Power amp and speaker cabinet
From Axe-Fx II Wiki
Power amp and speaker cabinet: introduction
- You can use a dedicated power amp and guitar cabinet with the Axe-Fx II and AX8. Some players prefer this to going direct and/or FR monitoring, in particular when using the Axe-Fx II or AX8 as a backline rig.
- You can also use a neutral power amp with FR speakers.
- Cliff: "I did some studies years ago and having a speaker in proximity to the guitar actually changes the final tone considerably. I compared the frequency response with the amp in isolation to the frequency response with the amp in proximity and measured several dB difference in the lows and mids. It was clearly audible when the recordings were played back." source
Power amp modeling
- Cliff: "If you use a tube power amp and don't turn off power amp modeling in the Axe-Fx you will get the impression that the tube power amp sounds "bigger" and "warmer". This is because the tube power amp will have more bass (and highs) than the solid-state power amp since a tube power amp's response follows the speaker impedance. People will ALWAYS find that more bass and treble sounds "better" when listening alone but in a band context that tone will get lost. Speaker designers have been exploiting this fact of human perception for decades. Many "hi-fi" speakers exaggerate the bass and treble because the uneducated customer will think they sound "better". A truly flat speaker will sound dull in comparison to one with exaggerated lows and highs. Over time, however, those exaggerated frequencies lead to fatigue. It's only in comparison that exaggerated bass and treble sound "better". In an isolated context this aspect of human perception is not evident." source
Configuring power amp and speaker cabinet
- When connecting to a (non-neutral) tube power amp, it's probably best to switch off the power amp simulation. Either globally (Global menu) or in the Amp block (Sag at 0). Cliff: "If you shut the power amp modeling off from the Global menu it is not exactly the same as turning it off by setting Supply Sag to zero. This is because the virtual power amp always runs. So if you shut the power amp modeling off from the Global menu the supply will still sag resulting in a more compressed response. If the Master is set high the sag can be quite pronounced." source
- Cliff's comments:
- "If you turn off power amp modeling always check the presence control. It changes from a "classic" control to a shelving type where 5.00 is neutral. I just spent an hour trying to figure out why this preamp model I am working on wasn't matching. Forgot to set the presence control to 5.00." source
- "If you are using a tube power amp you should set any Presence, Depth, Resonance, etc. controls to their minimum positions on the amp (assuming they are conventional controls). On a Mesa power amp, set them to noon. The Presence control on Mesa amps is most neutral around noon. If you turn it up it boosts the highs, if you turn it down it cuts the highs. On most other power amps it only boosts. source
- The gain-staging information and procedure below is provided by forum member s0c9. source Gain Staging Explained
- Gain staging:
- The "knobs" on a power amp are NOT volume levels, they are input signal attenuators. They control how much input signal voltage is allowed to pass to the power stage. The amp can reach its rated output wattage with the input attenuators wide open, or almost closed. Its totally dependent on the strength of the input signal. If the amp knobs are down, but the Fractal outputs are cranked, that amp can still produced its full power output. If you have the amp wide open, the noise floor will be much higher and if you mute the input signal you will get "noise/buzz" thru your system. That noise may be quite loud/annoying, especially if you have 50/60 cycle hum added thru bad grounding.
- NOTE: The following steps ONLY apply to SOLID-STATE amps. Opening up a tube amp (wide open) with no speakers connected can PERMANENTLY DAMAGE the amp!! They are also not valid for active devices (such as the QSC K12) that have the amp built in.
- To properly gain-stage your system, take the following steps:
- Turn down the output level (assuming OUT1 going to amp) on the Fractal.
- Turn down the input attenuators on the amp.
- Disconnect the cab from the amp (only with solid-state amps! Tube amps need a load at all times).
- Open up the amps input levels all the way.
- While playing a relatively "loud" preset, SLOWLY turn up the OUT1 level on the Fractal until the CLIP lights on the amp light up.
- Now, (while still playing) back down the input level on the amp just hair - that could be one click of the knob or one measurement - so that the CLIP light goes off.
- Now play real hard and make sure the CLIP light does not come on. if it does, back down the amps input levels another notch.
- Turn off power amp, turn off the Fractal.
- Reconnect speakers.
- Turn on the Fractal, then turn on amp (amps should ALWAYS be switched on LAST and powered down FIRST!).
- You have now gain-staged your setup. Will this be the "loudest" it can be? No. But you will not be in danger of blowing speakers, and you should not be introducing any more "noise" in the system than already existed. You should use the Tuner mute to silence the system or the volume knob on your guitar. You should not need to adjust the OUT1 level on the Fractal, nor the knobs on the amp.
- Cliff's comments:
- "Optimal gain staging would be with the level knob around noon. Higher than this and you risk clipping the inputs of the downstream device. With the level knob at full the Axe-Fx II will probably incinerate a Soundblaster or other low-cost stuff. The max level out of the Axe-Fx II is +20 dBu. Most pro gear can easily handle that but lots of gear cannot and the trend in newer gear is towards lower and lower maximum input levels (due to single-ended designs and low-voltage/low-power constraints). In the old days, +20 dBu was routine. Everything could put out and handle +20. Not so much anymore." source
- "The II actually has more output than the I. The II can do about +20 dBu, the I was about +18." source
- "Start with amp volume at noon. Bring up Axe-Fx volume until desired level is reached. If you need more, turn up amp. With the Axe-Fx volume all the way up you would be pushing +20 dBu into the amp which could clip the inputs to the amp." source
Speaker impedance and low frequency resonance (LFR)
- This page explains the importance of resonance. Finding and setting the correct Low Frequency Resonance value improves the interaction between Axe-Fx or AX8 and speaker cabinet, bass response will be better.
- Here's how to find the correct resonating frequency:
- Put a Filter block after the Amp block.
- Set the type to Peaking, Q to 5 or so and Gain to 10 dB.
- Start with a Frequency of around 50 Hz. Play some chugga-chugga and slowly adjust the Frequency until you hear and feel the cabinet resonate. You need to do this at loud volume level to notice it. Make a note of the frequency.
- Remove the Filter block and set the Amp block SRF to match.
Combining FRFR with a power amp and guitar speaker
- The Axe-Fx II and AX8 let you combine amplification methods, such as sending a signal with cabinet modeling (FRFR) to Output 1, and a signal without cabinet modeling to Output 2 (power amp and guitar speaker). Axe-Fx II connections and levels
Amp channel switching
- The Axe-Fx II and AX8 have no relay switches so they can't switch amp channels themselves. You can use a MIDI foot controller and an external MIDI switcher to accomplish this. More information