I'm having an issue with the first (bass) EQ stage on the Para-EQ. I'm only getting half the boost on that stage. The cut works fine, as does the boost and cut on all the subsequent stages. All other parameters also work as expected. Also, when I pull the 1st TL074, the problem moves to the second (now first) EQ stage. In other words, I pull the Bass IC, and the previously working midrange section inherits the problem. I pull the second IC, and the Treble section then inherits the problem. The first stage always has the issue, regardless of where and what it is.
I tested the EQ response with a white noise loop and Voxengo Span spectrum analyzer. I also used an external sine wave generator set to the EQ's center frequency and poked around with my Tektronix scope. This did reveal something interesting. The input signal to the first stage (post R4) drops down when the EQ is boosted, as if negative feedback is getting into it. This doesn't happen on the subsequent stages (post R6, R19). The input signals are rock solid regardless of boost.
Has anyone else noticed this type of behavior? I've been doing DIY for a very long time and went over my build multiple times, checking and rechecking. So I'm pretty confident that I've eliminated the obvious, like bad joints, bridges, or misplaced components. Plus the problem can move to a completely different set of components. Any insights would be appreciated, especially if anyone with a working build can test their bass boost level compared to the cuts and other boosts.
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