Dumb lo-fi question.

Hey @n80
The watts don't increase. It would mean the power would get split among all speakers.

Do you get internet access at your cabin? If so, Sonos may be your best option.

Your amp doesn't "send" watts anywhere. Nor does it even "make" 25 watts! That number, 25 wpc, is merely a reference based on a particular set of circumstances- typically (not always!) being specified as a load of 8 ohms, across a frequency range of (again, typically) 20 to 20kHz, after having been warmed up, and delivering the rated load continuously and as measured (typically, not always!) as the root mean square (RMS) of the sine wave amplitude. 

The relevance of all this technical detail is in the fact that neither of your speakers is 8 ohms, at least not across their full response band, nor any of the other stuff. (Measurement standards now considered typical only started to become the standard right around that 1980 time frame!) Anyway, it all gets much worse when you try and drive TWO sets. Because then the power equation changes dramatically depending on how the speakers are connected- in series or in parallel. Two sets of nominal 8 ohm speakers can present a load of 4 ohms, or 16 ohms, series or parallel. Amps typically would measure twice (50 watts) at 4, half (12.5 watts) at 16. 

In any case this is almost entirely academic. What matters in the real world is how it sounds. You will turn it up until either its loud enough, or distorted enough, to stop. Or maybe until it gets hot enough (or the load drops low enough) to trip protective shutoff. A lot of gear from back then (especially when sold with multiple speaker terminals) would have thermal and load protection.

Wow thats a tough one dude 12.5 in each  maybe??

Yeah depends on many factors of course - much like millercarbon wrote. You really need to know how much power your speakers will draw from your receiver. The speakers have a sensitivity rating and an ohm rating, which is a good start. Just follow the passage from millercarbon about that ohm rating and find the sensitivity rating on the speakers.  Also, older receivers tend to need parts like capacitors.... so it may need serviced too!!

Thanks guys. That's the info I needed. 

I guess a better way to phrase my question is did these old receivers ever come with two separate amps inside...one for speakers A and one for speakers B. I assume the answer is no.....but it would have been a neat feature.

I believe you are assuming correctly. Receivers of that era, and probably now also, would parallel two sets of speaker terminals.  Generally, most speakers, I say most, not all, run a nominal impedance of 6 to 8 ohms. If you run four speakers that are 8 ohms nominal (average), your total impedance would be approx. 4 ohms. The receiver might put out more than it's rated power of 25 watts per channel, which should be ok for that receiver. If you have the owner's manual they might caution you about running four speakers with impedances lower than 8 ohms. To low of a speaker impedance load, and you will wind up frying that receiver, especially if you crank up the volume.

I have a Harmon Kardon receiver with the same capability to run  2 sets of speakers, and it pretty much told me what I have told you.
Good luck!

The last paragraph of what millercarbon said is what matters. The amp will work harder driving 4 speakers compared to 2 so if you try to go as loud with 4 versus two, you will probably find the sound quality deteriorates and the amp will start to clip sooner, so be more careful in that over-driving any amp into clipping not only affects sound quality but can also damage speakers.

I ran across a review of the old Toshiba (SA-2500) that I have. It is a contemporary review and says a lot of good things about this receiver. 

http://www.hifi-classic.net/review/toshiba-sa-2500-322.html

I guess I wonder why there are two sets of outputs on a unit of that vintage? Is that a holdover from the old idea of "quadraphonic" sound?

falconquest,

Many amplifiers, and I guess receivers, from late 1970s and early 1980s had two sets of speaker outputs. Such machines usually came with many buttons on the front plate, too. As for the reason, it was probably what mental just mentioned. Two different zones/areas. Idea of Sonos and similar, but with limitations of technology that was available then.

These days, there are still amplifiers with two sets of speaker outputs. Not majority, but they are around.

n80,

Now I understand why you are not letting that receiver go. It is a pretty piece.

As far as technology goes, others have provided much more explanation than I could, so I would just like to emphasize to pay attention to speaker impedance. Usually, in your case very likely, there is a note just under terminals "A or B 4-16 Ohm, A+B 8-16 Ohm"

glupson, good catch. I never noticed that. My only real interest in this was possibly putting a couple of small towers in the middle of the room and my existing Bose 2.2s on the flanks. Totally unnecessary for a room that small but just a thought. Given the limitations of the receiver I have no intention of doing that now.

