Bridged lm3886 suggestions Please

[Scott_drake]

Hi -

Thanks, could you please tell me what inverting topolgy is, in regards to the passive pot?

I know a higher resistance pot would cause less of a problem to the source signal. But is the promblem to do with the signal impedance seen by the power amp?

The source will see the impedance of the pot in parallel with Rin of the amp. So surely if i keep both impedances as high as possible then it would be ok?

I am obviously wrong so please tell me where i am?

I have attatched some pics to show you the ways i would have configured it, please tell me which one is better?

[Scott_drake]

Second pic

[AndrewT]

Hi,
both your posts show non-inverting topology.
Both show DC block on the NFB leg.
Both show DC coupling on the input.
The result is that the inputs of both amps see each others impedance and see the pot. The impedance of the pot varies with wiper position.
At the top, maximum volume, the amp input sees the source impedance in series with the DC blocking cap//potvalue//Zin (if fitted).
At the bottom, minimum volume, the amp sees the zero ohms (0r0) // all the others but they are effectively irrelevant.
At very near the middle, about -6db on volume, the amp input sees [potvalue/4]//Zin (if fitted).
The range of resistances seen by the non-inverting input of the amp is from 0r0 to 12k5 (for a 50k pot).
The inverting input sees 20k//1k+series cap.
The unbalanced resistances cause the input offset current to generate an input offset voltage, which also appears at the output as a DC output offset.
As you adjust the volume pot the output offset varies.
The way to cure this is to DC block the input after the pot. The amp then sees Zin//other amp input impedance.
The amp as shown has no RF filter on it's input. That 1k0 is crying out for you to fit a single pole RC filter there on each amp. You can choose anywhere between 220pF and 1.5nF and adjust until you hear the correct amount of treble for your ears and speakers.

Now that you have the filter in place, you will find that it's effect varies as the source impedance (not resistance) increases from zero. At zero volume or maximum volume, the filter is 1k0+C,
at -6db volume the filter is 12k5+1k0 =13k5+C. It has moved down the frequency range by a factor of 13.5 times. This is effectively a variable treble cut filter controlled by the passive pre pot position.

Select a pot <<20% of Zin to ameliorate the variable effect. For Zin~=50k I would try a 2k pot. This presents a maximum source impedance of 500ohms. The RF filter now varies from 1k0 to 1k5 +C. Much better.

Now, can your source drive 2k properly and still produce good quality sound?

Sorry this is so long winded.

[Scott_drake]

It sounds like i might be best to place a buffer between the pot and the amp, then the source will only see the 50k impedance of the pot, and the amp will only see one common low impedance source.

What are some reccomended op-amps or circuits to use for this. Is it a good idea to have some gain in the buffer circuit or just a standard unity gain buffer and leave all the gain to the power amp?

Thanks
Scott

[AndrewT]

Originally posted by Scott_drake
......It sounds like i might be best to place a buffer between the pot and the amp......

Hi,
not necessarily.
It might be better to use a passive pre-amp.
Just use the lowest pot value that your source can drive well and keep the cables low capacitance and short.
Listening to this low cost set up may convince you not to start experimenting with IC and discrete opamps.

[Scott_drake]

As far as my source driving capabilities, i have a audigy sound card which should be able to drive anything as it can drive headphones. But my main concern is my cd player, i am using the commonly used playstation 1 model spch-1002 with modded output. The outputs come directly from the DAC through 3.3uf wima capacitors so i doubt it will be able to drive very much current at all.

What do you reccomend for lowest impedance for this type of setup, i have no idea of the current capabilities from a DAC. I would like to be able to connect the amp to any source without worrying about what can drive it.