new (to me) all-pass circuit

On 09/22/2018 09:18 PM, Phil Allison wrote:
> 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
> 
> 

Right I meant the PH-2:

<https://www.youtube.com/watch?v=3w2yAb7rog4>

On Saturday, September 22, 2018 at 6:48:45 PM UTC-4, 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

Phil, thanks for the circuit and explanation.
How does that oscillator work? (around IC6)
I don't see anything to reverse the current.

George H.   

On 09/23/2018 02:36 PM, George Herold wrote:
> On Saturday, September 22, 2018 at 6:48:45 PM UTC-4, 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
> 
> Phil, thanks for the circuit and explanation.
> How does that oscillator work? (around IC6)
> I don't see anything to reverse the current.
> 
> George H.
> 

keep in mind they're using a "virtual ground" so the ground symbol is 
not signal ground but the negative terminal of the 9V battery, the 
signal ground is at the 4.5V midpoint (everything connected to bus 
connected to the JFET sources)

The triangle wave generator is just a variation of this circuit:

<http://www.eeeguide.com/wp-content/uploads/2016/09/Triangular-Wave-Generator-Using-Op-amp.jpg>

where they've put the R3 and C integration capacitor inside the loop to 
save an extra op amp section

On Sunday, September 23, 2018 at 3:31:54 PM UTC-4, bitrex wrote:
> On 09/23/2018 02:36 PM, George Herold wrote:
> > On Saturday, September 22, 2018 at 6:48:45 PM UTC-4, 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
> > 
> > Phil, thanks for the circuit and explanation.
> > How does that oscillator work? (around IC6)
> > I don't see anything to reverse the current.
> > 
> > George H.
> > 
> 
> keep in mind they're using a "virtual ground" so the ground symbol is 
> not signal ground but the negative terminal of the 9V battery, the 
> signal ground is at the 4.5V midpoint (everything connected to bus 
> connected to the JFET sources)
> 
> The triangle wave generator is just a variation of this circuit:
> 
> <http://www.eeeguide.com/wp-content/uploads/2016/09/Triangular-Wave-Generator-Using-Op-amp.jpg>

Thanks... I saw the two caps, but didn't know how it was working.
(I did see the split ground.) 

George H. 
> 
> where they've put the R3 and C integration capacitor inside the loop to 
> save an extra op amp section

George Herold 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, thanks for the circuit and explanation.
> How does that oscillator work? (around IC6)
> I don't see anything to reverse the current.
> 

** It's an astable, so produces a square wave output at a frequency determined by the 15uF cap ( C7) and the setting of the 500k speed pot. 

The charging and discharging voltage signal on C7 is used for the JFETS  -  so not really a triangle wave but does the job.

I suspect R19 and C8 are there to supress clicking noises from IC6. 


....  Phil 

On 09/23/2018 04:05 PM, George Herold wrote:
> On Sunday, September 23, 2018 at 3:31:54 PM UTC-4, bitrex wrote:
>> On 09/23/2018 02:36 PM, George Herold wrote:
>>> On Saturday, September 22, 2018 at 6:48:45 PM UTC-4, 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
>>>
>>> Phil, thanks for the circuit and explanation.
>>> How does that oscillator work? (around IC6)
>>> I don't see anything to reverse the current.
>>>
>>> George H.
>>>
>>
>> keep in mind they're using a "virtual ground" so the ground symbol is
>> not signal ground but the negative terminal of the 9V battery, the
>> signal ground is at the 4.5V midpoint (everything connected to bus
>> connected to the JFET sources)
>>
>> The triangle wave generator is just a variation of this circuit:
>>
>> <http://www.eeeguide.com/wp-content/uploads/2016/09/Triangular-Wave-Generator-Using-Op-amp.jpg>
> 
> Thanks... I saw the two caps, but didn't know how it was working.
> (I did see the split ground.)
> 
> George H.

It's a bit more nuanced than I thought at first glance, it looks like 
there's also a bootstrapping arrangement involving the Q5 
emitter-follower load resistor and JFET source and gates to keep the 
JFETs operating in the triode/linear-resistance region as the LFO output 
ramps above and below "ground."

R22 trims the symmetry so they stay in that region on both the 
negative-going and positive-going half-cycles, while R23 gets 
bootstrapped so behaves like a CCS load for the output summer regardless.

bitrex wrote:

> 
> > 
> > Thanks... I saw the two caps, but didn't know how it was working.
> > (I did see the split ground.)
> > 
> > George H.
> 
> It's a bit more nuanced than I thought at first glance, it looks like 
> there's also a bootstrapping arrangement involving the Q5 
> emitter-follower load resistor and JFET source and gates to keep the 
> JFETs operating in the triode/linear-resistance region as the LFO output 
> ramps above and below "ground."
> 


** Look more carefully !!!

The emitter of Q5 and the sources of the JFETS connect to D2, a +5.1V *ZENER regulated* half supply rail. 


> while R23 gets 
> bootstrapped so behaves like a CCS load for the output summer regardless.

**  Nonsense.
 
R23 simply connects to the same 5.1V half rail supply mentioned above.  

The circuit is actually a lot simpler than you think. 
 


....   Phil 
             

On 09/24/2018 12:04 AM, Phil Allison wrote:
> bitrex wrote:
> 
>>
>>>
>>> Thanks... I saw the two caps, but didn't know how it was working.
>>> (I did see the split ground.)
>>>
>>> George H.
>>
>> It's a bit more nuanced than I thought at first glance, it looks like
>> there's also a bootstrapping arrangement involving the Q5
>> emitter-follower load resistor and JFET source and gates to keep the
>> JFETs operating in the triode/linear-resistance region as the LFO output
>> ramps above and below "ground."
>>
> 
> 
> ** Look more carefully !!!
> 
> The emitter of Q5 and the sources of the JFETS connect to D2, a +5.1V *ZENER regulated* half supply rail.
> 
> 
>> while R23 gets
>> bootstrapped so behaves like a CCS load for the output summer regardless.
> 
> **  Nonsense.
>   
> R23 simply connects to the same 5.1V half rail supply mentioned above.
> 
> The circuit is actually a lot simpler than you think.
>   
> 
> 
> ....   Phil
>               
> 

I see it, I had to look for a different schematic that one isn't doing 
me any favors. I like this one where signal flow is more clear and the 
bias and reference voltages are labeled buses:

<https://www.electrosmash.com/images/tech/phase-90/mxr-phase-90-script-logo-schematic-parts.png>

Analysis:

<https://www.electrosmash.com/mxr-phase90>

Q5 is a standard common emitter PNP summer with feedback resistor and 
the emitter connected directly to the 5.1 V bus.

as you say the LFO wave form is "vaguely triangular"

On 23/09/18 08:48, 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,

I'm sure you'll remember the ETI447 phaser,
<http://www.spontis.se/pdf/eti447.pdf>.

I still have the twelve-stage one I built back then,
with two CD4049's and four quad op-amps, all built
on Veroboard. Mine ran off two 9v batteries.
Or maybe it was my wah pedal that needed two, I forget.

Clifford Heath.

Clifford Heath wrote:

> 
> 
> I'm sure you'll remember the ETI447 phaser,
> <http://www.spontis.se/pdf/eti447.pdf>.
> 


** I remember building one for a friend from a Dick Smith kit.

   Half the supplied 741s did not work. 




....  Phil