8w practice amp tone control

[El Heisenberg]

I built this amp a while ago, and decided to add a tone control to it. I just stuck it in there not really knowing what Im doing. I had to up the gain of the circuit by upping R2 in the amp section to 2.7 instead of 780 ohms, where I had it before. It works, but the last part of the rotation is where I get most of the usable sound. when it's backed off, there is alot of treble. I want to tweak it to be more usable, but since I built it in point-to-point, it would be helpfull if someone could maybe tell me what is wrong with the design, or help me to understand what's going on with the tone control. Maybe I should move the tone control?

The amp is just craig andertons 8 watt, using a TDA2003, and 18v of DC power. I used a fetzer valve for the preamp. here's the schem:

http://i633.photobucket.com/albums/uu56/el_heisenberg/amp.jpg

[El Heisenberg]

Also, in the origibal schem, R3 is 22-270 ohms. R2 is 220-4k7. What does raising or lowering R3 do???

[davidallancole]

If you could change the volume pot to a 10k log and put a buffer between the tone and volume pot, it would probably work better for you.  Also take out R1, its not required.  It is loading down the volume pot and changing its logarithmic curve.  As well, the volume pot is loading down the tone pot and changing its curve as well.  Usually loads on pots should be ~10 times larger in resistance than the pot itself.

[El Heisenberg]

Ok ill try that

[El Heisenberg]

Is this good for the buffer? My power supply is 12-18 volts so will this work on that higher voltage?

http://www.generalguitargadgets.com/pdf/ggg_buffer_jfet_sc.pdf


Actually my tone control was in front of the fetzer valve. I kept switchijgbit back forth from before and after the fetzer valve and forgot where id left it. Changing the pot to 10k totally fixed the response of the tone control. I also took R1 out, and that fixed the volume pots response. I dont need the buffer if the tone control is before the fetzer valve right?

[brett]

Hi

I dont need the buffer if the tone control is before the fetzer valve right?

Not necessarily.  I'd use a JFET buffer to be sure.  The one you show is ok, but I'd delete R1 and R6 and make R2 lower.  R2 sets the input impedance.  Somewhere between 220k and 1M makes for a quieter front end.  High impedances increase the pickup of interference.  330k is a good compromise between treble loss and noise gain IMO.
cheers

[davidallancole]

Thats correct El.  If your tone control is before your Fetzer and your happy with it there, then you don't need a buffer, but if you are running your guitar straight into the tone controls, you might want to buffer the guitar signal before it goes into the tone controls.

[Gus]

Find the duncan tone sim.  Model the tone control you have.  Make sure you adjust the source and load resistance for the BMP type tone control.

NS app note 32
bottom of page 8
http://www.national.com/an/AN/AN-32.pdf