Problem of subwoofer amplifier

[Nikolai Petrenko]

Hi all members. I am making a low power home theater amplifier. Satellite speakers drive by 1 LA4440. But the problem in the subwoofer amplifier. I used 2 Tda2003 to drive subwoofer speaker. I also built a low-pass filter is active. Filter circuit from construyasuvideorockola. When i connect the filter to the TDA board , subwoofer hum too loud and some sound like "ugg" or "ups". "Ugg"' then hum for 2 second and "ugg" again .... I'm trying to use passive filters , noise and "ugg" no more. What is wrong about active filter ???. Constryasu using TL071 circuit , they say can replace by uA741. But when I buy components , I forget it. The store did not have TL071 so I ask for Tl072 so they gave me JRC 072D. I only use one of its channels .Link of the circuit https://construyasuvideorockola.com/downloads/pre_subw_car.pdf

 

[audioguru]

Datasheet archive dot com has never heard of a JRC 072D so I do not know what it is. A TL071 or TL072 is very common and is available everywhere except not on the moon, are you there?
The circuit you found has a HORRIBLE lowpass filter.

 

[audioguru]

I found it. JRC was Japan Radio Corporation but today they make products not parts. The parts are made by New Japan Radio company and the IC is called NJR072. The datasheet says that at low gain it will probably oscillate.
I recall that the Japanese copied the Texas Instruments TL072 but "made it better". Then it oscillates.

 

[Nikolai Petrenko]

why you call this is horrible low-pass filter

 

[audioguru]

why you call this is horrible low-pass filter

Because it has two RC networks but they are not used together in a Butterworth circuit:
1) The 0.22uF input capacitor makes a passive filter with the two stereo to mono 10k resistors causing a gradual cutoff at 146Hz.
2) The 100k negative feedback capacitor has another 0.22uF capacitor in parallel producing another gradual cutoff at 7.3Hz.
Why does it have two gradual cutoff frequencies that are completely separate?

Most active crossover circuits use a Sallen and Key filter circuit designed for a Butterworth response. The Butterworth response has a flat response then a sharp cutoff.

 

[chemelec]

The TL071 (TL) is made by Texas Instruments.
The JRC 072D (JRC) is probably the Same IC, just made by a Different Company.

But I Agree, that is NOT a good filter circuit.

 

[audioguru]

New Japan Radio says on its datasheet for the NJM072 that in certain applications like a follower (low gain) it may oscillate. The TL072 from Texas Instruments and copied by many English speaking companies never oscillates because it has proper frequency compensation.
The 072 "improved" by Toshiba oscillated so badly that all that were sold were recalled and replaced with something else. I think the New Japan Radio opamp uses Toshiba's design.

 

[Nikolai Petrenko]

i throwed alway active filter , just use passive. I use 1K resistor and 1uF, and and variable resistor to control cutoff frequency point, but the variable seem be not effect. bass to low, so I want to make an preamp with s9014, 8050, 8550 , any problem???

 

[Nikolai Petrenko]

oh, audioguru can you post formula to calculate cutoff frequency when resistor parallel with capacitor, and when only a capacitor

 

[Nigel Goodwin]

i throwed alway active filter , just use passive. I use 1K resistor and 1uF, and and variable resistor to control cutoff frequency point, but the variable seem be not effect. bass to low, so I want to make an preamp with s9014, 8050, 8550 , any problem???

You don't use passive filters, they are too ineffective, use an active one.

There are numerous sub-woofer examples on the Internet, just copy one of those.

 

[chemelec]

Go to This LINK. It will give you all the Info for a Good Low Pass Filter.
Or many other types also.
**broken link removed**

 

[audioguru]

oh, audioguru can you post formula to calculate cutoff frequency when resistor parallel with capacitor, and when only a capacitor

The cutoff frequency of an RC lowpass filter is the frequency that has the output level reduced -3dB which is 0.707 times the signal voltage. Then higher frequencies are reduced -6dB per octave or reduced -20dB per decade of frequencies.
The formula is 1 divided by (pi x R x C). So 1k and a 1uF capacitor produce a cutoff frequency of 160Hz. 320Hz is cut -6dB and 640Hz is cut -12dB. 1600Hz is cut -20dB so it is still produced and 16kHz is cut -40dB so it is at a low level but is still heard.
It is a very poor and simple filter.

 

[Ziddik]

Hi, use tda 2030A bridged Power-Amp ICs, they make decent subwoofer amp and gives 36W out. You will need dual power supply

 

[audioguru]

Hi, use tda 2030A bridged Power-Amp ICs, they make decent subwoofer amp and gives 36W out. You will need dual power supply

He already has a bridged amplifier, he wants a lowpass filter for it to drive his subwoofer.

 

[crutschow]

For a 2nd-order rolloff you can use a Sallen-Key filter with a dual-pot to vary the rolloff frequency such as shown here.
Digital potentiometers are shown but a dual-ganged manual pot will also work.

 

[audioguru]

For an accurate crossover the satellite amplifiers need a highpass filter that matches the lowpass filter for the subwoofer.

 

[Tony Stewart]

You can certainly use the NJM072 parts. Design formula are pretty easy in attached.

 

[Nikolai Petrenko]

ok, thank a lot. But I concern to reduce noise. noise not loud but make me annoy when sit near it. What shuld I do??? When I touch the ground, noise on satellite reduce, but not in subwoofer

 

[Tony Stewart]

ok, thank a lot. But I concern to reduce noise. noise not loud but make me annoy when sit near it. What shuld I do??? When I touch the ground, noise on satellite reduce, but not in subwoofer

Without photos and wiring details, video ... hard to see

 

[audioguru]

Now you say there is noise but you did not describe the noise. Low frequency hum (from the 50Hz or 60Hz electricity), high frequency hiss or radio interference??
You must have a wiring or shielding problem if there is noise that is reduced when you touch its ground.

The digital modem for my cable internet causes a clicking noise in my nearby stereo and in my wireless telephone.