2.1 Subwoofer amplifier

[slnuwan]

Hi All,

I wanted to make a high power 2.1 audio amplifier which gives 100W+100W+200W.
Then I ordered 200W class-D power amplifier and 100W+100W class-D power amplifier via online store.

I thought it can be connected to my old 2.1 sub woofer pre-amp.
But unfortunately it could not be connected as preamp out is not mach with power amplifiers and audio is destroyed.
Someone please tell me, are there any specific preamplifier for class-D power amps?
Can you kindly share a high quality 2.1 pre amplifier please?

And 200w class-D amplifier dissipate high heat even though there is a heat shrink , Please tell me a solution for this also.

Thanks,
Nuwan.

 

[Fidelis Anyanti]

First, Any good Preamp with good gain and fidelity can be connected to a class D amplifier. If your sub-woofer has a gain that is too much ( in other words it can play a speaker on its own), then proper impedance matching is an issue.

If you'd like to do it yourself, then try this:



Goodluck

 

[chuckey]

" And 200w class-D amplifier dissipate high heat even though there is a heat shrink , Please tell me a solution for this also. " A good class d amp should have an efficiency better then 80%, so for 100 W output the output devices dissipate 20% X 100W = 20W, which is a small heat sink. No way around this. Beware the drive to the output devices is often done via some form of boot strapping with a capacitor or a charge pump and a capacitor. if this capacitor is not large enough, the boot strapped voltage falls off at low frequencies which results in the output devices over heating and a lack of output. Have you tested the frequency response at full power right down to 10HZ or so?
Frank

 

[Fidelis Anyanti]

Yes, A class-D amplifier or switching amplifier is an electronic
amplifier in which the amplifying devices (transistors,
usually MOSFETs) operate as electronic switches, instead of as
linear gain devices as in other amplifiers. The signal to be
amplified is a train of constant amplitude pulses, so the
active devices switch rapidly back and forth between a fully
conductive and nonconductive state. The analog signal to be
amplified is converted to a series of pulses by pulse width
modulation, pulse density modulation or other method before
being applied to the amplifier. After amplification, the
output pulse train can be converted back to an analog signal
by passing through a passive low pass filter consisting of
inductors and capacitors. The major advantage of a class-D
amplifier is that it can be more efficient than analog
amplifiers, with less power dissipated as heat in the active
devices.

 

[Audioguru]

DO NOT use a lousy old (designed 46 years ago) 741 opamp for audio. It has hiss, its high audio frequency response and slew rate are horrible and it produces distortion.

Audio power amplifiers can be class AB or class-D and have a specified input level. Your preamp output must produce that level. If the input level is too high then the output of the amplifier clips and produces square waves.

Impedance matching is almost NEVER done for a preamp output to a power amp input. The preamp output is a low impedance and the power amp input is a high impedance.

A 2.1 preamp has a very low frequency output to feed a sub-woofer power amp and two low, mid and high frequency outputs to feed the left and right power amps. The preamp might have the frequency crossover circuits or the power amps might have them.

- - - Updated - - -

The tone controls preamp with the awful 741 opamp will not work anyway because it is biased WRONG:

 

[slnuwan]

Thanks All,

I think 741 can be work with stereo amplifier, But I want a high quality low pass filter with preamp for subwoofer.
Can you please share a circuit diagram?

 

[Audioguru]

The subwoofer amp needs a lowpass filter and the stereo amp needs highpass filters.
Here is an electronic crossover with 18dB/octave slopes. Its crossover frequency is 300Hz and for your subwoofer you might want to change it to 150Hz by increasing the *11k resistors to 22k and increasing the *51k resistors to 100k. Make two of the highpass filters for the stereo amp.

 

[slnuwan]

Thanks Audioguru,

I will make this cct.
Thanks a lot.

Nuwan.

- - - Updated - - -

Hi Audioguru,

can you kindly explain why it should be 150hz?
If we want to vary the frequency response I mean adjustable frequency, please tell me how it should be modified.

Thanks,
Nuwan.

 

[Fidelis Anyanti]

Well I'm sure that even OVERSABI 'RU Knows that the 741 opamp has proved itself to be of great value over the years.

however, a two way crossover network will barely span over the Audio frequency. A superb System will use a 3 way crossover

The circuit below shows the way to connect a 3-Way crossover. This unit produces excellent results, with good phase coherency and a flat response across the entire frequency band.
That means, a third power amp will be used to handle the midrange frequency.

I know the circuits look complicated, but each is basically repetition of a common circuit block - the filter section. Since the opamps are all used as unity gain buffers, the use of premium devices is not really essential, so the TL072 type would be quite serviceable in this role. Needless to say, if you want to use better devices (even discrete opamps) you can easily do so. Make sure that any device used is stable for unity gain - this is not always the case with some devices, especially when external compensation is used. In this case, use the manufacturer's recommended value of stability cap for unity gain operation.



Cheers

 

[Audioguru]

A subwoofer plays from about 15Hz to about 150Hz and the woofers in your stereo produce sounds from about 150Hz and higher.
The resistors and capacitors marked with a (*) have values that must be in the same ratios as they are now but changing their values changes the crossover frequency.

My computer uses one woofer that they call a subwoofer but it is small and cannot produce very low frequencies. It has two tiny satellite speakers for stereo. Its crossover is at about 500Hz.

 

[slnuwan]

Thanks Audioguru,

One more question, do not we change any capacitor value for 150 Hz ?

Thanks,
Nuwan.

 

[Audioguru]

One more question, do not we change any capacitor value for 150 Hz ?

It is easy to change the 11k resistors to 22k and change the 51k resistors to 100k for a 150Hz crossover.

But to change the capacitor values instead you will need capacitor values of 94nF and 20nF that are not available.