Speaker "mismatching", 1x16ohm plus 1x8ohm speakers into same amp (nerd content :D)

santiall

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Hi,
there has been an ongoing discussion about how to connect speakers of different impedances to an amplifier and, apart from the obvious direct match (a speaker to the output of the same impedance) it often comes the question of how to connect a 16ohm and an 8 ohm speaker to the same amp. The answer is to connect the 16ohm speaker to the 8 ohm out and the 8ohm speaker to the 4 ohm output, which is a perfect match.

I understand it is confusing and some faith is needed to believe it. We've been justifying it by explaining how the power is split in halves, etc. so I've plotted the impedances of some speakers and a pure resistive load with an impedance analizer hoping that something more visual makes everything clear.

What we see below is the Anode-to-Anode impedance, in other words, what the output tubes see when looking into the primary of the transformer. The transformer used is from a 2203, a C1998 with an expected A-A impedance of 1700-1800ohm.

- The pink dashed line is a 16ohm pure resistor connected to the 16 ohm transformer output. Below 300Hz or so, the transformer primary inductance dominates and above 10kHz the parasitics start to play and the impedance rises till it resonates with the internal capacitance around 45kHz.
The 'important' bit is the middle, which is an impedance of approx. 1800-1900ohm which is what is expected. Technically should be a bit lower but the analyzer also measures all the resistances (for example the primary resistance) so instead of 1800 it can be 1900 or a bit more, I haven't measured the resistance). Anyway, that's is like a reference of what's is expected.

- The solid lines are speaker impedances. I've used Celestions Creamback G12H75 on the bench, they aren't mounted to any baffle or anything so the impedance at the low frequency resonance is a bit different to what can be expected inside of a cabinet but good enough to illustrate the impedance matching.
- Orange: 16ohm speaker into the 16ohm output.
- Green: 2 x 16ohm speakers in parallel into the 8ohm output. Pretty much same as above, probably the secondary is slightly mismatched due to the way the transformer is wound and/or one of the crocodile clips is a bit loose / not making full contact so the impedance is a bit higher than the others. Still pretty much the same anyway.
- Red: 1 x 16ohm speaker into the 8ohm output plus 2 x 16ohm speakers into the 4ohm output. As anticipated, same as above

speaker mismatch.png

So, in short, the tubes "see" the same impedance regardless of the apparent mismatch. Hope that after seeing this everything is clearer. Cheers!
 

fitz288

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- Orange: 16ohm speaker into the 16ohm output.
- Green: 2 x 16ohm speakers in parallel into the 8ohm output. Pretty much same as above, probably the secondary is slightly mismatched due to the way the transformer is wound and/or one of the crocodile clips is a bit loose / not making full contact so the impedance is a bit higher than the others. Still pretty much the same anyway.
- Red: 1 x 16ohm speaker into the 8ohm output plus 2 x 16ohm speakers into the 4ohm output. As anticipated, same as above
OK, am I reading this correctly?

- Orange: 16 into 16 = normal, no mismatch.
- Green: 2x16 into 8 = normal, no mismatch.
- Red: 16 into 8 & 8 into 4 = the known 1/2 load + 1/2 load into a 5 jack amp thing = not exactly normal, but verified as no mismatch.

I was hoping for some science on what happens when some wing-nut plugs a 16 & an 8 into their 2 jack amp = mismatch.
 

PelliX

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OK, am I reading this correctly?

- Orange: 16 into 16 = normal, no mismatch.
- Green: 2x16 into 8 = normal, no mismatch.
- Red: 16 into 8 & 8 into 4 = the known 1/2 load + 1/2 load into a 5 jack amp thing = not exactly normal, but verified as no mismatch.

I was hoping for some science on what happens when some wing-nut plugs a 16 & an 8 into their 2 jack amp = mismatch.

Well, combine the 16 and 8 Ohm speakers on one tap and you get ~5.3 Ohms - which would be fine for a 4 Ohm tap.

R = (1/R1 + 1/R2)^-1
R = (R1xR2)/R1+R2

Note of course that the 8 Ohm speaker will be doing more 'work' than the 16 Ohm, so you wouldn't get a balanced sound from both, as it were. Put them on different taps, like outlined and the difference should be negligable, taking the speaker response into consideration, obviously.
 

Jethro Rocker

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Wait, @santiall is Santiago, the Marshall master amp guru!!

Thanks so much for this response man, I’ve seen this issue go back and forth with no resolution.

Scozz
This was posted a few years ago albeit without tbe graphs for us technical know nots.
For me, that WAS the resolution as it too came from Santiago.
Good to hear from him again.
 

spacerocker

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Thanks for this @santiall Hopefully this will put this question to bed once and for all!

Calculating the impedances of the speakers as seen by the primary using "nominal" speaker impedances shows that this can be done safely and without mis-match, but seeing the real-world speaker responses compared over a frequency range is even more convincing! It can clearly be seen that the Orange (16Ohm>16Ohm) and Red (16>8, 8>4) apparent mis-match give almost identical results!
 


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