TDA7294 based logitech z 5500

[Edmunds]

Hi all,

I am looking for some assistance with converting my logitech z 5500 TDA7294 based system. My intention is to run as much of it as possible on 12VDC from the car battery. I can also use 5VDC, because that I already have for other equipment. Worst case, I can also go up to, say 18 or 36 volts, but I have not yet looked in how difficult it would be to do myself and a quick look did not give me any ready-made products that would do this.

I have taken the thing apart to find the transformer is 2x26.4V and 1x14.5V. Obviously, I am not interested in the transformer anymore, but that gives me an idea what I'm dealing with.

The 2x26.4V secondary goes out to a rectifier with the middle 0V going in between the capacitors. It looks like a version of the schematics in the bottom lowest picture on this guide Building a Gainclone chip amp power supply., but I do not get it fully, as the whole thing is not grounded in the mains. So maybe it is just 2x36V after the rectifier.

I can also see two 78M18A and 79M18A chips side by side with the capacitors, so I'm quite confused about all of this.

The TDA datasheet says, the chip would run happily on as little as 10V, but the question is how do I locate where do I apply my 12V to get it working and what more components are likely in the way and need to be bypassed/removed.

The 14.5V secondary is connected to a small rectifier on the board. I have an idea it is used for running the remote control of the board.

I thought I cannot ruin much by applying 12VDC just after the big rectifier if that would go through and things would just run. I don't think I damaged anything, but it did not run, of course.

I then went on to re-connect everything with the big ugly transformer. Everything still worked fine. Huh. Nothing burnt. Now, I checked the voltages all over the place and it is even more confusing.

The 26.4VAC becomes 36VDC after the rectifier. Ok, so I am dealing with 2x36VDC here. I cannot expect that to run on 12V, right? Still, if the chips actually run on 12VDC, it feels like I just need to bypass the ugly stuff I do not need.

Then I tried 12VAC on the remote control supply - works. Then I tested what would be the outcome of 12VAC applied to a rectifier I had at hand (same look as the one on the board) - ~17VDC. A bit much, but what a heck - nothing got fried in a couple of minutes at least. I did not find any voltage regulator near the input, so I think it just said, ok, 17 will it be or something.

All in all, I posted a message to the z5500 thread, but no smart guys replied. It seems there are people out there, who have figured out what the circuit looks like and also know what components do what (which I do not - audio is not my comfort zone) and thus could help me figure out how to modify it for 12VDC. If you happen to be just such person, please help!

Thank you in advance,

/Edmunds

[theAnonymous1]

The TDA7294 requires a minimum +/-10V for the power supply, which means 20V from negative supply to positive supply.

This chip is not suitable to be run from a single 12V supply. Either find another amplifier IC to use (preferably with bridged outputs), or get an appropriate SMPS to power it with.

[Edmunds]

Well, thx anonymous . Any suggestion on a replacement amp chip? I have no problem of re- soldering the chips, but I have no idea how to recognize chips that do the same with the same outputs, but at different voltages.

Thank you,

/Edmunds

[theAnonymous1]

You mean a different IC that you can just swap into the existing TDA7294 circuit and use @ 12V? I don't think that will be possible.

[Edmunds]

Originally Posted by theAnonymous1

You mean a different IC that you can just swap into the existing TDA7294 circuit and use @ 12V? I don't think that will be possible.

That was what I meant, yeah. Nevermind, I guess . I've searched half the world by now for a suitable step-up converter, but I seem to be getting somewhere on the second half. I'll let you know if I find something interesting.

Thank's,

/Edmunds

[theAnonymous1]

There is a China based seller on e*ay that sells a 12V to +/-35V switching supply for between 20-30USD.

Do a search for "Switching boost Power Supply board DC12V to DC±32V 180W".

I have one of these but haven't used it for anything yet. It say its 32V but mine measures 35V.

[Edmunds]

Thank's a million!

I ordered one for even less than you suggested including shipping to my corner of the World . Great. I'm not sure about the 180W max power output, but it should be enough for some 50% power and I cannot use any more than that anyway, so I'm pretty optimistic here.

Is there an easy way to work out the current requirement at the input of 12VDC at least for the max output i.e. 180W? P=IV and factor in 90% efficiency of the inverter or so?


/Edmunds

[inVain]

dear Edmunds,
why don't you just use two car batteries in series?
this setup will give you 24watt DC or 12-0-12

you also need to multiply AC by 1.414 to estimate it's DC value after rectifier

[Edmunds]

Originally Posted by inVain

dear Edmunds,
why don't you just use two car batteries in series?
this setup will give you 24watt DC or 12-0-12

you also need to multiply AC by 1.414 to estimate it's DC value after rectifier

1) The two batteries are already connected in parallel. Adding a third one is extra weight, extra space, extra charger and extra fat cables from wherever I put it. As I have 2 times 90Ah at the moment, I would probably need to duplicate that and that much of all the extra- is just no way. I was thinking at one point, maybe I should run one side of it from the starter battery as these do run on separate circuits with living area battery, but then I realized that since the starter battery is much smaller than the 2 times 90Ah, there would be high risk of running it dry and thus not being able to start the car when needed.

2) Thank you for the number, I knew it was around that, but it is good to confirm.

/Edmunds

[Formula350]

I know you already ordered something, but had you considered running a simple automotive power inverter and just leave it AC? In the US you can get them at any car parts store or almost any of the places like Wal-Mart. My uncle picked up a used one for $15 that was rated at 1500W peak, 1000W continual :P