Subwoofer locking up AVR

[theJman]

I just encountered an issue that has me stumped so I wanted to see if anyone else has run across something like it, and if so what you did to rectify the problem.

A subwoofer was recently submitted to me for testing. I plugged it into the surge protector that houses all my HT equipment, attached the RCA cable to the LFE in and then flipped on the power, just like I've done at least a dozen times before. But something new happened; my AVR (a Sherwood R904N, and yes I know it's a POS) froze. Completely locked up. I shut off the power to the sub and the AVR started working again. This is with the sub in standby - it wasn't even on yet.

Figuring it was a problem with the amp I contacted the manufacturer who promptly sent another amp and torodial transformer. When I got the new parts I swapped them in, hooked everything up again, flipped the switch and the same exact thing happened. That doesn't make a lot of sense, obviously, so I checked to see if the driver was shorted. Disconnected I got a reading of 3.9 ohms across the terminals, so the problem wasn't the driver either. But there's nothing left really. Now here's where it gets weird...

Using both the new and old parts -- in pairs and various combinations -- I tried every conceivable manner in which to connect it. Single LFE in, single low level (first right, then left) both low levels using a Y splitter, etc. Every time I flipped on the power my AVR froze. I checked the electrical outlet the surge protector is connected to, which is three prong, and it's properly grounded, so that's not the issue. Troubleshooting further I plugged the sub into a separate outlet (which is still on the same circuit, but bypasses the surge protector). Flipped on the power, but this time the AVR keeps working. Progress, finally! Flip it into "always on" to wake it from standby, and then the AVR locks up. Back to square one.

To summarize... if the power cord to the sub is plugged into the surge protector, where all my HT gear is, then as soon as I flip the switch my AVR locks up. That's in standby mode with the RCA cable disconnected. If I plug the sub into a different outlet I can power it up, but when I connect the RCA cable -- to LFE in or low level -- the instant it comes out of standby the AVR freezes.

Counting the units I own personally this AVR/RCA cable combination has been used with 11 different subwoofers, so the possibility something is wrong their is virtually nonexistent. However, it doesn't appear to be the sub either. When the manufacturer got the unit back he tested it on three systems -- Denon, Onkyo and I don't know the third -- and in each instance it worked flawlessly, and that was with the original transformer and amp. I've spoken with this person on the phone several times, so I do believe he's being completely truthful with me.

So, it's not the AVR or cable I use and it's not the sub or amp. What's left? I have to be missing something, but for the life of me I can't see what it is. Any ideas what might be causing this?

 

[TLS Guy]

I just encountered an issue that has me stumped so I wanted to see if anyone else has run across something like it, and if so what you did to rectify the problem.

A subwoofer was recently submitted to me for testing. I plugged it into the surge protector that houses all my HT equipment, attached the RCA cable to the LFE in and then flipped on the power, just like I've done at least a dozen times before. But something new happened; my AVR (a Sherwood R904N, and yes I know it's a POS) froze. Completely locked up. I shut off the power to the sub and the AVR started working again. This is with the sub in standby - it wasn't even on yet.

Figuring it was a problem with the amp I contacted the manufacturer who promptly sent another amp and torodial transformer. When I got the new parts I swapped them in, hooked everything up again, flipped the switch and the same exact thing happened. That doesn't make a lot of sense, obviously, so I checked to see if the driver was shorted. Disconnected I got a reading of 3.9 ohms across the terminals, so the problem wasn't the driver either. But there's nothing left really. Now here's where it gets weird...

Using both the new and old parts -- in pairs and various combinations -- I tried every conceivable manner in which to connect it. Single LFE in, single low level (first right, then left) both low levels using a Y splitter, etc. Every time I flipped on the power my AVR froze. I checked the electrical outlet the surge protector is connected to, which is three prong, and it's properly grounded, so that's not the issue. Troubleshooting further I plugged the sub into a separate outlet (which is still on the same circuit, but bypasses the surge protector). Flipped on the power, but this time the AVR keeps working. Progress, finally! Flip it into "always on" to wake it from standby, and then the AVR locks up. Back to square one.

