Photocells rather than Jfets?

[Denio]

Hi,all!
I have this weird idea-what if the Jfets in the phase 90 circuit(or phase 45) were replaced with photocells,illuminated by a bulb or by individual LED's?Has anyone ever done this?Will it even work?

Thank you for your time!

[duck_arse]

I think that basic idea is called a uni-vibe. replace the transistor stages w/ opamp stages, and they should look fairly similar.


you might need to apply a little imagination tho .....

[Denio]

The phase 90 circuit already used opamps for the phase stages so I think I could give it a try.I am going to have to find the range of the variable resistance that the jfet and resistor connected parallel to it provide.
Thanks!

[lapsteelman]

The general idea will work.. you might have to play around with some of the values... I've made an op-amp phase shifter I based on Craig Anderton's circuit.. I seem to remember seeing others use photocells and op-amps as well.

[Mark Hammer]

1) Photocells do not distort, such that one is able to add some gain at the input of a phaser (complemented by some attenuation at the output) to achieve a better S/N ratio.

2) Photocells vary in their responsiveness and recovery-time, such that their characteristics can change at fast speeds.  That can be good or limiting, depending on your needs.  In some cases, it's a good thing in that we prefer to reduce sweep width at faster speeds, and photocells can do that automatically, rather than requiring the user to adjust two controls.  That is, at fast-enough speeds (using slow-enough photocells), the photocell does not go back all the way to maximum dark resistance before responding to illumination and heading towards minimum light resistance again.

3) Photocells should probably be in the ballpark (i.e., use units that are nominally spec'd the same), but do NOT need to be matched.  The sound achieved is a function of the *total* phase shift across however many stages you have, and as long as the total keeps moving/changing over the sweep cycle, you're okay.  FETs need to be reasonably matched because their sweep range is fairly limited and they need to be aligned to ensure that they all respond to the LFO at all times and none of them runs into a point where they choke and do not change their drain-source resistance.

[Denio]

To make the change in resistance as equal as possible in all the photocells I will buy identical ones,and I am planning on illuminating all four of them by using a single bulb,and the whole contraption will be sealed in some kind of housing to make it light proof.
You said that photocells behave a bit slow at high frequencies.In our case the maximum frequency is,I guess about 20Hz.Would that cause a noticeable drop in depth of the phased sound?

[Mark Hammer]

Again, they don't have to be "matched", but yes, 2 or 4 or 6 of the same part number are fine.

Depending on where you get them from, the specs can indicate how long it takes for the photocell's resistance to "settle" after a brief period of illumination.  If it takes 200msec for full dark resistance to be reached, then sweeping faster than 3-4hz will likely result in the sweep width being reduced a bit.  My understanding is that responding to illumination is usually faster than settling back down after dimming of the light source.  In the case of the Uni-Vibe, or any other pedal employing an incandescent bulb,  such light sources also tend to lag a bit as the filament cools down after the LFO voltage goes low.

By contrast, direct current-control of JFETs is instantaneous, with no lag, which means that whatever the LFO "says" is what the JFETs are doing.  Though it is not necessarily a given, one might find that use of some photocells needs some tailoring of the LFO waveshape to provide the best feel to the sweep.  That's no reason to avoid photocells, just something to be mindful of.

[duck_arse]

Denio - you can watch/map the behaviour/response of a led/lamp/photocell if you have an oscilloscope. or an [needle-type] analogue voltmeter.

\ I need a slash. /

[pinkjimiphoton]

hahah,
you guys are describing earthquaker devices "the depths".

i built one. its absolutely killer, replaced all my other vibes. fits in a crybaby. almost as good as the black cat vibe.
can be very transparent.

there are layouts and schems on google. and its not hard, and works great.