Re: mic preamp modification
- From: Pooh Bear <rabbitsfriendsandrelations@xxxxxxxxxxx>
- Date: Fri, 08 Apr 2005 09:23:30 +0100
rex wrote:
> 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.)
I'll guess it's 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:
> http://www.oade.com/digital_recorders/hard_disc_recorders/PMD-670.html
> He only does it on stuff he sells and I already have mine. I asked him
> if he would share anything about the nature of his change, but he
> declined.
Tuh ! Silly wanker.
> 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?
NO. The 'front end' is what matters.
> If so, what might be good choices for replacements?
>
> Anyone know what the 62Z devices are?
That's a typical SMD code. There isn't enough space to fit the full part
number. Sorry !
> I did some searching but didn't
> find anything that matched. Do you think that bears looking at?
NO.
> What are the chances that the caps are contributing?
So close as next to nil that you might as well forget it.
> 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?
105 means 105 degrees C rating - that's fine. 0402 is normally an SMD
footprint size.
> Any thoughts about the HC4052A?
Nope.
> I know this isn't much to go on without a schematic. I don't have much
> experience with quality audio circuits.
I do !
> I thought I would start by
> asking opinions here about whether swagging out any of these parts is
> worth the effort.
Unlikely. Once the circuit topology is fixed there's usually bugger all that
you can do to improve it.
The Eastern guys don't have a clue how to make a good mic amp - apart from
Yamaha maybe ?
Graham
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