portable pa hack gone wrong!

[newjazz]

hello,

i recently bought a pretty good portable pa to use busking and to do cafe restaurant gigging. the product i am speaking of is a nano lucas 300 which i bought it for a really good price on a french version of craigslists.
my plan from the beginning was to mod it to work on a lithium ion battery.
initially, i was thinking of connecting directly to the circuit board at 12v but, when i opened it up it i quickly realized that it was NOT running a simple 12v power supply and there was no transformer so naturally, i decided to just buy a dc to ac pure sine wave inverter.
the problem started when i had the fancy idea of taking the inverter out of its casing and install it directly into the amplifier in order to simplify transporting it. at first everything went fine, i made the rectangle cutouts for both the input dc and the ac output displays and it looked fantastic. i admit was quite impressed with my drummel hacking. the surprised happened when i plugged in the battery and heard a loud hum that wasnt present when it was working on mains.
after, spending literally all night splicing in wires to extend the display wires, cutting out the holes to mount them and a place inside for the circuit board. of course, i was quite disappointed by the hum and figured it was the result of some sort of ground noise but, i wasnt able to totally eliminate it by changing where i put it in the case.
the really bad part was made when i unplugged the speaker wire while the amp was still powered. the wire fell on the amplifier board and i heard a slight pop and that was "all she wrote"..........any suggestions as to how to get myself out of this mess grounding or where i should start with diagnosing the blown amplifier board would be greatly appreciated.
i am guessing that i toasted the amplifier IC but, i cant b sure until the ordered part comes in..........thanks!

 

[crutschow]

It's only a guess as to what you zapped without a schematic.
My condolences.

As far as the hum, it would seem your "pure sinewave" converter may not be so pure.
Or you have some sort of ground problem.
Again, hard to tell from your description.

 

[dr pepper]

The first thing to do is put the amp back as original as far as wiring is concerned, then you know your working with something that will operate.
You probably have blown the o/p Ic, what is its number?, you might be able to check the supply voltages of said Ic with it out of the board to test the power supply, might also be worth ohming the speaker, you get get 6 to 10 ohms for an 8 ohm speaker.
As for the hum you might need to put the inverter in its own metal enclosure, there are 2 ways hum can get into the amp, one is conducted noise which you can deal with by carefull wiring, and the other is magnetic which you need to shield.

 

[alec_t]

I quickly realized that it was NOT running a simple 12v power supply and there was no transformer so naturally, I decided to just buy a dc to ac pure sine wave inverter.

This is all the manual says about the power supply :-

So clearly the Nano has a SMPS internally. Feeding that with another SMPS may cause one or other to throw a tantrum.

 

[dr pepper]

The Op did say he's got a sine wave inverter, less likely to have trouble if it really is a pure sine wave.

 

[alec_t]

True, providing it can cope with the 48A inrush current.

 

[newjazz]

guys, thanks for the suggestions!...by the way, the amplifier IC is a L4780TA
https://www.farnell.com/datasheets/1994371.pdf

 

[newjazz]

ok, guys dont laugh but, there is more to the story! I used my lithium ion battery pack that i built for my ebike which is made from 18650 cells. it is quite larger than a standard ebike setup in that it is a 48v system (13s 6p). i also forgot to mention that i used the amplifier, inverter and battery for almost a week with no audible problems. in fact, i had enough power in my pack to commute to and from downtown with running the amp for 3.5 hours in between!

 

[tomizett]

i used the amplifier, inverter and battery for almost a week with no audible problems

So, just to be clear, are you saying that the amplifier was fine running from the inverter while the inverter was external (in its own original case), and the trouble only started once you mounted the amplifier and inverter together in the same case?

 

[newjazz]

exactly!........there was no loud hum that i noticed when i was outside! i am presently reading some stuff online about the possibility og the magnetic interference from the speaker on the inverter particularly the transformer on the inverter's circuit board.

