Notch Filter - HELP

[Rayman]

I added a notch filter to a circuit I'm playing with.  I am using a 10k resistor in series with a 22k resistor.  I have a .01 cap in parallel with both resistors, and I have a cap to ground between the resistors.  At the output of the notch filter I am going to an outside lug of a pot.

In parallel with the notch filter I have a 33k resistor which goes to the other outside lug of the pot.  This is to even out the volume when I use the pot to go between notch filter and "no" notch filter.

I have tried a B50k and a B100K pot.  The problem with the B50K is that it lets too much of the path I'm not using bleed through.  The problem with increasing to B100K is that I get a dip in output level in the middle of the pot.

Does anyone have an idea on how to be able to dial in purely one path, the other, or a mixture of both with a constant output across the entire sweep of the pot?  I hope I explained this clear enough

[bioroids]

Hi!

Check out this tone control, may suit your needs. Is not totally constant volume, but still quite usable I think.

http://dedalofx.com/bioroids/ideas/idea_01.html

May be simpler ways though.

Luck

Miguel

[Rayman]

Thanks Miguel,

Do I have to use a transistor?  Will this work without inverting one of the signals?  In other words, just use this circuit without the FET.

Ray

[bioroids]

The way this circuit works needs the transistor

Where are you putting your circuit anyway?
What is before and after it? Maybe there's a simpler way of doing it taking into account the rest of the circuit.

Luck

Miguel

[Rayman]

I'm putting it on the end of a Vulcan stage.  There is a 2N5089 with Rc of 10K and Re of 1K.  Can I take off from the collector and emittier to get a phase split the same way as the FET?

[bioroids]

Yes you can, but the signal at the emmiter is about ten times weaker than at the collector. On the other hand the notch filter will attenuate the signal, so you should try it on the collector side.

Is there a capacitor at the emmiter, pointing to ground? If that's the case, it wont work because there will be no signal at the emmiter at all.

Miguel

[Rayman]

Miguel,

Actually I'm not using an emitter bypass cap so this might work.  I'll put the notch on the collector and re-bias to get the two signals closer.  If I go to 10k and 2K2 it will match what is on your schematic.  Hopefully I will still be able to get enough gain.

Thanks fo your help, this looks promising.

Ray

[bioroids]

Let us know how it goes!

Also, what is the impedance that follows the tone control? If it's too low, may have an impact on the behaviour too.

Luck

Miguel

[Rayman]

Impedance is something I seem to struggle with (figuring it out that is).  After the tone control I am going directly to the volume pot.  There is so much signal coming out of the last gain stage that a tone control on the end brings it down enough that I don't have to use a voltage divider at the output.

Ray

[bioroids]

Hi!

Consider that when you are at the middle of the rotation of the tone control, you have in fact a voltage divider between the 50K (asumming a 100K pot) and the value of the volume pot. Maybe using a 1M volume control gives you better results (if you are not using one actually).

Luck

Miguel

[Rayman]

Miguel

I added the filter off the last 2N5089 stage with the notch filter off the emitter and the mid boost filter off the collector.  I re-biased Rc and Re to match the filter levels.  It sounded pretty good. 

Next I tried both filters off the collector and for some reason the mid boost (Wein?) sounded amazing.  I then replaced the notch filter values with a 22k and 10K resistor.  Both bypassed with a .01 cap, and a .0047 cap to ground from between the resistors.  I added a 33K resistor before the pot on the notch filter side to balance out the volumes.  It sounds fantastic.  The mid boost I got from your web page really adds a buttery smoothness to the sound.  When I rotate to the notch filter I get a good metal sound. 

Thanks for your help.  This is exactly what I was looking for.

Ray

[bioroids]

Hi!

I'm glad it worked out!

How do you feel the level across the pot rotation?
Also, how is the sound at the middle of the pot? Is it more or less flat?

Luck!

Miguel

[Rayman]

the sound level is pretty even.  The middle of the pot sounds even, but for some reason I like the extreme ends of the pot sweep better than the sound sin between.  I wish I had a scope to see what was happening graphically.

Ray

[Rayman]

Miguel,

How did you get the Wien Bridge config in your schematic?  I'm very interested in how that is giving a mid boost.

Ray

[bioroids]

Hi!

I dont have too much knowledge about how it works, but to me looks like a low pass filter in series with a high pass filter. If you cut lows and highs you're left with the mids! Is not a real boost, as you are loosing signal, but as there is plenty of output from previous stages, it doesn't really matter.

It's mostly used to make sinewave oscilators (that's where I heard about it first time)

Regards

Miguel