What's that C for?

[Bassterror]

http://www.audio.webd.pl/pdf/lm4780_sch.pdf

C1 and 2.

[Bazukaz]

DC blocking caps.

[davidallancole]

These capacitors will bring your DC gain down to unity.

If you removed these from the circuit, then with a 1 mV DC offset on your input signal, the amp would amplify it 32 times so you have a DC offset of 32 mV at your output.

So with these capacitors in circuit your gain would be 1 at DC, so 1 mV offset at the input would corolate to a 1 mV offset at your output.

[Bassterror]

Thank you

So if I have an input cap, i do not need them, right?

[davidallancole]

I think you could get away with that.

[ifrythings]

Originally posted by Bassterror
Thank you

So if I have an input cap, i do not need them, right?

It depends also, if you have large DC offset at the output put one in, it also may add sability to the amp so it doesn't oscilate.(Anyone care to verify?)

Simple addvice, try it with out and if you have problems add it later.

[Bassterror]

I'll try.
Another thing:
What do you think is going to have a greater effect on sound, the input cap or the mentioned one?

Is it worth to use panasonic FCs at this place?


greetings,
Stephan

[AndrewT]

Hi,
the input cap you require is quite small, 1uF to 10uF, so can be Polypropylene or smaller and cheaper use Polyester.

The NFB cap needs to be a lot bigger and you have to use an electrolytic.

On that basis, I think the NFB cap will have more influence on the sound.

The NFB cap, as said before, reduces the amp DC gain to one (1). but the output offset will vary with temperature.

You may find that the offset will vary with ambient conditions, with signal output level, and with heatsink temperature. The designer may have made a good job of compensating for these and reduced offset drift to neglible proportions, but be careful to check the offset regularly in the early days.

If you fit an input DC blocking cap, remember to put in ground referencing rsistors on BOTH sides of the cap. The one on the RCA input side can be 1M0 to 2M2 to RCA ground. The one after the cap is your input impedance setting resistor and should match the upper leg of the NFB loop i.e. r1=r6.
I note that the PCB does not attempt to balance the impedances seen by the inverting and non-inverting inputs. This unbalance will exaggerate output offset drift.

[davidallancole]

I am only asking because I don't know, but what is the purpose of putting resistors on both sides of the input capacitor?

[lohk]

The sound difference with or without the FB cap is huge. I tried all variants, but a cap in the position always sounds worse than no cap at all. If you have no input offset voltage the "three resistor version" (Rin, Rfbg, Rfb) is not to beat.