Impedance question from hard of thinking old duffer!

[Karma Train]

Hi there,
Can anyone help me out here please?
I'm in the throes of putting a vintage system together because I'm of an age that nostalgia sets in I suppose!
Technically speaking I suppose this topic should be posted in the 'Past Masters' section because the gear in question is as old as the hills but as my particular query is pertaining to speaker impedance which is a general subject, I'm hoping I can get away with posting it here.
I just picked up a circa 1978 JVC JA-S44 integrated which works a treat and sounds lovely and I'm currently running it through a pair of Technics SB-6000s also from 1978, it's a sweet combination and I'm enjoying it greatly.
However as the Technics speakers are the size of small wardrobes and I have neighbours, I want to add a second pair of speakers to the amp for low level late night listening.
The JVC amp has connections on the back for two pairs of speakers and a selector switch on the front which can switch between speakers 1, speakers 2 and speakers 1+2.
Underneath these speaker connections there is a printed warning which reads 'System 1+2, 8~16 ohms - Others 4 ~16 ohms'.
The Technics SB-6000s run at 6 ohms and the second pair of speakers I want to add run at 8 ohms.
Now I realise that this setup would be perfectly fine if all I ever did was switch between the two pairs of speakers but would engaging both pairs of speakers in the 1+2 position fry my amplifier in a second even if I only hit that position by accident?
I've googled this scenario for ages and although there's a heap of info out there not much of it is sinking in I'm afraid.
In short would I turn my amp inside out if I switched on both pairs of speakers even accidentally? Could I possibly even run both pairs of speakers at a low level without damaging anything?
Sorry if this an idiotic question but I'm suffering from Google fatigue and thought it far better to come to the font of all hi-fi knowledge.
Thanks for bearing with me.

[struth]

Will be ok

[Karma Train]

Will be ok

Thanks, that's reassuring, I was kind of hoping for something with a little more theory behind it though,
i.e. what load does a pair of 6 ohm speakers coupled with a pair of 8 ohm speakers present to the amplifier,
and if it IS OK to run all four speakers at once at a low level, the figures and the reasoning behind it?
Sorry to be a pain but I'd hate to damage this fine old amp if I could possibly avoid it.

[jollyfix]

Much of my youth was spent with big old Amps with speakers A&B, always with 2 sets of speakers used at the same time. the wall of sound my mates used to call it. Funny but could still get a stereo image, not focused, but hear things from everywhere type of sound. Used to love messing around with placement 2 high, 2 low etc ( always book shelf spks).
Took a long time to get used to a 2 speaker set up ,when i changed. Just missed that wall of sound, not HIFI i suppose, but loads of fun, and to this day some tunes i play just don't cut it, and sound lacking.
Never blew an amp ( pardon the expression), maybe i was lucky, sorry no help on the tech side of things, just reading this thread brought back some good memories.

[Karma Train]

Much of my youth was spent with big old Amps with speakers A&B, always with 2 sets of speakers used at the same time. the wall of sound my mates used to call it. Funny but could still get a stereo image, not focused, but hear things from everywhere type of sound. Used to love messing around with placement 2 high, 2 low etc ( always book shelf spks).
Took a long time to get used to a 2 speaker set up ,when i changed. Just missed that wall of sound, not HIFI i suppose, but loads of fun, and to this day some tunes i play just don't cut it, and sound lacking.
Never blew an amp ( pardon the expression), maybe i was lucky, sorry no help on the tech side of things, just reading this thread brought back some good memories.

Never blew an amp....fnaar, fnaar! :lol:
Glad I brought back some nice memories for you, get to my age and that's pretty much all I've got left!
I don't want to run 2 pairs of speakers constantly of course but it would be nice to do occasionally for the wall of sound effect you mention,
really don't want to damage the amp though as it really is a lovely old thing, twin illuminated power meters 'an all!
Someone has got know the maths behind this kind of setup though.
2 at 6 ohms plus 2 at 8 ohms shows what impedance to the amp and is it safe at lowish volumes?

[Pharos]

Your speakers are quoted as 8 and 6 ohms respectively. In parallel this would give product/sum value of 3.42 ohms, a little below the 4 quoted.

