B&K ST1400 II

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I decided to take my amp to work and see how it looked on our scope, I found some 15 ohm 900w resistors I used to make a load bank at 7.5 and 5 ohms. I used my phone hooked to my audio quest dragon fly as the source. The amp exceeded the rated power just fine, but when I ran a square wave through it i see what could be ringing. Can someone tell me what could be the issue. I'm still in the learning phase of this hobby. I replaced the power supply caps and replaced the rectifer with some high current shockley diodes we had laying around. Thanks.
 

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So it's the dragon fly. I don't have my power amp with me today to test it again but the dragon fly was clearly causing the ringing. I tried it with and without a resistor and the ringing looked better with a 56 ohm resistor. Is ringing something that can be audible? Could that explain why I feel like I wasted 200 bucks on a the dragon fly? Lol
 

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The ringing is I believe due to an IIR brickwall filter - its clearly ringing at about 20kHz right on the filter edge, looks like a high order Chebyshev filter or similar. Audio DACs can't produce square waves outside their bandwidth limit, for this you need a pulse generator or arb. function generator.
 
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