My educated guess from stereo and guitar amp experience--if they are specifying 8 ohms I'm betting the amp circuit will draw too much power and/or run too hot on 4 ohms. It will pump more power into a 4 ohm load than it would an 8 ohm load. Maybe it'll work on a pin, maybe not, I'm sure someone must have tried it.

Its mentioned in the first thread that the amp/s are reported to be 4 ohm capable

http://pinside.com/pinball/forum/topic/stock-stern-speakers-vs-ff-frequency-response

Other reading / reference threads-

http://pinside.com/pinball/forum/topic/massively-improve-your-acdc-premiumle-sound-under-80

http://pinside.com/pinball/forum/topic/new-speakers-on-my-ac-dcahhhhhhh

I have put 4 ohm speakers in my spiderman.
What I did ( warning somebody told this to me I do not have background knowledge myself):
Normally stern speakers are wired paralel if you want to use 4 ohm speakers you have to wire them in serie so that the resistance for the amp is stil 8 ohm.
One wire to left speaker plus, one wire to right speaker min, on wire from left speaker min to right speaker plus and you are good to go.
Now also buy one of those cheap car bass speakers ( no rewiring needed) and the sound quality is greatly improved for €60,-

Series is the bottom diagram-

http://www.dziedzic.us/wpc_speaker_sound_info.html#newwiring

Correct. Chuckle nailed it. Two 4 ohm speakers wired in parallel will result in a 2 ohm load at the sound amp. Wiring them in series is what you need. Plus it will lower the bil slughy so that if you have an upgraded sub, it can get a more balance of bass.

You should either use an 8 ohm speaker or two 4 ohm speakers wired in series to make use of the available power and keep the amplifier happy with the impedance it 'sees'. Some may suggest using a high power 4 ohm resistor to soak up the power, but this will waste power and cast it off as heat, so it is not a very good idea - possible, but not the best option. If you add additional 4 ohm speakers in series you can either add them to the game cabinet, or run them out to an external speaker box. The amp doesn't care where the speaker lives as long as it is wired correctly.

okay if you use 4 ohm wired in serial for the backbox, do you need to use an 8 ohm sub also? or is a 4 ohm sub okay??

If you use a passive subwoofer (unpowered), the impedance will depend on the amplifier output you choose. You cannot run a passive subwoofer without an amplifier.

If you use a powered subwoofer, you will be connecting your speaker outputs from the game to the high powered inputs of the subs amplifier. The games amp won't care what the subs speaker impedance is, because the speaker is connected to the subs internal amplifier, and the necessary impedance matching circuit is on the subs front end.

i was going to run a passive sub with a crossover. Does anyone know what ohm the stock stern sub is?

The sub amp is its own amp channel on all stern games. So you get dedicated 4 ohms regardless of what's happening in the backbox.

You will want to lower the vol of the backbox a bit with the sub to increase the bass. So wiring in series is a good start. Depending on the efficiency of the sub you choose, you may want to add 20-30 ohms of additional resistance to the backbox if install an lpad.

No this is wrong. Having built tons of guitar cabs speaker rules are simple.

Parallel is the ohms of your highest speaker. So if you have 10 4ohm speakers in Parallel it will be 4 ohms. It will not add or subtract from it.

If you series them then it will increase.

For solid state the ohm rating is not as critical. You want to be close. For older tubes its critical.

And the general rule is always have the speaker equal or greater than the amp.

Running a 4ohm speaker on a 16 ohm amp will cause damage as it needs the resistance. Also lower the ohm the louder/power. On amps it shows the power vs ohms., etc.

For pins I would not worry one bit. I have car speakers in all my pins at all different ohms.

Good Luck

I'm pretty sure that is incorrect

Here's a site with calculators, choose parallel or series and it does the math for ya
http://www.speakerimpedance.co.uk/

NO! this is WRONG. 10 4 ohm speakers in parallel is .4 (four TENTHS of an ohm!)

