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new (to me) all-pass circuit

Started by John Larkin September 22, 2018

It occurred to me that, in the conventional opamp all-pass 90 degree
phase shifter circuit,

https://www.dropbox.com/s/nfwd32p3z61g15y/90_degree.JPG?raw=1

a fast signal passes through all the opamp stages, so all the amps
have to be fast. In this one,

https://www.dropbox.com/s/rulyvlt072k49cp/New_All-Pass_1.jpg?raw=1

the fast signals just rip through the big caps in the slow stages. So
in a high-order shifter we could use slow, cheap, low-noise amps in
the first few stages and just use fast opamps towards the end. That
will make less HF distortion too.

The down side is that the output impedance isn't low, like an opamp
would be, so cascading stages gets a little more difficult, or more
interesting depending on your attitude.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

On 09/21/2018 11:39 PM, John Larkin wrote:
> > > It occurred to me that, in the conventional opamp all-pass 90 degree > phase shifter circuit, > > https://www.dropbox.com/s/nfwd32p3z61g15y/90_degree.JPG?raw=1 > > a fast signal passes through all the opamp stages, so all the amps > have to be fast. In this one, > > https://www.dropbox.com/s/rulyvlt072k49cp/New_All-Pass_1.jpg?raw=1 > > the fast signals just rip through the big caps in the slow stages. So > in a high-order shifter we could use slow, cheap, low-noise amps in > the first few stages and just use fast opamps towards the end. That > will make less HF distortion too. > > The down side is that the output impedance isn't low, like an opamp > would be, so cascading stages gets a little more difficult, or more > interesting depending on your attitude. > >
If you split the capacitor into say 2/3rds on the left and 1/3rd on the right and picking an "approrpiately sized" resistor from the center tap to the inverting input it should improve the frequency response into a load, at the cost of it phase-shifting the opposite way. e.g. with 0.68uF on the left 0.22 on the right, 47k resistor from the tap, into a 10k load, with LT1014 the output amplitude hasn't changed at all at 10 Hz vs down 100mV with the stock circuit
John Larkin
>It occurred to me that, in the conventional opamp all-pass 90 degree >phase shifter circuit, > >https://www.dropbox.com/s/nfwd32p3z61g15y/90_degree.JPG?raw=1
Dunno what book that comes from, but it strongly reminds me of radio ham audio 90 degrees phase shifter for a SSB modulator. This is fun: http://qrp-popcorn.blogspot.com/2015/11/that-90-degree-phase-shift.html Also see his reference list at the bottom of the page.
On Sat, 22 Sep 2018 07:06:53 GMT, <698839253X6D445TD@nospam.org>
wrote:

>John Larkin >>It occurred to me that, in the conventional opamp all-pass 90 degree >>phase shifter circuit, >> >>https://www.dropbox.com/s/nfwd32p3z61g15y/90_degree.JPG?raw=1 > >Dunno what book that comes from, >but it strongly reminds me of radio ham audio 90 degrees phase shifter for a SSB modulator.
That's the Williams&Taylor filter book.
> >This is fun: > http://qrp-popcorn.blogspot.com/2015/11/that-90-degree-phase-shift.html > >Also see his reference list at the bottom of the page.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
On Friday, September 21, 2018 at 11:39:40 PM UTC-4, John Larkin wrote:
> It occurred to me that, in the conventional opamp all-pass 90 degree > phase shifter circuit, > > https://www.dropbox.com/s/nfwd32p3z61g15y/90_degree.JPG?raw=1 > > a fast signal passes through all the opamp stages, so all the amps > have to be fast. In this one, > > https://www.dropbox.com/s/rulyvlt072k49cp/New_All-Pass_1.jpg?raw=1 > > the fast signals just rip through the big caps in the slow stages. So > in a high-order shifter we could use slow, cheap, low-noise amps in > the first few stages and just use fast opamps towards the end. That > will make less HF distortion too. > > The down side is that the output impedance isn't low, like an opamp > would be, so cascading stages gets a little more difficult, or more > interesting depending on your attitude.
For the just RC phase sequence filter the order of the R's and C's don't matter. I'm not sure if that's true for the all-pass version. But it might help you deal with the output impedance issue, when cascading. George H.
> > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
On 09/22/2018 11:42 AM, John Larkin wrote:
> On Sat, 22 Sep 2018 07:06:53 GMT, <698839253X6D445TD@nospam.org> > wrote: > >> John Larkin >>> It occurred to me that, in the conventional opamp all-pass 90 degree >>> phase shifter circuit, >>> >>> https://www.dropbox.com/s/nfwd32p3z61g15y/90_degree.JPG?raw=1 >> >> Dunno what book that comes from, >> but it strongly reminds me of radio ham audio 90 degrees phase shifter for a SSB modulator. > > That's the Williams&Taylor filter book.
It's also more-or-less the circuit for the MXR Phase 90 (with the resistors from non-inverting to ground replaced with JFETs as VCRs and fed with a triangle oscillator) <https://www.youtube.com/watch?v=OJfFt2i2Yd4> Sounds like Purple Haze, far out man...
>> This is fun: >> http://qrp-popcorn.blogspot.com/2015/11/that-90-degree-phase-shift.html >> >> Also see his reference list at the bottom of the page. >
On a sunny day (Sat, 22 Sep 2018 08:42:53 -0700) it happened John Larkin
<jjlarkin@highlandtechnology.com> wrote in
<ndlcqdpip7n5r86t2pgmq6dtghiptr2ras@4ax.com>:

