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In Reply to: RE: Forget everything I have posted for the last 5 years! posted by neolith on October 22, 2015 at 17:40:32
If your phono input did not have its 47k-ohm load connected, it probably had much higher impedance -- perhaps as high as 1M-ohm. Since you were using a step-up transformer, this would cause a high-frequency peak that would probably not be audible, so you might not hear any difference. It would also cause that channel to play a little louder.
Below is a frequency response graph of a CineMag 3440A SUT terminated with 1M-ohm in the upper trace producing a noticeable peak at 38-kHz and terminated with 47k-ohms in the lower trace. The signal with 47k-ohm termination is about 1.5-dB lower in volume.
Follow Ups:
Thanks John. At least I haven't been deluding myself for the last 6 decades. Although my upper limit is about 13-14 KHz (not bad for 70) and missing the effect is excusable.One other the question -- the schematic I have (which has clear errors) shows a bypass cap across the resistor (150 or 180 pf) but these were never Installed on the PCB. I know the designer (Rodger Modjeski) used a Denon 103 so maybe he found the cartridge sounded better without the cap. What is the effect of including capacitance loading on the cartridge? And yes I use a SUT (26db) for a DV 17D2 without any parallel resistance across the primaries.
Edits: 10/23/15
I would recommend leaving out the capacitors, especially when using a step-up transformer. The SUT reflects capacitance back to the cartridge multiplied by the square of its turns-ratio. Therefore, 180-pF on the transformer secondary will be reflected back to the cartridge as 180 x 26 2 = 0.122-uF. This is why you should always use a short, low-capacitance interconnect to attach the SUT to your phono stage. Capacitance on the secondary of a step-up transformer is not at all desirable.
Best regards,
John Elison
Great, one more thing not to worry about. l
The cap is for MM cartridges, which often specify an optimum load capacitance (cable plus auxiliary cap plus the tube's Miller capacitance - about 50pF for a 6DJ8). With an SUT, lower capacitance is better unless the SUT maker has other ideas.
In both cases there is a large contingent who feel that any added capacitance causes a sonic loss. I'm agnostic at this point.
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