a digital chip

Well, it's funny you should mention that
One of the projects I have kicking around on my desk at the moment is a three-phase LFO based on the 16F1778, which usefully has three 10-bit DACs on the chip. The firmware currently does four modes inspired by various historical ensemble chorus designs: TriphaseLFO, SolinaLFO, TripleLFO, and HarmonicLFO.

Triphase LFO is a single LFO with triphase (0,120,240 degree) outputs.

SolinaLFO is similar but includes a 'Fast' LFO and a 'Slow' LFO each at three phases and mixed at each output.

TripleLFO is three independent LFOs running at unrelated frequencies, one "slow", one "medium", and one "fast".

HarmonicLFO is three LFOs running at F/2, F, and 2F. This was an old Roland design.

My chip lets you alter the frequency, which the Solina never did, and includes a "Spread" control for the multi-LFO modes (so you can alter how far apart the slow and fast Solina LFOs are, or how different the three independent LFOs are etc etc). Plus there are four waveforms, sine plus three others yet to be finally determined.

Having done it, it was pointed out to me that it would save circuitry if it could output biphase clock signals for driving BBDs (so "instant triple chorus, just add BBDs!") so I've been experimenting to see if that's possible (doesn't look like it with the current chip, but there's an 18F chip which might be able to do it).

So...yeah, I have one done and running on a stripboard prototype. Give me a shout if you'd like one for testing.

[tubegeek]

So...yeah, I have one done and running on a stripboard prototype. Give me a shout if you'd like one for testing.

.... hand raised ....

[Mark Hammer]

How is it that aperiodic was hip in the '70s, and then we forgot all about it?  Everything old is new again!

[Scruffie]

Speaking of phase shifting and quadratures, I've been wondering when you're going to recreate a Solina style LFO on a digital chip

Well, it's funny you should mention that
One of the projects I have kicking around on my desk at the moment is a three-phase LFO based on the 16F1778, which usefully has three 10-bit DACs on the chip. The firmware currently does four modes inspired by various historical ensemble chorus designs: TriphaseLFO, SolinaLFO, TripleLFO, and HarmonicLFO.

Triphase LFO is a single LFO with triphase (0,120,240 degree) outputs.

SolinaLFO is similar but includes a 'Fast' LFO and a 'Slow' LFO each at three phases and mixed at each output.

TripleLFO is three independent LFOs running at unrelated frequencies, one "slow", one "medium", and one "fast".

HarmonicLFO is three LFOs running at F/2, F, and 2F. This was an old Roland design.

My chip lets you alter the frequency, which the Solina never did, and includes a "Spread" control for the multi-LFO modes (so you can alter how far apart the slow and fast Solina LFOs are, or how different the three independent LFOs are etc etc). Plus there are four waveforms, sine plus three others yet to be finally determined.

Having done it, it was pointed out to me that it would save circuitry if it could output biphase clock signals for driving BBDs (so "instant triple chorus, just add BBDs!") so I've been experimenting to see if that's possible (doesn't look like it with the current chip, but there's an 18F chip which might be able to do it).

So...yeah, I have one done and running on a stripboard prototype. Give me a shout if you'd like one for testing.

I should have known you'd do it and kick it up a notch.

If it can't do bi-phase for BBD's, single phase would still be pretty useful if that's a more realistic option, just add inverters or hook it up to your favourite bi-phase clock etc. You'd already have 3 BBD paths, an extra 4069 isn't going to break the bank or board space and if there was PWM outputs, you could use them for a switched resistor ensemble phaser 

Well, it would be rude not to

So...yeah, I have one done and running on a stripboard prototype. Give me a shout if you'd like one for testing.

.... hand raised ....

*Smacks hand down* Mine! 

[tubegeek]

*Smacks hand down* Mine! 

I'm just looking to be 2nd in line!

[anotherjim]

Korg did a 3 phase ensemble LFO in one 4069. It's a pair of ring oscillators.
Funnily enough, the rate and depth of a string ensemble chorus do not really benefit from being fully adjustable. The slow chorale speed and the faster vibrato rate need to be never far from 0.6Hz and 6Hz up to 0.8Hz to 8Hz - notice the vibrato is always x10 the chorale speed in these things?
http://urekarm.tripod.com/synth/enslfo_p6.pdf