Power attenuators have been a staple in rock and roll music for a long time. Musicians used them on stage, during recording sessions, and even at home. Some attenuators are designed to work with a specific impedance, but many support the most commonly used impedances: 4, 8, and 16 Ohm. Typically, the speaker impedance, the attenuator impedance, and the output impedance of the tube amplifier should all match. In this post, I’ll share my thoughts on matching impedances while using power attenuators, especially when dealing with different cabinets that have varying speaker impedances. I’ll also address some common questions musicians have asked me in the past.

Where goes the Energy?

As you may know, a power attenuator is placed between the amplifier and the speaker cabinet to reduce volume while allowing the amplifier to be cranked up to achieve a desirable tone. This means that some of the energy is converted to heat by the attenuator, and the rest by the speaker itself. As you cranck your amplifier, the attenuator absorbs more load, which is why it gets hot.

The energy inside the attenuator is dissipated by power resistors. Here is a simple circuit of how attenuators work:

Guitar attenuator schematics

If the attenuator operates at 4 Ohm, the total impedance of the circuit after the attenuator must also be 4 Ohm. The resistance is calculated using the formula:

\( \frac{1}{R_{\text{total}}} = \frac{1}{R_1} + \frac{1}{(R_2 + R_{\text{speaker}})} \)

For this example where 𝑅1=8 Ohm, 𝑅2=4 Ohm, and the speaker is 4 Ohm, we get a total impedance of 4 Ohm. If the speaker impedance is changed to 8 Ohms, the output impedance becomes 4.8 Ohm, which is very close to 4 Ohm. With a 16 Ohm speaker, the total impedance is 5.7 Ohm – still closer to 4 Ohm than 8 Ohm. These calculations are based on 50% speaker load. The table below shows the impedance with different speaker loads with an ideal 4 Ohm power attenuator:

4 Ohm Attenuator Speaker load: 10% Speaker load: 25% Speaker load: 50%
With 4 Ohm Speaker 4 Ohm 4 Ohm 4 Ohm
With 8 Ohm Speaker 4.03 Ohm 4.14 Ohm 4.8 Ohm
With 16 Ohm Speaker
4.09 Ohm 4.32 Ohm 5.7 Ohms

If you examine the numbers in the table closely, you’ll see that the impedance seen by the amplifier remains close to the impedance of the attenuator, even when using speakers with different impedances. This means the amplifier stays within the safe operating range.
These calculations assume perfectly set resistors for the attenuator. In reality, manufacturers use fixed resistors available on the market, which can introduce additional impedance mismatches into the system.

In summary: When using an amp cabinet switcher to connect multiple amplifiers and a power attenuator, ensure all amplifiers match the attenuator’s impedance setting. The speaker’s impedance is less critical when the attenuator is in use, especially at lower volumes.

The information above applies only to passive attenuators, which do not have built-in power amplifiers. An example of this type is also the Universal Audio OX Amp Top Box. For attenuators with built-in amplifiers, such as the Fryette Power Station or Boss Waza Tube Amp Expander, see below.

Can I use a power attenuator connected to the speaker output of my amp switcher?

Yes, you can. The N-audio 8X8 Switcher and 4X4 Switcher have a dedicated insert for a power attenuator, which you can enable or disable from the front panel or via MIDI. If you want to use  attenuator, such as for recording quietly at night, you can connect it to any of the speaker cabinet outputs as if it were a speaker. Ensure your attenuator operates safely without a speaker connected, but generally, any attenuator should be capable of this.

Can I use one attenuator for a specific amplifier only with my amp cabinet switcher?

Yes, you can. Connect the output of your amplifier to the input of the attenuator. Then, connect the output of the attenuator to the amp cabinet switcher, where you would normally connect the speaker cable for that amplifier. This allows you to use a dedicated attenuator for that amplifier seamlessly.

Power attenuator with tube amplifier

Are All Power Attenuators on the market Suitable for Amp Cabinet Switchers?

Absolutely! You can use any active or passive power attenuator with your rig, whether it consists of a single amplifier and cabinet or a more complex setup with multiple amplifiers and an amp cabinet switcher.

Some of my clients have expressed concerns about Fryette Power Station attenuators since they aren’t traditional attenuators. Instead, they are tube power amps with a built-in attenuator. To ensure everything works perfectly, I reached out to Fryette for clarification. They confirmed that their attenuators are indeed compatible with any amp cabinet switcher.

What About the Fryette Power Station and Boss Waza Tube Amp Expander?

These two guitar power attenuators contain built-in power amplifiers. The Fryette Power Station features a tube amplifier, while the Boss Waza Tube Amp Expander uses solid-state amplifiers.

When using the Fryette Power Station with any amp cabinet switcher or your guitar rig, it’s crucial to match the output impedance of your Fryette with the impedance of your cabinet. Think of the Fryette Power Station as a real tube amplifier; careful impedance matching is necessary to ensure optimal performance and avoid damage.

As mentioned above, the Boss Waza Tube Amp Expander has a solid-state amplifier. In this case, matching the impedance is not necessary because solid-state amplifiers are not affected by the impedance of the connected speaker. However, remember that a solid-state amplifier will be less powerful at higher impedances and more powerful at lower impedances. Use your Tube Amp Expander with a 4 or 8 Ohm speaker cabinet for maximum power. The only concern is the minimum speaker impedance. According to the Tube Amp Expander’s specifications, the minimum impedance is 4 ohms. Keep this in mind if you plan to use more than one cabinet in parallel with this attenuator. Ensure the total speaker impedance does not drop below 4 ohms. This rule also applies to any solid-state amplifier!

Summary

I understand I may not have covered all your concerns or questions, but I’ve tried to address some of the most common ones I’ve received. Since N-audio is a small company, I personally promise to answer any questions you have. Please feel free to reach out for assistance, and I will do my best to help you.

Niki Hristov