Bridging class D amps..pseudo AB?

[dschwartz]

Hello fellow amp masters..
Rring post about his 100w amp in a stomp got me thinking.
I've been researching class D amps chips..and i would like to get about 100W @8 ohms, 1% thd with 24v and 50w per channel in stereo @4 ohms (trimode)
Most datasheets of stereo class D amps with the specs i want have btl amps per channel.and for mono they describe PBTL configuration..wich i don't like  because they need lower impedance speakers for desired power..

My question is..
It is safe to BTL the already BTL channels? It shouldn't be a problem in theory, but maybe you know more..

Moreso, what if i use btl configuration in an AB fashion, splitting the positive and negative part into each channel..my intuition tells​ me that at the crossover section, the amps will see a different impedance and funny stuff may happen.

What do you think?

[PRR]

> It is safe to BTL the already BTL channels?

Basically: No.

If you must, it is possible to do it, but multiple isolated power supplies and floating signal sources. Not worth the trouble.

What is your objection to low impedances? Powers this large probably need to be split over several speakers. Which can be 4 Ohm, four of those parallel is 1 Ohm, and you can get a couple hundred Watts that way.

"Most" of the class D amps I have glanced at run 24V supply. This is probably because IC devices have serious compromises with breakdown voltage.

[dschwartz]

> It is safe to BTL the already BTL channels?

Basically: No.

If you must, it is possible to do it, but multiple isolated power supplies and floating signal sources. Not worth the trouble.

What is your objection to low impedances? Powers this large probably need to be split over several speakers. Which can be 4 Ohm, four of those parallel is 1 Ohm, and you can get a couple hundred Watts that way.

"Most" of the class D amps I have glanced at run 24V supply. This is probably because IC devices have serious compromises with breakdown voltage.

I just sat down to think and realized it is not simple...you need 2 SE for BTL.. that's why PBTL is the obvious way...

My goal is to make a pedalboard amplifier, so i ( or potential customer) can use any guitar cabinet (or full range speaker) . Guitar cabs are usually 4 or 8 ohms, so that's why.

The tda7298e looks interesting..at 36V..no problem

[anotherjim]

Needs a dual secondary output transformer - a somewhat backward step. I think that has been done in car audio (not necessarily class D).

[dschwartz]

What about the class AB idea?
Using a BTL output where each side carries half of the wave?

[rezzonics]

I think you can easily calculate the maximum power of a class D amp on one channel from speaker impedance and power supply:
P = V^2/R/2. So to get 100W output on a single channel you need 40VDC power supply. 40*40/8/2= 100W.
If you want to use 24VDC power supply, the maximum power you can get on 8 ohm speaker from one channel is 24*24/8/2= 36W
But you can parallel the BTL output of a stereo power amp and double the power on a single speaker.
But still to get 100W, you need 50W per channel, so the required power supply is at least 28VDC.
You can parallel the BTL output before or after the LC filter.
I guess you could parallel two stereo amps with 25W per output.
You can check how to parallel the outputs in this TI TPA3245 datasheet, section 10.2.4 shows outputs paralleled before LC filter and section 10.2.5 shows outputs paralleled after LC filter.

[rezzonics]

For 10$ you could buy a 2*100W dual TPA3116D amp and see how it works on 8 ohms speakers.
I think that 2x100W is for 4 ohms speaker, but you should be able to get 2x50W on a 8 ohms speaker.

[PRR]

> tda7298e looks interesting..

16 Watts is way short of the 100W you were asking for.

> What about the class AB idea?

You gotta draw me a picture with waveforms; no idea what advantage you expect.

For a given supply voltage and load connection, the power output is about the same AB, B, or D. The D-class chips are often multipled for easy bridging, though bridge is common in 14V car-audio also.

[dschwartz]

> tda7298e looks interesting..

16 Watts is way short of the 100W you were asking for.

> What about the class AB idea?

You gotta draw me a picture with waveforms; no idea what advantage you expect.

For a given supply voltage and load connection, the power output is about the same AB, B, or D. The D-class chips are often multipled for easy bridging, though bridge is common in 14V car-audio also.

Lol, I meant tda7498e...160W @4ohm 36v BTL stereo

The advantage (in theory) of using a BTL with push pull config is that when one amp is pushing, the other is not pulling, so the impedance seen by the amp is not half..that way you wouldn't need to double the impedance on that config..also, lets you play with bias shifting and other tubey stuff

[PRR]

> the other is not pulling, so the impedance seen by the amp is not half..

In a world where voltage is limited, not sucking half impedance means not getting double current, so you just get half the power.

I would not sneer at 80W in 8 Ohms. This is Big Amp turf.

A bass player would indeed want all 320W split over two speakers, chosen for 4 Ohms (or four 8-Ohm cones).