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phri 27th May 2019 04:55 AM

elktor preamp prelude march 1983
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Currently I am trying to getting some of my old builds in action again. Back in 1984 I build a pre-amp based on the line amp (and MC) of the original prelude.
btw, I avoided the other overly complicated printboards of this design.

The line amp in a discrete build opamp see attachment. Back then I replaced the input electrolytic capacitors with MKT types and removed the output elco completely. I included a small circuit to adjust offset on the output manually.

While the sound of the unit is fairly good in my ears it has been build with standard none-audio grade parts, inclusive a big electrolytic capacitor in the feed back line. And of course it's a very old design.

Would it be worth it to try to improve the pre-amp? Could I use an auto offset correction as implemented in the crescendo millennium? (if yes how?)
Any ideas which other changes would give me a worthy sound improvement?

I don't mind spending some $$$ but don't want to go over the top (like 2 X 220 uF foil capacitors).

phri 28th May 2019 03:51 AM

I also want to lower the gain of this pre-amp as this is now 22x

The ratio of R4 and R3 determine gain. I want to roughly halve the gain. Easiest way seems to be lowering R4 from 68K to 34K. However not sure if I have to change Cx Any feedback?

Kay Pirinha 9th July 2019 06:22 AM

Sorry for the delay. I've just seen your question.
I've also built this preamp in the 1980ies and yes, the line amp's gain is waaay too big. If you change R4 for 33k, gain will be 11x. Cx roughly has to be doubled (22p -> 47p) then. To get best results, this capacitor hat to be determined, using a square wave generator and an o'scope.
Good luck!

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