# Need help with guitar amp diagram

#### dingari

Joined Jan 25, 2014
10
Hi guys, I am building this portable headphone guitar amp as a project for school. I am stuck on choosing values for the capacitors though (just ignore the values I have assigned).

The lowest string on a guitar is about 70Hz so anything above that should be let through. How should I choose the capacitors? The idea with the pot and capacitor next to the input is to be able to control tone by shunting high frequencies to ground (like a guitar tone control). Any ideas on a better tone control?

Anyway, could you guys look at my circuit and advise me? Thanks in advance

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#### Veracohr

Joined Jan 3, 2011
695
I'd think you'd want the filter after the distortion, not before it.

#### dingari

Joined Jan 25, 2014
10
Thanks for the reply, I will try that. But I'm wondering about how the rest of the circuitry will affect the choice of capacitor value?

#### bountyhunter

Joined Sep 7, 2009
2,512
Thanks for the reply, I will try that. But I'm wondering about how the rest of the circuitry will affect the choice of capacitor value?
In audio amps:

anytime you have a coupling cap in the signal path (like C3 in yours), the low frequency corner pole (which is the -3dB point for low freq rolloff) is calculated only by knowing whatever resistance or impedance value follows the cap. Then, you calculate the corner freq by setting that resistance equal to the impedance of the cap which is:

Xc = 1 / ( 2 x pi x freq x c )

When Xc equals the resistance after the cap, that is the frequency where the low freq gain is -3 dB. You can size the cap to set that corner where you want it.

#### bountyhunter

Joined Sep 7, 2009
2,512
Hi guys, I am building this portable headphone guitar amp as a project for school. I am stuck on choosing values for the capacitors though (just ignore the values I have assigned).

The lowest string on a guitar is about 70Hz so anything above that should be let through. How should I choose the capacitors? The idea with the pot and capacitor next to the input is to be able to control tone by shunting high frequencies to ground (like a guitar tone control). Any ideas on a better tone control?

Anyway, could you guys look at my circuit and advise me? Thanks in advance
I think U1A needs some value of resistance from the + input to ground or it won't work.