Thank you very much Mr Ted.
This is indeed what I need, basic concepts.
I am everything but an educated diyer.
By definition opamps operate with DC on the inputs and outputs, that is the purpose of an opamp to amplify DC and they do it very well, the DC output is very predictable. For example at the input is a 100mV DC signal, the gain is 100x so the output is 10V DC.
When the input goes from +100mV to 0V DC the output also goes to 0VDC that is called a linear amplifier.
So if I wire the opamp as a buffer (no voltage gain) the offset at the input is the same offset at the output.
I use sources (dvd players) that usually have no output offset, being cap coupled at the output.
By the way i would like to confirm that the problem of caps in series with the signal has been "re-sized"
I have seen lab reports of cap coupled units passing quite well square wave. So caps indeed are not that bad for signals.
Anyway ,,
The general case of the DC circuit is no input capacitor and no output capacitor.
The DC supply to the opamp must be greater than the maximum output voltage required and less than the maximum allowed for the IC
I understand. OK.
The normal type of DC supply for the opamp is with a single DC supply, there are two connections, the positive DC supply and the ground of the DC supply.
The input signal and output signal use the DC ground as the input to output common connection.
Some opamps can also be used to amplify audio, which is a form of AC signal. But to begin you must add a DC bias voltage to the input signal equal to half the DC supply voltage. You must also arrange to eliminate DC gain while keeping AC gain. So if the DC supply is 20V, the input must be at 10V DC. This allows the output to swing between 0V DC and +20V DC. For example there is 100mV sine wave 1kHz at the input. The gain is 100x so the output is 10V sine wave. The output of the opamp is now two signals combined, it has a positive DC voltage equal to half the DC supply voltage and the 10V sine wave is superimposed on it. The DC voltage can be got rid of with a blocking capacitor.
This is not as I thought.
I believed dual supply was more common actually.
Nevertheless I am very interested in a single supply solution.
Maybe with one 12V SLA battery, or two for a 24V total.
Audio is not the general DC situation, it is a special case.
It is usual to operate the opamp with two DC supplies, one the symmetrical mirror of the other, and there is a third connection the midpoint of the two DC supplies at 0V DC.
For example +15V DC and -15V DC, the center point is also used as the input and output common.
Now signals can pass which have positive amplitude and negative amplitude, which is what audio is.
Ok. I had to read this one. Actually dual supplies are much more common for audio.
In this arrangement capacitors are not needed at the input or output, with one exception.
There are small DC voltages on the opamp input and output terminals, a few millivolts.
Often these are not a problem but they can cause noise on pots (volume control) and switches so even though the opamp does not need capacitors at the input and output they are sometimes included to stop noise from pots and switches.
Very interesting . Thank you for the advice.
So, as the input impedance of an opamp is usually quite high it would be better to put a small high quality film cap after the pot before the opamp.
Then I can measure the output offset to decide if I have to place another cap at the output.
By the way after all these very kind and useful advice I see the caps as more user friendly.
They are better than what I thought at the beginning,
When you look for opamps that are intended for audio use chose the ones that show operation from dual DC supplies, the symmetrical +/-15V DC arrangement is the most common in professional audio, +/- 12V or +/-10V in hifi sometimes.
Avoid the opamps that are intended for use with single DC supplies, (it says so in the technical description)
OK. Only dual supply intended OAs then. All right.
There is whole new generation of opamps specially for audio with logic chips that use single DC supply +3 or +5V DC.
For these applications capacitors are sometimes needed to block DC
well, I think this is too complex for me.
To be clear I was thinking to start with a nice NE5534 used as unity gain buffer with a nice attenuator in front of it to drive properly my power amp.
The only thing "high-endish" would be a dual supply built around two 12V sla rechargeable batteries.
I select this opamp on the basis of these consideration:
1) it can be found even in series at the signal in very well reviewed units, like dacs and preamps. So it must be good for audio
2) it draws very little current so the batteries' charge should last a bit
3) last but not least is quite cheap. Actually unbelievably cheap.
Hope this helps
Ted
Dear Mr Ted, your very kind advice has helped me a lot indeed.
It prevents me to do the wrong things at the start.
Sometimes I read about the limitation of opamps, but I am not that convinced.
I see more and more units (dvd players, dacs, preamps, etc.) that used them and that have very well performance during listening tests.
Thank you very much indeed.
Kindest regards,
beppe
P.S. as i am about to leave i will be able to reply only by friday evening.
I have no access to the web during the working days.
Thanks to everyone.