 

[dr pepper]

Good idea theres enough energy in an ebike battery to power the thing for ages.
However you really need circuit protection, ie fuse inline with the battery.
The speaker wont cause much if any magnetic issues, your on the right line with the transformer in the inverter though, even wires can cause magnetic issues.
Looks like the Ic has 90v across it from what you've said and the datasheet, I wouldnt recommend poking around with a meter on that voltage, its enough to give you a zap.

 

[newjazz]

So, just to be clear, are you saying that the amplifier was fine running from the inverter while the inverter was external (in its own original case), and the trouble only started once you mounted the amplifier and inverter together in the same case?

yes!

 

[newjazz]

Good idea theres enough energy in an ebike battery to power the thing for ages.
However you really need circuit protection, ie fuse inline with the battery.
The speaker wont cause much if any magnetic issues, your on the right line with the transformer in the inverter though, even wires can cause magnetic issues.
Looks like the Ic has 90v across it from what you've said and the datasheet, I wouldnt recommend poking around with a meter on that voltage, its enough to give you a zap.

yeah, i heard greater than 72v is enough to get shocked! Lol
are you suggesting i add a fuse even if i have a BMS on the battery pack?

 

[dr pepper]

They say 50v is the 'safe' limit.
Yes add a fuse, your amp will not pull anything like as much juice as your ebike, if it does it'll be loud.

 

[newjazz]

They say 50v is the 'safe' limit.
Yes add a fuse, your amp will not pull anything like as much juice as your ebike, if it does it'll be loud.

Dam sure will do!.........i ordered the IC amplifier chip and one of the linear voltage regulators so, hopefully the problem is one of those! Then, once i get it running again i can sus out that terrible hum!
I am thinking to try sheilding the transformer on the inverter board and then the wires. Any suggestions as to what products would work?

 

[dr pepper]

Try the amp as original first once reapired, see if the hum persists, if not connect it back to the inverter & repeat, if the hum returns put the inverter on longer leads (24") away from the amp & see what happens, if the amp still hums its most likely conducted hum, if not then it could be magnetic.
If its conducted then configuring the grounds in a star pattern might improve things, maybe some ferrite rings if the issue is high freq (which is still possible even if it sounds low freq).
If magnetic then using some copper clad board as a means of segregating the amp/inverter might be a good start.

 

[KJ6EAD]

I keep thinking this whole AC hum mess could have been avoided had the OP been able to identify and isolate the DC supply initially and simply replace the existing AC/DC with a DC/DC system instead of the less efficient and noisy DC/AC/DC system. A lot of these types of devices operate on 12 or 24 volts internally, sometimes with a split supply for the op amps. The power amplifier in this case would be quite happy with a 24V (±12V) supply. The mixer circuits would likely work from the same but since there's a dearth of documented or measured pre-hack data, who knows?

 

[newjazz]

So what youre saying is i could have gone directly to the board with a dc voltage?......thats what i was hoping to do but, got a bit intimidated when i didnt see a transformer!
First, how do i decide on the correct dc input? Thats the question!
I put a link to the amplifier IC, its a "L4780TA"......
If this works i will still need a buck converter, right?

 

[KJ6EAD]

Recognizing a switch mode power supply topology, identifying and measuring it's output would have been a logical first step. Since we've not even seen the device, we can only make assumptions and guesses. We know three actual facts: the brand name and model of the initially functional unit, the brand and model of the integrated power amplifier and that some damage has been done. I won't speculate further; I'll only attempt to advise based on more information.

 

[newjazz]

I keep thinking this whole AC hum mess could have been avoided had the OP been able to identify and isolate the DC supply initially and simply replace the existing AC/DC with a DC/DC system instead of the less efficient and noisy DC/AC/DC system. A lot of these types of devices operate on 12 or 24 volts internally, sometimes with a split supply for the op amps. The power amplifier in this case would be quite happy with a 24V (±12V) supply. The mixer circuits would likely work from the same but since there's a dearth of documented or measured pre-hack data, who knows?