To summarize... if the power cord to the sub is plugged into the surge protector, where all my HT gear is, then as soon as I flip the switch my AVR locks up. That's in standby mode with the RCA cable disconnected. If I plug the sub into a different outlet I can power it up, but when I connect the RCA cable -- to LFE in or low level -- the instant it comes out of standby the AVR freezes.

Counting the units I own personally this AVR/RCA cable combination has been used with 11 different subwoofers, so the possibility something is wrong their is virtually nonexistent. However, it doesn't appear to be the sub either. When the manufacturer got the unit back he tested it on three systems -- Denon, Onkyo and I don't know the third -- and in each instance it worked flawlessly, and that was with the original transformer and amp. I've spoken with this person on the phone several times, so I do believe he's being completely truthful with me.

So, it's not the AVR or cable I use and it's not the sub or amp. What's left? I have to be missing something, but for the life of me I can't see what it is. Any ideas what might be causing this?

I would put my money on the live and neutral being reversed in the outlet that gives trouble.

When it lock up you need to see if there is a leakage DC voltage the live pin you plug into the receiver. You will need an amplified meter or a scope, not a cheapo meter though.

Also get an outlet tester from a home improvement store an check that outlet.

The only thing that would explain the lock up, is DC phantom voltage leaking to the line level input of the sub. That is the only way plugging in the sub cable would affect the receiver.

By the way from now on if you ever encounter a problem like that use test instruments to find the problem, not the receiver. You are lucky it still works and if you keep testing that way it won't work much longer.

 

[j_garcia]

Never even heard of something like that happening. So it was only when the sub received signal? Has to have something to do with the line level connection or the power on the sub. Was the sub set for 120VAC? Really weird that it affected your receiver and not the ones they tested, so means something is maybe not isolated on your AVR?

 

[theJman]

I would put my money on the live and neutral being reversed in the outlet that gives trouble.

Also get an outlet tester from a home improvement store an check that outlet.

Every homeowner should have one of these, and I'm certainly no exception. When I moved into this house I tested all the outlets almost immediately, and every one of them checked out OK. But you're right, this was one of the first things I thought of as well.

When it lock up you need to see if there is a leakage DC voltage the live pin you plug into the receiver. You will need an amplified meter or a scope, not a cheapo meter though.

My DMM is actually pretty good; it cost me around $150, so it's not a cheapo. I don't have a scope though, nor do I know how to effectively use one, so I couldn't really do much their.

The only thing that would explain the lock up, is DC phantom voltage leaking to the line level input of the sub. That is the only way plugging in the sub cable would affect the receiver.

Not sure I fully understand what you mean. Is the leakage you're referring to coming from the amps power supply and "shorting" (for lack of a better word) the inputs?

By the way from now on if you ever encounter a problem like that use test instruments to find the problem, not the receiver. You are lucky it still works and if you keep testing that way it won't work much longer.

Nah, I'm not too worried about this AVR. I don't really like it, to be honest, so if it blew up that would only force me to get off my @ss and buy a new one.

 

[theJman]

Never even heard of something like that happening. So it was only when the sub received signal? Has to have something to do with the line level connection or the power on the sub. Was the sub set for 120VAC? Really weird that it affected your receiver and not the ones they tested, so means something is maybe not isolated on your AVR?

It was when I plugged the sub into the surge protector where the AVR is too -- even before the RCA cable was attached -- or when the sub was plugged into a different outlet and turned on, with the RCA cable connected. And I'm totally with you; after all the testing I still walked away thinking "huh?". Got me bewildered, that's for sure.

 

[TLS Guy]

Every homeowner should have one of these, and I'm certainly no exception. When I moved into this house I tested all the outlets almost immediately, and every one of them checked out OK. But you're right, this was one of the first things I thought of as well.




