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Old 21st March 2006, 06:34 PM   #1
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Default Question: I need to build an AB switch for testing speakers

I know this is a stupidly simple question, but I really do not know anything about what would be safest for my equipment in this instance.

Since my amp died, I no longer have the ability to do fast AB switching of single speakers. I have now ordered a new amp (SI Super-T), and I need to build a switch box so I can continue doing AB testing of my prototypes. When not in use, the switch will be disconnected.

Because I'm only switching between two speakers, I need to sum the stereo out from the amp into a mono signal, so I will splice two blacks plugs for the neg terminals and two red plugs for the pos terminals on the amp. That gives me a mono signal. I then want that wire to go to a switch box that will switch between two separate outs, one going to each speaker. So one speaker wire in (- and +) and two wires out.

My question is what kind of switch do I get? I'm mostly concerned about protecting the amp and the speakers from damage, and I want it to be as sonically neutral as possible.

Thanks for any advice on this.

Doug
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Old 21st March 2006, 07:21 PM   #2
dnsey is offline dnsey  United Kingdom
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It's not a good idea to parallel your amp's L & R outputs.
Instead, link L & R inputs (for whatever source you're using), and connect one speaker to each amp output. You can then use the balance control to swap speakers.
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Old 21st March 2006, 10:14 PM   #3
chrisb is offline chrisb  Canada
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if the test signal is mono'd before the (pre)amp - I usually use FM tuner, then just plug each channel into alternate channels on sequential inputs ( tuner/ CD. etc) - then the balance control could be used to adjust for sensitivity differences, and the selector will give you the A / B
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Old 22nd March 2006, 03:14 AM   #4
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Thanks for your input so far, but are you guys saying that I shouldn't run both left and right speaker channels to one speaker? What exactly is wrong with having both negative speaker terminals and both positive terminals from the amp going to one speaker? That just makes a mono signal, right?. The driver is a Fostex FE 108 Sigma, 8 ohm. The amp specs are 4 ohm to 16 ohm, so I think I am okay there (or am I?)

All I want to do then is put a switch on that wire to switch from one driver to the other driver SAFELY! I don't want to damage the amp or the driver.

The Super-T has an on/off button and volume knob in the front and L and R RCA's and a pair of speaker jacks in the back. That's it. There is no balance knob. No way to get a mono signal there. The only way I can get a mono signal is via the speaker wires.

Doug

(Maybe this shouldn't be done at all. Hopefully, I can get something that works).
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Old 22nd March 2006, 07:50 PM   #5
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Quote:
What exactly is wrong with having both negative speaker terminals and both positive terminals from the amp going to one speaker?
Maybe this explains why your other amp died. Probably other people can tell you with more details why this should not be done, the only thing I can say for sure is that the outputs of an amp have a very low impedance, so if the output of one is channel is higher than the other, current will flow from one chanel to the other... Feeding a mono signal to the amp is not a solution, because the gain of the amp can be slightly different from one channel to the other.

Erik
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Old 22nd March 2006, 08:49 PM   #6
dnsey is offline dnsey  United Kingdom
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Quote:
the gain of the amp can be slightly different from one channel to the other.
Agreed, but there shouldn't be enough difference to matter for a basic comparison. It would always be possible to calibrate the balance control to allow for any difference.
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Old 22nd March 2006, 10:35 PM   #7
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Quote:
Originally posted by ErikdeBest


Maybe this explains why your other amp died. Probably other people can tell you with more details why this should not be done, the only thing I can say for sure is that the outputs of an amp have a very low impedance, so if the output of one is channel is higher than the other, current will flow from one chanel to the other... Feeding a mono signal to the amp is not a solution, because the gain of the amp can be slightly different from one channel to the other.

Erik
Erik, that thought did cross my mind. However, the amp itself had A and B speaker binding posts (A left and right and B left and right, for a total of eight) and a speaker AB switch and a mono switch. I engaged the mono switch and plugged one speaker into one channel of the A posts and the other into one channel of the B posts and switched between the two. It worked fine this way for a couple of years.

I then got the "brilliant" idea that the amp was "unbalanced", so I spliced the wires to have four banana plugs for each speaker so that all binding posts were used. It seemed to work fine for a while anyway. If I screwed it up, I'll take full responsiblity. Just don't tell the wife

If you say that feeding a mono signal to the amp is also bad, then just how the heck can I do this? How is a mono switch inside an amp wired? I have this same question asked on the Class-D forum, you might check that out. They so far assure me it won't hurt the amp or speakers, but I want to be absolutely sure.

Thanks for your input.

Doug
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Old 23rd March 2006, 01:26 AM   #8
MuaDibb is offline MuaDibb  United States
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Look into a Niles SPS-4, when I built my house I did speakers in different rooms and this little box will let you choose between 4 sets of speakers or any combination of all 4. It works very well and you run from the amp to the box and plug all the speakers into the box. I think it was $99 when I bought it.
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Old 23rd March 2006, 01:41 AM   #9
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Get a double pole double throw switch. Connect the amp output to the center terminals, and the speakers to the outer terminals. The picture just illustrates how to hook up the switch. You can use any dpdt switch for that; it doesn't have to be as big as the one in the picture. And as everybody else said, don't connect the 2 channels (outputs) together. I don't see why you couldn't do a/b testing using the signal from only one channel.
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Old 23rd March 2006, 01:51 AM   #10
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But does it let you hook one speaker to the left and right channels? That has been the big sticking point here. Everyone tells me that doing this is not a good idea, and now I have a dying amp to possibly back that up.

I looked at the Niles and other units. I figured I could just build one, but now everyone tells me that doing it the way I need is bad for the amp. So I don't know what the answer is right now.

If I build a switch for the L and R inputs as suggested, I will most likely use the old computer speaker amp I'm now using instead of the Super-T. It doesn't sound terribly bad, just not as detailed. If I blow that one, it's no big deal.

It's amazing how something that to me is so simple could actually be so complicated to implement. Perhaps the answer is to reduce the stereo recordings I want to use to mono recordings (which is another thing I know absolutely nothing about)

Doug
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