Input Capacitor choice vs frequency (LPB II)

[bdevlin]

For a simple booster circuit, like the LPB I built (link below), isn't there a basic formula that would tell you what the frequency rolloff would be for a given capacitor?
My LPB is not a "clean" boost.  It boosts the bass too much.  So far, I tried a .022 uF input and that still was not quite to the sweet spot.  Do I really need to change the output cap as well?

http://www.generalguitargadgets.com/index.php?+option=content&task=view&id=72&Itemid=103

[petemoore]

For a simple booster circuit, like the LPB I built (link below), isn't there a basic formula that would tell you what the frequency rolloff would be for a given capacitor?
My LPB is not a "clean" boost.  It boosts the bass too much.  So far, I tried a .022 uF input and that still was not quite to the sweet spot.  Do I really need to change the output cap as well?
  One way to find the capacitor which suits the application is socket the cap.
  For smaller values though, pulling the existing cap isn't necessary, a second series cap in that position can do the same rolloff as a 1 smaller capacitor.
  This also offers the option to jumper either of the two capacitors for the value of the one left doing the DC blocking.
  Done at the circuit input [say right in the wire or on the bypass switch, perhaps on an added switch for input cap values switching], means no need to lift to access the bottom of the board and do surgery.
  As far as the math...yes it exists. Whether the math vs. sound  translates into the value you would choose by ear for tone [recommended], is different than being able to work the equations for other practical reasons [I would have to study the equations and then ballpark, then compare to the sum I found to see if I think I worked them correctly...

[ZeroGravity]

If you really want to do the math, here goes:

For a desired frequency response, the value of the cap depends on the impedance that is following.
The basic formula to calculate the frequency roll-off point is   f=1/(2*pi*R*C).
In the case of your booster schematic, the impedance (R in the formula) is formed by R1, R2 and R4*hFE. (hFE is the amplification factor of the transistor)
You'll have to parallel those resistor values first. So R=1/[(1/R1) + (1/R2) + (1/R4*hFE)].
hFE can be looked up in de datasheet of the transistor.
Now you have all variables to complete the basic formula for the frequency.
To calculate the value of the cap: C=1/(2*pi*f*R).

It's best to leave the output cap as it is. That way you are sure the frequency response doesn't change too much depending on what you connect on the output.

[bdevlin]

Very cool to play with the formula to see how different values for the different components change the frequency.  So my calculation shows the frequency at 52Hz based on the values in the schematic.  I have not verified my actual values.  Last night I popped in a .01 uF.  That worked but perhaps a bit too much.

Thanks! 

[soulsonic]

what you really need to do is add a scalarizer!

[Krinor]

For a simple booster circuit, like the LPB I built (link below), isn't there a basic formula that would tell you what the frequency rolloff would be for a given capacitor?
My LPB is not a "clean" boost.  It boosts the bass too much.  So far, I tried a .022 uF input and that still was not quite to the sweet spot.  Do I really need to change the output cap as well?

http://www.generalguitargadgets.com/index.php?+option=content&task=view&id=72&Itemid=103

My entry into pedal building was the need for "something" which would drive the 12ax7 of my Champ amp into overdrive at low volume. I soon discovered the LPB-2 at GGG. After finishing it I felt it had a bit too much low end (just like yours bdevlin), but I didn't want to modify it so simply I built another. In this second pedal I put sockets for both the input and output caps and also for the transistor. Instead of "doing the maths" I finetuned the sound by ear/emotion by switching caps and transistors untill I found exactely the tone and frequenzy respons I wanted. In retrospect I realize that this was a very rewarding beginners experience, and I highly recommend it to others who are just starting out.

[aron]

>what you really need to do is add a scalarizer!

No, I looked this up and someone tried it and it reduced highs. :-)