KaHOnas

Gretschie
Aug 29, 2021
304
Chattanooga, Tennessee, USA
I got a hold of an old busted AC30 cab. No speakers and no amplifier. Just the wood box. But I did have a little 50W Vox MV50 microamp laying around doing nothing so I figured I'd get the two together. I picked up a pair of 8Ω Peavey Blue Marvels relatively cheap, mounted them with minimal modification to the cabinet, and wired it all up.

0002471B-CD9A-4162-89E8-C1255988052F.jpeg

B75E214E-F6E9-438F-934F-1D849D8CADB3.jpeg

89B3ABC0-06B5-484A-AA6B-FB2C2CCBF296.jpeg

Now to my question: the output switch on the amp has selections for 4, 8, and 16Ω so I can wire these however I like. So, what's the advantage of series over parallel or vice versa? Right now I'm running them in series and have the amp set for 16Ω. Unless my math is wrong, if I were to run them in parallel, the total impedance would be 4Ω, right?

MV50 output label:

50W @ 4Ω
25W @ 8Ω
12.5W @ 16Ω

D012A7D5-6B0E-4D96-B9E0-6953E656DB9A.jpeg

Any input?

Thanks everyone.
 
Last edited:

wabash slim

I Bleed Orange
Feb 10, 2010
17,601
lafayette in
It depends on how loud you want to be. With two speakers, you can do 4 Ohms in parallel, or 16 Ohms in series. One speaker alone will give you 8 Ohms. What also matters is what wattage your speakers are rated for.
 

hcsterg

Friend of Fred
Feb 13, 2012
6,567
France
Attn: @hcsterg ( thanks buddy ) ;)

Sure ! If I can help, it's always with pleasure... ;)

Now to my question: the output switch on the amp has selections for 4, 8, and 16Ω so I can wire these however I like. So, what's the advantage of series over parallel or vice versa? Right now I'm running them in series and have the amp set for 16Ω. Unless my math is wrong, if I were to run them in parallel, the total impedance would be 4Ω, right?

You are right, @KaHOnas. :cool:

In the case of your 2x12" with two 8R speakers, if you want maximum efficiency in dB/watt, you should wire them in parallel, and hook them to the 4R output of the amp.

Doing so, the global output level in dB will be increased by +6dB versus the single speaker :
* +3dB due to the doubling of the cone surface
and :
* +3dB due to halving the impedance.

If you choose instead to wire the two speakers in serie, and hook them in the 16R output of the amp, then the efficiency will not be changed versus the single speaker. The global output level in dB will be :
* increased by +3dB due to the doubling of the cone surface
but :
* minored by -3dB due to doubling the impedance.

That said, as long as the output impedance selection of the amp matches the resulting impedance of the speaker association, there will be no difference (and no issue) than the resulting dB loudness per watt.

I mean, in you case, if you play at say 5W output on your amp, the loudest solution will be to wire the 2x12 in parallel, and hook it on the matching Z output of your amp. Doing this, you will have a gain in loudness of +6dB versus the single speaker arrangement.

Many years ago, for my pal the boss of our local music shop (who was a bit lost on the subject), I wrote some tables about speaker association concerning the resulting impedance, and resulting efficiency - sorry Guys, it's in French :oops:, but I think that's understandable nevertheless... If not, of course : feel free to ask ! :)

The usual cases :
yO1iNb-SPKR-Association-3.jpg


The math rules :
xO1iNb-SPKR-Association-1.jpg


The resulting efficiency in dB :
xO1iNb-SPKR-Association-2.jpg


I hope I'm clear... o_O:D

A+!
 
Last edited:

GlenP

Country Gent
Jul 23, 2019
2,364
WA
How do you know which poles on the speaker should connect to the tip and ring/shield of the speaker cable? Is there a standard convention for that? If the speaker has + and - poles, which one goes to the jack tip and which one goes to the “shield” on a TS two conductor speaker jack & cable?

planning a cabinet build myself, one of these days and just wondering.
 

j.s.c

Country Gent
Aug 19, 2008
3,625
france
How do you know which poles on the speaker should connect to the tip and ring/shield of the speaker cable? Is there a standard convention for that? If the speaker has + and - poles, which one goes to the jack tip and which one goes to the “shield” on a TS two conductor speaker jack & cable?

planning a cabinet build myself, one of these days and just wondering.

Ha! I think I could help here with the old 9V battery trick, just contact the speaker with both + & - of the battery if the bumper cone goes in your face (move ahead) then the polarity of the speaker in the same as the battery. In reverse, if it goes to the back side, than you could inverse the result. Am I right ?
 

hcsterg

Friend of Fred
Feb 13, 2012
6,567
France
Ha! I think I could help here with the old 9V battery trick, just contact the speaker with both + & - of the battery if the bumper cone goes in your face (move ahead) then the polarity of the speaker in the same as the battery. In reverse, if it goes to the back side, than you could inverse the result. Am I right ?

Yes. Still simple and reliable.

Right, but I was not sure which one goes to the jack connectors. I found a diagram here, looks like the speaker + or red goes to the tip of the jack, the - or black wire goes to the ring. Of course, my old Peavey amp has blue and yellow wires on the speaker.

https://celestion.com/blog/speaker-wiring-configurations/

View attachment 173819

Yes. That's usually what is done - but the battery trick as mentioned by j.s.c will confirm or not the speaker polarity.

Assuming a 2x12" cab where the speakers are wired in reverse polarity (= reverse phase). Usually :
- the output level in dB will be neatly decreased.
- the bass level will be strongly reduced.
- the tone will be rather nasal.

There's no danger for the amp, as long as the impedance matches : it's only a matter of loudness ans tone - or taste !

I remember a Punk-Rock player that much preferred the reverse phase tone than the correct phase tone on his 1964 VOX AC30 Twin Top Boost that I restored for him.

A+!
 


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