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 Post subject: Combining three speaker outputs into two
PostPosted: May Sat 25, 2019 4:47 pm 
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Joined: Apr Sun 14, 2019 12:42 am
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I have an old Motorola console stereo with L, R, and center base outputs. I'd rather not have to run three speakers to get full range sound. Running just the L and R I lose nearly all the base. Can I combine the three channels into two and recover the lost base output from the center channel?


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 Post subject: Re: Combining three speaker outputs into two
PostPosted: May Sat 25, 2019 5:19 pm 
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Joined: Feb Thu 28, 2019 7:48 pm
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Location: Lawrenceville, Illinois 62439
If it were me, I'd find an inexpensive bass speaker and hook it up - you can park it most anywhere; under the unit if room, or even inside a louvered cabinet or nearby closet. Your ears can't detect where bass (low) notes are coming from (within a certain distance).
Dan


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 Post subject: Re: Combining three speaker outputs into two
PostPosted: May Sat 25, 2019 7:06 pm 
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Joined: Jul Fri 18, 2008 10:02 am
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Location: near ST Louis Mo 62002
You ill need two new larger output transformers and larger coupling caps to make the L & R channels full range .
The L & R channels are single ended so this would take some work to combine them to PP , and use the bass channel as the the second channel .
It is a lot of work to convert it to a full range stereo amp with good fidelity .

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 Post subject: Re: Combining three speaker outputs into two
PostPosted: May Sun 26, 2019 2:22 pm 
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I'm going with just adding a bass speaker. Thanks for the advice.


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 Post subject: Re: Combining three speaker outputs into two
PostPosted: May Sun 26, 2019 3:51 pm 
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battradio@ wrote:
You ill need two new larger output transformers and larger coupling caps to make the L & R channels full range .
The L & R channels are single ended so this would take some work to combine them to PP , and use the bass channel as the the second channel .
It is a lot of work to convert it to a full range stereo amp with good fidelity .



Mine is PP X 3, all 6V6s.

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 Post subject: Re: Combining three speaker outputs into two
PostPosted: May Sun 26, 2019 9:42 pm 
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Location: Rochester NY USA
And mine is four channels: P-P 6BQ5 for the center, two P-P 6BM8 for R & L, and a separate P-P 6BM8 chassis for reverb. Moto found lots of ways to do the same thing... they had three-channel amps with two, four, six output tubes.

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 Post subject: Re: Combining three speaker outputs into two
PostPosted: May Mon 27, 2019 6:50 am 
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Location: Powell River BC Canada
quote: Running just the L and R I lose nearly all the base.

Try reversing the polarity on the right or left speaker, and see if the bass improves.

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 Post subject: Re: Combining three speaker outputs into two
PostPosted: May Mon 27, 2019 9:57 pm 
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radiotechnician wrote:
quote: Running just the L and R I lose nearly all the base.

Try reversing the polarity on the right or left speaker, and see if the bass improves.


It won't help it has been filtered out ad directed to the center channel .

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 Post subject: Re: Combining three speaker outputs into two
PostPosted: May Tue 28, 2019 1:30 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
battradio@ wrote:
radiotechnician wrote:
quote: Running just the L and R I lose nearly all the base.

Try reversing the polarity on the right or left speaker, and see if the bass improves.


It won't help it has been filtered out ad directed to the center channel .



Cant hurt to just try it.

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 Post subject: Re: Combining three speaker outputs into two
PostPosted: May Tue 28, 2019 4:11 pm 
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Joined: Aug Wed 04, 2010 5:40 pm
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Location: Tyngsborough, Massachusetts
I have a Motorola SP-30-1-GL phonograph that has the three-channel setup. The left and right channel signals go through a filter to remove the bass frequencies before the output amplifier, and the center, bass, channel passes through a filter to remove the higher frequencies before going to the driver, phase inverter and output amplifier.

To get full response from the left and right channels, it would seem that the high-pass filter network must be removed or bypassed. The output transformers also may not be adequate for the lower frequencies.

Jeff


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 Post subject: Re: Combining three speaker outputs into two
PostPosted: May Tue 28, 2019 9:21 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11628
Location: Powell River BC Canada
In centre bass systems the left and right speakers are intended to present an illusion of
separation. The bass fills in to give the same effect, as if there were two speaker systems.

With a two speaker stereo setup, if the phasing is wrong, you will hear it immediately.
Thin sound from two sides. When corrected, the bass centre is heard with just two speakers.

Ping-Pong stereo recording vanished very soon after the , "this is the left" , "this is the
right", demo records came out. 60 years ago.


Bass is filtered with steep slope filters to protect the cone from burning out with small
drivers.

The OT was to try and make the bass come back. As I said, just try reversing one
speaker. It will work, or it will not.

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