I will point you to the formulas:
http://www.dummies.com/how-to/content/electronics-components-parallel-resistors.html

Looking at a Stern schematic the audio amp chip is a Pentawat TDA2030 / 18 Watt .

Looking at one data sheet:

The TDA2030A is a single-channel, low-frequency class AB audio power amplifier.

It can deliver 14 Watts into a 4 Ohm load.

It comes in the 5-pin PENTAWATT package.

so are you saying I can run them in parallel and get 14 watts out of them or run them in series for 18 watts?

I suggest all who want to play with speakers look at AND USE the excellent calculator on the site suggested by SteveP3:

http://www.speakerimpedance.co.uk/

Some of the misinformation in this thread is mind boggling.......

No. A single 4 ohm load (a single four ohm speaker, or two 8 ohm speakers in parallel) that is capable of handling at least 14 watts is required is what is meant.

I'm wondering about your statement here. If the Stern speakers are 8 ohm each and wired in parallel as you say, that presents a nominal impedance of 4 ohms to the amp (this appears consistent with what cal50 said about the TDA2030A amplifier earlier in the thread). Therefore, if you are replacing with 4 ohm speakers you need only one 4 ohm speaker to present a nominal impedance of 4 ohms, not two 4 ohm speakers in series.

You can't do this - a subwoofer requires an amplifier I think you are confusing the terms subwoofer and woofer.

I should have looked at cal50s links early in the thread.

"The Stern SAM systems are driven by two 18w 4 ohm amps. One is used for the backbox hooked up with the red wires, the other is used for the cabinet sub, hooked up with yellow wires. The cabinet sub is running full range. The speakers used in the back cabinet are cheap 4" speakers. These are run in series."

So...we are talking about 2 existing amps in this series of pin:
one amp drives 2 speakers in series (anyone know exactly what impedance they are?)
and the other drives a single subwoofer at 18w of power, probably also 4 ohm impedance.

This is a simplified wiring diagram with load values.

http://www.eminence.com/support/wiring-diagrams/

In my STLE, the backbox speakers are hm and they are wired in series.
What also puzzles me is that they seem to be in counter-phase. The A+ goes to the + of 1 speaker, A- goes to the + of the other speaker. Then both - are connected together with a 22µF capacitor.

I'm thinking of replacing them with 4Ohm speakers, also wired in series (but in Phase).

Any thoughts?

This is an old audiophile trick called "poor man's surround sound". http://monkeyfighter.com/surroundsound.html

I just don't have enough technical information to advise on changing the impedance of speakers on STLE. Usually, there is a bit of research done to choose a certain design. Even though deviating from the ratings may work, the safest best is to replace with ratings called out in the design.

The TDA 2030A is pretty much a bulletproof amplifier, and is capable of delivering much, much higher quality sound than a pinball machine needs.

Although specified for 4 ohms, it will run happily - and more efficiently - with higher impedance speakers. I'm sure there's a minimal - and undetectable in a pinball machine - tradeoff in quality. I prefer running the speakers at 8 ohms if I can because reliability goes up. In WPC machines, the audio electrolytics are usually pretty much destroyed. This destruction happens more slowly at 8 ohms.

The bottom line is:
- You can use 4 ohm or 8 ohm speakers in the cabinet
- You can use 4 ohms or 8 ohms in the head. Wiring two car speakers in series is a great solution

For the cab, pick your speaker on efficiency - 93dB or more - and a frequency response rated from 40 -50Hz. Some efficient speakers start at 90Hz, and that's no good.

For the head, you might need to add a series resistor of anything from 8 - 20 ohms to balance with the cab.

Thanks, I never heard of that. I wonder if they actually encoded the sound that way...

Yes, they did. Dolby surround coded four audio channels – Left, Center, Right, Surround (LCRS), into two channels referred to as Right Total and Left Total.

http://en.wikipedia.org/wiki/Dolby_Surround

Ok, then I better keep it wired that way...

