ADA Final Phase PWM?

[daverdave]

Just been looking at the ADA final phase schematic, I've been told that it's PWM controlled. But the 4047 seems set up to vary frequency, not duty cycle, to the bilateral switches.

Am I missing something here?

[Mark Hammer]

If I've understood it correctly, varying the frequency of a switched system is appropriate for switching a capacitor, while varying the pulse-width is appropriate for simulating a variable resistance.  Since the locus of the phase shift depends on both the resistance and cap value on the non-inverting pin, phase shifters can use either approach.

[slacker]

According to ADAdepot there's two versions, neither of them appear to use PWM. The version with the CD4047 uses it in the same way as in the ADA Flanger which, like you say, varies the switching frequency. The other version looks like it uses the LFO to drive LDRs, but again nothing that looks like PWM going on.

Maybe who ever told you it used PWM had misunderstood what the "Sweep modulation" did?

The LFO in it looks pretty interesting I've never looked at it before, might be worth some further investigation.

[Mark Hammer]

Yep, I've only seen LDR-based Final Phase schematics, myself, but had no basis for rejecting the possibility that there might be another version out there.

[slacker]

Good point, maybe there's another version out there that does use PWM.

[daverdave]

Off the top of my head I've a fealing that I read it in a book, could just have been a mixup between the two.
The ad claims the allpass stages have a Q of 1, whereas from the schematic it looks like only the two second order stages are, which are fixed.
don't know the component values off the schem so it's hard to say.

It's the only commercial phaser I've seen that employs both electronic switching to control the shift and second order allpass filters,
maybe I'm just not looking hard enough.

[StephenGiles]

Version 5 uses LDR/LEDs and version 9 uses the 4007/4047/4016 combination, which looks very much to me like resistances being varied by the transmission gates of the jolly old 4016.........so PWM it is - I'm sure

[daverdave]

The lfo looks to be changing the frequency of the 4047 astable. Surely if the 4016's are seeing the same duty cycle, ie, 50% or whatever, then there's no variation when the duty cycle is integrated by the cap. It's get the same current through the resistor at the same pulse width whatever the frequency within reason.

[earthtonesaudio]

I only just glanced at the schem but it's definitely varying the frequency and probably not PWM.  It is strange though, frequency-varying switched capacitor filters typically use SPDT switches, not SPST.

[StephenGiles]

Why do you say they are frequency-varying switched capacitor filters?

[daverdave]

From what I understand, the switched capacitor acts as a resistor, as the current is controlled by the charge on the cap at a rate determined by the frequency of the switching. A higher frequency means a lower resistance since more charge is transferred.

On the subject of PWM, if the resistance is doubled as the pulse width is halved, so for a 1k resistor a 50% pulse width gives the appearance of 2k and a 10% gives 10k, would this mean the change is resistance isn't particularly linear over the change in duty cycle? Plus, am I right in thinking that the resistance range would be quite small, say 1k to 100k for a 100% to 1% duty cycle range, which I could see  being difficult to achieve?

[Mark Hammer]

The cap is not acting like a varied resistor.  Rather, the HF switching permits the cap different opportunity to charge up, which is essentially the same thing as changing cap value.

[daverdave]

So it's not a switched capacitor in the traditional sense, ie, with a cap to ground and two spst switches controlling the charging?

Would you say that the switch is controlling the ammount of current the capacitor discharges similarly to how a varied PWM resistor works?

[Mark Hammer]

If the rate at which an SPST switch, connected to the "open" end of a cap, switched on and off was slow enough, you'd hear exactly what you hear when there is an input or output cap on a pedal without any terminating resistor.  It would be a pop.  If you ratchet up the rate at which the switching occurs so that it was above human hearing range, you'd have a cap that keeps trying to fully charge up, but only gets brief intermittent chances to do so.  In a sense, the net effect is like a variable series resistance that slows down how quickly the cap charges up from some current source.

Make sense?

[daverdave]

Yeah, it makes sense. I'm just trying to get my head around the effect of varying the frequency of switching.
I've got it in my head as the charge on the cap is C=QV, I=Qf, so the apparent capacitance change will be C=I/fV?

I think I might be way off here.

[earthtonesaudio]

Why do you say they are frequency-varying switched capacitor filters?

The frequency of the VCO is varied, but the on:off ratio remains about 1:1 always.  It is not PWM.

[earthtonesaudio]

Yeah, it makes sense. I'm just trying to get my head around the effect of varying the frequency of switching.
I've got it in my head as the charge on the cap is C=QV, I=Qf, so the apparent capacitance change will be C=I/fV?

I think I might be way off here.

Correct except it should be "Q=CV" for the first relationship.

[daverdave]

Yeah cheers, forgot the / in there. Makes alot more sense, thanks for the help guys, really appreciate it.

I do really like the sound of the final phase, sounds quite deep sounding to what I expected from a frequency switched circuit.
Plus I've never seen that type of second order allpass used before, looks like the + input of the opamp is fed from the output via a multiple feedback bandpass like arrangement, giving your typical 1 - 2BP 2nd order allpass, dunno how much the sound is affected by the Q from hearing it, I'm picturing a narrower notch/peak in the frequency spectrum from the two 2nd order stages.