elktor preamp prelude march 1983

[phri]

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]

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]

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. C_x 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!

[Kay Pirinha]

Are you still there?

Well, after another one and a half years I stumbled over this thread again and found that I need to rethink my above posting. The values of R2 and R4 need to be the same in order to minimize DC offset. So get both resistors to 33 kohms.

If others don't contradict, you could also increase R3's value. With 6.8 kohms the amp's gain also will be 11.

Looking even closer at the schematics reveals there's no need for that big value of C2. With R4 = 3.3 kohms, the resulting LF roll-off is just 0.24 Hz. You could decrease to 22 µF and get sufficiently low 2,4 Hz roll-off. With R4 = 6.8 kohms C2 might be 10 µF - ready for a low voltage foil capacitor.

To get more sophisticated, you can add an integrator using a FET input opamp. A TL071 will do. This way you can get rid of the output coupling capacitor as well as C2.

Best regards!

[jean-paul]

Ha ha rethinking and replying to your own 1.5 year old last post. Class!

I would advise to use way higher voltage rating for input and output caps. C2 should be bipolar or simply 2 x 470 µF 16V back to back (I would leave LF roll off out of audible area). C7 could be smaller in value but at least 16V rated, better even 25V. This device begs for nonpolar Nichicon MUSE caps.