My DMM is actually pretty good; it cost me around $150, so it's not a cheapo. I don't have a scope though, nor do I know how to effectively use one, so I couldn't really do much their.




Not sure I fully understand what you mean. Is the leakage you're referring to coming from the amps power supply and "shorting" (for lack of a better word) the inputs?




Nah, I'm not too worried about this AVR. I don't really like it, to be honest, so if it blew up that would only force me to get off my @ss and buy a new one.

I suppose the surge protector could have leakage, and damaged the plate amp.

It could also be that the plate amp design is no good.

Most plate amps are in the classic POS category, and would never connect one to any equipment I cared about. In general they have horrid switching power supplies and DC voltage leaks are not uncommon.

In order to prove this you must have a volt meter with at least 1 meg ohm input impedance, other wise the phantom voltage will disappear as soon as you connect your volt meter.

For whatever reason, there has to be a DC voltage between the live and ground on your sub input, or a voltage between the ground of the sub and the ground of the receiver. One thing I am absolutely certain of is that this is a DC voltage leak problem.

Since I'm not there you will have to find it.

So the sequence is test the outlet.

Test the surge protector for a voltage between neutral and ground.

With a high impedance amplified voltmeter (minimum 1 meg ohm) check for a voltage between the sub ground and the receiver chassis.

With the same meter check for a DC voltage at the sub line input terminal between the signal pin and ground.

There should be no measurable DC voltage at all, except the surge protector live neutral test. There should be no more than a volt and should be a lot less. You could also test your wall outlets, and if they have over a volt between live and neutral, hire an electrician pronto.

There is nothing else that could possibly cause this and that test sequence will identify it.

We have had a post with this problem before and it was DC leakage to the live pin of the line input of the sub if I recall.

 

[theJman]

I suppose the surge protector could have leakage, and damaged the plate amp.

It could also be that the plate amp design is no good.

Most plate amps are in the classic POS category, and would never connect one to any equipment I cared about. In general they have horrid switching power supplies and DC voltage leaks are not uncommon.

That was my first thought too, but it turns out there are several hundred of these amps already in use so it sure looks like there's a problem on my end. But I've used about a dozen different subs already, so it sure looks like it's not a problem on my end.

In order to prove this you must have a volt meter with at least 1 meg ohm input impedance, other wise the phantom voltage will disappear as soon as you connect your volt meter.

My meter scales that high, so I'm good their.

For whatever reason, there has to be a DC voltage between the live and ground on your sub input, or a voltage between the ground of the sub and the ground of the receiver. One thing I am absolutely certain of is that this is a DC voltage leak problem.

Since I'm not there you will have to find it. So the sequence is test the outlet.

Test the surge protector for a voltage between neutral and ground.

That ones easy enough to do. Well, once I drag all the HT gear out so I can get to the surge protector.

With a high impedance amplified voltmeter (minimum 1 meg ohm) check for a voltage between the sub ground and the receiver chassis.

By this you mean the amps ground to the AVR chassis? With it powered up, or just plugged in?

With the same meter check for a DC voltage at the sub line input terminal between the signal pin and ground.

This one I'm not sure I understand. You want me to check across the amps electrical ground to the pin on the RCA input connector on the amp?

 

[TLS Guy]

That was my first thought too, but it turns out there are several hundred of these amps already in use so it sure looks like there's a problem on my end. But I've used about a dozen different subs already, so it sure looks like it's not a problem on my end.

My meter scales that high, so I'm good their.

The ohm scale of your meter is not the issue. The issue is the input impedance on the voltage scale of your meter. Meters with an input impedance of 1 meg ohm on the voltage scale input are amplified meters, usually FET VOMs or tube amplified meters. You need to look at the spec, of your meter.

That ones easy enough to do. Well, once I drag all the HT gear out so I can get to the surge protector.

By this you mean the amps ground to the AVR chassis? With it powered up, or just plugged in?

With it powered.

