## Which Impedance is the Right Impedance?

Choosing the Right Attenuator

TL;DR: Use an attenuator that matches the impedance of the input you connect to on the cabinet. It will be written on the back next to the connector.

Amp heads typically output at 4, 8 or 16 Ohms. This referrers to the load the amplifier is expecting to see. Often there is more than one output, For example you might find a single 8 Ohm output (to drive a single 8 Ohm load) and two 4 Ohm outputs to drive two 8 Ohm cabinets in parallel or one 4 Ohm cabinet. The Mesa Mark V 25 is an example. The two 4 Ohm outputs are tapped off the midpoint of the output transformer, so a 4 Ohm load at that tap would look similar to an 8 Ohm load across the full transformer.

**What's on the back of the amp?**Amp heads typically output at 4, 8 or 16 Ohms. This referrers to the load the amplifier is expecting to see. Often there is more than one output, For example you might find a single 8 Ohm output (to drive a single 8 Ohm load) and two 4 Ohm outputs to drive two 8 Ohm cabinets in parallel or one 4 Ohm cabinet. The Mesa Mark V 25 is an example. The two 4 Ohm outputs are tapped off the midpoint of the output transformer, so a 4 Ohm load at that tap would look similar to an 8 Ohm load across the full transformer.

On some amps you get the same series setup with two outputs for an 8 Ohm load or one 16 Ohm load. Here's an Orange Micro Terror for example:

Some amps might have a single output labeled '8+' or '8 Minimum'. So this can drive an 8 or 16 Ohm load. The amp will be fine. The total output power will reduce as the impedance of the speaker cabinet goes up and the tone will change, but the amp will survive just fine.

So what impedance should I use? If the output of the amp says 8 Ohms and the input of the cabinet says 8 Ohm, it's obvious it needs to be an 8 Ohm attenuator. However if the output is labelled 8+ Ohms and the cabinet is labelled 16 Ohms which do you choose?

If you have some quality of noise coming out of your speaker and as far as possible you would like to get the same noise, just at a lower volume, you want to match the attenuator impedance to the impedance of the cabinet. So the amplifier will see the same load that the speaker alone would present and deliver the same signal, but the speaker will receive the same signal at a lower amplitude. If the amps see a different load, there will be a different current through the tubes and a different resulting tone.

In the case of daisy-chained cabinets (where the cabinet has an output connector to chain to a second cabinet, the two cabinets are connected in parallel. So two daisy chained 8 Ohm cabinets would actually present 4 Ohms to the amplifier and two daisy chained 16 Ohm cabinets would present 8 Ohms to the amplifier, so you would need to use an attenuator that is half the stated impedance of two cabinets daisy chained.

In the case of connecting two cabinets to the two outputs (E.G. two 8 Ohm cabinets to the two 4 Ohm outputs, or two 16 Ohm cabinets to the two 8 Ohm outputs) you would need to use two attenuators, but if the goal of using the attenuator is to achieve lower volume, you wouldn't want to be plugging in both cabinets. Just use one.

The are some cabinets that have both a single (mono) input or two (stereo) inputs. E.G. the Orange PPC412HP8 or the 1936 and 1932 models from Marshall. In that case, match the impedance to the connector or connectors you connect to.

Here's a list of a few cabinets from Orange, Mesa and Marshall and the input impedance. You will typically find it written next to the input connector or connectors on the back of the cabinet.

