This is the Kenwood SW-30 subwoofer which I have updated with a new driver and a new port.
http://i1186.photobucket.com/albums/z379/mgalabrun/DSC_7912_modified.jpg
I had to edit the signal path to eliminate the "subsonic filter" which was a bass boost feature, by running a wire from pin 1 of IC 502 1/2 to R 522 through a 10 mf capacitor. It worked great and everything sounded great for a while, but now there is a loud humming noise.
There is nothing if it is cold, but once it has warmed up it gets worse. It is also really bad on return from standby. The standby feature only cuts power to the main board, it does not turn off the transformer, so it stays warm even in standby.
I am hoping that there is something obvious that is mostly likely in need of replacement, like one of the capacitors.
Please advise, Thank you,
Here is the schematic.
SW-30 schematic.jpg - Google Drive
http://i1186.photobucket.com/albums/z379/mgalabrun/DSC_7912_modified.jpg
I had to edit the signal path to eliminate the "subsonic filter" which was a bass boost feature, by running a wire from pin 1 of IC 502 1/2 to R 522 through a 10 mf capacitor. It worked great and everything sounded great for a while, but now there is a loud humming noise.
There is nothing if it is cold, but once it has warmed up it gets worse. It is also really bad on return from standby. The standby feature only cuts power to the main board, it does not turn off the transformer, so it stays warm even in standby.
I am hoping that there is something obvious that is mostly likely in need of replacement, like one of the capacitors.
Please advise, Thank you,
Here is the schematic.
SW-30 schematic.jpg - Google Drive
Thank you for your reply. I just tried that and there is no humming noise when the inputs are shorted. It wasn't warm though, so it wasn't humming anyway.
Check for bad solder joints on 330R dropper resistors R801/802 (this unit uses a simple zener-regulated supply for the preamp portion), as well as on board interconnects (especially grounds). If nothing found, look at all the boards (also watchng out for cracks in the PCB), and monitor voltage drop over said resistors just in case there is oscillation going on.
Also, a subsonic is not a bass boost, it's primarily a highpass. While it can negatively impact bass quality if set too high, it also improves level handling, so be sure you know exactly what you're doing. The average American home (drywall galore) has a habit of gobbling up bass like there's no tomorrow, and that amp is none too powerful. I would rather simulate the response and see where it gets us, and if there is overshoot, modify part values for a clean rolloff.
Also, a subsonic is not a bass boost, it's primarily a highpass. While it can negatively impact bass quality if set too high, it also improves level handling, so be sure you know exactly what you're doing. The average American home (drywall galore) has a habit of gobbling up bass like there's no tomorrow, and that amp is none too powerful. I would rather simulate the response and see where it gets us, and if there is overshoot, modify part values for a clean rolloff.
Yes, just checked the Highpass and even without simulation (which I don't do anyway) I see it just cuts off anything under 8Hz ... way below any audible frequency, so should have no audible effect at all.
So just put things back to normal and check whether your hum/buzz disappears.
If anything, it´s caused by something else.
EDIT: when it starts humming touch heatsinks, do they significantly heat up?
"Delayed humming" stinks of thermal instability>runaway with "old power supply caps about to burst" as a second possibility.
So just put things back to normal and check whether your hum/buzz disappears.
If anything, it´s caused by something else.
EDIT: when it starts humming touch heatsinks, do they significantly heat up?
"Delayed humming" stinks of thermal instability>runaway with "old power supply caps about to burst" as a second possibility.
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8 Hz actually seems rather too low to be of much use. Depending on level requirements, I would have gone with 20-30ish.
I think that result is actually incorrect though. For C1=C2=47n, R1=12k, R2=430k I'm getting fc = 47 Hz. Seems more like it. A bit high for something considered a sub, but then again, the power amp is running on +/-32 V, can't be more than 30-ish Wpc or so (or 50 into 4 ohms)? Not sure how efficient the driver is, of course, might still be something a bit more sensitive that doesn't go quite as deep... I'm thinking maybe 90-ish dB / W / m. How old is that sub, early/mid 1980s? Keep in mind that woofers in those days didn't go as deep in return for being more sensitive.
