Hi
OK so they probably should be selected. With a decent collection it
should not be difficult to get some similar pairs. If they don't match
it will be slightly more distorted but will still work fine.
It is probably possible to make the transistors noisy by shorting inputs
to ground with the phantom power on, if the mood takes it.
Matt S
Ike Zimbel wrote:
Hi Matt,
All of the documentation that I have seen says "factory selected"
on the 2SB737's, which are the only one of these transistors that I
have seen (or at least noticed) in a Neotek mic pre. Incidentally, I
have never seen one fail in a mic pre yet. Jared, I agree with Matt
that you should check all of the other components 1st, both for
soldering issues and value. Have the caps been done?
best,
Ike
On 1/26/2010 12:36 PM, Matt Syson wrote:
Hi Jared
All or some of these transistors are approaching obsolete but should
be obtainable by post from the internet.
I would suggest getting a bag of them (they are not expensive) then
you can match them using a multimeter for similar Hfe.
(note, don't hold them for long as the finger temperature affects
gain!).
You could simply fit one and it will work but distortion would be
reduced by matching. I don't suppose Neotek matched them originally
though.
It may be worth running the multimeter over the whole circuit
comparing it to others (DC conditions) as there may be something else
amiss.
Matt S
Jared wrote:
Hello everyone
I'm having a problem with a series 1 mic pre (just the mic pre, line
in is fine). I'm getting a lot of noise and extremely distorted
signal. Everything looks fine visually. I swapped the chip and that
didn't fix it. My next thought was that it could be caused by a bad
transistor on the front end of the circuit. So I did a little
looking and now I'm a bit confused.
It seems that I need either of these transistors: 2SAB737, 2SA991,
or 2SA1084
I'm sure their are others but this is what is in my console and the
2SA1084 was written in some of the documentation that I have. Does
anyone have a lead on where these can be sourced or other
replacement options? Also what is the best way to match them? Can it
be a simple DMM hFE test?
Also maybe I'm going down the wrong path, any other thoughts about
what could be causing this?
Sorry for the long post,
Thanks,
Jared
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