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Old 26th March 2006, 01:39 PM   #1
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Default 4780 troubles

lm4780 kit from chipamp.com kit. Found this to be considerably more difficult than 3886 or 3875 due to closer pins, and the need for insulator to heatsink combined with screw holes that are barely accessible behind the capacitors.

Finally got it together and now I need to take it apart to find out why I have -24V on one of the -ve power rails, the others are all +/-35V.

When I disconnect the amp board the voltage is correct, so I'm guessing a short between 2 chip pins. I'm off to find a magnifying glass now, in the meantime - can anyone work out what the problems might be from the symptoms?

ps. channelA appears to work fine, although it gets hot very quickly.
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Old 26th March 2006, 06:30 PM   #2
pete.a is offline pete.a  United Kingdom
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Default Magnifying Pcb's

It sounds like youv'e got a short some where along the way. Ive made the same kit and found it to be rather fiddley !
I always use a magnifyer now, even though iv'e got very good eyesight . Burnt too manty projects !
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Old 27th March 2006, 03:52 AM   #3
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Diverted answer

But u need filter capacitors. i don't see any except on the GC boards.

Gajanan Phadte
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Old 27th March 2006, 08:24 AM   #4
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The rectifier board has little electrolytics on it. The -24V could indicate that the smoothing cap is bad.

My voltmeter (on DC) reads 35V with the amp removed so is it possible that the cap only work at low load, or does my meter smooth the AC to give the expected DC value?

Or is it the bigger cap on the amp board that might be the problem? What do those little caps do? They can't store much energy. Are they just there so the voltage from the psu reads right?

I'm pretty sure there are no chip pin shorts. Tonight I will swap out some capacitors.
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Old 27th March 2006, 09:18 AM   #5
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hi jimbo1968,

IMHO, the little cap on the PSU PCB can't really be considered a smoothing cap. I think, to measure accurate DC voltage with a DMM directly after the diode bridge you need some reasonable sized filter caps.

For example: On my 22-0-22VAC BrianGT LM3875 GC, without amp PCB connected, I get 1.5 and 4 volts at the PSU PCB using one DMM and 33 volts using another DMM. With the PCB connected (i.e. 1500uF filter cap connected) I get 32 volts with both DMMs.

You can safely run you amp without those little PSU caps if you find them the problem.

You didn't mention DC offset.

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Old 27th March 2006, 09:50 AM   #6
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So, assuming no short, it may be a problem with my 1500uf cap on the amp board.

What's DC offset again? DC at the output with input shorted?
edit: i've just read the other post on dc offset.
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Old 27th March 2006, 12:34 PM   #7
RDV is offline RDV  United States
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There are spaces on the bottom of your rectifier boards to put 10,000F caps. I would put some in them.

These from Mouser are reasonably priced and have worked fine for me.

RDV
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Old 27th March 2006, 07:10 PM   #8
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Default voltage Ok, now humming and oscillating !!!

After re-soldering some chip pins (not that I could see a problem) and removing and replacing the power and output wires the -ve rail on channel B is now OK.

Now I have hum/motorboat(?) that is quite a lot louder than the noise on my 3886 amp Also channel A gets very hot very quickly and on top of the hum makes occasional noises like static interference.

I'm off to search on oscillation. Is it odd that it would effect one channel only?
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Old 27th March 2006, 07:46 PM   #9
RDV is offline RDV  United States
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Default Re: voltage Ok, now humming and oscillating !!!

Quote:
Originally posted by jimbo1968
After re-soldering some chip pins (not that I could see a problem) and removing and replacing the power and output wires the -ve rail on channel B is now OK.

Now I have hum/motorboat(?) that is quite a lot louder than the noise on my 3886 amp Also channel A gets very hot very quickly and on top of the hum makes occasional noises like static interference.

I'm off to search on oscillation. Is it odd that it would effect one channel only?
In addition to adding the 10,000F filter caps you should also add .1F ceramic caps in parallel with C1 & C2 to prevent high frequency oscillation.

RDV
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Old 27th March 2006, 10:33 PM   #10
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I was hoping to build the kit as standard.

I notice that the kit is different from the datasheet circuit which has 1000/10/0.1uf on the supply. This kit has 1500/10

Looks like adding Ci might stop the oscillation (and I want a 12Hz high pass for my subwoofer anyway).
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