can anybody here explain this audio circuit

[ankan]

hi everybody!!!!
i'am currently working on a set top box design.Recently am into the part to design the audio schematic for the box.I came across a reference design(check the attached file) for the audio using LM833dt.Can anybody explain how this preamplifier circuit (using resistors and capacitors ) works?I am facing a problem of low loudness in the output audio.Can i modify this circuit to increase loudness of the audio or is it a limitation of LM833?

will be highly obliged if anybody could answer.

[pacificblue]

This preamplifier expects a balanced input signal. It needs two virtual grounds, one because it is working from a single supply to reference the input signal to 1/2 the supply voltage and another to reference the inverted and non-inverted signal to. In the end those two virtual grounds have to be combined. The circuit formed by RA2-RA6 and CA3, CA5-CA7 does that.

If you want more loudness, you have to increase the gain. The gain setting resistors a RA9/RA6 for the right channel and RA12/RA8 for the left channel. If you increase the gain by increasing RA9 and RA12, you will have to adapt CA8 and CA9 accordingly. If you do it by decreasing RA6 and RA8, you will have to change RA5 and RA7 as well to maintain the virtual ground centered.

[gabor]

Hi,
Yes, it seems to be a simple op amp stage fed from differential signal that gets 150 Ohms load at the inputs. The amp gets assymetric psu with 12V DC that means the output would be at half of that (ie. 6V DC).
To prevent this, the designer made a half DC jump to + inputs to hold the output around 0V with a kind of HF filtering. In commercial designs the main reason to do this is to cut the cost but when it comes to non-commercial way (DIY) it is always better to make it as it should be, dual psu, etc. LM833 is another question, it is inferior compared to say, AD825 in sound quality and distortion if such things may have any importance in a set-top box.
Regards,

Paul G. Budapest Hungary

[gabor]

Ah, yes, you can modify it replacing the 300 Ohms resistors by a slightly higher value say, 430 R or 510Rgetting higher gain, but this also means a need of altering the input load resistors accordingly. However, it will work if you leave them as they are, as well.

Regards.

[ankan]

Thanks Qabor and Pacificblue for your valuable replies and feedback.

[ankan]

can i replace this dual opamp (LM833) with another dual opamp with good sound qaulity and low noise distortion at output without modifying the circuit.can you people suggest few of them .