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ppia600 22nd June 2009 01:40 AM

Need output chokes for power acoustick a2400db
I got this amp over a year ago and the guy claimed it worked but made noises. After pulling it apart I found both output chokes were a little dark and one had unsoldered from the board on one end. This is the type with the two coils after the "output" transistors and a small fan that cools the drive circuit and the two chokes. After soldering the choke back in and epoxying them to keep them secure the amp worked fine so I was baffled.

Jump to a few days ago and a friend knew someone who wanted an amp. I sold it hoping the guy would connect it correctly and not fry it. Sure enough my friend texts me at about 2am telling me the guy says the amp doesn't work. What it was doing was blowing the underhood fuse (100amp) after powering on. One of the output chokes is pretty fried, apparely it was somehow shorting to itself. The power supply transistors read ok and so did the outputs. I understand the output chokes have to complete the circuit for the two sets of transistors to send signal to the woofers but why would them being shorted internally be an issue? They are chokes with a single circuit. Would it be possible to jump the circuit where the chokes were with two thick filters like on the input circuits of some amps or do I have to use coils with the same exact uH measurement? They do seem pretty heavy duty with four smaller wires twisted into one and then wrapped around the cores.

I have no idea how to get the original specs for these chokes obviously, does anyone have this amp (or its twin) that they could rob the chokes from or even just measure one out of circuit? Thanks

Perry Babin 22nd June 2009 05:36 AM

These inductors will short in virtually every amp that uses them. I've even seen them short in the latest version that uses the black fixative to hold them firmly. You have to insert an insulator between the windings to prevent them from shorting.

The schematic lists them as 50uh each.

You should probably replace both of them if you can find replacements. They need to be matched as closely as possible.

The fans fail (either don't turn or turn too slowly) and that can cause the inductors to overheat. They have a thermistor to protect them but it's generally not in contact with the inductor so it's essentially useless.

If the entire inductor overheated, it probably didn't short to itself. It's more likely that the other inductor is shorted and cause the darkened one to overheat.

The inductors need to block the high frequencies from the output filter capacitors. If they short, they cause excessive current flow which can cause damage to various components.

The inductors from the 12v side of the amp will likely saturate if you try to use them on the high voltage used in the output section of the amplifier. This would make them appear as a short circuit which would likely cause the output filter capacitors to fail.

ppia600 23rd June 2009 01:14 AM

For 50uH, all I could find were part # 807-PTKM50R-894H on Mouser. They are 9.9amp, .014 ohm, 4mhz and about $13 each I believe. That doesn't seem like high enough current capability for 1800-2200 watts rms even with the high voltage of the power supply.I searched API Delevan's site and the higher inductance chokes have relatively low current. Are you sure they are 50uH? Its very difficult to find a high power inductor in that rating, unless I'm searching for too much current.

I also noticed in similar models on ampguts that the output toroids are using the single thicker wire wound only once vs the multiple twisted strands this one uses. I wonder if I could just rewind the cores with single thicker strand wire or just use one out of a similar cheapo high power class d? I have a lightning audio 1000.1 but the toroids are smaller than I need.

djQUAN 23rd June 2009 02:06 AM

you could try rewinding it but with my experience repairing another class D that had failed output inductors, once they are toast, the core parameters change so it won't work anymore. it would for a little bit but fail after a few minutes.

ended up buying MPP cores from epay. the cores I got were a bit smaller so I used two cores one on top of the other and it's still working to this day. even works better than the original.

Perry Babin 23rd June 2009 10:30 AM

Try contacting power acoustik to see if they can supply a pair of inductors. If not, I may be willing to purchase and test a few different cores but that would take at least a couple of weeks (to get the cores and test the inductors in an amp).

Yes, 50uH. I've measured some in the past and they were just over 51uH.

I agree with djQUAN, many times when the cores are overheated, their specs will change and they will not work properly. You can try it but if the cores got too hot, they will cause the amp to fail again.

ppia600 23rd June 2009 11:38 AM

The reason I'm asking about using a different type choke is because the single larger wire design would be almost impossible to short anywhere considering the distance between strands. And it could be lightly laminated like in many other class "d" amps to prevent movement and rubbing through. I've looked on ampguts and most class "d"s seem to use the single wire style output chokes.

Perry Babin 23rd June 2009 11:54 AM

In these amps, the single winding types short as often as the litz wound type. In a few of the clones, they remove one wrap and that solves the problem. In the majority of these clones with the single wire, they try to get one wrap too many and they terminal windings short together.

ppia600 28th June 2009 06:47 AM

I believe I'll just buy some magnet wire and wind the chokes myself. Probably use 14awg and put the winds about 1/8" apart then use a light coat of epoxy. I'll also use some of that cool fabric tape around them and push the sensor against one with some thermally conductive adhesive. Surely the inductance measurement will be way off but maybe it will work fine.

ppia600 26th July 2009 03:39 PM

Here is my diy choke and the original on right. I'm sure the original has more surface area because of using six fine strands vs one large. If mine don't work I'll buy some fine strand wire and wrap it like the original, I just wanted to try solid. It was difficult to get the wire close to the core, that was as good as I could get it by hand.

ppia600 27th July 2009 02:08 AM

The amp lives once again :). I epoxied both chokes on top and bottom and in the middle. They're mounted in the amp and doing their job. It plays fine on my 4ohm woofer on small power supply now I just have to load it down in a car and see how it goes. The coils were wrapped too loose so they hit the bottom plate which I'll be cutting for clearance. Here's my sloppy QUICK setting epoxy job... probably should have used the 5 minute version instead.

The gray is just some silicone I put in between to keep the coils from bumping against eachother/rubbing.

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