What's Going On With My Amp?

[Peter P.]

My amp has the ability to handle 2 channels, A and B.

 

My Heresy's are connected to Channel A; my subwoofer is connected to Channel B.

 

My subwoofer has a Standby Mode; after 5 minutes of no signal, the blue LED turns red.

 

I just noticed today that if I have the Channel B speakers (subwoofer input) turned off at the amp, the subwoofer won't go into Standby Mode, even with the amp turned off.

 

If I put my ear close to the subwoofer I can hear a low level rumbling. So I got the bright idea to put a voltmeter at the speaker leads to the subwoofer and with the Channel shut OFF, I measure 135mV of signal! If I turn ON the channel, the signal drops to essentially 0mV and the subwoofer successfully enters Standby Mode.

 

So why would I measure a signal with the amp shut off AND the channel turned off, but with the amp off and the channel turned on, there's no signal?

[fmalloy]

Sounds like turning off Channel B leaves the speaker output "floating" and not tied down, and it is picking up 0.135V of a stray signal from nearby components on the PCB, and it's enough to keep the sub thinking there's a signal. When Channel B is on, there is a driver with a series capacitor/inductor that is keeping any stray signal from reaching the output.

 

I think it's common for speaker switches to leave the speaker terminals floating when switched off; I don' t know if 0.1V at the terminals when off is within spec or not. 

 

Can you just use the same speaker output to drive your main speakers and your sub?

[glens]

3 hours ago, Peter P. said:

So why would I measure a signal with the amp shut off AND the channel turned off, but with the amp off and the channel turned on, there's no signal?

 

The lead(s) from output B to the subwoofer are picking up an induced voltage, likely from a power cord.  When you turn on B, amp on or off - doesn't matter - so long as A is on and there's a load on it, as was said above the speaker snubs the induced voltage (you could try turning A off and it likely would act just like it does now whatever any other combination).  Try putting some twists in the wires feeding the sub or at least route them so they don't run alongside a power cord.  If that doesn't get it, check for a potential between chassis of the amp and sub.

[Peter P.]

I think I found the source of my mysterious voltage.

 

I unplugged all my stereo equipment from the wall and STILL measured a voltage-and 60Hz line frequency!

 

I took a length of speaker wire roughly 15 ft. long, and the width of my room, from subwoofer to amp, and laid it along the floor.

 

I put a voltmeter across the wires at one end and immediately read an AC voltage in the 0.XXX range. Then I set the voltmeter to frequency and it picked up line frequency-60Hz!

 

So I held the wire up in the air, in free space, and the frequency went to zero. But when I laid  the speaker wire down, near the 220V outlet for the airconditioner, which sits adjacent to the subwoofer, the voltage and frequency appeared.

 

Mystery solved! I suppose I could perform one more test and lay the wire down near the 220V outlet, connect it to the subwoofer, and see if I hear the rumble, but I have to run to work.

 

Thanks for the replies!

[glens]

https://www.nema.org/Technical/Documents/Bull_88_reaffirmed_12_15_11.pdf

[dtr20]

On 5/28/2019 at 7:28 AM, Peter P. said:

I think I found the source of my mysterious voltage.

 

I unplugged all my stereo equipment from the wall and STILL measured a voltage-and 60Hz line frequency!

 

I took a length of speaker wire roughly 15 ft. long, and the width of my room, from subwoofer to amp, and laid it along the floor.

 

I put a voltmeter across the wires at one end and immediately read an AC voltage in the 0.XXX range. Then I set the voltmeter to frequency and it picked up line frequency-60Hz!

 

So I held the wire up in the air, in free space, and the frequency went to zero. But when I laid  the speaker wire down, near the 220V outlet for the airconditioner, which sits adjacent to the subwoofer, the voltage and frequency appeared.

 

Mystery solved! I suppose I could perform one more test and lay the wire down near the 220V outlet, connect it to the subwoofer, and see if I hear the rumble, but I have to run to work.

 

Thanks for the replies!

Great, he just gave credit to the stupid bridges that the crazy over the top audiophiles use to keep their wires off of the floor.