Do I need a dummy load?

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daveError

Member
2017-10-25 11:03 pm
Michigan, USA
Alright, I'll start by saying that I don't know whether this is the appropriate place or the Class D forums, but here goes...

I'm working on a proof-of-concept idea for a mobile karaoke setup. I have two cheap, Chinese class D amps that cost me $22 and $30 respectively, the first one based on the TPA3116 that I bought a couple years ago and a new one based on the TDA7498 that I literally just got in the mail today. They both share the same extruded aluminum "Breeze Audio" case, but I suppose that's not terribly relevant. I'm sure many of you are familiar with these. The new one is definitely more hefty.

Anyway, my proof-of-concept idea goes as follows:
I can back my Jeep Cherokee up into any random field, parking lot, etc. and pop the tailgate, and within five minutes have a working PA set up. The back of the Cherokee is the "table" where I set my laptop and mixer and the two amps, the two 12" speakers (Panasonic RAMSA WS-A200's if anyone cares) go up on some cheap stands, a mic stand is set up with two mics on it, and an 8" monitor speaker (RAMSA WS-A80) gets set up on some kind of stand (still working this part out) with a USB 3.0 screen connected to my laptop in front of the mics. Boom.

So here's my question:

The two amps are both set up stereo. Obviously I want to use the more powerful of the two (TDA7498) to drive the mains. The monitor only needs to be mono though, so should I consider attaching a dummy load to the unused channel of the TPA3116? I don't want to hurt it, but these little amps actually seem fairly resilient. Let me stress that no signal will be put through that channel.


Add-ons:

If anyone knows anything about wiring extra power into this system via a couple deep-cycle batteries so I don't have to run my engine/alternator all the time and still have enough juice to get me through a night without killing my car battery, any info on that would be awesome. Ideally I'd like the extra batteries to charge from the alternator while I'm driving. Even better would be if I could run the power from them back into my car in case I have trouble starting it in the winter, but I have no idea how plausible that is.

I'm already taking the car power from a cigarette lighter and running it into a 24v up-converter to drive the amps. I've yet to attempt driving both at once, so I suppose another question would be whether I should invest in another up-converter that puts out like 10 amps and drive both amps off that, or if I can use the two I already own that put out 3 and 5 amps. Will that make a difference? What's the more efficient way to handle this? The mixer will also need to derive its power from the vehicle but it takes 15v so I know I'll have to buy a 12-15v converter, too.

The mixer is a cheap Chinese model as well. It's an "ammoon" mixer with five mic channels and one stereo channel, along with some on-board delay/echo. I entertained the idea of using a small Behringer mixer I already own but those take AC power and I wasn't sure if there was a workable DC solution.

Any comments/info would be appreciated. I'll do my best to answer follow-up questions.
 
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tataylino

Member
2017-04-05 8:06 am
Philippines
www.tataylino.com
1.Dummy load not needed. It is only a practice on tube amplifiers before, solid state amplifiers don't need this.

2. I would suggest using lithium iron batteries(LifePo4). They are rated 3.2V so you need 4 in series for 12.8V output. I use them a lot because it is safer than Litium ion batteries(Lithium ion batteries can get on fire when not used properly while lithium iron don't). They also have higher power density than lead acid batteries so it is better for mobile solution where space and weight is a big factor. There are 12V LifePo4 batteries already in the market(basically a multiple cells in a package) which you can check. If you decided to use 2 12V LifePo4 battery and estimate your average power would be around 180W, you would need around 75AH battery to keep it running for 10Hrs. I suggest buying 2 12V 100AH(series to make 24V) batteries is more than enough for the whole night. Battery capacity degrades as you use them so you need to have more than what you need in the first place so you have more years to use them.

3. The datasheet of TDA7498 says it can deliver up to 100W x 2 with 36V supply and 6 ohms load. By using 24V supply, you may only get around 60Wx2. I would say you need around 10A 24V supply for better reliability. It is better to have more power than you need. Power supply runs hotter as you use its full power output capability. It is more reliable when you use less than 80% of its full power.

Note:
* These are conservative estimates. I prioritize reliability than cost.
* Music average power depends on the type of music and how much you boost the low end frequency(bass).
* LifePo4 battery charging voltage is 3.65V, For 12V LifePo4 battery pack would be 14.6V.
 
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PetruV

Member
2012-12-09 7:56 pm
Bucuresti
Lithium batteries are not that dangerous,yeah,there are freak accidents,but I used them a lot and have a hundreds of cells,not one time did one even get warm.As long as you use protection circuitry and don't stab them you'll be fine.Powering the setup from the cars battery is also a good idea as long as you keep the engine on or turn it on when the voltage gets too low so you don't get stranded.There are dc-dc converters on ebay that can do a lot of power.Average music power is 1/8 of peak power,So if your amps total 150 watts (a conservative number since you aren't running them at their limits),you'll draw 20-30 w continous max,so you'll get decent runtime from the battery without having to run the engine to often.It would also be great if you could tap the power directly from the battery to avoid using the long and thin wiring from the lighter socket.
 
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