The LPB-1, caps and frequencies?

[Beros]

Hi there everybody.

Im building the LPB-1. It's a nice booster but im thinking about tweaking it a bit.
I'd like it to be more of in between a treble booster and regular booster. I'd like it to cut more bass.

Should i switch both caps, or only the input cap?



Also, how wold the circuit react if i changed the 100k input resistor for a larger one?

What values would be good for it to cut around 200Khz? How do i calculate it myself?

[blooze_man]

I have built the LPB1 and the Screaming Bird. For the LPB1 I used .1uf and for the Screaming Bird I used .0022uf which is VERY trebly. So the only advice I have is to experiment with values in between.

[Beros]

I have built the LPB1 and the Screaming Bird. For the LPB1 I used .1uf and for the Screaming Bird I used .0022uf which is VERY trebly. So the only advice I have is to experiment with values in between.

Those two cicuits are very similar, but use different transistors. Does the transistor affect the tone very much? Or is the only real difference the caps?

[skiraly017]

I don't think I've ever seen a LPB-1 with those values.

[CynicalMan]

Should i switch both caps, or only the input cap?

If I were you, I'd only switch the input cap.

Also, how wold the circuit react if i changed the 100k input resistor for a larger one?

You'd get a bit more treble response (unless you're running a buffer or booster in front of the LPB), but it might start clipping. That resistor sets to transistor bias and if you increase it, the bias would go up and the top side of the signal might clip.

What values would be good for it to cut around 200Khz? How do i calculate it myself?

I don't think you mean 200kHz. That's 10 times higher than the limit of human hearing.
The equation to calculate the rolloff is 1/(2*pi*Cin*(R1||R2)) where R1 and R2 are the bias resistors. They are now at 830k||100k, or around 89k. To make things a bit easier:
www.muzique.com/schem/filter.htm
The cutoff is currently at 11.9Hz. You'd probably want it at around 500Hz, which would use a 4n capacitor (3.9n is the closest standard value). I suggest using that as a starting point and tweaking from there based on how it sounds.

[Beros]

Should i switch both caps, or only the input cap?

If I were you, I'd only switch the input cap.

Also, how wold the circuit react if i changed the 100k input resistor for a larger one?

You'd get a bit more treble response (unless you're running a buffer or booster in front of the LPB), but it might start clipping. That resistor sets to transistor bias and if you increase it, the bias would go up and the top side of the signal might clip.

What values would be good for it to cut around 200Khz? How do i calculate it myself?

I don't think you mean 200kHz. That's 10 times higher than the limit of human hearing.
The equation to calculate the rolloff is 1/(2*pi*Cin*(R1||R2)) where R1 and R2 are the bias resistors. They are now at 830k||100k, or around 89k. To make things a bit easier:
www.muzique.com/schem/filter.htm
The cutoff is currently at 11.9Hz. You'd probably want it at around 500Hz, which would use a 4n capacitor (3.9n is the closest standard value). I suggest using that as a starting point and tweaking from there based on how it sounds.

Thanks. Yeah, I didnt mean 200Khz, I ment 200 hz.

[earthtonesaudio]

The input resistance is the two input bias resistor and the resistance looking into the base, all in parallel.

The base resistance is approximately the transistor's beta times the emitter resistor, or about 108k.  So for the total resistance after the input cap you've got 108k||100k||830k which is about 49k.

With a .15u cap your corner frequency is around 20Hz.

If you lower the value of this cap you can increase the bass roll-off frequency by the formula:

cutoff freq = 1/(2*pi*R*C)


I personally would advise decreasing the input cap versus the output cap, because a smaller cap presents a higher series impedance, and raising the output impedance of the boost reduces its boosting ability.



Another thing to consider is that decreasing the input cap value increases the overall effect input impedance, so if you're the type of player who uses the guitar volume a lot, a higher input impedance means the change you get from the guitar's volume knob has more to do with resistive attenuation and less to do with loading on the pickup.  This makes a subtle, but noticeable change in how the volume pot "responds" to the circuit.  It will change the apparent taper of the pot.

[John Lyons]

I don't think I've ever seen a LPB-1 with those values.

I bet there are a bunch of others with other values as well.
After all, EH built them right.

[CynicalMan]

The base resistance is approximately the transistor's beta times the emitter resistor, or about 108k.  So for the total resistance after the input cap you've got 108k||100k||830k which is about 49k.

Sorry, I forgot about the base resistance. 
So for 200Hz, you'd need a 16nF capacitor. 15n would work.

[Quackzed]

you could use an input cap 'blend' pot.i added one to a different booster i made and its a good addition to any boost.
                            .001uf
                        ,----l l--------------.
                   ---l                  1uf    l---
                        '-0    0----0---l l---'
                           100k pot

100k pot wired as a variable resistor, adds from 0 to 100k resistance in series with the big cap and dials in how much bass gets in, so you can go from full frequency to treble boost or anywhere in between.

[mwynwood]

I've been messing around with my LPB-1 clone a bit... I ended up adding a switch that goes between:
(C1: 100nF C2: 100nF) and (C1: 2.2nF C2: 27nF)
I'm not sure if these are the best values, it sounds ok but it might be a little too 'bighty' still...
What do people recommend as good values for a nice treble boost?

[WGTP]

There are 2 easy ways for me to think about frequencies.  The "0" method and the "1/2 - Double" method which corresponds to octaves.

If 15n=200Hz, then 150n=20Hz and 1.5n=2000Hz.

If 15n=200hz then 30n=100Hz and 7.5n=400Hz.

Once you have a frequency to start with, you can figure it at other frequencies. 

[mwynwood]

Ah! That's really cool 
So is 15nF really 200Hz, or was that just for your example?
This is gonna be great.

By the way, here's how I hooked up the switch to select between two different sets of components:

[WGTP]

I used 15n from above.  

Impedance in these circuits is more complex than the simple formula, so things may not be exact, but it gives you a good idea about what's going on.  Once you hear it, you may decide you need something different.  Socket the strategic caps.  

[PRR]

> is 15nF really 200Hz,

No. Depends on Resistance the cap works with/against.

15nFd against 53K is 200Hz.

In this particular case, the input cap drives 100K, 820K, and the transistor. The combination works out to 50K-55K.

Other caps in other places in other circuits would be different.

With 5.3K, 15nF would be 2,000Hz.

What WGTP is telling you: use a likely cap. Listen. If you think the cap-effect should be an Octave higher (2X frequency), try a cap of half the value.

While you can also change resistors, very often the resistors are picked for other reasons. DC bias, interface impedance, ratio with available pot values. You can usually change capacitors freely/recklessly. You just need a concept of which-way (smaller larger) and how-much (2X per Octave).

[WGTP]

My latest scientific technique is leaning a single coild guitar up against my amp so it hums real good.  Then you can hear the changes the caps make.  Once I can no longer hear the changes either high or low, I can estimate the frequencies and tweak to taste.