# Need help with guitar amp diagram

Discussion in 'The Projects Forum' started by dingari, Feb 19, 2014.

1. ### dingari Thread Starter New Member

Jan 25, 2014
10
0
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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2. ### Veracohr Well-Known Member

Jan 3, 2011
559
77
I'd think you'd want the filter after the distortion, not before it.

3. ### dingari Thread Starter New Member

Jan 25, 2014
10
0
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?

4. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
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.

5. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
I think U1A needs some value of resistance from the + input to ground or it won't work.