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Re: Jensen Transformers

Hello
this is a wild guess on my part:
Most active RIAA stages consist of two small (pf range) caps in series and two resistors in paralell with those caps. Those parts are connected from preamp output to the cathode of the first tube.
Also, if the RIAA stage is connected between two coupling caps at the output, you may remove the first cap.
"The guy at Jensen said it could become tricky" ...that will happen most likely due to excessive gain, because you removed the Riaa feedback. In that case, install a feedback resistor between 2nd stage output and 1st stage cathode. For starters, you can use the resistor values used in the ARC SP6-A preamp and a DPDT toggle switch. This will give you high-gain and low-gain option.

To add variable gain between stage 1 and 2, substitute a 100K attenuator for the 2nd 12au7 grid leak resistor.
Connect as follows:
Pin 1 of attenuator to signal in: from 1st 12au7 plate (you may need a coupling cap, a .47 uF 400V high-quality film cap is fine);
Pin 2 (wiper) to 2nd 12au7 grid; use shielded cable.
Pin 3 to signal ground; make it a short connection.
If the volume control works backwards, reverse the connections to pin 1 and 3.
I hope this helps.
If you send me the URL of the schematics, I can try to be more specific, though I am not a designer, just a former newbie.
Good Luck an have a merry Xmas
Carlos


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  • Re: Jensen Transformers - Carlos 12/24/0204:43:08 12/24/02 (0)


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