Orange, CRPRO412 16 Ohms 240W

Orange, PPC212 16 Ohms 120W

Orange, PPC108 8 Ohms 20W

Orange, PPC412, 16 Ohms, 240W

Orange, PPC412HP8, 2x16 Ohm or 1x8 Ohm, 200W or 400W

Orange, PPC412A, 16 Ohm, 240W

Orange, PPC112, 16 Ohm, 60W

Orange, PPC212, 16 Ohm, 120W

Orange, PPC2120B, 16 Ohm, 120W

Mesa Boogie, 1X12 WIDEBODY™ CLOSED BACK, 8 Ohm, 90W

Mesa Boogie, 1x12 WideBody™ Compact Cabinet, 8 Ohm, 90W

Mesa Boogie, 1x12 WideBody™ Compact Cabinet, 8 Ohm, 90W

Marshall, JVMC212, 16 Ohm, 140W

Marshall, 1936V, 8 Ohm Mono 16 Ohm Stereo, 140W

Marshall, 1936, 8 Ohm Mono 16 Ohm Stereo, 150W

Marshall, 1922, 8 Ohm Mono 16 Ohm Stereo, 150W

Marshall, 1912, 8 Ohm, 150W

See Also:

How Does an Attenuator Work? Looking at how attenuators work with tube amplifiers

So what impedance should I use? If the output of the amp says 8 Ohms and the input of the cabinet says 8 Ohm, it's obvious it needs to be an 8 Ohm attenuator. However if the output is labelled 8+ Ohms and the cabinet is labelled 16 Ohms which do you choose?

If you have some quality of noise coming out of your speaker and as far as possible you would like to get the same noise, just at a lower volume, you want to match the attenuator impedance to the impedance of the cabinet. So the amplifier will see the same load that the speaker alone would present and deliver the same signal, but the speaker will receive the same signal at a lower amplitude. If the amps see a different load, there will be a different current through the tubes and a different resulting tone.

In the case of daisy-chained cabinets (where the cabinet has an output connector to chain to a second cabinet, the two cabinets are connected in parallel. So two daisy chained 8 Ohm cabinets would actually present 4 Ohms to the amplifier and two daisy chained 16 Ohm cabinets would present 8 Ohms to the amplifier, so you would need to use an attenuator that is half the stated impedance of two cabinets daisy chained.

In the case of connecting two cabinets to the two outputs (E.G. two 8 Ohm cabinets to the two 4 Ohm outputs, or two 16 Ohm cabinets to the two 8 Ohm outputs) you would need to use two attenuators, but if the goal of using the attenuator is to achieve lower volume, you wouldn't want to be plugging in both cabinets. Just use one.

The are some cabinets that have both a single (mono) input or two (stereo) inputs. E.G. the Orange PPC412HP8 or the 1936 and 1932 models from Marshall. In that case, match the impedance to the connector or connectors you connect to.

Here's a list of a few cabinets from Orange, Mesa and Marshall and the input impedance. You will typically find it written next to the input connector or connectors on the back of the cabinet.

**Brand****Name,****Ohms,****Power**Orange, CRPRO412 16 Ohms 240W

Orange, PPC212 16 Ohms 120W

Orange, PPC108 8 Ohms 20W

Orange, PPC412, 16 Ohms, 240W

Orange, PPC412HP8, 2x16 Ohm or 1x8 Ohm, 200W or 400W

Orange, PPC412A, 16 Ohm, 240W

Orange, PPC112, 16 Ohm, 60W

Orange, PPC212, 16 Ohm, 120W

Orange, PPC2120B, 16 Ohm, 120W

Mesa Boogie, 1X12 WIDEBODY™ CLOSED BACK, 8 Ohm, 90W

Mesa Boogie, 1x12 WideBody™ Compact Cabinet, 8 Ohm, 90W

Mesa Boogie, 1x12 WideBody™ Compact Cabinet, 8 Ohm, 90W

Marshall, JVMC212, 16 Ohm, 140W

Marshall, 1936V, 8 Ohm Mono 16 Ohm Stereo, 140W

Marshall, 1936, 8 Ohm Mono 16 Ohm Stereo, 150W

Marshall, 1922, 8 Ohm Mono 16 Ohm Stereo, 150W

Marshall, 1912, 8 Ohm, 150W

See Also:

How Does an Attenuator Work? Looking at how attenuators work with tube amplifiers