Amplifier auto sense modification (subwoofer)

[pjug]

I have a subwoofer with no defeat on the autosense function, forcing me to use autosense. It isn't sensitive enough so I have to turn up the input to get it to come on and then it often turns itself off while I am listening.

I don't have a schematic for this, but looking at other ones it seems the autosense circuit idea includes a comparator that simply compares the audio signal to a level determined by a voltage divider off the positive rail. So I just need to find this and reduce the divider voltage by a factor of 10 or so? Or maybe just make it 0V? I don't care if the thing just stays on since I switch it off anyway.

Anybody have experience messing around with this? @restorer-john @Doodski ?

 

[Doodski]

I have a subwoofer with no defeat on the autosense function, forcing me to use autosense. It isn't sensitive enough so I have to turn up the input to get it to come on and then it often turns itself off while I am listening.

I don't have a schematic for this, but looking at other ones it seems the autosense circuit idea includes a comparator that simply compares the audio signal to a level determined by a voltage divider off the positive rail. So I just need to find this and reduce the divider voltage by a factor of 10 or so? Or maybe just make it 0V? I don't care if the thing just stays on since I switch it off anyway.

Anybody have experience messing around with this? @restorer-john @Doodski ?

I have not modified the input sense circuitry on a subwoofer and without a schematic we are kind of lost as per what can be done at this time. Most of those sub plate amps I've seen had issues with had blown outputs, broken solder, bad caps or where entirely swapped out by the manufacturer.

 

[DonH56]

Sub manufacturer and model?

Rythmik, and maybe others, can take a Y cable for the RCA inputs to double the input signal and help with the auto-on feature.

 

[pjug]

It's a pinnacle baby boomer (the dual 8" one), purchased for cheap on ebay. The company is out of business and I can't find a schematic, probably not to be found anywhere. Attached is a schematic having what I assume is a similar circuit. I circled in blue the voltage divider where I was thinking an easy mid could be made (assuming my sub indeed has circuit that uses the same idea).

I was using the sub output from a Sonos Amp for the input signal.

 

[pjug]

Part of my question is: if I can get some modification to work, how much change should I try to begin with? Given that this is shutting off pretty frequently I was thinking a factor of 10 to start. Or else just trying to defeat it completely.

 

[Deleted member 2944]

In that particular schematic I would change the value of R42 vice fiddling with the divider.
However, that schematic is a different scheme than most I've seen on auto-sensing subwoofer plate amps.

But, if you just switch the thing off anyway, why mess with it?

Dave.

 

[Doodski]

It's a pinnacle baby boomer (the dual 8" one), purchased for cheap on ebay. The company is out of business and I can't find a schematic, probably not to be found anywhere. Attached is a schematic having what I assume is a similar circuit. I circled in blue the voltage divider where I was thinking an easy mid could be made (assuming my sub indeed has circuit that uses the same idea).

I was using the sub output from a Sonos Amp for the input signal.

View attachment 115017

If this is a similar circuit to your sub then one option would be to supply +15V DC across a 10K resistor to the pin 5 input of IC2:B and at the same time disable the connection from C24. Basically emulate the SW1 in position "Power" that is shown in this schematic. We are assuming this schematic is similar though and there are no guarantees it is even closely similar. Otherwise change the voltage divider at pin 6 of IC2:B or even put in a trim pot so you can adjust it to conditions as required.

 

[pjug]

In that particular schematic I would change the value of R42 vice fiddling with the divider.
However, that schematic is a different scheme than most I've seen on auto-sensing subwoofer plate amps.

But, if you just switch the thing off anyway, why mess with it?

Dave.

I don't know if my writing in OP is clear, but the problem is that autosense is shutting the sub off during listening when I want it to stay on.

 

[pjug]

If this is a similar circuit to your sub then one option would be to supply +15V DC across a 10K resistor to the pin 5 input of IC2:B and at the same time disable the connection from C24. Basically emulate the SW1 in position "Power" that is shown in this schematic. We are assuming this schematic is similar though and there are no guarantees it is even closely similar. Otherwise change the voltage divider at pin 6 of IC2:B or even put in a trim pot so you can adjust it to conditions as required.

Trim pot is a good idea if I can figure out the circuit. For some reason I hadn't even thought about making it adjustable. Pain to open up the sub to adjust it though, and I wouldn't go through the trouble of trying to mount something for external control.

 

[Doodski]

Trim pot is a good idea if I can figure out the circuit. For some reason I hadn't even thought about making it adjustable. Pain to open up the sub to adjust it though, and I wouldn't go through the trouble of trying to mount something for external control.

If you go the trim pot method then adjust for always power on or if you want to get fussy adjust for proper level sensing and run with that until you are sure you have it set and then screw everything back together and voila.. done. Don't change R42 it is for purposes other than the switching sensitivity and it is required to be ~100K Ohm.

 

[Deleted member 2944]

I don't know if my writing in OP is clear, but the problem is that autosense is shutting the sub off during listening when I want it to stay on.

Yes, I understand. Increasing R42 will do exactly what you want to do. For this particular schematic, that would be the easiest/best modification to accomplish your goal.
But, you obviously need to ascertain whether your unit is even similar to the schematic you posted.

