Author Topic: Rhodes MK I Janus preamp upgrade  (Read 42 times)

Offline Will

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Rhodes MK I Janus preamp upgrade
« on: June 29, 2020, 01:45:29 PM »
Hello

I have the early version of Janus preamp ref. PART NO.013487 which I intend to upgrade. You can find the schematic diagram here.
These are my thoughts and questions about its upgrade which I submit to your comments.
You can find the part list here to follow the subject.

The LM1458 OpAmp on signal path can be upgraded. I have a pair of Burr Brown OPA2134PA which I intend to use. They have the same pinout config. And add their package.

The electrolytic capacitors need replacement after more than 40 years of services.  I will choose Panasonic (FC series), Nichicon (KA, FG, KW), ELNA (not Silmic II)... The relevant capacitors are C8 C20 C12 C13 C14 C15 C3  and non-polarised C4 C6 C16.

Black and round shaped capacitors seem to be polypropylic. If that is the case, they are good quality caps and age well so they must be kept in place, right ? Those are C11 C1 C9 C5 C7 C10.

Small and flat caps are ceramic. From what I know, they are not suitable on audio signal path. On the schematic, they seem to help stabilise the OpAmp (C2) and to suppress unwanted high frequencies (C17 C18 C19). Am I right ? Are they worth to be changed ? For what type ?

I don't know the quality of the present resistors and I am wondering if a change to modern low noise metal oxyde resistors would be beneficial. I have an assortment of them.

The transistor and diodes are not in the signal path and don't need to be changed, unless faulty. Same for the vibrato speed pot.

Linear bass and treble pots and vibrato intensity pot are on the signal path. I don't plan to change them as they make no crack.