As a side note...I mentioned this in another thread...a while back several of the pots would cause static and distortion when turned, particularly the volume and I was about to toss it when someone on another web site mentioned a spray for pots and terminals and such. I bought some and opened up the receiver and sprayed the various controls and now everything works perfectly. The only thing that does not function is the tiny light on the channel slider so to change radio stations you have to look very closely or get a flashlight out. The inside is pristine. A miracle since this thing went to college with me in the balmy south in rooms with no A/C for 4 years.

First encounter with stereo equipment was 1968, Yokuska Japan, building A33.  Every Japanese Stereo manufacturer was on display, clean, well lit.  They all had lots of knobs and switches, dazzling backlit displays.  The display stands  were large round circles that drew me into their glowing wonders.  Sleek black/chrome blue light wonders.  Some had terminals and switches for up to 3 sets of speakers.  Abundant brochures from each company, touting the virtues and attributes of each unit.The American manufacturers were haphazardly placed on a table, unlit.  There was a total of 3 pieces on the table, an Acoustic Research receiver and integrated unit.  One piece by Lafayette or Harmon Kardon.  All were transistor, there were no brochures.I chose a Pioneer SX525 over a JVC Nivico.  There was zero McIntosh on display.  Years later the disparity of the displays has me still wondering about the difference in the displays.  5 years later I replaced the Pioneer with a Acoustic Research integrated unit, a big upgrade at the time. 
N80,  integrated puts it all under one hood, separates give you a lot of flexibility.  No mater what choice you make, enjoy the music, that is why we are here.

To answer the op’s question...... 25wpc divided by 4 speakers is 12.5 wpc in 4 channel mode. 
Ive had plenty of vintage equipment and the power drops dramatically with 4 speakers in a plus b mode. Also, watch the speakers rated impedance. Running 2 or more 4 ohm speakers in ab mode can fry the amp..

The amplifier will still be rated at 25 watts per channel no matter if one or both pairs of speakers are connected.

The sound output level will correspond to 25 watts of power being consumed. If one pair is connected and playing at full power, the SPL is determined by the 25 watts. If two pairs are connected, the power consumed by each pair is 12.5 watts (assuming identical speakers) and the resultant SPL drop is 3dB per pair. But when you add the SPL of each of the speaker pairs, that raises the SPL by 3 db. Conservation of energy.

I have some older amps with the A, B or A+B switching.  The best way to use the speaker options, is to use a set of speakers in two seperate spaces (rooms/areas). They would normally be switched either A or B, depending on which room/area you were in.  The other option of A+B can be used for bi-wiring one set of speakers if that is desired.

If you don’t want to take a chance on cutting your impedance in half and possibly fry the amp or destroy the speakers, get an external speaker selector box with matching impedance so the amp sees an 8 ohm load even with 6 speakers playing. PM me and I can tell you what units I have used for whole house (master bedroom and bathroom, library room, outside, living room) audio using Yamaha or onkyo receivers or go search for matching impedance selector boxes for 4 or 6 pairs of speakers.

organicsound, you mention bi-wiring one set of speakers by using A+B.

I just put in an offer on a set of low end Polk Audio towers with have double connectors on the back and claim to have a separate crossover for the two woofers (that they call subwoofers) while there is another crossover for the two mids and the tweeter. They are 8 ohm speakers.

On such low end speakers and with my old receiver is there any chance that bi-wiring this way would provide any benefit?

I'm assuming that this would not put any additional strain on the receiver?

Bi-wiring is a waste of effort on a receiver of this sort. If you did  bi-wire the speakers, you would only use one speaker output (A or B) and not both. 

Good to know, thanks.

To complete this thread, I did get the used Polk Audio Monitor 70s to replace my old Bose 2.2s. Got the Polks for $180, near mint condition.

I’m really amazed how well this old receiver drives these large speakers which sound great for Polk’s lower end line. Of course they sound way better than the Bose. Now I will enjoy listening to music down at the cabin a lot more.

But, I have no plans to run them and the Bose at the same time. However, I probably will keep the Bose down there and put them on the porch when the weather is nice.