I recently bought a Marantz PMD670 -- a digital audio recorder. It seems pretty good except the mic preamps have a pretty high noise level for a recorder that is good otherwise. I'd like to see if I can make it better. (It isn't broken, its a design issue.)
As a general description, the mic inputs are balanced XLR with switchable phantom power. It only seems to work decently with pretty high-level condenser mics. A lower dynamic seems to require cranking the gain way up and the crappy noise floor becomes apparent.
One guy offers a mod for the preamps that improves things considerably. link:
I don't have any schematic, but I opened the thing up and had a look at what is in the preamp area. Assuming the signal flows sensibly across the board, there are (per channel) two largish electrolytics. Then a couple sot-23 devices (transistors I assume) marked 62Z. Then an HC4052A analog mux. Then a JRC 2068 op amp.
I was surprized to see that 4052 so close to the input. Not sure what exactly it is doing.
If I was just to replace the 2068 with something else, do you think it would help? If so, what might be good choices for replacements?
Anyone know what the 62Z devices are? I did some searching but didn't find anything that matched. Do you think that bears looking at?
What are the chances that the caps are contributing? I'm not sure what these are. They have a symbol that is an S in a circle. They are marked CE105 C above 0402. Is 402 the value? What does it mean?
Any thoughts about the HC4052A?
I know this isn't much to go on without a schematic. I don't have much experience with quality audio circuits. I thought I would start by asking opinions here about whether swagging out any of these parts is worth the effort.
Thanks for any opinions.