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.
How to Connect a Power Attenuator to an Amp Switcher?
Connecting a power attenuator to an amp cabinet switcher is simple. All you need are two speaker cables to connect the attenuator to the switcher. A detailed wiring diagram is also included in the user manual for reference.
To make things even easier, I’ve included the sketch below. It shows how to connect some of the most popular power attenuators available today. This method works universally, so you can use it to connect any power attenuator.
Click here to view it in full size
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 goes to the speaker. 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:
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:
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 signal: 10% | Speaker signal: 25% | Speaker signal: 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 conclusion: 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 only for that amplifier.
Solid-State and Bridged Mono Amplifiers with an Attenuator
While most guitar players prefer tube amplifiers, which are the standard choice, there’s still value in vintage solid-state amps and modern Class D bridged mono amplifiers. Some solid-state amps, such as the Marshall Valvestate and Peavey Bandit, use a technique called “positive feedback.” This involves using a relatively low-value resistor as a shunt, slightly elevating the negative output to the speaker. This smart trick reduces the output impedance, but it also means that the speaker’s negative wire is slightly above the amplifier’s ground.
Bridged mono amplifiers take this a step further, with both the positive and negative speaker wires being “hot.” While all types of amplifiers work with N-audio amp cabinet switchers, they are not compatible with all speaker attenuators on the market!
Types of Power Attenuators:
Passive Attenuators: These attenuators do not require a power supply, meaning they aren’t connected to the mains power. This makes them suitable for all types of amplifiers.
Active Power Attenuators: Some active attenuators are powered by an external isolated power supply, while others are grounded via the mains earth. When using an active attenuator, it’s essential to confirm with the manufacturer whether it supports your amplifier type.
Unsupported power attenuators can short the “hot” (sleeve) negative wire to ground, potentially damaging your amp. If you plan to use a solid-state or bridged mono amplifier, ensure that your attenuator is compatible. The engineers at Two Notes, for example, have addressed this issue in an article, confirming that their Torpedo supports all amplifier types. Read the full article here.
I also reached out to Fryette, and here’s what Dave from Fryette had to say:
Hi Niki,
Generally, we advise against this, but with some care and attention to the rig setup, it can be done. This may not work in every case, so you will need to use the ground lift switch on the rear of the unit and see if that works for your application.
Other companies like Universal Audio with their OX and Boss with the Tube Amp Expander haven’t responded to my inquiry.
In Summary: When using N-audio amp cabinet switchers, remember that the sleeve (speaker ground) is always connected to the negative of your external power attenuator. If the attenuator is active and doesn’t support bridged mono amps or solid-state amps with a positive feedback sense resistor, you have two options: switch to a different power attenuator or only use supported by the attenuator amps.
Since amplifier topologies vary between brands and models, it’s impossible to cover all scenarios in this post. In my opinion, choosing an attenuator that supports all types of amplifiers is the best solution.
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! Pay attention when using those amplifiers with bridged mono and solid-state amps!
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
