Measurements > Levels


Signal levels in the Flex 3 are subject of debate. What should you do? Turn up the MASTER to 10 and use VOLUME to adjust your levels? Or should you keep the MASTER at 5 and avoid clipping?

The decision is yours, but here are some facts to help you make it.

All measurements carried out by myself on my Flex 3 XL amp.

Signal levels

(all dB values with +4dBu as ref)

block diagram #1The Flex3 can bee seen as a signal processing unit (SPU), followed by a power amp (POWAMP) with attenuator (MASTER). The signal processing unit takes care of modeling and FX. The power amp is there to amplify the signal for the speaker(s).

The output of the SPU is controlled by the VOLUME control, while the POWAMP has the MASTER control to control volume coming from the speakers.

SPU levels

The SPU uses instrument levels at the input. A guitar typically produces between 0.5V and 1V peaks (-2dB).

The output of the SPU is capable of delivering 2.5V (+6dB) before it starts clipping. You could say the SPU roughly has unity gain (a gain of 1). The FX outputs are also roughly at instrument level, so a stomp box can in fact be used in the FX loop without too many problems.

When operated as a unity gain device, it has a headroom of 12dB. Typically that is what you have with VOLUME set between 5 and 7. Some FX, like the sweep echo, add significant gain at certain frequencies, so some headroom is welcome.

MASTERmax input

POWAMP levels

The power amp has a gain of about 100 and is capable of delivering 32V peaks to the speaker (measured at my XL, YMMV). Therefore 300mV (-12dB) peaks at the input are capable of driving it into clipping if the MASTER is set to 10.

The table shows the relationship between the MASTER setting and the max input in dB when the output starts clipping.

With an input signal of 2.5V (+6dB, the max coming from the SPU), a MASTER setting of 2.5 and higher WILL SEND THE OUTPUT INTO CLIPPING.

Signal levels for factory defaults

It's always a good idea to see what the manufacturer gives you for default settings. Well, the Flex 3 default patches are well tweaked, level wise. The SPU output is always free of clipping.

When you run with the factory default settings, a MASTER setting of 5 keeps you away from clipping the POWAMP. For some patches it has to be turned back though. For patches 4C Deep vibrato, 6D Awake Sleeper, 9A Jazz box, and 9D Sweepy tone the MASTER should be at 3.

Thoughts and conclusions

If you want to record the amp via the XLR outputs, or you plan a show using a PA connected to the XLRs, ideally you'd want the average signal levels from the SPU to be around .5 to 1 V, leaving sufficient headroom while retaining good dynamic range and a low noise level. Putting the MASTER between 4 and 5 would limit clipping of the POWAMP to occasional peaks. You might not even hear those very well as the speaker is a low-pass filter (limited to 5 kHz). If you are very critical of your stage sound coming from the amp, you'd need to run the MASTER even lower. Note that, during clipping, the peak output of the amp is still 60W per channel.

So there is really a trade-off to be made here. When you rely on a PA to deliver your sound, or you are recording, you should go for optimum levels from the SPU. And when you work a room with just the amp, you might prefer lower levels coming out of the SPU, leaving you more control with the MASTER volume.

I believe it would be good to use a peak detector on the power amp at all times. See clipping detector for this. During tweaking, you can use a mixer, or VU meters, to monitor the SPU's level at the FX SEND to make sure your peaks are never above 6dB.

My personal choice would be to keep the MASTER at 5 or less, and not to use VOLUME settings above 7 for the loudest patch.

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