>On Sat, 22 Sep 2018 07:06:53 GMT, <698839253X6D445TD@nospam.org> >wrote: > >>John Larkin >>>It occurred to me that, in the conventional opamp all-pass 90 degree >>>phase shifter circuit, >>> >>>https://www.dropbox.com/s/nfwd32p3z61g15y/90_degree.JPG?raw=1 >> >>Dunno what book that comes from, >>but it strongly reminds me of radio ham audio 90 degrees phase shifter for a SSB modulator. > >That's the Williams&Taylor filter book.
Thank you, did never read it.
bitrex wrote:

> > > > > That's the Williams&Taylor filter book. > > It's also more-or-less the circuit for the MXR Phase 90 (with the > resistors from non-inverting to ground replaced with JFETs as VCRs and > fed with a triangle oscillator) > > <https://www.youtube.com/watch?v=OJfFt2i2Yd4> >
** See schem: https://scfxguide.files.wordpress.com/2014/02/phase-90.png The arrangement mixes direct and phase shifted signals so deep notches are created in the audio band that are sweept up and down in frequency by the triangle wave applied to the JFET gates. There is also quite a lot of 2nd harmonic distortion produced by the same JFETs that adds character and some compression to the resulting sound. .... Phil
On 09/22/2018 06:48 PM, Phil Allison wrote:
> bitrex wrote: > >> >>> >>> That's the Williams&Taylor filter book. >> >> It's also more-or-less the circuit for the MXR Phase 90 (with the >> resistors from non-inverting to ground replaced with JFETs as VCRs and >> fed with a triangle oscillator) >> >> <https://www.youtube.com/watch?v=OJfFt2i2Yd4> >> > > ** See schem: > > https://scfxguide.files.wordpress.com/2014/02/phase-90.png > > The arrangement mixes direct and phase shifted signals so deep notches are created in the audio band that are sweept up and down in frequency by the triangle wave applied to the JFET gates. > > There is also quite a lot of 2nd harmonic distortion produced by the same JFETs that adds character and some compression to the resulting sound. > > > > .... Phil >
My "effects chain" such as it was back in the day (1997 or so) was a Dunlop crybaby -> Boss Super Distortion -> Boss PS-2 phaser - > DM-2 analog delay. all except the wah pedal were pretty well worn by the time I got my hands on them. The PS-2 was a bit more refined design than the Phase 90 it had some extra controls and operating modes and IIRC used a compander for noise reduction in phase signal path. Sometimes I used a chorus pedal instead of the phaser but I remember I didn't like Boss chorus pedals very much I think it was a DOD pedal of some type, can't remember the exact model. Don't have any pedals anymore I switched over to amp + cab simulators/Line 6 PD-type products quite a while back.
bitrex wrote:

> > > ** See schem: > > > > https://scfxguide.files.wordpress.com/2014/02/phase-90.png > > > > The arrangement mixes direct and phase shifted signals so deep notches are created in the audio band that are sweept up and down in frequency by the triangle wave applied to the JFET gates. > > > > There is also quite a lot of 2nd harmonic distortion produced by the same JFETs that adds character and some compression to the resulting sound. > > > > > > > > .... Phil > > > > My "effects chain" such as it was back in the day (1997 or so) was a > Dunlop crybaby -> Boss Super Distortion -> Boss PS-2 phaser - > DM-2 > analog delay. all except the wah pedal were pretty well worn by the time > I got my hands on them. The PS-2 was a bit more refined design than the > Phase 90 it had some extra controls and operating modes and IIRC used a > compander for noise reduction in phase signal path. > >
** The PS-2 is not a phaser - it is a digital delay / pitch shifter. It is capable of 2 second time delays and one octave up and down shifts. https://guitarloverblog.files.wordpress.com/2010/02/boss_ps2.jpg .... Phil