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Boost Low Frequencies

audioguru

Well-Known Member
Most Helpful Member
40kHz is not audio, instead it is an ultrasonic frequency that some dogs might hear. The 6.75" woofer might produce a very faint 40Hz.
A speaker specification should say +/- 2dB from 20Hz to 20kHz.

I have seen some 3-way speakers that have a photo of a tweeter instead of a real ultra-tweeter.
 

audioguru

Well-Known Member
Most Helpful Member
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.
 

Suraj143

Active Member
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.
You two are correct.The boost is un-noticable.I am repeatedly changing values and make checking...
 

Suraj143

Active Member
Hi,

I have a small doubt how this happens.In below circuit the overoll gain is 1.42 (Non bass gain). But in simulator shows 3dB. 3dB is the gain of 2.How this 1.42 gain comes to 3dB?

Thanks
 

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audioguru

Well-Known Member
Most Helpful Member
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.
 

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Suraj143

Active Member
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.
 

Suraj143

Active Member
Hi,

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??

1613616727928.png
 

audioguru

Well-Known Member
Most Helpful Member
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.
 

Suraj143

Active Member
Hi sorry for messing up.My amplifier is TDA7375 in dual bridged mode configuration.

I was reading an article which explains the gain in chip amps.But it uses STA540.But I use TDA7375.

Everything is clear now except below equation.I cannot understand the formula.

V= \/8*4
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Well if P=(V^2)/R, then P*R = V^2; so V=sqrt(P*R); V=sqrt(8*4); V=sqrt(32)

Not sure where the 8 ohms = 4 ohms bridged comes from.

Bridging increases the voltage. It does not increase the current. You really need a 4 ohm specification for 8 ohms bridged.

Audio amps are wierd in the sense that R, I and V could constrain the maximum output. You cn make R=1 ohm, but the amp probably can;t drive 1 ohm. Bridging, usually keeps R, the same as unbridged.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Not sure where the 8 ohms = 4 ohms bridged comes from.
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.
 

audioguru

Well-Known Member
Most Helpful Member
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.
 

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