Re: Adding a selectable line input to valve radiogram
I don't really understand what you are getting at when you talk about "adjustment of the RIAA curve from the cartridge and similarly inverting this effect from the modern line signal to the amp" or "RIAA bias adjustment".
For a start if you have lots of 78s from before 1954 they won't have been cut with RIAA pre-emphasis but with each record company's patent pre-emphasis and the same goes for pre-1954 microgroove discs. Which de-emphasis to use is a complete minefield which is why the RIAA (Radio Industry Association of America) introduced their standard in 1954 and succeded in having it adopted by most of the world's record-issuing companies by the time stereo came along.
As far as I can see the only components in the disc input circuit are a couple of 1M resistors which are presumably the correct loading for the two ceramic/crystal cartridges. Is there any circuitry further down the line before the main amplifier which could contribute to equalization of the signal from these cartridges?
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