I saw that they use basically the same design of amplifier as in the WPC systems.
The wear on the electrolytics is not a big concern. They are easy to replace...
I'll be doing some experimenting with the speakers soon now that I know I can use 4Ohm speakers.

Always good to proceed with a little caution. Have fun!

There is so much misinformation in this thread that I want to throw up.

WRONG. That's how you wire speakers in series. They are neither out of phase nor wired for some fake surround sound in that configuration. They are wired in series to lower the volume of the backbox.

WRONG as everyone already told you. Lol.

Both Pam from pinbits and cal50 made some sense here and provided good advise. If you want to run two 4 ohm backbox speakers on a stern, wire them in series to protect your amp. It really is that simple.

Series wiring the negatives must be connected to the positives...from tantaka1961s post he says the negatives are connected to each other through a cap. Is there something I am missing here?
http://www.eminence.com/support/wiring-diagrams/
http://cie-wc.edu/Series_Parallel_9_14.pdf
http://www.audiokarma.org/forums/archive/index.php/t-103793.html

Show me a an example of a diagram that shows you series wiring has both negatives connected to each other. I've been in audio electronics for over 30 years I have never seen series wiring with the speaker polarity reversed, except for using phase reversal to pull out surround.

The question was not whether to wire them in series or not, but that originally they are wired 'out of phase' (from what I understand)
I never saw any schematic wiring them like this except for the reason wayout440 explained.
Currently, I have them wired in series (and 'in phase') A shame Stern didn't use 3 real channels like Data East.

The negatives are not "connected to each other". The cap is used as a high pass crossover only. The entire 2 speaker "circuit" gets crossed over. The cap can go on either terminal it doesn't matter. And since the speakers are wired in series, it crosses over the set together. Wiring is something like:
Amp to speaker 1 +, speaker 1 - to cap terminal 1, cap terminal 2 to speaker 2 +, speaker 2 - to amp. That's in series and the load shows as 16 ohm at the amp. Stern uses the cap to help clean up the back box voices. I've also seen: amp to cap term 1, cap term 2 to speaker 1 +, speaker 1 - to speaker 2 +, speaker 2 - to amp.

Its possible that the guys at stern wired one of your speakers backwards. They actually make mistakes a lot with it also for the cabinet speaker. Its unlikely you can hear the difference on those crossed over crappy 4" they use either way.

But we should not forget this thread was started by someone asking about speaker impedance and wiring for experimenting with speaker replacements safely.

Sorry if I've gotten a bit 'off-topic', but this is actually the reason why I asked this. I want to replace those ''crappy 4" " and I also want to do the wiring properly.
So the question remains if they reversed 1 speaker on purpose or by accident (but this last one is off-topic I guess)

That makes more sense, thanks Markmon. I was only going by the text as typed. Tantaka's could be installed wrong?

I think PamPinbits answered the OPs original question, and stated you can use 4 or 8 ohm...and some other specs that you might take into consideration.

The 4 ohms speaker will sound 2 times lower than a 8 ohms and to compensate you will turn the sound up and by the same way use more power from your power transistor. It will heat more so be sure the heatsink is good and dissipate heat properly.

Really recommended to use an 8 ohms speakers or two 4 ohms speakers in line.

Yes mine MET pro is wired the SAME way. In series, but 1 speaker the reverse the polarity.

in mine cse i have exacly the same:
2 red/white wires are connected to the left speaker +
Then from the - of left speaker to one end of this elco, then to the - of right speaker.
Then 2 red black wires are connected to the + of right speaker.

In this case they used 2 8ohm 4watt speakers in serie.

Yes it is in serie, but one speaker is reversed polarity. (surround job?)

I look in the manual, and there you see. The guys at stern did nog follow the original schematics for series wiring, for some reasons. Dont no why, or it was or wasnot intended to do so in the first case.
In factory schematics it shows an clear original series wiring, with crossover (elco)

See olso page 57 of the metallica user manual @ stern website.