As the handle suggests, I'm new here. This is my first question, and I hope I'm following the rules here. I have a PSB century 2i 10" powered subwoofer. It has no option to remain on all the time, and the "auto on" feature is a nuisance, as it often falls asleep and doesn't wake when it should. I've adjusted its level, so that the AVR driving its input is +2db. I may lower its trim even more, so that the AVR has to send a hotter signal, in the hopes of keeping it awake. I've also turned on "extra bass" in the AVR, meaning the sub should be receiving a signal all the time, and not just when there's something in the LFE channel.
All this to say, would it be safe to add a small DC offset to the input of this powered sub, in order to keep it on all the time? I was thinking literally 1 volt or so. I'm hoping there's a decoupling capacitor in the signal path somewhere, so that DC offset never actually reaches the speaker. It's an older discreet amp, with 2 output transistors and a huge 40V dual rail supply, rather than a chip amp. Thanks in advance.
All this to say, would it be safe to add a small DC offset to the input of this powered sub, in order to keep it on all the time? I was thinking literally 1 volt or so. I'm hoping there's a decoupling capacitor in the signal path somewhere, so that DC offset never actually reaches the speaker. It's an older discreet amp, with 2 output transistors and a huge 40V dual rail supply, rather than a chip amp. Thanks in advance.
I can't answer your question specifically, however as to adding a DC offset...
It doesn't sound a great idea tbh. Your amp 'probably' has a gain of unity (1) at DC due to the standard configuration of feedback networks. So 1 volt DC offset at the input would give 1 volt at the speaker.
It may well have other AC coupling in the signal path particularly around any input circuitry and processing. That would remove DC from the speaker.
The whole thing may well be AC coupled right through and so an added DC offset would do nothing.
It doesn't sound a great idea tbh. Your amp 'probably' has a gain of unity (1) at DC due to the standard configuration of feedback networks. So 1 volt DC offset at the input would give 1 volt at the speaker.
It may well have other AC coupling in the signal path particularly around any input circuitry and processing. That would remove DC from the speaker.
The whole thing may well be AC coupled right through and so an added DC offset would do nothing.
After 20 years it may just be caps in the signal path that have gone bad... Have you examined the board and components?
Thanks for the responses so far folks. Everything on the circuit board looks fine. The electrolytics aren't tented up, and I only see some browning on the board where there are (2) TO-220 case transistors. I can't quite read them, but they are TPICs of some sort. (power supply regulators?). The output transistors attached to the heatsink look fine also. There's nothing black and burned, and screaming "hey...over here". I suppose I could just attach a 1.5V battery to the line input, and see if the woofer stays out. I'm just trying to keep this thing "wake", without keeping the speaker coil energized. Thanks again
I would be very wary of just connecting a 1.5 battery and seeing what happens... not something I would do tbh.
If you really really want to investigate what adding a DC input does then you should rig up a preset resistor across the battery and then you can get an adjustable 0 to 1.5 volts DC level.
Having said that I would still not advise using added DC as a means of keeping it on.
If you really really want to investigate what adding a DC input does then you should rig up a preset resistor across the battery and then you can get an adjustable 0 to 1.5 volts DC level.
Having said that I would still not advise using added DC as a means of keeping it on.
Oh for sure... keeping the speaker coil energized sounds like a terrible idea. I just wish I knew enough to bypass the "auto on" feature. This powered sub is in a home theatre room, where if anyone is in the room, it should be on, and then the power is cut from the entire room upon exiting. (A double pole light switch, as if a lightning strike can't jump an 1/8 of an inch). In other words, on is on, and off is off, with no in-between state. I don't mean to keep this thing powered 24/7.
If we had circuit details we might be able to advise on doing that. Difficult without... perhaps there is a relay switching mains to the main circuitry that could be bypassed.
I've had no luck in finding a schematic unfortunately. There's no relay, other than the one I installed to disconnected the speaker from the turn-on and turn-off thuds. These pops may not damage the speaker in any way, but it annoyed me to no end. Seems simple enough to implement a muting circuit in what was NOT an inexpensive piece 20 years ago or so.
Worse comes to worse, I may replace the innards entirely with a chimp amp. Yes.... "chimp" amp. As in, something that someone like me (primate with a screwdriver), could accomplish. It would likely be a step down compared to a discreet circuit from back in the day, but I could live with it. I repaired an 8" Polk powered sub using a TDA7293 chip amp just recently. Ironically, it was the original chip in that newer/cheaper sub anyway. It now lacks level, phase, and cutoff control, but I reckon that's what the AVR is for. Works remarkably well IMO.
Worse comes to worse, I may replace the innards entirely with a chimp amp. Yes.... "chimp" amp. As in, something that someone like me (primate with a screwdriver), could accomplish. It would likely be a step down compared to a discreet circuit from back in the day, but I could live with it. I repaired an 8" Polk powered sub using a TDA7293 chip amp just recently. Ironically, it was the original chip in that newer/cheaper sub anyway. It now lacks level, phase, and cutoff control, but I reckon that's what the AVR is for. Works remarkably well IMO.
Only you can decide how you proceed with this but I suppose what you suggest is an option.
Almost impossible to advise on the existing design without knowing the details I'm afraid. You would need an understanding of circuit configurations but it might be worth seeing how the line input is processed 'to keep it awake'. If it is opamp based circuitry then you might increase the gain of that section to enable it to keep powered. Also look at any time constants that determine how long it stays on in the absence of any signal.
Almost impossible to advise on the existing design without knowing the details I'm afraid. You would need an understanding of circuit configurations but it might be worth seeing how the line input is processed 'to keep it awake'. If it is opamp based circuitry then you might increase the gain of that section to enable it to keep powered. Also look at any time constants that determine how long it stays on in the absence of any signal.
Thanks Mooly. I see no DIP package opamps inside, just a few TO-92 case transistors. There's a balance to be struck between investing time to "fix" what is already more or less working, and finding an excuse to "build" something new. Nothing like a pandemic to help me find the time.
Only you can make that choice 🙂
If its all discrete then we definitely would need diagrams to advise you. A chi(m)p amp could be a way to go if you fancy building one. Lots to choose from, just make sure you check supply voltages first if using the existing power supply.
If its all discrete then we definitely would need diagrams to advise you. A chi(m)p amp could be a way to go if you fancy building one. Lots to choose from, just make sure you check supply voltages first if using the existing power supply.