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The way minimum phase eq works is by shifting the relative phase relationships of the audio being equalized, (we know that out of phase signals cancel entirely and a partial phase shift of say 90 degrees will have a partially additive effect) depending on what you are doing (say cutting 1kHz as an example) then the phase shifts that are occurring result predominantly as an attenuation of 1kHz. As such sometimes in a filter (which is not linear phase) you get are highly like to get additive phase relationships at certain frequencies above the HPF cut off which can result in a net increase in amplitude. This is why when you HPF your lows an increased peak value is evident.

To follow up on phase, I wrote this some time back, it explains phase in the easiest way I could relative to a recording situation, the same applies to artificial waves in a synthesizer.

Understanding phase

Hope that explains it.

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But I'm still sceptic about it actually raising the signal's level by 3dB though.

Well you said you are using a steep slope on the EQ, and as far as I know most EQ's boost at the cutoff point creating a peak at the particular frequency, and how steeper the curve how bigger the peak is going to be. This in combination with the phase thing explained earlier might explain the increase in volume.

AxeD wrote:But I'm still sceptic about it actually raising the signal's level by 3dB though.

It can up to 6 if not more. When I switch to linear phase (I also use Pro-Q), it may add LESS volume, but still kill headroom.

Right now I just ignore it. It used to did my head in though

Edit: now, with the EQ cutting freqs, it's about 7 db louder than the dry signal...

ARTFX wrote:Well you said you are using a steep slope on the EQ, and as far as I know most EQ's boost at the cutoff point creating a peak at the particular frequency, and how steeper the curve how bigger the peak is going to be. This in combination with the phase thing explained earlier might explain the increase in volume.

I made sure there was no boost at the cut-off point, it was flat. It did the same thing 12 db slope too

This answer has probably been said, but I haven't read through the responses, so:

It's a side-effect of minimum-phase equilization. Since different areas in the frequency spectrum are altered different amounts, the phase of different harmonics change by different amounts, as well. That's as basic as I know how to put it. Linear phase equilization doesn't have this effect, but linear-phase equilizers tend to introduce latency.

There's a great article here that goes into detail about the different types of equalizer, their limitations and the problems with FFT.