But those figures are quite nominal, most speakers have impedances varying quite a lot, and it is the very low values which are really of concern, these at often different points on the frequency spectrum.

The worst case is when two paralleled speakers have low impedances which coincide, and this is relatively unlikely, and 3.42 ohms is only a little below 4 ohms, so I think you will be OK, especially at low levels.

[walpurgis]

My thinking too.

[Karma Train]

Your speakers are quoted as 8 and 6 ohms respectively. In parallel this would give product/sum value of 3.42 ohms, a little below the 4 quoted.

But those figures are quite nominal, most speakers have impedances varying quite a lot, and it is the very low values which are really of concern, these at often different points on the frequency spectrum.

The worst case is when two paralleled speakers have low impedances which coincide, and this is relatively unlikely, and 3.42 ohms is only a little below 4 ohms, so I think you will be OK, especially at low levels.

Thank you so much for that comprehensive answer, much appreciated.
As I said in the OP this old amp has two sets of speaker connections and the selector switch which allows for selection of system 1 or 2 or 1+2.
Using the provided connectors would I be wiring them up in parallel?
Also, there is a warning written on the back of the amp which states 'System 1+2' - 8 ~ 16 ohms'.
So if you say that the sum value is 3.42 ohms surely that would be far too low for the required 8 ~ 16 ohms requested by the legend on the amplifier?

[Audio Al]

I think you need to ask Paul at RFC

http://theartofsound.net/forum/forumdisplay.php?78-Reference-Fidelity-Components

He likes things like this and should be able to help with a technical answer :)

the link is to his trade page on AOS

[jollyfix]

Just to ad, the speakers i used were all 8ohm. Amps used Akai AM-2950, Sansui 990.

[Firebottle]

So if you say that the sum value is 3.42 ohms surely that would be far too low for the required 8 ~ 16 ohms requested by the legend on the amplifier?

Hi Charlie, the reason the spec for 1+2 is 8-16 ohm is because with both speakers selected/connected they are in parallel.
This means that both speakers are presenting a load to the amp, in parallel 2 x 8ohm loads works out to 4 ohms.

This minimum suggested load of 4 ohms is to not stress the amplifier beyond its current rating, when used at maximum output.
Halving the load on the amp doubles the current required to provide the same power output. Excess current in the output transistors is what will likely kill the amplifier. Keeping the output/volume some way below maximum will keep the current levels in order and will be safe to use.

Pm is you want more detailed explanation,
:)

[struth]

if you ever want to do math again, its a*b / a+b. You originally said if you accidentally pressed both buttons for a second etc, so didnt think you wanted the calc. sorry. Its only 1/2ohm below so as Alan says and you intimated, if volume is not at max it will be fine.

[Karma Train]

Hi Charlie, the reason the spec for 1+2 is 8-16 ohm is because with both speakers selected/connected they are in parallel.
This means that both speakers are presenting a load to the amp, in parallel 2 x 8ohm loads works out to 4 ohms.

This minimum suggested load of 4 ohms is to not stress the amplifier beyond its current rating, when used at maximum output.
Halving the load on the amp doubles the current required to provide the same power output. Excess current in the output transistors is what will likely kill the amplifier. Keeping the output/volume some way below maximum will keep the current levels in order and will be safe to use.

Pm is you want more detailed explanation,
:)

Thank you most kindly, I get it now....(apologies for my apparent thickness).
I did so much reading about this and yet was still left totally baffled.
One of the things that confused me most was that in all the online explanations when the term 'both speakers' was used they actually meant both 'pairs' of speakers, just knowing that clears up most of the fogginess for me, thanks again.
This old JVC amp as I mentioned before has twin illuminated power meters on the front, perhaps watching what happens with those when both sets of speakers are engaged will also give me a visual clue as to what's going on inside the amp as well.

[Karma Train]

if you ever want to do math again, its a*b / a+b. You originally said if you accidentally pressed both buttons for a second etc, so didnt think you wanted the calc. sorry. Its only 1/2ohm below so as Alan says and you intimated, if volume is not at max it will be fine.

Thanks Grant, that really is a great help.
I'll hold on to that little formula probably until I snuff it now.
:)

[Pharos]

"Halving the load on the amp doubles the current required to provide the same power output."
I think it is I squared that doubles.