Amp Impedence Question

[Harker1440]

I have a 2 Channel H head and 16 ohm 1x12 cab and i just picked up another vcab but its 8 ohm where do I set the impedence
8 ohm or 4 ohm when using both cabs?

[SpHj]

Personally I'd replace one of the speakers so they are either both 8ohm, or both 16ohm. Mismatching speakers could lead to a world full of heartache

[Em7]

You would select the 4-Ohm tap on the output transformer.

parallel impedance = 1 / ( 1 / cabinet_a_impedance + 1 / cabinet_b_impedance)

If cabinet_a_impedance = 8 Ohms and cabinet_b_impedance = 16 Ohms, then the parallel impedance = 1 / ( 1 / 8 + 1 / 16) ~= 5.33 Ohms

The problem with wiring an 8-Ohm cabinet in parallel with a 16-Ohm cabinet is uneven power distribution. The 8-Ohm cabinet will receive 2/3rds of the power.

cabinet_a_power_percentage = cabinet_b_impedance / (cabinet_a_impedance + cabinet_b_impedance) x 100

cabinet_b_power_percentage = cabinet_a_impedance / (cabinet_a_impedance + cabinet_b_impedance) x 100


cabinet_a_power_percentage = 16 / (8 + 16) x 100 ~= 66.7%

cabinet_b_power_percentage = 8 / (8 +16) x 100 ~= 33.3%

If both cabinets share the same impedance rating, then the power is divided evenly between the cabinets and the parallel impedance is 1/2 of the cabinet impedance.

[dr johnsonstein]

You would select the 4-Ohm tap on the output transformer.

parallel impedance = 1 / ( 1 / cabinet_a_impedance + 1 / cabinet_b_impedance)

If cabinet_a_impedance = 8 Ohms and cabinet_b_impedance = 16 Ohms, then the parallel impedance = 1 / ( 1 / 8 + 1 / 16) ~= 5.33 Ohms

The problem with wiring an 8-Ohm cabinet in parallel with a 16-Ohm cabinet is uneven power distribution. The 8-Ohm cabinet will receive 2/3rds of the power.

cabinet_a_power_percentage = cabinet_b_impedance / (cabinet_a_impedance + cabinet_b_impedance) x 100

cabinet_b_power_percentage = cabinet_a_impedance / (cabinet_a_impedance + cabinet_b_impedance) x 100


cabinet_a_power_percentage = 16 / (8 + 16) x 100 ~= 66.7%

cabinet_b_power_percentage = 8 / (8 +16) x 100 ~= 33.3%

If both cabinets share the same impedance rating, then the power is divided evenly between the cabinets and the parallel impedance is 1/2 of the cabinet impedance.

I was gonna say that, really. Now, my head hurts....which way sound sounds better? Em7, your depth of knowledge never ceases to amaze.

[DirtyMoonsRJT]

Very good post!

[PRSHB2]

I was gonna say that, really. Now, my head hurts....which way sound sounds better? Em7, your depth of knowledge never ceases to amaze.

I had typed all the same info up, but figured why double post.

[Em7]

Which way sound sounds better?

Sound is subjective. The reason why I brought up the uneven power distribution issue is due to the fact that guitarists will often opt to plug a 16-Ohm 4x12 cabinet into a combo with one 8-Ohm speaker. The best thing to do in this situation is to disconnect the internal speaker when plugged into the 4x12.

With that said, one can take advantage of uneven power distribution to be able to mix and match high power and low power speakers. For example, a Celestion Blue is a 15-Watt speaker. However, we can safely use a 16-Ohm Celestion Blue in 2x12 cabinet with a 40 to 50-Watt amp if we wire it in parallel with an 8-Ohm Vintage 30 or G12-65 (that is, if we refrain from "diming" the amp). The cabinet impedance will be 5.33 Ohms. The Blue will receive one-third of the power.

your depth of knowledge never ceases to amaze.

I have a long way to go before I reach mad scientist level!

[Harker1440]

Thanks Mark for the Info!!

[Haycock_Fan]

You always get the maximum clean headroom when you have the correct impedance. It is also less taxing on the tubes to have the proper match.

I love the speaker outputs on my Mesa Blue Angel:


Blue Angel Manual Text On Output Jacks:

Two 8 ohm, two 4 ohm and one 16 ohm speaker jacks are provided. The BLUE ANGEL is not very sensitive to speaker mismatches and will not be damaged by them, except that very low ohmage loads will cause the power tubes to wear faster. A single twelve-inch 8 ohm speaker should generally be connected to the 8 ohm output. When using two 8 ohm speakers, connect them to the 4 ohm jacks to insure proper impedance matching. Mesa/Boogie 4X12 and 4X10 cabinets come standard wired to 8 ohms. They have four 8 ohm speakers wired in series-parallel. Some cabinets other than Mesa/Boogie may be 4, 8 or 6 ohms; to confirm how your cabinet may be wired and the impedance rating of your speakers, you may need to remove the back panel to verify this in formation in regards to proper speaker matching. Feel free to experiment with the speaker jacks, trying different loads. Keep in mind the greatest clean headroom is always achieved with proper impedance loads...i.e. an 8 ohm speaker connect ed to an 8 ohm jack. Always insure that a speaker load of some kind is connected when using the amplifier.