How to identify FETs

[Bill Ruys]

I have a very early (circa 1993) Rode large diaphragm condensor mic. I am
trying to identify a couple transistors, which I believe to be FETs.
Problem is, I cannot find any data on them at all.

The two transistors in question have the numbers:
3DJ7F
3CX5B

Do these numbers mean anything to anyone out there? I have tried searching
on the net, and the few on-line cross reference guides I can find, but I'm
drawing blanks.

Any suggestions would be greatly appreciated.

Bill Ruys.

 

[Phil Allison]

"Bill Ruys" <billruys@xtra.co.nz>

I have a very early (circa 1993) Rode large diaphragm condensor mic. I am
trying to identify a couple transistors, which I believe to be FETs.
Problem is, I cannot find any data on them at all.

The two transistors in question have the numbers:
3DJ7F
3CX5B

Do these numbers mean anything to anyone out there? I have tried
searching
on the net, and the few on-line cross reference guides I can find, but I'm
drawing blanks.

** Those FETS come from China, same as the mic's capsule and the 2 Gohm
resistors. They have generic Chinese part numbers so you will not likely
find them on the net.

The devices are however N channel junction fets - vaguely similar to the
2SK30A available from WES components for A$0.86.



........... Phil

 

[Bill Ruys]

"Phil Allison" <philallison@optusnet.com.au> wrote in message
news:407e25a8$0$4544$afc38c87@news.optusnet.com.au...

"Bill Ruys" <billruys@xtra.co.nz

I have a very early (circa 1993) Rode large diaphragm condensor mic. I
am
trying to identify a couple transistors, which I believe to be FETs.
Problem is, I cannot find any data on them at all.

The two transistors in question have the numbers:
3DJ7F
3CX5B

Do these numbers mean anything to anyone out there? I have tried
searching
on the net, and the few on-line cross reference guides I can find, but
I'm
drawing blanks.



** Those FETS come from China, same as the mic's capsule and the 2 Gohm
resistors. They have generic Chinese part numbers so you will not likely
find them on the net.

The devices are however N channel junction fets - vaguely similar to the
2SK30A available from WES components for A$0.86.



.......... Phil

Thanks, Phil. Could I replace them both with the same FET? I have been
recommended the Toshiba 2sk170, which is also an N channel FET.

Cheers,
Bill.

 

[Phil Allison]

"Bill Ruys" <billruys@xtra.co.nz> wrote in message
news:g5qfc.15590$d%6.279440@news.xtra.co.nz...

"Phil Allison" <philallison@optusnet.com.au> wrote in message



Thanks, Phil. Could I replace them both with the same FET? I have been
recommended the Toshiba 2sk170, which is also an N channel FET.

** Are they both faulty ??

The 2SK30A is low noise and ideal for the job - you may well have to
select devices for drain current or tweak the biasing circuits to use them
as substitutes.





............ Phil

 

[Bill Ruys]

"Phil Allison" <philallison@optusnet.com.au> wrote in message
news:407e30b8$0$441$afc38c87@news.optusnet.com.au...

"Bill Ruys" <billruys@xtra.co.nz> wrote in message
news:g5qfc.15590$d%6.279440@news.xtra.co.nz...
"Phil Allison" <philallison@optusnet.com.au> wrote in message



Thanks, Phil. Could I replace them both with the same FET? I have been
recommended the Toshiba 2sk170, which is also an N channel FET.


** Are they both faulty ??

The 2SK30A is low noise and ideal for the job - you may well have to
select devices for drain current or tweak the biasing circuits to use them
as substitutes.

The problem with the mic is that it is quite noisey (constant low level
hiss). I have been told that the FETs are probably part of the problem.
But also I plan on replacing the low quality electrolytics with some kind of
poly film capacitors, and most of the resistors with metal film types. The
goal is to get lower self-noise overall.

The mic is the very first NT1 that was only sold in small numbers in NZ &
Aust.
I have some pictures here.
http://www.siliconaudio.co.nz/nt1.htm
(one of the transistors, & one of the caps is hard to see due to poor
lighting)
Have you seen one of these before, Phil?

Basically my background was in board repairs of electronic typewriters,
photocopiers, etc. The only transistors I've ever delt with were bi-polar,
I've virtually never touched FETs, and I've done little with audio, so I'm a
little out of my depth. I've designed a few basic circuits of my own over
the years but little in the audio arena.

Am I headed in the right direction to improve the self-noise characteristics
of this mic? Or am I pushing the smelly stuff up hill with a pointed stick?

Thanks again Phil, I'm learning as I go on this one

Bill.

 

[Phil Allison]

"Bill Ruys" <
"Phil Allison" <

** Are they both faulty ??

The 2SK30A is low noise and ideal for the job - you may well have
to
select devices for drain current or tweak the biasing circuits to use
them
as substitutes.


The problem with the mic is that it is quite noisey (constant low level
hiss). I have been told that the FETs are probably part of the problem.

** That is ***very*** unlikely to be true.

But also I plan on replacing the low quality electrolytics with some kind
of
poly film capacitors, and most of the resistors with metal film types.
The
goal is to get lower self-noise overall.


Have you seen one of these before, Phil?

** Yep - seen quite a few "Rodents" aka Rode NTs !

Am I headed in the right direction to improve the self-noise
characteristics
of this mic?

** I fear not. As sold the mics have low self noise - but it often
deteriorates due to moisture and gunk contaminating the diaphragm. This is
guaranteed to happen if dopes use the mic for vocals without an effective
breath shield.

Cleaning of the diaphragm is possible, but risky - I have has success by
carefully flushing it with metho and then drying with warm air from a hair
drier to re-tension the plastic film.

