Noob ? BrianGT Lm 3875 with 10k pot?

[bender460]

Sorry for the noob question but I have built the amps Lm 3875 basic kit witch work good with a 100k "radioshack pot". I have some 10k pots I would like to try but do not get much volume. What do I need to change? I did try to search for it but all the formulas concerning gain I am lost. Any help would be great.

 

[carlosfm]

You don't get much volume with a 10k pot because you have a 22k resistor on the PCB, from NI input to ground.
Change that resistor for 100k.

You will have better results with a 10k pot than with a 100k pot IMHO.

 

[bender460]

Thanks for the response . The 22k is where I have the pot (R2) no resistor just the pot. R1 is a 220ohm. Do I need the pot wired in different?

 

[mcs]

How is it wired now?

You should have one end to the input, the other end to ground and the wiper to the chip input pin.

Best regards,

Mikkel C. Simonsen

 

[bender460]

Mikkel , that is how I have it wired in. I still havn't figured it out.

 

[Vikash]

I'm using a 10k pot and I've left the 22k from NI to ground as is without any volume issues.

Link to pot wiring just in case.

 

[carlosfm]

I suspect there may be a mistake with the gain setting resistors on the chip.

 

[bender460]

I'm using the pot as a preamp, with my cd player connected directly. If I leave the 22k to ground I get no control with the pot. VERY loud (like full volume), works fine with my preamp with the 22k but not with the pot alone. If one of you could post a diagram that would be great "pics are worth a thousand words espacialy for newbies like me. Thanks again guys.

 

[Vikash]

have you connected something in place of the removed 22k resistor?

 

[carlosfm]

There's definitely (or so it seams) a mistake with the way you wired the pot.
Can someone make a sketch, as I don't have the time now?
A new amp is calling me, it's sounding good.

 

[bender460]

I replaced the 22k resistor with the pot. There is a 220ohm resistor after the pot in line as per Brian's diagram. The 100k pot works fine this way but the 10k there is not enough volume.

 

[mcs]

What is your source?

If your source has a high output impedance it may not be able to handle a 10k load.

Best regards,

Mikkel C. Simonsen

 

[bender460]

Source is a cheap sony cd player. Is it the feedback resistor I need to change?

Thanks for the help guys I feel a solution coming soon.

 

[bender460]

Well I've been searching and searching, reading and reading I've found one schemaitic take a look and tell me what you think. Its a little different than brians. Brians has a 220ohm at RB "input" and also Ri is 620ohm. Also there is no Ci 22uf cap. I really would like the best sound I can get out of them. I hope the pic is attached.