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Micpre of Graham

B

Ban

Jan 1, 1970
0
Phil said:
"Ban"


** Good pre-amp ?

With a pair of 2.2 uF film caps ( = 1.1 uF ) in series with the mic
input ?

Not too good for the noise figure at or below 1kHz.
Why do you think a reactive element increases the noise?
 
B

Ban

Jan 1, 1970
0
YD said:
Late at night, by candle light, John Larkin


Looks like something's wrong with the feedback connections of IC23/R88
and IC30/R115. Maybe I'm just missing something.

- YD.

Bingo, it is drawn wrongly. THX for pointing that out. Should be like in the
stage before.
 
E

Eeyore

Jan 1, 1970
0
Ban said:
Why do you think a reactive element increases the noise?

Come on, what predominantly determines the noise of that stage at high gain ?
Hint: it's not those transistors.

Graham
 
B

Ban

Jan 1, 1970
0
Eeyore said:
Exactly.

I was hoping that Ban would see his error himself.

Well, the thing is there is a divider formed by the 4k7 from 17V and the 6k8
from the opamp output. In the first moment the opamps have 0V, when the
transient arrives, one opamp goes to the pos rail and forces the other to
the negative. Now there are 4k7 || 5k(pot) with the 470u cap voltage, which
can be as high as 4V, depending on the history. That makes 1.5V across the
first junction and around 10V across the other one.
 
C

cledus

Jan 1, 1970
0
Jim said:
I got ~960nV/rt-Hz

I substituted 50 ohms for the 18k input resistors (R3 & R5). I forgot to
point this out. This may, in part, account for the differences in our
results. Not sure what makes up the rest. Perhaps the noise model for
the 2N2907's?
I suspect you made a mis-entry.

You may be right. I don't trust the model I have for the LT1028. I
entered a different high bandwidth op amp that seems a bit more
trustworthy (MAX4106). It appears to be stable with this component. In
fact the peak in freq response moves from 1 MHz using LM833 to greater
than 10 MHz with the MAX4106.

From LTC ?:)

When I feel ambitious, I will dowload their freebie SPICE app and give
it a try. I have used CircuitMaker2K for awhile now and need to get
over the inertia to learn something new.
...Jim Thompson


TNX,
-c
 
J

Jim Thompson

Jan 1, 1970
0
Jim Thompson wrote: [snip]
I got ~960nV/rt-Hz

I substituted 50 ohms for the 18k input resistors (R3 & R5). I forgot to
point this out. This may, in part, account for the differences in our
results. Not sure what makes up the rest. Perhaps the noise model for
the 2N2907's?

The BJT model IS the noise model.

[snip]

...Jim Thompson
 
E

Eeyore

Jan 1, 1970
0
Ban said:
Well, the thing is there is a divider formed by the 4k7 from 17V and the 6k8
from the opamp output.

Yes, that's equivalent to 2k8 from 10.23V.

In the first moment the opamps have 0V, when the
transient arrives, one opamp goes to the pos rail and forces the other to
the negative.

What makes you think that happens ? The 'transient' appears in common mode on
*both* inputs. Of course there won't be a transient with a properly ramped 48V
supply of course.

Now there are 4k7 || 5k(pot) with the 470u cap voltage, which
can be as high as 4V, depending on the history. That makes 1.5V across the
first junction and around 10V across the other one.

You've missed something very basic.

Read Phalluson's post and see if the number he quotes rings any bells for you.
You *are* wong about this I have to tell you.

Graham
 
K

Kevin Aylward

Jan 1, 1970
0
Ban said:
Why do you think a reactive element increases the noise?


Input shot noise droped accross the capacitor. Say Ic=2ma, hfe=200,
In=sqrt(2.Ic/hfe.q) = 2pa/rthz. At 1k, Xc = 160 ohms, giving 0.3nv/rthz.
 
J

John Larkin

Jan 1, 1970
0
Input shot noise droped accross the capacitor. Say Ic=2ma, hfe=200,
In=sqrt(2.Ic/hfe.q) = 2pa/rthz. At 1k, Xc = 160 ohms, giving 0.3nv/rthz.

Does Ib have full shot noise? If Ie is set by the voltage drop across
a metal-film resistor, it has no shot noise. So I'd expect Ib, some
fraction of Ie, to be shot-noise free as well.

