Soundstream SA 245 noisy subwoofer channel

Dr Zeus

Member
2010-10-17 5:12 am
I got this amp in from a collector after it had been colecting dust for a number of years. This, like my MC140x, is a fairly simplistic and comprehensive design. The main difference seems to be that this amp is more or less manufactured rather than the MC140x's asembled layout. Both amps are more or less very similar in component selection which makes me pretty happy to fix this one. I've been running the MC140x in my car since fixing it and have been overly happy with it's quality and I'd like this amp to be the same for someone in the future.

First, all the switches needed cleaning. A little WD40 goes a long way in fixing the slider switches - a little trick I learned in the computer business when fixing older Dell laptop keyboards. WD40 is ever-so mildly conductive and and also cleans the corosion off the switches. its about the only thing I use WD40 ever for these days. So many uses....

Anywho, I got all 5 channels to play. Channels 1-4 are prefect and dont need anything. The subwoofer channel has a pretty large amount of white noise or hissing going on. This amp has a crossover but its kind of confusing what to do with the resistor network; which is set to low-pass at about 80hz. Despite the LP crossover, the subwoofer channel will actually play any frequency fairly well; which tells me there might be something up with the crossover or opAmp network

The subwoofer channel has one TL074CN, TL072CP, and two NE5532 opamps directly on the PCB, plus 2 x 6-Legged spider opAmps with 3087 on the backs.

The crossover works fine in channels 1-4.
 

Dr Zeus

Member
2010-10-17 5:12 am
On the SA245, the subwoofer crossover is permanently set to low-pass per the instruction manual. The SIP has 19.2k resistors which I checked and are within tollerance. Accoring to the manual that is about 85hz crossing point; however a 10,000hz frequency is just as audible as a 30hz frequency. I think its a 24db/octave crossover.

Using internal RCA switches and connections to RCAs of different channels does not change how the sub channel makes white noise or crosses frequencies; except when the switches are all set to external - then each channel has its own discrete RCA input but the sub channel still has both problems. I think the RCA/Input network is working just fine.

I was thinking of detaching the boards for Channels 1 through 4 to take some stuff out of the equasion; plus I'll likely have to solder a replacement part in the sub channel once I find what it needs.
 
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