Custom Switch For Center Speaker

[jonnyz123]

I have a 500C connecting to a sub from the center speaker terminals. Problem is that I want to cancel that signal to the sub when listening to earphones, or “Speaker 2” (connected to 500C but in different room than sub). Unlike my 202B, there’s no center channel switch built into the 500C.

My system spans two rooms, and suffice it to say that I don’t want to drag my lazy butt over to the other side of the house to turn off the sub directly, at the sub itself, and then turn it on again when playing speaker 1.

Was thinking I could somehow rig a little toggle switch along the speaker wire, near the receiver. Would that cause issues? Of the smoke-producing variety, or otherwise?

If not, any guidance you might provide to a non-technical guy, for rigging such a switch, would be much appreciated. I can solder...barely...and ugly like.

Thanks in advance!

 

[larryderouin]

Cut one of the wires near the 500c and install a SPST toggle. Put it in an enclosure (small box screwed to the 500c?) or just use one of those line switches that you can press together and has a thumbwheel for on/off. Like adding an on/off to an extension or lamp cord.

 

[dcgillespie]

Jonny -- Using the center channel output of the 500C to drive a sub is not an optimal method to use, as that output is a "difference" output -- not a "summing" output. Note that with your current connection, you will get no signal to the sub at all when a balance mono signal is played through your system.

Here is an approach that will solve all of your issues:

1. Use the line level input on your Sub.

2. At the Fisher, you will need two 1000 Ohm 1/2 watt resistors to make the connection between the cable going to the sub, and the speaker terminal connections.

3. Assuming your main speakers are connected to Spkr 1, connect one lead of a resistor to one Spkr 1 terminal, and one lead of the other resistor to the other Spkr 1 terminal in the other channel.

4. Join the free end of the two resistors together. This connection becomes the "hot" signal to the sub. The chassis of the Fisher is the ground lead to the sub.

With this arrangement, you will now have a summing signal going to the sub, meaning that it makes no difference which channel the bass frequency appears in and whether the signal is stereo or mono or not, and, the sub will turn off when you turn off the speakers connected to Spkr 1 by the Fisher Speaker Switch.

I hope this helps!

Dave

 

[jonnyz123]

Thanks for this Dave. Seems easy enough, even for me! I will let you know the results, and your advice is much appreciated!

 

[thornev]

Dave - I made a Frankenstein circuit board per your suggestion and it works great. At first I thought there was some kind of time shift in that the subwoofer was getting the signal slightly later than the main speakers, but after listening for a few hours I don't hear it anymore. I think I had to adjust the balance of sound between the sub and the main speakers.

I'm wondering if you know how to apply different value resistors to affect at which frequency the receiver will let pass specific bass frequencies. For example, say I want my subwoofer to receive frequencies below only 100Hz. Is there such a thing as a specific resistor that will do that?

If so, would I insert this circuit after the two 1000 ohm resistors attached to the "+" L and R speaker outputs, or replace the two 1000 ohm resistors and add the capacitor?

Thanks, Thorne

PS - I found this info and formula:

The cutoff frequency for a low-pass filter is that frequency at which the output (load) voltage equals 70.7% of the input (source) voltage. Above the cutoff frequency, the output voltage is lower than 70.7% of the input, and vice versa.

For the capacitive low-pass filter with R = 500 Ω and C = 7 µF, the Output should be 70.7% at 45.473 Hz.

fcutoff = 1/(2πRC) = 1/(2π(500 Ω)(7 µF)) = 45.473 Hz

Would this math work if I were to plug in a final value of 100 and work backwards?

This web page seems to explain it well for me:
http://www.learningaboutelectronics.com/Articles/Low-pass-filter.php

And this page does the calculations for me !
http://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php

 

[jonnyz123]

Hi @dcgillespie. I hope you are well, and healthy. It’s been a while since this subwoofer reco you provided, and with some time on my hands I recently asked my local repair guy to help me with the hookup (i’m not technical). I wanted to get your thoughts on his response:

“The 500C is already designed to drive a mono signal to the center channel terminals. It’s explained on p. 6 of the pdf of the instruction manual at hifiengine.com. Your friend may have been thinking of the gimmick back in the ‘70s of deriving pseudo-quadraphonic sound where you’d connect a third speaker directly between the two red speaker terminals? Which would indeed provide the difference between left and right signals for a spacey effect.

A 1K resistor would probably not reduce the output enough to avoid damaging the line inputs of the subwoofer.”

Thank you much Dave,

Jonathan

 

[dcgillespie]

Hi Jonathan -- Well, either I was in a drunken stupor at the time, or (more likely) had too many irons in the fire -- but between my original comments (which I see now were incomplete and also contained an error), and your tech's comments (which were right on but didn't address your issue) -- I'm sure it left you with your head spinning. I surely apologize for the misinformation on my part!! Let me see if I can straighten it all out, and I invite you to share my comments with your tech to make sure your goal is achieved if need be.

Your question was how to turn on and off the sub signal at the receiver, either by adding a switch in the sub feed or by some other means so you weren't running all over the house when you wanted to use headphones. This is because the Center Channel output on the 500C is hot all the time regardless of the position of the speaker switch.

My error came in when I explained that the Center Channel output was not optimal to use for a sub because it was a "difference" output. I'm not sure where my head was, but that was a bunch of hooey -- the Center Channel output on the 500C is in fact a summing output -- which is the appropriate type of signal to drive your sub -- but that output on the 500C has two problems for your application: It is hot all the time, and also potentially presents ground problems (depending on the design of your sub) as the two Center Channel output terminals both operate above ground level. If the ground of your sub high level input is connected to the AC ground system, that would present problems to your 500C if you also have its chassis as well.

The remedy I provided is the correct way to go (providing a summing output that is referenced to ground and controlled by speaker #1), but I omitted one little detail: At the point where the two 1000Ω resistors connect together, there should also be connected a 220Ω (1/2 watt) resistor to ground.

With the addition of this resistor, the maximum output of the center channel feed to your sub will be (assuming an 8Ω impedance tap connection):

1. With either channel (only) producing a full power (30 watt) signal, the feed to the sub in this scenario will be a maximum of 2.36 vac.

2. With both channels producing a full power signal, the feed to the sub in this scenario will be a maximum of 4.74 vac.

Neither of these levels should be so great as to overdrive your sub to the point of causing any harm using its high level inputs, and are only achieved of course when your Fisher is delivering full power output as described (based on 8Ω). If you desire, the 220Ω resistor could be replaced with a 500Ω control to provide a variable center channel output. That way, the center channel output level could be set to an optimum level based on the sensitivity of your sub's line level inputs. In this case, the "top" of the 500Ω control would connect to the point where the two 1000Ω resistors connect, the bottom of the control would be grounded, and the output to your sub would come from the wiper of the control. In either case however, the input side of the two 1000Ω resistors would still connect to the hot terminals of speaker #1 in each channel. That way, when that speaker set is de-energized, the sub will be de-energized also.

I hope this helps!

Dave

 

[thornev]

Adding a variable resistor (pot) is a great idea. When I built the suggested board, I had to add a resistor whose value I found through trial and error to reduce the signal level sent to the sub in my system as the volume control on the sub dialed in too loud a signal. Thorne