Early vs Mid 70's DR504 preamps....

[JeffreyC]

Ok, so I've been looking over the schematics on Mark's site and I can see some component value differences here and there,
but I'm wondering what are the main differences in these two preamps and how do they effect tone? Also, when did they change
to the mid 70's version and what's the simplest way to identify which preamp you're looking at?

Sorry if this has been answered - couldn't find anything on it.

[mhuss]

The main difference is the PI, which changed how it does its voltage reference. The telltale for the earlier PI is a short yellow wire connected directly from V3 to V4.

I don't think the two PIs sound very different.

--mark