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Started by GuyB, January 27, 2014, 08:33:38 AM
Quote from: J M Fahey on February 11, 2014, 02:49:17 AMVery interesting and thanks for experimenting and posting.I think one reason for the behaviour you noticed is not that the intput impedance "is not that high" , (by the way you don't show any input impedance graph or table), but the *output* impedance is way too high.The output impedance of a classic 12AX7 is around 40K , coming from a 100K load resistor in parallel with the tube's internal Rp, around 68K .While here you have the *very* high load impedance provided by a constant current source (which in theory should be infinite, go figure), in parallel with the internal output impedance of a FET, much more similar to a pentode than a triode.So gain is much more dependent on the complex impedance presented by the tone stack.Which for a reason I don't understand you downscaled 10X in impedance.I wonder why, since :a) it still drives a 1M input impedance recovery stageb) it's driven by a source impedance at least as high as that of the original triode stage, and probably much higher.I'd retest with standard Fender values and see/hear what happens.I'd also retest using TL072 gain stages, around 10X to 20X gain each, and standard Fender values, *or* in this case, with 10X reduced impedance one if you wish, since the 1st stage will easily drive that and then some.I have used that for ages with excellent results."Tubey"?Dunno, define "tubey"
Quote from: Loudthud on February 10, 2014, 07:07:06 PMI built up a preamp, made some changes and ran into an interesting problem. As near as I can figure the input impedance of the JFET with Soruce grounded is not as high as you would think. It causes the frequency response to slowly dribble down about 2dB from 100Hz to 10KHz before the dominate pole really kicks in. I didn't notice until I actually plotted the response. The MPF102 was the worst, a J201 gave the flattest response but less overall gain and bandwidth. I tried several JFETs that I just had laying around.Sound is warm and somewhat tube like. I installed the preamp in an old amp I built some years ago with +/- 20V rails for the power amp. The preamp worked fine with no adjustments, I had used a 30V rail to get it running.There are better sounding preamps, but they are more complicated.