I think that result is actually incorrect though. For C1=C2=47n, R1=12k, R2=430k I'm getting fc = 47 Hz. Seems more like it. A bit high for something considered a sub, but then again, the power amp is running on +/-32 V, can't be more than 30-ish Wpc or so (or 50 into 4 ohms)? Not sure how efficient the driver is, of course, might still be something a bit more sensitive that doesn't go quite as deep... I'm thinking maybe 90-ish dB / W / m. How old is that sub, early/mid 1980s? Keep in mind that woofers in those days didn't go as deep in return for being more sensitive.
Okay, Thank you for responses!
I am not at all knowledgeable in circuits and electricity, just a little. I had some help from this posting to eliminate the subsonic filter.
editing old Kenwood amp-board
The improvement really worked. I think that this sub is from the early 80's, probably for use in a home theater system, and from what I understand, they used to put in a bass boost in the 50Hz range to improve the sounds of voices. It sounded terrible for music, only producing one baggy bass note. The new driver is 80w RMS. At first it was no better, but with the improvement, it sounds great, and it's not using too much power🙂.
I do have a multi-meter, and I am daring. You will have to be very specific. Did you expect me to power on the unit and test resistors? What should I be looking for? Oscillation? I don't know what that is.
If I had to try something, what would be the most likely candidates for replacement, i.e. capacitors?
I am not at all knowledgeable in circuits and electricity, just a little. I had some help from this posting to eliminate the subsonic filter.
editing old Kenwood amp-board
The improvement really worked. I think that this sub is from the early 80's, probably for use in a home theater system, and from what I understand, they used to put in a bass boost in the 50Hz range to improve the sounds of voices. It sounded terrible for music, only producing one baggy bass note. The new driver is 80w RMS. At first it was no better, but with the improvement, it sounds great, and it's not using too much power🙂.
I do have a multi-meter, and I am daring. You will have to be very specific. Did you expect me to power on the unit and test resistors? What should I be looking for? Oscillation? I don't know what that is.
If I had to try something, what would be the most likely candidates for replacement, i.e. capacitors?
The humming is slow to increase and once the unit is warm it tends to stay warm.
Can you help me to identify which caps I should try to replace? I have already replaced the two big ones on the amp board.
Hi
Suggestion 1. You said that humming is absent when the unit is cold and slowly increases. Did you try shorting the inputs and waiting the usual time till the humming starts?
Suggestion 2. why not back track the modifications done to the circuit and see if it still hums.. If it doesnt , you would know where to concentrate your efforts. If it hums still, you will know the problem is somewhere else. If I understand correctly , you jumpered from Pin 1 to Pin 7 of IC502 via a 10u cap, right? and no other modifications to the circuit?
regards
Prasi
Suggestion 1. You said that humming is absent when the unit is cold and slowly increases. Did you try shorting the inputs and waiting the usual time till the humming starts?
Suggestion 2. why not back track the modifications done to the circuit and see if it still hums.. If it doesnt , you would know where to concentrate your efforts. If it hums still, you will know the problem is somewhere else. If I understand correctly , you jumpered from Pin 1 to Pin 7 of IC502 via a 10u cap, right? and no other modifications to the circuit?
regards
Prasi
1 - Thank you, I did try shorting the inputs but not for that long. What does this show?
2 - I didn't want to undo the modifications that I have already made because they were so successful at eliminating the sound problem that I was having. Also it looks like that is an isolated portion of the signal path not having to do with the heat developing in the more heavily powered boards, but I am a total beginner and haven't many insights into the problem. Cool web-page by the way! PCB board design is very cool.
I would like to try switching out a few electrical components to see if anything would work.
1. It shows if the problem persists even if inputs are shorted.
2. this will show if the humming is caused by mods done and if everything works fine if the ckt is brought to its original state.
another, you said you changed out the big caps.. The new caps, were they from ebay or some other unknown vendors and from very old stock. Did you measure the actual capacitance?
Check if there is a cold joint when the caps were replaced and check the orientation too.
2. this will show if the humming is caused by mods done and if everything works fine if the ckt is brought to its original state.
another, you said you changed out the big caps.. The new caps, were they from ebay or some other unknown vendors and from very old stock. Did you measure the actual capacitance?
Check if there is a cold joint when the caps were replaced and check the orientation too.
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