This one I'm not sure I understand. You want me to check across the amps electrical ground to the pin on the RCA input connector on the amp?

Yes, that is correct and most likely to be the source of your problem.

 

[theJman]

Round two will begin shortly... I spoke with my contact at the company -- who's been reading these threads too (I've posted this on several forums) -- and he agreed to send me another unit. Now I'll be able to test some of the suggestions and see if I'm able to resolve this.

 

[TLS Guy]

Round two will begin shortly... I spoke with my contact at the company -- who's been reading these threads too (I've posted this on several forums) -- and he agreed to send me another unit. Now I'll be able to test some of the suggestions and see if I'm able to resolve this.

Let's hope they test it for DC offset at the input before they ship it!

 

[theJman]

Let's hope they test it for DC offset at the input before they ship it!

One of the things we discussed was that it would be a brand new unit that he tested before shipping. Not sure he'll be taking any measurements like that, but at least I'll know it was personally checked.

 

[TLS Guy]

One of the things we discussed was that it would be a brand new unit that he tested before shipping. Not sure he'll be taking any measurements like that, but at least I'll know it was personally checked.

Well, pretty much the only thing he needs to test is DC offset on the input.

It would only take a few minutes, a scope would be best, but a FET VOM would suffice. I assume they have a scope.

 

[theJman]

Problem solved!

I was sent a second brand new subwoofer from the manufacturer, plugged it in using the same RCA cable and power cord I have with all the other units I've reviewed, and fired it up. As soon as it came out of standby the TV locked up, just like the original unit. This was getting ridiculous, so I stepped back to evaluate what else to do. Then, almost by accident, I figured it out...

When I get a new sub to test I allow it to break in for at least 12-15 hours before doing any critical listening. The way I accomplish that is by using the weather channel on my cable system. They scroll the 5 day weather repeated while a jazz soundtrack plays in the background. I merely leave the TV on that channel the entire time I'm breaking the sub in, varying the volume from time to time. Simple, easy, effective. Most of the time, anyway. Never having done anything other then that in the past I essentially overlooked the possibility that something associated to that process could be the culprit. Breaking that pattern was the breakthrough I needed.

Although I had tried it previously I decided a new RCA cable was in order, as was using an electrical socket on a different circuit (which is physically located within a few feet). Same problem - TV locks up. I then used an extension cord to plug the sub into a kitchen outlet, 2 rooms away, and finally made some progress; the TV would lock up still, but only if I raised the volume a little beyond normal listening level. If it was low the TV now worked! Granted that makes no sense, but at least I was getting somewhere.

Now here's the "by accident" part... I shut off the cable STB, fired up the DVD player and dropped in a CD. I was attempting to determine how loud I could get the CD before it started breaking up the TV picture too, but much to my surprise the problem was completely gone! No matter how loud I made it the sub -- and TV -- worked flawlessly. So then I dropped in a movie, and had the same result; it worked perfectly. OK, now at least I know it has something to do with cable.

I checked for a proper ground on the coupler outside the house, just to be sure the wire coming in from the street was being grounded at the junction on the house. Ground was fine there. Inside the house there's a splitter (an expensive one, not some cheapy) so I can get separate feeds for the cable modem, TV in the living room and TV in the bedroom. That wasn't grounded, so I attached one to the cold water pipe (not sure if that was actually necessary, but it couldn't hurt). I also went out and bought a line isolator and placed that strictly on the feed off the splitter to the TV.

Eureka, it works beautifully now! With the sub hooked up exactly as I have done with all the others -- using the original RCA cable and the power cord plugged into the surge protector -- I have no more problem. So in short it appears to have been related to a previously undetected ground loop issue with my cable system, combined with a subwoofer amp that seems to be rather sensitive to such things.

Thanks for everyone's help. It was a learning experience for me, and an interesting problem (to say the least), but in the end it has been successfully resolved.

 

[GlocksRock]

Glad you finally got it all figured out.