A/B/X Speaker Selector With Level Matching for <$100

[toddalin]

The part list is included on the schematic. Change R6 to 220K ohm so you don't have to wait quite as long for it to reset each time.

Right now it's operating on a "breadboard." I need to solder the pieces on a perf board and pull the power supply from an old Altec computer speaker system.

Also, I made the sides and front for a cabinet (with the holes for the 12 speaker jacks spaced for dual banana plugs in one side) but the face came out a bit too small. These were done in clear acrylic and cut on the laser cutter. The back and bottom of the chassis are from an old CD player. The two RCA plugs on the back of the chassis (from the CD player) will serve as plugs for two lights that can go on the speakers to indicate which is playing, if so desired.

For some reason Corel re-scaled the face a bit and I need to redo it. Anyway, the second itteration will have the hole for the Mute LED and the Mute and speaker selector switch, which I didn't include the first time. It will be more convenient having these features on the face for set-up or just when standing next to the system, as well as on the remote, though I don't really see the need for the randomizer on the face plate.

I'll get pics when done.

[BMWCCA]

The part list is included on the schematic. Change R6 to 220K ohm so you don't have to wait quite as long for it to reset each time.

Thanks, I see it now! Had to zoom in on the laptop (and take my glasses off!). I appreciate your work. It's something I could certainly use. An early Christmas present from you to us!

[Odd]

Thanks, I see it now! Had to zoom in on the laptop (and take my glasses off!). I appreciate your work. It's something I could certainly use. An early Christmas present from you to us!

The man in the tree, waving the Norwegian flag?

[toddalin]

Thanks, I see it now! Had to zoom in on the laptop (and take my glasses off!). I appreciate your work. It's something I could certainly use. An early Christmas present from you to us!

ItwascommentedthattheuseoftheLpadscouldcompromiset hedampningoftheamp.Assuch,Iwillmodifythecircuittoi ncludepassiveattenuatorstogobetweenthesourceandrec eiverorpreampandamp.Thedevicewillallowforeithermet hodofattenuation.Thelogicisinplaceandit'sjustamatt erofaddingacouplemorerelays/wires/plugs/pots.

Oh, I the space bar is doing it's weirdness again. Interesting that the first line ran together, but not this one. WEIRD!!!

[toddalin]

Thanks, I see it now! Had to zoom in on the laptop (and take my glasses off!). I appreciate your work. It's something I could certainly use. An early Christmas present from you to us!

Unfortunately, my schematic program doesn't output to a format that can be read by the forum, so I end up using ScreenGrabber to capture the image and put it on my desktop. That puts it into jpg, but I loose resolution.

Let me know if ther is anything you can't read.

BTW, I am modifying the circuit so that it can use the L-pads and/or passive attenuators in the line level audio path. I believe I even have the necessary parts and relays to do this. Back to the bread board!

[toddalin]

I've included a revised schematic that allows you to either/both attenuate between the source/preamp and power amp and/or between the power amp output and the speakers. The passive attenuator for the source/preamp uses stereo 100K pots in a voltage divider arrangement and triggers to the appropriate set with the speakers. This gets around any speaker damping issues. This can be defeated using the switch.

[toddalin]

Todd,

Thanks for that. I hope I have the time to build one some day. A parts list would help me, too!

Is it possible to see a shot of your finished product?

Awaiting parts from China. Meanwhile, the working breadboard:

[toddalin]

The board is now all soldered up and the A/B/X and wireless remote functions are really cool. I made some changes to the original schematic and included the revised version.

[BMWCCA]

Thanks for the update!

[toddalin]

I've now mounted all of the components onto an old CD chassis. I made a new faceplate and sides from acrylic and still need to cut the top with the laser.


This works really cool except that the relay that switches the line level signal was an old surplus piece and it has high resistance (corrosion) on the N/O contacts leading to reduced volume in that position. Of course the two channels can still be made to be the same level, but there is a loss of several dB when the attenuators are switched in. I'll order a quality piece to replace this relay and life is good. When done, I'll get some pics and maybe a video.


This will be available for forum member to use in any forum demos in Orange County CA area. I designed it so that someone can monitor the panel and there is a switch such that a 12 volt light can be placed on the speakers that will come on when that speaker plays. But the panel switch can also turn off the speaker lights and or the panel lights so blind and double blind testing may be performed.

[toddalin]

I finished out the case today and snapped some pics.


The chassis/back are from an old cd player. The original RCA jacks on the back now send 12 volts to a pair of lamps that can be placed on/near the speaker to indicate which is playing. This is also displayed on the panel and both are defeatable. The new RCA jacks are for the source attenuators.

[toddalin]

All done.


I had to replace the audio relays because the pieces I originally used were less than steller quality adding as much as a few hundred ohms and reducing the volume. The replacements were used in a professional audio mixing board and are sealed with gold contacts.


Switches and pots from left to right: Power (Off/On), Panel Lights (Off/Panel/Panel and Rear RCA Jacks), "A" Channel Line Attenuator, "A" Channel L-pad Selector (Off/On), "A" Channel L-pad, "B" Channel L-pad, "B" Channel L-pad Selector, "B" Channel Line Attenuator, A/B Channel Selector, Line Attenuators (Off/On)


LEDs in center of panel: "A" Channel (Green), Mute (Upper Yellow), Power (Lower Red), "B" Channel (Green)


Switches on Remote: "A" ("A" Channel) "B" ("B" Channel) "C" ("A" or "B" Channel at Random) , "D" (Mute)

If anyone in So Cal wants to do a speaker demo and use this, I'm good with that.





The original CD outputs now provide 12 volts in accordance with which channel (A/B) is playing and switchable on the front panel. The gold jacks are for the line attenuators that go anywhere in the audio chain between the source and amp. When "Random" or "Mute" is selected the amplifier's outputs goes to 10 ohm, 100 watt resistors until released.

[toddalin]

This thing works great and the only way to compare speakers!

I was running my L112s against the modified L200/300s. I noted that the L112s had a much more centered "center" whereas the L200/300 seemed more "ambiguous" in this regard.

Soooo, I found it has another use for the box..., phasing of the components.

I disconnected the horn leads and using the "+" terminals of the box, I was able to use the box to instantly flip the polarity of the horn driver and listen/compare it both ways. As is was, I had the polarity with the driver's + on the crossover's - because this is what the 61-band RTA seemed to show as having the flatter response in the room when I set them up. Recognize that the JBL 2235 woofer has "reversed polarity" and I run amp's - wire with the the speaker's + lead, accordingly to maintain phase with the system, so connecting the horn driver in this manner actually puts it out of phase with the woofer, but in phase with the system.

With the abillity to instantly switch, I found that I had this wrong and changing the polarity to the horn driver (2420 with tangential diaphragms on the longer L300 horns) increased focus, intelligability, and crispness enough that I could easily tell the difference, even in A/B/X testing.

Then I did the same for the 2405 tweeter and also found that I preferred the sound, again with the polarity reversed from what it was (previously crossover's - on tweeter's + to maintain system phase), but this was very hard to A/B/X and tell the difference consistently.