Tweaked Pre

[Rothchild]

I've had this pre kicking about for a while purchased as a lucky-dip on ebay, I could hear it had decent quality but found it to be neither 'fish nor fowl' insofar as it always seemed to kick out a bit too much gain for mm and not enough for mc.

So I finally got my old man to take a look at it for me and make some adjustments to both the input and output stages which are switchable with the microswitches. It now has a 15ohm input as well as the 47k and range of attenuations on the output side.



Here's a rough schematic, I'm interested to know if anyone knows where it comes from as we've been unable to find anything else exactly like it online.



I've manage to spin a couple of sides through it since I got it back and it's sounding good but I've not had enough time to form a proper opinion yet or to compare it to the other pre-channels I have access to.

[RothwellAudio]

Where it comes from? Pretty much out of any text book.
There are three time constants in the RIAA playback curve.
https://en.wikipedia.org/wiki/RIAA_equalization

The time constants for 50Hz and 500Hz are handled by the feedback network around the first stage. It looks like the capacitors to ground are just rolling of the gain to unity at DC, though without seeing values I can't be sure - they might be an additional 20Hz roll-off as prescribed by the IEC standard.
The 2122Hz roll-off is handled passively by the RC filter before the second stage.

BTW, 15 ohms and 30 ohms are in my opinion a bit on the low side for an LOMC input. About 100 ohms would be better for a wide range of cartridges.

[Rothchild]

BTW, 15 ohms and 30 ohms are in my opinion a bit on the low side for an LOMC input. About 100 ohms would be better for a wide range of cartridges.

Thanks Andrew, yeah it's similar to a bunch of stuff we found but not identical.

We went for those on the input as the spec on my Ortofon Salsa said >10ohms (and the next one up the range (samba) said >20ohms) but certainly worth knowing, if I arrive at a point where I can actually afford a decent cart it shouldn't be too hard to switch out one of these to 100ohms.

At the moment I've got my V15 IV on and it's sounding pretty sweet - it's the kind of cart we made the mods for, with a 4mv output it sounded a bit too hot before.