# ~100+ watts mono board without on/off thump?

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#### CZ Eddie

Hey folks, do you know what full-range (40hz-18k at least) board I should buy?
I'm looking for mono 100-120 watts Class D.
Cannot have any on/off thump like my 60W mono board has.
Or a stereo board that can be bridged to 100+ watts.

Or bonus points if you can find me a Bluetooth board with built in amp that can be bridged mono.

Will be using a 21 volt LiPo battery pack.

I'm okay buying from China but would prefer a supplier stateside.

#### rayma

what board I should buy? Will be using a 21 volt LiPo battery pack.

A 21V supply on a mono amp with bridged outputs gives only 28W output into 8 Ohms.
A 4 Ohm speaker would get to 56W, if the amp can output enough current.
You'd need a 40V supply to get 100W into 8 Ohms.

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#### CZ Eddie

A 21V supply on an amp with bridged outputs gives only 28W output.
It takes around a 36V supply to get 100W on a bridged output amp.

Oh dangit. Thank you.
Do you know of an online calculator for me to figure this out?
I'm not too good with the math behind it.

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#### rayma

Oh dangit. Thank you.
Do you know of an online calculator for me to figure this out?
I'm not too good with the math behind it.

Vsupply / 2.818 = Vrmsmax (maximum rms voltage output, single ended amp )

Vrmsmax x Vrmsmax / Rspeaker = POWERmax (single ended amp)

Multiply POWERmax by 4, if the amplifier has bridged outputs (since the output voltage is doubled)

Also there's the question of how long the battery can last powering the amplifier.

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#### CZ Eddie

Vsupply / 2.818 = Vrmsmax

Vrmsmax x Vrmsmax / Rspeaker = POWERmax

Multiply POWERmax by 4, if the amplifier has bridged outputs

Okay, I think most stereo boards out there are already bridged internally to provide the stereo output?
So am I correct in interpreting your calculations?

Vsupply = 21V LiPo battery pack
Rspeaker = 4 ohm speaker

So...

21 / 2.818 = 7.45

7.45 x 7.45 / 4 = 13.88

13.88 x 4 = 55 watts

Is that correct?
If so, thank you.

I'm looking for a board but thanks. I can Google the TDA7498 chip to find one that doesn't have on/off thump.

#### CZ Eddie

Actually, that TDA7498 board is a monster.
Since I only have 21V to work with, I'll have to just find another 60W+ mono/bridged amp without on/off thumps.
Since apparently my 21V limits me to 55 watts anyhow.

#### rayma

13.88 x 4 = 55 watts Is that correct?

Yes, that's right. Make sure that the bridged amplifier is rated for 4 Ohms,
since each half will see an effective load of 2 Ohms.

Now think about how long the amplifier will work, on the particular battery that you have.

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#### rayma

Would I be correct with this?

12.6 / 2.818 = 4.47
4.47 x 4.47 / 4 = 5.0
5.0 x 4 = 20 Watts that my 4ohm NS3 is getting

Yes, if the amplifier is bridged, and can drive the 4 Ohm load, and the battery is good enough.

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#### CZ Eddie

Thank you so much!

The battery only needs to last a couple hours at medium-high volume.
It's five 18650 LiPo batteries.

So right now I'm researching to find a Bluetooth/Amp board which can be bridged.
I've already got a fancy Sure JAB2-50 (TPA3116) bluetooth/amp board but Sure won't tell me how to mod it for PBTL. ugh.

http://store3.sure-electronics.com/aa-ja32171-1521

#### J. L.

Thank you so much!

The battery only needs to last a couple hours at medium-high volume.
It's five 18650 LiPo batteries.

Assuming those batteries are somewhat standard in capacity, they will be approx 2250mAh. ( 2.25 Ah )

They should therefore be capable of supplying 750 mA ( .75 Amps) for three hours.

My concern is that 3.6 volts * 5 = 18 volts.

18 volts * .75 A = 13.5 watts

If the average current drawn by your 100 watt capable amplifier is under 13.5 watts, the battery should last the duration of your event.
(assuming it is 3 hours or less)

However, if, you are actually drawing 20 watts of power that equates to 1100 mA being drawn from the battery pack. (18 volts * 1.1 Amp = 20 watts)

At 20 watts, the battery might last 2 hours. (Tough to tell exactly, it will depend on lots of factors, but to me, you are cutting it too close for comfort.)

If you are really drawing 100 watts from the battery pack you propose, it will draw 5.5 Amps from the battery and run for 20-30 minutes at best.

I know you are looking for a 100 watt amplifier. If playing most music, and if Class-D, the average current draw will most likely be 10 watts or less, but who knows...

You might want to perform some tests with your current amplifier and the proposed battery pack, and your specific program material to see how long it will play. It might give you some comfort the alternative amplifier board you eventually find (with no "on/off thump) might work for the duration of your event with the battery capacity you've supplied.

Joe L.

#### Buehgemeiste

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