4780 troubles

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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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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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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.

regards
 
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?
 
Re: voltage Ok, now humming and oscillating !!!

jimbo1968 said:
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 10,000µF filter caps you should also add .1µF ceramic caps in parallel with C1 & C2 to prevent high frequency oscillation.

RDV
 
So it's 4 years later, and I'm hoping to finish this project!

Problem 1 is hum.

Since the photo I have removed one of the rectifier boards and I'm sad to say that it didn't help and now the wiring looks a mess! I don't have a good star grounding scheme yet so I expect this can be fixed. Also the hum only occurs when I plug in both RCAs from my source, either alone is OK. So for subwoofer use it should be fine with both channels driven from one input.

Problem 2 is heat.

This is either heatsinking or oscillation problem. The datasheet also warns of oscillation if Ri is high and inputs are floating, but the 22K used is in the middle of the recommended values. I don't have an input filter capacitor so intend to add one. For 15Hz HiPass I make it 0.5uF ? What types can I use? Who thinks this will solve it ?
 
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