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What in the world does "Thump" mean? Does it go away when "RES LEVEL" goes to zero? And just what is "RES LEVEL"? I took my HD500 into the lab to find out. Since I didn't know what I was getting myself into I wanted to try to find an amp with flat frequency response, a mic with flat frequency response, and a cab with flat frequency response. If not flat then at least smoothly varying. If not over the whole frequency range, then at least over the range where Thump and Res Level do something interesting. So I picked: Class A-15, adjusted for +/-2dB variation over a wide frequency range 121 Ribbon mic 1x12 Blackface 'Lux Cab The results are in the PDF attached here. You can download the entire spreadsheet of data here. I then set Thump = 0% and adjusted Res Level over it's range. You can see it basically acts as a boost below 1 kHz. It especially invokes a resonance at 80 Hz (for this cab at least). I then set Res Level to 50% (the default setting) and adjusted Thump over it's entire range. It has a similar effect but more restricted around the 80 Hz resonance. I suspect different cabs will have different resonance frequencies but the effect will be similar. I then set Res Level to 0 to see what the Thump control does. Although it's effect is less, it does NOT go away. If you want to remove these effects then both Res Level AND Thump need to be set to 0. Note that I did not measure DEP Decay. It is a slow time domain effect and can cause havoc in frequency domain measurements. I would get different measurements as I changed the sweep rate. The decayed signal would mix with the original signal and add constructively/destructively, depending on which frequency I was measuring and how long it took the sweep generator to get there. The measurement artifacts swamped any meaningful frequency response measurements. So, I *think* this ends my adventures into HD500 amp/mic/cab DEP measurements. There should be total of 4 threads on this: Amp/Pre frequency response and THD measurements. Cab frequency response measurements Mic frequency response measurements And this one on Cab DEP measurements Cab DEP Parameters.pdf