MXR Flanger "Manual" knob

[Bainzy]

What type is it? Logarithmic (audio) or linear?

The schematic doesn't show it. 

[Mark Hammer]

Looking at the schematic ( http://experimentalistsanonymous.com/diy/Schematics/Flangers/MXR%20Flanger.gif ) it looks like some parts have been aded to achieve a special taper.  In which case, my guess it that they started out with a linear taper, and added on things to make it behave a little differently.

[Peter Snowberg]

You want linear there.

[Bainzy]

Dammit - I ordered Log! 

Looks like the clone will take a bit longer to build now... 

[phaeton]

Looking at the schematic ( http://experimentalistsanonymous.com/diy/Schematics/Flangers/MXR%20Flanger.gif ) it looks like some parts have been aded to achieve a special taper.  In which case, my guess it that they started out with a linear taper, and added on things to make it behave a little differently.

You mean the 470k and 100k resistors on either side being the "parts added"?  Does that affect the taper of the pot? I just thought it moved its operational range up or down (like, from 470k to 520k instead of 0k to 50k).

[Mark Hammer]

I'm sure you'll be able to find a use for a 50k log pot eventually.  Use of a log pot will not prevent the unit from functioning.  It will simply squash portions of the range of adjustments into a small amount of rotation and make it harder to dial in some settings tht you may like to repeat again.

As for the added components changing the taper, ask yourself if the bias that the op-amp sees at pin 3 is *exclusively* the result of the pot setting or a slightly different bias voltage/current consisting of what comes from the 470k resistor and the 100k resistor and pot.

[Bainzy]

Right - in that case I'll build it with the log pot and replace it later when the linear one arrives.

[Bainzy]

One thing I don't understand in the schematic is why there are 2 polarized 15uF capacitors connected in opposite directions? (see top right hand corner)

[Bainzy]

Fortunately I got in touch with Small Bear before my original order shipped - so the log 50k pot is gonna be replaced with a linear one! 

[Peter Snowberg]

One thing I don't understand in the schematic is why there are 2 polarized 15uF capacitors connected in opposite directions? (see top right hand corner)

That's a trick to make a non-polarized cap out of two polarized aluminum electrolytic cap. Don't try the same trick with tantalum caps.

Congrats on getting the substitution. It's great to deal with small vendors who give great service. 

Welcoe to the forum too!

[Bainzy]

Thanks Peter 

I actually ordered 15uF Tantalum caps for that purpose - what cap options would I have other than to go out and buy a pair of 15uF electrolytic caps? Trying to find 15uF electrolytic caps was my first choice, but I could find any on the sites I was ordering from.

[Bainzy]

bump

[Bainzy]

...-ety bump

[Bainzy]

What cap options would I have other than to go out and buy a pair of 15uF electrolytic caps? Trying to find 15uF electrolytic caps was my first choice, but I could find any on the sites I was ordering from.

[Mark Hammer]

I can't see the schematic at the moment but the goal of two back to back 15uf caps is to simulate a 7.5uf non-polarized unit.  If you use a pair of 22uf caps instead, you mimic an 11uf NP unit.  I can't imagine that a difference that small will either prevent the flanger from functioning properly, or produce a huge decrement in the quality of effect.  Certainly won't make it blow up, so it's worth a try.

I will confess to ignorance here for a moment.  I know that caps in series behave the same way that resistors in parallel do, for computation purposes.  And that doesn't change regardless of whether the caps in series are of equal value or not.  But that has always involved small value nonpolarized units in my experience.  All attempts I have ever seen to use a back-to-back polarized pair to simulate a large value non-polarized unit have involved equal value components.

So allow me to ask the more formally trained folks: if aiming for a simulated nonpolarized value, are the two members of a back-to-back pair required to be equal value?  For example, in principle a 33uf cap and 10uf capin series produce a combined effective capacitance of  7.67uf.  Could such a back to back pair be used to mimic what a 15+15 does?

[Bainzy]

Right I've just worked out a formula for adding the caps in series:

r = ab / a+b

r being the resultant, a and b being the 2 capacitor values.

With 10u and 22u, I get 6.875. (10 and 22 are the only values my local electronics shop stocks)

So does anyone know if the trick to non-polarise an electrolytic works with 2 different values?

[George Giblet]

>As for the added components changing the taper,

The 470k + 100k makes sure the voltage at following the opamp doesn't below the opamp's useable input range.  Without the resistors a considerable portion of the manual pots sweep would have no effect on the delay setting - effectively producing a dead spot.