Dave.

 

[DonH56]

R42 appears to be the input resistor that sets the load (input) impedance of the auto-on circuit, and is essentially a DC reference for the ON divider (R52/R42) or the capacitively-coupled output of IC5:B that provides the audio signal to the auto-on circuit. Unless the thing driving it has very high impedance, that would not do anything.

The auto-on circuit input appears to be the diode/capacitor D2/C4 that provides a rectified average version of the input (buffered by IC2-B) to the sense amplifier IC2-A. You could change the resistors you circled, but IC2:B is basically open-loop already. Instead just disable the entire auto-on circuit by setting the threshold on the (+) input of IC2:A to ground on something like that. You could also disconnect the signal input (C24) and just pull the (+) input of IC2:B high as your schematic shows in the "ON" position. Your circuit may be different, including different polarity, so without a schematic you're shooting in the dark.

 

[pjug]

Thanks much to all who replied. I grabbed the thing and will look at it at work. Maybe this afternoon, more likely over the weekend.

 

[DonH56]

I would start with more searching to see if there is a schematic someplace (may not be for an inexpensive and old sub).

 

[pjug]

I would start with more searching to see if there is a schematic someplace (may not be for an inexpensive and old sub).

Yep, I couldn't find anything.

I was able to find the op amp with the divider circuit, and just changed a resistor for about an order of magnitude change. I didn't have time today to attempt tracing out the circuit and try to figure out exactly how it is designed. It seemed maybe a little more than a simple comparator. Anyway I just went for it and made the change.

This seems to be making a difference. A click or two difference using the volume on my phone. Now I need to see if it works better at home.

 

[dasdoing]

thank god my KEF KUBE 12b wont turn off ever even at very low volume,
but on a sidenote: is this auto sense stuff not snake oil? I asked here once in the forum and people told me that an amp with no signal wont draw much current anyways

 

[DonH56]

thank god my KEF KUBE 12b wont turn off ever even at very low volume,
but on a sidenote: is this auto sense stuff not snake oil? I asked here once in the forum and people told me that an amp with no signal wont draw much current anyways

The current drawn depends very much on the amplifier topology. Some may draw only a few watts, others may draw 10's of watts. You'll be reducing the lifetime of your electronics leaving them on all the time, and wasting energy for which you must pay. It does not sound like much, but I seem to recall studies from the IEEE and elsewhere showing anywhere from 10% to 30% of the average home's monthly power use is from "standby" and leakage energy drawn by all those instant-on devices like TVs and audio electronics as well as wall-warts (e.g. phone chargers) and power supplies (e.g. notebook PC supplies) sitting idle but plugged in. In my case, I have additional concerns because our power is not the most reliable, and I don't want things on during power outages any more than I have to. It could still happen, but I'd rather I was there when a power glitch blew up my sub than find out at the end of the day when I sat down to relax (or worst-case watching my house burn -- seems far-fetched, but happened last winter in our area, though it was a power amplifier and not a sub that was at fault).

I do not use auto-on in my subs but instead use trigger circuits. I do leave my processor in standby, so pay the extra cost for convenience, but it is also on a UPS (my subs and power amps are not).

FWIWFM - Don

 

[dasdoing]

The current drawn depends very much on the amplifier topology. Some may draw only a few watts, others may draw 10's of watts. You'll be reducing the lifetime of your electronics leaving them on all the time, and wasting energy for which you must pay. It does not sound like much, but I seem to recall studies from the IEEE and elsewhere showing anywhere from 10% to 30% of the average home's monthly power use is from "standby" and leakage energy drawn by all those instant-on devices like TVs and audio electronics as well as wall-warts (e.g. phone chargers) and power supplies (e.g. notebook PC supplies) sitting idle but plugged in. In my case, I have additional concerns because our power is not the most reliable, and I don't want things on during power outages any more than I have to. It could still happen, but I'd rather I was there when a power glitch blew up my sub than find out at the end of the day when I sat down to relax (or worst-case watching my house burn -- seems far-fetched, but happened last winter in our area, though it was a power amplifier and not a sub that was at fault).

I do not use auto-on in my subs but instead use trigger circuits. I do leave my processor in standby, so pay the extra cost for convenience, but it is also on a UPS (my subs and power amps are not).

FWIWFM - Don

I use a power strip with protection circuits. don't rember the tecnical details, but I reasearched a lot when I bought it.
I won't let my amps on all day, but my PC has to run 24/7 because I live on a tropical beach and it's the only way to protect it from humidity.
thanks for reply

 

[pjug]

thank god my KEF KUBE 12b wont turn off ever even at very low volume,
but on a sidenote: is this auto sense stuff not snake oil? I asked here once in the forum and people told me that an amp with no signal wont draw much current anyways

This particular sub amp is Class AB so yes it would draw power if not put into standby. Still I wish it has an always on option (although so far I don't want to defeat the autosense completely).

 

[boXem]

Even a class D amplifier with an SMPS will easily draw 5 to 10 W while idling. This kind of mechanism allows to divide the consumption by 10 to 20.