DC offset and gain on GC LM3886

[avbuilder]

Buy these.

http://www.ebay.com.my/itm/320802386...84.m1497.l2649

[pacificblue]

You have two issues. One is that the amps input bias current is connected to the potentiometer. As any current through a resistance produces a voltage drop across it, you get varying DC voltage as you turn the potentiometer. The solution for this is to use the input cap between potentiometer and amp, and to use an input resistor to ground that provides a stable path for the bias current to ground.

Now your non-inverting input has a fixed DC offset and you need to make sure that this is not amplified. For that purpose you need another capacitor in the feedback loop, which is called Ci in the datasheet.

[woody]

I doubt that 32 mv on the output will be any cause for concern. Just think about it
how much power would that be into a 8 ohm speaker ? Just for fun do the math and
I think you will agree.

[AndrewT]

I'll do the maths for you.
32mVdc into 6r0 (assumed DCR) is ~0.17mW

Maximum amplifier output is assumed to be 28.3Vac (100W into 8r0).
Normal average listening level is ~1W allowing for a 20dB overhead for unclipped transient peaks.

A quiet passage during that listening session could be 20dB below your 1W average and the S/N ratio down there would be just 0.17mW/10mW = 17.7dB

The 32mV offset, when thought of as a noise source, is only 17dB below a quiet passage on a normal recording.

Very quiet passages or lower than 1W average listening could easily bring the wanted music signal to a level that is lower than the offset voltage.
Yes, the amplitude of the quieter passages in the music can be considerably lower than the displacement due to a 32mVdc output offset.

[Xerxes75]

Before you think where is hè gone? I am still here but because of my illness it is not my week so to say. But i am reading your Posts and i am thinking about it. I also ordered some things on THE internet. So that i van try THE things you mentioned.

I can't wait

[pacificblue]

Originally Posted by woody

I doubt that 32 mv on the output will be any cause for concern.

Will it destroy the amp or the speaker? No.

Will the DC offset be audible? No.

Will it have audible effects? Yes.

It will displace the woofer or full-range speaker from its center. Excursion will not be symmetric anymore, leading to higher distortion from the speakers.

It will not displace the other drivers in a passive multi-way speaker, because the crossover will filter DC components out for them. The woofer will be misaligned in relation to the other drivers, producing phase shifts, leading to a more rippled frequency response around the crossing frequency, if you don't redesign the crossover. The smaller the woofer and the higher the crossing frequency, the worse is the effect.

It heats up the woofer coil, leading to power compression. As a result the woofer will produce less SPL for the same voltage, making it quieter than the other drivers in a multi-way system. Again the crossover needs to be adapted to the new situation.

[Xerxes75]

So until now if have the following:

Is everything in the right place
Yes
Place a log potentiometer
An Alps 50k is going to be placed
Increase the resistor R3 (680=original)
Trying different once starting at 1k
Place a coupling cap
Laying around are some 2mfd ones
Buy speaker protection
Ordered the ones from EBay
Place the feedback cap at the Ci position
These came with the kit I will put them in place

From one of the technicians off a high end audio store I got a tip to place to Allen and Bradley resistors to middle the input...

A circuit is coming

[Xerxes75]

I dismantled the hole thing and put I all back together with the writing above.
And it works!!

The dc offset is now 0.1 that is mV... The problems with the gain are also sort off over.

The problem now is that the amp has is bit to low on gain and it sounds a bit dull. But it is all detailed. Mmmhh perhaps a few tweaks will do....

[GloBug]

The volume pot will suck tone. Get rid of it unless you need it. Most sources will have a volume control.

It's not worth the loss of tone unless you really need a volume control.

For future reference, the higher ohm pots will suck less tone, but will still suck tone nonetheless.