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Old 4th March 2009, 11:17 PM   #1
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Default DC from INPUT!

Ok, i've been reading on these forums for about a year now, but finally come across a problem, that seacrh has no answers too.

I have a class D car amplifier, with 14.4 volts poaring from its RCA inputs.
I've looked for inline caps and found no shorts over any of the caps connected to the input stage.

If I power it up it works fine, but eventually fries headunits.
Not good!

So do you guys have any idea what this might be?
I was contemplating just getting some decent capacitors to check before the rca's, but i thought prevention is better than cure!

Pics of said amp. Will do close ups on request!
Can't find any schematics anywhere!!!!
http://ampguts.realmofexcursion.com/Powerbass_XA-3000D/

Thanks for any help
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Old 5th March 2009, 03:56 PM   #2
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How about a suggestion of some caps i could put inline. Worst case scenario is 14.4v (dc) with my 6v Hu pre outs (ac).
What sort of farad rating cap should i use? 1000uf or 1uf? I don't know which to pick?

It's a sub amplifier so i need it to allow everything down to 15hz.
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Old 6th March 2009, 04:03 PM   #3
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Quote:
Originally posted by agithegreat
How about a suggestion of some caps i could put inline. Worst case scenario is 14.4v (dc) with my 6v Hu pre outs (ac).
What sort of farad rating cap should i use? 1000uf or 1uf? I don't know which to pick?

It's a sub amplifier so i need it to allow everything down to 15hz.
You can calculate the exact value you need, if you know the input impedance of the amp. The value is probably going to be somewhere around 1-5uF. Many Tripath amps have a 22K input impedance, and with 3.3uF blocker caps, these amps are flat down to 3-10 Hz.
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Old 6th March 2009, 04:57 PM   #4
bver100 is offline bver100  United Kingdom
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Hi, while you can of course add additional caps to the inputs to block the DC, the fact that DC is there indicates a fault !

If this amp worked OK once upon a time I would endevour to find out why is doesn't now.
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Old 6th March 2009, 07:15 PM   #5
Tim__x is offline Tim__x  Canada
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Location: Edmonton area, Alberta
I agree with bver100.
If the amp still amplifies, and your post seems to imply it does.
My guess would be that the input caps in the amp are tantalum, failed short. Try to find the input caps, test them, and if needed replace them.
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Old 6th March 2009, 10:28 PM   #6
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Sorry, i'm a major newb to audio work on amplifiers, so excuse me...
The amplifier does still work! It still hits damn hard! Pushing what i believe to be 3kw rms.
I've had a look for some input caps, but with my simple multimeter i found nothing in the way of conductance from the rca inputs...
Roughly where will these capacitors be?
I've checked the 4 that are right next to the rca inputs, (which i thought would be the problem), but they seem fine... I whacked a 1.5 cell across them and they charged an discharged rapidly!

I know ideally it needs a fix, but as the amp is performing as should except for the dc leak through RCA, i was just looking for a fix to remove this.

Sorry i know little about how these work, but simply using a touch of common sense i followed the tracks but found nothing in the way of the RCA centre pin (singal pin).

I've ordered a couple of 10uF caps, i used f = 1/ 2 x pi x R x C to calc a C value, i hope that was correct.
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Old 9th March 2009, 11:15 AM   #7
bver100 is offline bver100  United Kingdom
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well i guess that if you are happy that is the main thing !

assuming that there is nothing else wrong, 10uF should be fine.

i think you said that the voltage in question was about 15V, in which case the cap should be rated at 25V or more.

make sure that you put in it the correct way round !
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Old 9th March 2009, 03:23 PM   #8
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Yeh ok, i bought unpolarised ones... so i shouldnt have to put them in any particular way i hope....
Also i got 35v ones.

I hope this was correct.
I'll try them as soon as they come and post up my results!
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Old 9th March 2009, 06:16 PM   #9
bver100 is offline bver100  United Kingdom
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they should be fine and yes, they can go in either way round.
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Old 9th March 2009, 06:58 PM   #10
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Still getting 8v DC across my RCA's folks...
I put the caps inline with a plug and socket either side...
But I'm still getting 8v on the socket side when the rca's are plugged in.
Anything else I can do???
It still works fine, but my RCA cables get melted! Also I'm sure it's not great for my HU!
That was with a 1mf cap, rated at 50v???
My 35v 10mF haven't come yet...


Does the DC get discharged from the capacitor as the AC passes???
Still not got a full grasp of how the capacitor works. It stores a small amount of DC charge, but will allow AC to pass through it yes.... But does the DC charge get discharged as the AC passes??
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