BTW The phantom power supply must be 48 volts for good results with
that early model.




.......... Phil

 

[Bill Ruys]

"Phil Allison" <philallison@optusnet.com.au> wrote in message
news:407e705b$0$4548$afc38c87@news.optusnet.com.au...

The problem with the mic is that it is quite noisey (constant low level
hiss). I have been told that the FETs are probably part of the problem.


** That is ***very*** unlikely to be true.



But also I plan on replacing the low quality electrolytics with some
kind
of
poly film capacitors, and most of the resistors with metal film types.
The
goal is to get lower self-noise overall.


Have you seen one of these before, Phil?


** Yep - seen quite a few "Rodents" aka Rode NTs !



Am I headed in the right direction to improve the self-noise
characteristics
of this mic?


** I fear not. As sold the mics have low self noise - but it
often
deteriorates due to moisture and gunk contaminating the diaphragm. This
is
guaranteed to happen if dopes use the mic for vocals without an effective
breath shield.

Cleaning of the diaphragm is possible, but risky - I have has success
by
carefully flushing it with metho and then drying with warm air from a hair
drier to re-tension the plastic film.

BTW The phantom power supply must be 48 volts for good results with
that early model.

Hmm, I've just had the grille off the mic for the first time. Looking at
the capsule it looks pretty good to me. If I look at it with light
reflecting off the mylar, I can see a few specks of dust, but nothing I
would call gunk.

An unloaded XLR on my desk shows about 46 volts at pins 2 & 3, but I've just
measured from the back of the mic XLR with the cover off and I'm reading
only 40 volts. The NT1 must draw a little current. Maybe this is my
problem !?!?

Bill.

 

[Phil Allison]

"Bill Ruys" <billruys@xtra.co.nz>

An unloaded XLR on my desk shows about 46 volts at pins 2 & 3, but I've
just
measured from the back of the mic XLR with the cover off and I'm reading
only 40 volts. The NT1 must draw a little current. Maybe this is my
problem !?!?

** No.


What makes you say the mic is noisy ?

Noisy compared to which other mic ??

How have you conducted noise level tests ?




........... Phil

 

[Bill Ruys]

"Phil Allison" <philallison@optusnet.com.au> wrote in message
news:407e8561$0$16964$afc38c87@news.optusnet.com.au...

"Bill Ruys" <billruys@xtra.co.nz


An unloaded XLR on my desk shows about 46 volts at pins 2 & 3, but I've
just
measured from the back of the mic XLR with the cover off and I'm reading
only 40 volts. The NT1 must draw a little current. Maybe this is my
problem !?!?

** No.


What makes you say the mic is noisy ?

Noisy compared to which other mic ??

How have you conducted noise level tests ?

I can only compare it to other condensor mics I have. Compared to my Studio
Projects C1, Audio Technica AT3035 (both LDC) and Rode NT5s, the old NT1 is
a lot noisier when set to match the levels recorded with the other mics.

Noise level tests are extremely un-scientific, A/B-ing against the other
mics with headphones on. Getting the level of my voice at the same volume
on each mic and then listening to the silence, soloing one mic at a time.
Have also tried each mic on the same channel of my desk to rule out a desk
problem.

I guess you could say that the SNR and noise floor is about equivalent to a
compact cassette played on a consumer level tape deck. It's high frequency
white noise, not unlike tape hiss actually. Acceptable if I was recording
reasonably loud source material, but I'd have to gate it if I was recording
anything intermittent or quiet.

Bill.

 

[Phil Allison]

"Bill Ruys" <billruys@xtra.co.nz>

"Phil Allison" <philallison@optusnet.com.au


What makes you say the mic is noisy ?

Noisy compared to which other mic ??

How have you conducted noise level tests ?


I can only compare it to other condensor mics I have. Compared to my
Studio
Projects C1, Audio Technica AT3035 (both LDC) and Rode NT5s, the old NT1
is
a lot noisier when set to match the levels recorded with the other mics.

Noise level tests are extremely un-scientific, A/B-ing against the other
mics with headphones on. Getting the level of my voice at the same volume
on each mic and then listening to the silence, soloing one mic at a time.
Have also tried each mic on the same channel of my desk to rule out a desk
problem.

I guess you could say that the SNR and noise floor is about equivalent to
a
compact cassette played on a consumer level tape deck. It's high
frequency
white noise, not unlike tape hiss actually. Acceptable if I was recording
reasonably loud source material, but I'd have to gate it if I was
recording
anything intermittent or quiet.

** Well, replace that first j-fet and see what happens.

The noise from a ( correctly functioning) condenser mic is not white
noise - as it is with dynamics. The spectrum is concentrated at the low
frequency end - like pink noise but even more so.





.............. Phil

 

[Bill Ruys]

"Phil Allison" <philallison@optusnet.com.au> wrote in message
news:407f309c$0$436$afc38c87@news.optusnet.com.au...

** Well, replace that first j-fet and see what happens.

The noise from a ( correctly functioning) condenser mic is not white
noise - as it is with dynamics. The spectrum is concentrated at the low
frequency end - like pink noise but even more so.

I should have a 2SK30A on Monday. I'll let you know how it goes.

There is lower frequency noise, but at a much lower level (expected
self-noise). It's the high frequency stuff that is making the mic unusable.
I tried putting some .47 uF caps across parts of the circuit to further
decouple the power supply in case there was some parasitic oscillations
going on, but that didn't help.

So the mic's back in the cabinet until my new FET turns up. Thanks for all
the help Phil.

Bill.