Why can't I use this ridiculously simple design?

[gnsdg]

Hey guys--since I come from the world of guitar amps, I don't know WHAT I don't know, but I was thinking about how to make the hi band frequency variable on this guy:

https://www.dropbox.com/s/hqq3aqwismac33u/M512YAMAHAServiceManual-EN.pdf

And I thought, why couldn't I just do this:

https://www.dropbox.com/s/qs0ju9u8poarmk8/Photo%20Feb%2022%2C%203%2056%2042%20PM.jpg

?  Too lossy?  Phase shift?  Or potentially passable in an already colorful unit?

 

[gnsdg]

No?  No one?

What did I say? 

 

[MagnetoSound]

Have you tried it? What happens?

 

[ruffrecords]

Looking at the original circuit, the 620 ohm resistor with the series cap sets the frequency at which the shelf occurs and the maximum depth of the shelf, and the cap with the original pot sets where the boost/cut begins. You could switch between different capacitors to set different shelf frequencies, and this is common practice,  but putting a pot in between two caps as in your sketch also alters where the boost starts and the depth of the shelf. Quite difficult to see exactly what it would do to the response curves but it is probably nothing like what you intended.

Cheers

Ian

 

[MagnetoSound]

Quite difficult to see exactly what it would do to the response curves but it is probably nothing like what you intended.

Yes, exactly. You are effectively overlapping two different shelves and blending them together - while shifting them at the same time! Unpredictable to say the least. 

Who's to say it might not sound cool though? Or not. Gotta suck it and see.

 

[gnsdg]

Thank you so much, gentleman.  I may try it anyway just for fun, but that's the answer I was expecting: probably not what I'm looking for.

Very much appreciated!

 

[gnsdg]

So I suppose this should've been my first question, but why not just vary the value of the 620 ohm resistor to vary the frequency as opposed to switching caps in and out?

I'm looking at the formula for a first-order low pass:

fc = 1/[(2pi)RC]

Mathematically, varying the R or varying the C seem equally effective.  I assume I am just being naive.

 

[MagnetoSound]

So I suppose this should've been my first question, but why not just vary the value of the 620 ohm resistor to vary the frequency as opposed to switching caps in and out?

Well, as Ian says, the 620 ohm resistor with the series cap sets the frequency at which the shelf occurs and the maximum depth of the shelf.

Meaning, if you vary the 620R, it affects the amount of feedback available through the network and therefore the amount of boost/cut available at the 50k pot.

In other words, as you increase the resistance the corner frequency drops, but the shelf becomes less effective, and vice versa.

You may be fine with this as the lower turnover point means the filter becomes more audible at upper frequencies anyway, but you may find it frustrating if you cannot achieve as much manipulation in the 5k area as you can at 10k.

 

[gnsdg]

Dan, thank you.  I think I'll go ahead and try it.

 

[abbey road d enfer]

And I thought, why couldn't I just do this:

Because it simply doesn't work. The potentiometer between the two capacitors introduces too much resistance in the circuit, making the amplitude of boost & cut simply tiny. If you use a lower value for this pot, it doesn't provide enough "independance" between the caps, so they will be seen as a single, almost invariable entity.
This is the kind of things you can clearly see by doing a sim; I've done a quick sim and it shows that you have negligible frequency variation and/or very smal boost/cut and/or unusable intermediate positions.
You could make the frequency variable by replacing the 62k res of the midrange controlby a pot; changing frequency would also change BW but it my suit you (I've seen a couple of commercially available product using this scheme...).