Pre Amp with Boost

Suraj143 · Jan 28, 2023

I have made a tone control preamp for my TDA7375A chip.It consist with a bass boost.I have noticed the boost does not effect a noticable different when boost ON/OFF.See the simulated graph.May be my speakers does not respond it..Any idea?

My speakers-Philips BTD7170

Speaker drivers
5.25" woofer
Loudspeaker types
Dome tweeter
Main Speaker
2 way

crutschow · Jan 28, 2023

What do the two graphs show?

A 5.25" woofer is not big enough to move much air at low frequencies, so you are not going to get a lot of bass from that.
You need at least a 10" woofer for reasonable bass.

Suraj143 · Jan 28, 2023

crutschow said:
What do the two graphs show?

A 5.25" woofer is not big enough to move much air at low frequencies, so you are not going to get a lot of bass from that.
You need at least a 10" woofer for reasonable bass.

Hi thanks.

The 1st graph is boost ON,
The 2nd graph is boost OFF.

Nigel Goodwin · Jan 29, 2023

Suraj143 said:
Hi thanks.

The 1st graph is boost ON,
The 2nd graph is boost OFF.

Well it's only about 6db boost, which isn't much, seeing as 3db is the smallest change you can hear.

Your speakers don't sound like they are much good either, but those are exactly the kind of speakers that benefit from a bass boost.

ronsimpson · Jan 29, 2023

Suraj143 said:
5.25" woofer

Your speakers probably can't produce sound below 100hz.

crutschow · Jan 29, 2023

Can you post a complete schematic of the boost circuit and the TDA7375A as you have them connected from input to speaker output?
The low-frequency rolloff without the boost seems excessive.

Suraj143 · Jan 29, 2023

crutschow said:
Can you post a complete schematic of the boost circuit and the TDA7375A as you have them connected from input to speaker output?
The low-frequency rolloff without the boost seems excessive.

Hi,
Here it is

And here is the amp

Suraj143 · Jan 29, 2023

The speakers are better than I expect.

Nigel Goodwin · Jan 29, 2023

Suraj143 said:
Hi,
Here it is View attachment 140195

And here is the amp

C5 seems rather low, as the preamp has a unusually low input impedance of only 12K, so this will start the bass rolling off lower than you'd like. Personally I'd probably increase the 12K to 120K, and change the feedback components accordingly, to give it a more sensible input impedance.

crutschow · Jan 29, 2023

Nigel Goodwin said:
C5 seems rather low, as the preamp has a unusually low input impedance of only 12K

Yes, two of the coupling capacitors combine to noticeably reduce the low frequency response.
Suggest C5 be increased to at least 2µF and C1 be increased to at least 5µF.

audioguru · Jan 29, 2023

I agree that your 5.25" woofers are too small for good bass.
The specs you attached of the audio system shows +/-3dB at 30Hz from the amplifier output but not one word about the little woofers. The specs I found for the system says +/-6dB at 30Hz for the amplifier output.

Your bass boost circuit has a slope of only +2dB per octave that is not audible. You need a +10dB slope like I use with my 8" woofers:

Suraj143 · Jan 29, 2023

audioguru said:
I agree that your 5.25" woofers are too small for good bass.
The specs you attached of the audio system shows +/-3dB at 30Hz from the amplifier output but not one word about the little woofers. The specs I found for the system says +/-6dB at 30Hz for the amplifier output.

Your bass boost circuit has a slope of only +2dB per octave that is not audible. You need a +10dB slope like I use with my 8" woofers:

Many thanks I'll try changing with your values.But where can be the BOOST OFF switch can be placed?

Suraj143 · Jan 31, 2023

I have a doubt where is the best place to fix this bass boost circuit?

Referring the Post 7 diagram.

1)Replace the first stage of pre amp "U2" by fixing Audioguru's boost circuit (Before the tone control part).
2)Leave the both U1 & U2 in pre amp as it is & connect this boost circuit externally.

Which is the best way to connect this?

crutschow · Jan 31, 2023

Suraj143 said:
But where can be the BOOST OFF switch can be placed?

I see no place to add a SPST switch in AG's circuit to turn it off.
Best would likely be to use a SPDT switch at its output to completely bypass the circuit (directly go from input to the tone-control circuit) when you want it off.

Suraj143 · Feb 1, 2023

Ok thanks.

Is my boost too much? Because after tone control section the signal will weak more.So I wanted to add some gain before going to AMP.Added the U2 Gain buffer.ANy idea?

crutschow · Feb 1, 2023

Suraj143 said:
s my boost too much?

Probably won't know that until you try it.

Suraj143 · Feb 3, 2023

Hi I built the boost part & add to the complete tone control circuit as in the post 15.

Results.
I can hear the subwoofer bass.But the problem is I cannot volume even 40%.The sound is clipping.I guess the boost circuit is messing up with the tone control circuit...Always I have to Set the Bass knob to middle.Then the quality is reduced & Mids get increased :-(.

Any idea to make the circuit to achieve full volume with a good bass control?

Nigel Goodwin · Feb 3, 2023

Suraj143 said:
Hi I built the boost part & add to the complete tone control circuit as in the post 15.

Results.
I can hear the subwoofer bass.But the problem is I cannot volume even 40%.The sound is clipping.I guess the boost circuit is messing up with the tone control circuit...Always I have to Set the Bass knob to middle.Then the quality is reduced & Mids get increased :-(.

Any idea to make the circuit to achieve full volume with a good bass control?

Well it's pretty obvious.

Drop the gain of U2, to less then one (use it as an attenuator if need be). It's basically just a low impedance buffer to feed the tone control stage anyway.

If that does do enough (I.E. U3 is clipping) then put an attenuator on the input - but if it's not clipping unless you turn the bass up, then altering U2 should be fine.

Suraj143 · Feb 3, 2023

Nigel Goodwin said:
Well it's pretty obvious.

Drop the gain of U2, to less then one (use it as an attenuator if need be). It's basically just a low impedance buffer to feed the tone control stage anyway.

If that does do enough (I.E. U3 is clipping) then put an attenuator on the input - but if it's not clipping unless you turn the bass up, then altering U2 should be fine.

Thanks.Attenuator means simply a Resistor & Capacitor parallel each other To the line IN in U3?

Nigel Goodwin · Feb 3, 2023

No, two resistors, one in series with the input, the other from the output of that resistor to ground.

Pre Amp with Boost

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