Rockford Power 1100a2

[shawnk]

Trying to finish up this amp and I'm getting stuck in the final stages.

I'm having an issue where there's audible noise present (buzzing) when I engage the crossover, even when the amp is idle. The noise is audible in both high and low pass settings and is only audible on the right channel. The left channel is silent and appears to be ok. With the crossover switch in "full range" position, the noise goes away.

I have tried my best to track down this noise. I believe there's something I must be missing within the high/low crossover circuits but I just can't seem to find it.

Thoughts?

Thanks

 

[seankane]

Do you have DC on any of the op-amp output pins?

Have you removed the switch and tested it or tried a different one?

 

[shawnk]

I have tried a new switch, and no there isn't any DC on the output pins for the op-amps.

This is driving me nutty..lol The noise is present ONLY in the right channel, and ONLY in the highpass, or lowpass setting. This narrows it down dramatically and yet I still can't find the culprit. It's as if there is a ground reference that is missing.

R163 & R159 do not have resistors present (factory spec), but if I place a 10k ohm resistor across the pads, thereby clamping the signal to the source ground, the noise goes away. But this of coarse reduces the overall signal, and is not a proper repair.

Thanks

 

[seankane]

You have checked the shield ground pins for the RCA jacks, and both 1N4003 Diodes?

You have confirmed there are no broken traces?

 

[shawnk]

I repaired the RCA shield before I even attempted to test the amp.

I'm not sure which diodes you are referring to... I don't see any in the signal chain

I don't believe any of the traces are bad to be honest. I've gone through just about every component in the chain and everything "appears" to be ok.

Thanks for the help Sean

 

[Perry Babin]

Can you see the buzzing part of the signal on the scope?

If so, where does it enter the signal chain?

Have you tried grounding the heatsink?

 

[shawnk]

Hi Perry,

I tried grounding the heatsink but it didn't make a difference.

Unfortunately I don't detect the noise on the oscilloscope. I have to say that the noise is not excessively loud... it's pretty quiet, but is certainly noticeable considering when the crossover is switched off (full range), the amp is dead silent.

I probably shouldn't have said the noise is a "buzzing" sound. I think a better way to describe it is more like "hissing"... very similar to what a high noise floor might sound like from a poor quality amp with the gains cranked.

It's really throwing me off since the amp appears to be just fine in full range mode, yet I don't see anything wrong within the two crossover circuits.

I don't know.

Thanks

 

[seankane]

There are 2 1N4003 diodes on the board.

They are not in the section you are looking at. Alot of times when the ground pins on the shield were broken one of those diodes was bad. It may have been a fluke thing, but I have seen it several times.

There is two of them on the board, and one of them can cause the amp to make wierd noises and such. May not be your problem but its a cheap and easy fix to try and resolve your problem.

I find it strange you cant see this noise on your scope.

Can you hear it through a speaker?

Does the noise change with a dummy load connected?

 

[Perry Babin]

If the diode between the RCA shield and the amp ground is shorted, you will read 0 ohms from the RCA shield (no RCAs plugged in) to the amp's ground terminal.

I've had trouble with these amps having a very noisy output (inaudible, hf noise). I ground the sink via a parallel 1k resistor and a 0.1uf capacitor. It generally cleans up the output significantly. This isn't your problem but if you have a lot of high frequency noise on the output, you should try it.