practice amp problem. Help needed

[jogina111]

I built a power amp from a tda2003 and its working. Now, I'm into putting a preamp for it. Maybe a simple opamp buffer to tonestack to opamp booster. Do I need to add another power supply for the preamp or just stick both grounds and +V together?   I've tried to put in a ROG fetzer valve but it resulted in a terrible hum...any suggestions?

[duck_arse]

try de-coupling the supply for the front end from the power stage. use a resistor, somewhere between 470R and 2k2 (?) between the positive supply for the amp and the positive supply of the pre-amp section, and add 220uF ~ 1000uF (? again) between your V+pre and ground. do for both V+ and V- if using split supplies. you can also beef-up whatever cap you have on any Vref lines you might have in the preamp section.

[jogina111]

dang! I cant visualize it. Mind if I request for a visual aid?

[GibsonGM]

V+ supply >>>>Preamp V+ -----/\/\/\/\470-2.2k resistor-------> to power amp
                  |
          220u-1000u
         Cap to ground
         "+" goes on 'top', toward V+
Hope that helps.

[duck_arse]

I woulda .....

V+supply >>>> power amp V+
      |
      |
      -----/VVV\------|--------> pre amp V+
         resistor        |
                           ==  electro
                            |
                            |
                           ground

but I'm not an expert.

[jogina111]

WHOAA! It made a significant loss in hum.. And WHOAA! there's still hum when  volume pot is about 50%..I've used a 470uf cap and 220k resistor... Can it be my power supply that is noisy?

[tca]

Try a 100 Ohm resistor and 100micro cap instead.

[GibsonGM]

Try a 100 Ohm resistor and 100micro cap instead.

+1

Glad to hear it's quieting down for you!

PS - yes, the way duck arse suggested running it is ok, too (in parallel) - in fact, I probably should have suggested that way.  The 'series' run is fine for pedals, but it you're pulling actual current, you might be better off his way.   Whichever one works best, is my rule   

[jogina111]

and that solved my problem.. Thanks guys. Whats this method called btw?

[tca]

I don't think there is a name for it... it is just a RC decouple network.

Glad it works.

Cheers.