Boost with HPF and LPF design

[subvader12]

Greetings everyone!

I'm actually designing a simple transistor booster with two filters after the amplification stage, one high-pass and one low-pass but I'm wondering what are some good cut off frequencies for each one of them? I know how to calculate it and how the cap-res and res-cap configuration work, but I just want a good range for it. Can you help me choosing the frequencies for each to make it versatile and have a good sound? I can experiment but any starting point would be appreciated! Thanks in advance! 

[ElectricDruid]

We can probably suggest some reasonable cutoff frequencies, but we need to know what you're aiming for! What is the *point* of these filters? Why do you need them at all? What sound are you after?

You can remove some bass with the highpass. At a minimum, you could use that to help reduce mains hum in your signal (but it won't make any odds if it's your amp that hums). You can remove treble with the lowpass. Once that goes below 12-15KHz or so, you'll start to hear the "brightness" of the guitar's tone removed. This can actually help if you're feeding some pedals that add more top end like drives or fuzzes.

So..what do you want your filters to achieve?

[antonis]

with two filters after the amplification stage, one high-pass and one low-pass

You must have in mind (in case you already haven't..) the fact that 2 cascaded passive filters exhibit interaction related to their individual roll-off frequencies..

[anotherjim]

LPF cut below 300Hz.
HPF cut over 700Hz (surprisingly low), but with a single RC filter, the cut off slope is so shallow it must start early and any compression of the signal afterwards will tend to undo it (level out the full bandwidth).
A dumb approach is to use the BMP model in the Duncantonestack calculator. That's LP and HP with a blend pot in between.

[idy]

It's often a good idea to filter out the bass before the gain stage (your transistor) because all those big waves tend to swamp the circuit. You get more gain before you get distortion.
Then cutting highs after also cuts any noise from the active device.

These two ideas mean not having to worry about interaction.

[subvader12]

We can probably suggest some reasonable cutoff frequencies, but we need to know what you're aiming for! What is the *point* of these filters? Why do you need them at all? What sound are you after?

You can remove some bass with the highpass. At a minimum, you could use that to help reduce mains hum in your signal (but it won't make any odds if it's your amp that hums). You can remove treble with the lowpass. Once that goes below 12-15KHz or so, you'll start to hear the "brightness" of the guitar's tone removed. This can actually help if you're feeding some pedals that add more top end like drives or fuzzes.

So..what do you want your filters to achieve?

Alright, for what I see, the tone control in any pedal is made filtering frequencies to make the sound brighter or darker. I'm implementing two filters to have the opportunity to control highs and lows independently with pots, am I getting this wrong?

And yes, it'll be feeding another drive/dist I built, but coming with more gain from this boost stage, and with the mentioned tone control. I'm making one filter just after the other, will idy's idea of having the HPF before the transistor, and LPF after the amplification make it viable?

[subvader12]

LPF cut below 300Hz.
HPF cut over 700Hz (surprisingly low), but with a single RC filter, the cut off slope is so shallow it must start early and any compression of the signal afterwards will tend to undo it (level out the full bandwidth).
A dumb approach is to use the BMP model in the Duncantonestack calculator. That's LP and HP with a blend pot in between.

I just searched for that model you mention and seems just what I want to achieve, the ability to cut highs and lows, and that blend is a plus I could implement. Having this in mind, will this be a better approach in terms of practicality, compared to simple RC and CR configurations in the circuit?

Also what idy says seems interesting to test.

[idy]

One way to work the high pass is with a pot panning between two input caps, one tiny, one big.

Easiest way to satisfy yourself and make a piece of useful kit for future experiments is to get a (couple of) rotary switch(es) and solder a selection of caps to it. I put mine inside of cans. With a label to remind me what values re inside. Whenever you have this kind of question (what's the best cap for my guitar tone control?) you just  pop this in. Click click click till you find the value that works....