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 Post subject: Building a Stereo Crossover to Work with a Single Woofer
PostPosted: Oct Fri 11, 2019 2:03 am 
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Joined: Apr Sat 30, 2011 2:14 am
Posts: 673
Location: Mississauga, Ontario
Greetings to All,
I'm a little out of my comfort zone here so thought I'd ask the opinion of the good members here.
I would like to incorporate a separate amplifier into my 1941 Seeburg jukebox. It will still work as original but I've built and added a switch box to divert the power from the jukebox motor to the auxiliary amplifier to allow playing music from a bluetooth device. I have added 2 small bookshelf speakers into the cabinet that are driven by this aux amp. My issue us that it lacks bass with the aux amp. I'd like to incorporate the 15" juke speaker wired mono to compliment the added speakers. I did it with a direct connection but the small amp cuts out after 20 seconds or so and I assume it is overloaded but sounds great!
I think I need to build or adapt a crossover to alleviate this problem but I'm not sure how to do it?
The other issue is that the small speakers probably have a built in crossover.
Can I do this?
Will the existing networks be affected?
I'm sure there is a simple solution as AMI jukeboxes are actually wired this way.
Any suggested or help would be greatly appreciated.
Thanks, Mauro


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 Post subject: Re: Building a Stereo Crossover to Work with a Single Woofer
PostPosted: Oct Fri 11, 2019 2:37 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5379
Location: Rochester NY USA
The small amp may not be able to handle the low impedance with all the speakers connected. How about this - connect the 15" speaker in series with the the two external speaker ground leads - a capacitor in parallel with the 15" will direct the bass to the big speaker. Assuming it's 8 Ohms, a 15 uF cap will cross over around 100 Hz. Since you probably don't have a 15 uF non-polar cap, you can use two 30 or 33 uF caps in series, plus to plus or minus to minus. Try 22 or 47 if you don't have the 33 uF.

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 Post subject: Re: Building a Stereo Crossover to Work with a Single Woofer
PostPosted: Oct Fri 11, 2019 2:49 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4165
Tom's idea above would be simplest. With an efficient speaker it doesn't take a lot of wattage for a woofer to be loud, so he's probably right about the impedance being too low for that amp.

Now if it still doesn't work, a standard LC crossover probably won't help, as it simply segregates highs and lows to tweeters and woofers, but the same power is still drawn (maybe more if the crossover is lossy.) If the amp can't cut it then next easiest, if you can fit it in, is to get a cheap powered woofer from Goodwill. Next easiest is to get one of those $5.00 amps off eBay and use it to drive the woofer.


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 Post subject: Re: Building a Stereo Crossover to Work with a Single Woofer
PostPosted: Oct Fri 11, 2019 5:30 am 
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Joined: Jun Sat 15, 2019 7:43 pm
Posts: 260
Why don't you use the jukebox amp for your bluetooth device?

DM


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 Post subject: Re: Building a Stereo Crossover to Work with a Single Woofer
PostPosted: Oct Sat 12, 2019 3:17 am 
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Joined: Apr Sat 30, 2011 2:14 am
Posts: 673
Location: Mississauga, Ontario
Thanks for the great suggestions!
I'm going to try Tom's idea first, now I don't know if I understand it correctly.
Here's what I think he means, left and right channel both go to the 15" speaker first then a np capacitor in series with with the + leads going to each of the bookshelf speakers.
Neg -------15" spkr-----------neg 1st speaker--- neg 2nd speaker
Pos -------15" spkr----np cap-----pos 1st speaker---pos 2nd speaker

Hmm, can't be right as it seems more like a parallel connection?
Let me know where I screwed up.
Thanks, Mauro


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 Post subject: Re: Building a Stereo Crossover to Work with a Single Woofer
PostPosted: Oct Sat 12, 2019 3:03 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5379
Location: Rochester NY USA
Like this:

Attachment:
spkr008.jpg
spkr008.jpg [ 276.55 KiB | Viewed 348 times ]

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 Post subject: Re: Building a Stereo Crossover to Work with a Single Woofer
PostPosted: Oct Sat 12, 2019 7:20 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
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Location: Annapolis, MD
I've never designed crossovers, so I get to ask dumb questions.....:)

In that circuit, the low frequencies pass through the "high frequency" speakers.
1. Do they care?
2. When processing a signal below their response rolloff, do they appear resistive or reactive?
3. Regardless of #2, doesn't their impedance need to be part of the rolloff calculation for the big LF speaker?

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 Post subject: Re: Building a Stereo Crossover to Work with a Single Woofer
PostPosted: Oct Sun 13, 2019 1:21 am 
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Joined: Apr Sat 30, 2011 2:14 am
Posts: 673
Location: Mississauga, Ontario
Tom, thanks for the schematic!
It helps a ton!!
Last question, I see one return connection and I'm assuming that's the negative terminal, but what do I do with the other negative terminal?
Do I tie them together?
I apologize for the spoon feeding needed but I'm really out of my element, but I really appreciate all the responses.
Mauro


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 Post subject: Re: Building a Stereo Crossover to Work with a Single Woofer
PostPosted: Oct Sun 13, 2019 1:28 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5379
Location: Rochester NY USA
Yes, low frequencies will pass through R / L speakers - but there will be less, and they're not reproducing low frequencies well now. They'll be mostly resistive at the low end. True, they affect the crossover frequency, so does the woofer impedance, but actual crossover frequency isn't that critical. Since both channels feed through a common woofer, there may be TOO much bass, but it also depends on relative speaker efficiency, which is unknown. Try and see...

As for the speaker return, use either right or left on amp , both are ground.

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 Post subject: Re: Building a Stereo Crossover to Work with a Single Woofer
PostPosted: Oct Fri 18, 2019 11:01 pm 
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Joined: Apr Sat 30, 2011 2:14 am
Posts: 673
Location: Mississauga, Ontario
So I'd like to thank Tom and all who contributed my education.
I wired up the speakers as per Tom's diagram but used a 22uF NP cap as that's what I had on hand. It sounds much better now with the 15" speaker in the mix.
Wish I'd ordered a more powerful amp than 50w now, lol.
Thanks to all, I'm quite pleased with how it turned out,
Mauro


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