Chipamp as (A)B output stage Driver?

[der Papst]

Have you ever considered using a Chip-Amp as driver for a (A)B output stage?

Chipamps work like OPamps with powerfull output stages and high voltage inputs.
And they have nice features like standby, mute and clipping detection.

Is there any reason why this would not work?

Are there special output stage drivers for (A)B-Amps with the same features?
I've searched the internet but i only find them for PWM stages for motor control...

BTW: Is class B fast enough for a Subwoofer Amp (~10-150Hz)

[der Papst]

Already >30 People read it, no answers.
Maybe i should describe my idea better:

Basics:
A powerfull (A)B-amp uses a array of complementary Transistors, their base/gate is connected to a differential amplifier which gets the audio signal and a (divided by voltage amplification) feedback and balances both values (audio signal = output/amplification)

My idea:
Simply replace the differential amplifier with a chip-amp.
The max. voltage of a chip-amp is high enough to drive the amp to its maximum, and we can also use the chip-amp's features: standby, mute, clipping detection.
Also the low current amplification of bipolar transistors is not a problem since our "driver" delivers up to 50Wrms into 4 Ohm (depending on the used chip-amp)

But i think, there are special driver-ics for audio-output stages...
Has anyone ever seen such a ic?

[pacificblue]

Originally Posted by der Papst

Are there special output stage drivers for (A)B-Amps with the same features?(~10-150Hz)

TDA7250, LME498xx

[multisync]

check out lme49810, lme49811 and lme49830 plus others. There are several threads on this site.

[der Papst]

When i build an Amp for subwoofers (10~150Hz) with a 498xx-driver, would a class b-topology be fast enough for it?
Even with MosFETs?

My target are 2x 250W into 2 Ohm, bridgeable to 1x500W into 4 Ohm

[multisync]

I recently built an LME49810 based amplifier with boards I bought on Ebay, frequency response is about -0.5db @2hz and 0.5db at 100khz, plenty fast for a sub. see the folowing posts #123 picture 13 and post 137 of this thread for a comment of the sound quality of the amplifier

Winter DIY OTTAWA MEET

[der Papst]

I consider building a class B amp.
Since i need it only for subwoofers, the frequency would be limited to ~120Hz.

The slew Rate of the LME498xx is very high (~100V/µs), so they're able to "jump" over the zero crossing in ~100ns (if you use Fets) and in ~10ns if you use bjts...
Because of the very low time (compared to a sinewave with <150Hz), i think this would not cause big dissortion?

Is there an error in my considerations?

[johnr66]

You can do this with some of the 5 pin chip amps as well. The older TDA2030A is "billed" as a stand alone amp and 35 watt driver.

[pacificblue]

Originally Posted by der Papst

When i build an Amp for subwoofers (10~150Hz) with a 498xx-driver, would a class b-topology be fast enough for it?
Even with MosFETs?

How fast is the amp you use above 150 Hz?

Originally Posted by der Papst

My target are 2x 250W into 2 Ohm, bridgeable to 1x500W into 4 Ohm

The datasheets specify the drivers to be capable of 300 W (49810, 49830) and 500 W (49811) into 8 Ohm. That means nominal output currents of 6,1 A and 7,9 A. 250 W into 2 Ohm means 11,2 A. Which of the ICs will be able to drive an output stage to deliver that current?

[der Papst]

Originally Posted by pacificblue

How fast is the amp you use above 150 Hz?

5xTDA7293 (i build a 5.1-Amplifier)

Originally Posted by pacificblue

The datasheets specify the drivers to be capable of 300 W (49810, 49830) and 500 W (49811) into 8 Ohm. That means nominal output currents of 6,1 A and 7,9 A. 250 W into 2 Ohm means 11,2 A. Which of the ICs will be able to drive an output stage to deliver that current?

I don't understand how the datasheet can specify the maximum current of the output stage although it isn't integrated into the IC?

The gate/base-current of the output stage strongly depends on the used transistors.

High power BJTs have a current amplification of only 50. Small BJTs between 100 and 1000.

I've never worked with FETs in non-switchmode aplications, so i will use BJTs for my example:
You build a darlington outputstage. The current amplification will be 50*1000 = 50.000

When the CE-Current is 15A, the Base-current is 15/50.000 = 300µA.


Maybe my calculation above is incorrect, if it is, please tell me why.