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Your new boost of only 4.2dB for 40Hz is barely noticeable and only 2dB for highs is not noticeable.
Your 220nF C2 feeding the 27k R3 resistor cuts 27Hz at 3dB so if C2 is increased to 470nF then the bass will be boosted more and go to deeper frequencies.
3dB is a gain of 1.414 times which is the root of two. 3dB is a power gain of 2 times.
6dB is a voltage gain of 2 times and a power gain of 4 times.
You probably want a lower cutoff frequency but since your resistor values are low then the capacitor C2 value will be high and be very large (film type) and more expensive. Your signal source might not be able to produce low frequencies when feeding the low value of only 12k ohms.
Thank you very much.Thats what I wanted to know.The reason is after the boosting part the signal will goto tone control circuit.and after it will goto TDA amplifier.so the input weak signals may weaker more if I do not amplify a bit.
I'm repeatedly reading articles to understand how to calculate the gain in chip amps. In this article it has come closer what I expect. But I cannot understand what is this highlighted formula..!! Never heard such a formula with a square root??
In post #12 you said my bass boost circuit in post #7 caused clipping when there was bass. Then the power amplifier does not have enough output power.
You need a lot of bass boost and you are hoping that your cheap crappy speakers will produce some bass. Maybe they will burn out instead.
If your speakers are 8 ohms and the STA540 amplifier is bridged with a 12V supply then the 6W to 7W at low distortion is a fairly low power.
As far as loading on the amp goes, each half of the amp effectively feeds half of the speaker - so with an 8 ohm speaker each amp is feeding 4 ohms.
A bridged amp doesn't give you any more power - it simply alters the impedance of the speaker you can feed.
It's probably simpler if you think of it this way.
Get a stereo amp, 10W per channel and two 4 ohm speakers. Wire it as stereo, you have a total of 20W. Now bridge the amplifier, and wire the two speakers in series (giving 8 ohms), you now have the exact same 20W, and the exact same 10W in each speaker.
Using a 14.4V charging car battery and a 4 ohms speaker, a single-ended TDA7375 amplifier produces 5.5W at 1% distortion or 5.2W when clipping begins.
Bridged into 4 ohms produces 20W at 1% distortion or 17W when clipping begins.
The datasheet does not mention 8 ohm speakers so I guess with 8 ohm speakers the power loss will be less and the power will be about 0.55 x 5.2W= 2.86W at 1% distortion single-ended or 0.55 x 17W= 9.35W when bridged and when clipping begins. Fairly low power.
Of course the speaker current increases when the bridging increases the speaker voltage, causing the output power to be almost 4 times higher when amplifiers are bridged.