John
 
B

Ban

Jan 1, 1970
0
Eeyore said:
Yes, that's equivalent to 2k8 from 10.23V.



What makes you think that happens ? The 'transient' appears in common
mode on *both* inputs. Of course there won't be a transient with a
properly ramped 48V supply of course.



You've missed something very basic.

Read Phalluson's post and see if the number he quotes rings any bells
for you. You *are* wong about this I have to tell you.

Why do you not take your simulation, give a 48V step function with say 100ms
risetime as phantom power and see what happens with the output and across
the transistor. Play a bit with values, reduce the 47uF to say 2.2u, or take
some higher slew rate opamp for the first OP like OP37. Check the voltage
across the transistors.
Maybe you check what a single pulse will do or even an adapter to an
unbalanced jack, the short Phil was talking about.
I could see some 300ms of saturation, rail to rail. That will pop out your
speakers quite a bit, man.
 
E

Eeyore

Jan 1, 1970
0
Ban said:
Eeyore wrote:

Why do you not take your simulation, give a 48V step function with say 100ms
risetime as phantom power and see what happens

First off since you can't apparently see the mistake you're making, I'll point
out that switching on the phantom power *does not* reverse bias any transistors.

There's a 3k resistor to ground on both inputs. The largest voltage that can
occur as a result of switching on the phantom is 48V * 3/(3+6.8) which is
14.69V.

with the output and across the transistor. Play a bit with values, reduce the
47uF to say 2.2u,

So you've also missed the point about compromising the input noise figure ?

or take some higher slew rate opamp for the first OP like OP37.

What exactly would be the point of that ?

Check the voltage across the transistors.

I can calculate that !

Maybe you check what a single pulse will do or even an adapter to an
unbalanced jack, the short Phil was talking about.

That's a pulse in the opposite direction. That one is actually the problematic
one.

I could see some 300ms of saturation, rail to rail.

Where did you see this ?
That will pop out your speakers quite a bit, man.

It's obviously inadvisable (and unnecessary) to switch the phantom on and off
when the channel is on.

Graham
 
P

Phil Allison

Jan 1, 1970
0
"Ban"
Why do you think a reactive element increases the noise?


** It raises the effective source impedance.

Can't you get ANYTHING right ?

Wog idiot.




....... Phil
 
K

Kevin Aylward

Jan 1, 1970
0
John said:
Does Ib have full shot noise? If Ie is set by the voltage drop across
a metal-film resistor, it has no shot noise. So I'd expect Ib, some
fraction of Ie, to be shot-noise free as well.

I am not sure what you are saying here. Base current and collector current
shot noise are modelled as inherent current sources across the respective
junctions. External components can't change these values. However, external
components, might modify the effect of the inherent shot noise.
 
J

John Larkin

Jan 1, 1970
0
I am not sure what you are saying here. Base current and collector current
shot noise are modelled as inherent current sources across the respective
junctions. External components can't change these values. However, external
components, might modify the effect of the inherent shot noise.

OK, simplest case first, an emitter follower:


+10
|
|
|
c
+5----------------b
e
|
|
1K metal film
|
|
|
GND


Does Ib or Ic have full shot noise? Ie sure doesn't.

John
 
F

Fred Bartoli

Jan 1, 1970
0
John Larkin a écrit :
OK, simplest case first, an emitter follower:


+10
|
|
|
c
+5----------------b
e
|
|
1K metal film
|
|
|
GND


Does Ib or Ic have full shot noise? Ie sure doesn't.

Actually that's pretty easy to check with an LF SA (1M input)

15V >---.
|
.-.
Rc | |
| |1K
'-'
|
| ||
Adjust +---||--> SA
| ||
for Ve=5V |
___ |/
10.7V >----|___|--+-----|
| |>
Rb --- |
--- | ||
| +---||--> SA
SA <---' | ||
|
.-.
Re | |
| |1K
'-'
|
|
===
GND


For shot noise to dominate you need:
2q.I.R > 4kT or R.I > 52mV
That's easily satisfied.


Now your SA has about 10nv/rtHz noise floor so you want the measurement
to be higher.

Shot noise across R will be en^2=R^2.2q.I = 2q.R.RI
or en= sqrt(R) sqrt(2q.R_drop).

In the above test, we have 5V across each resistor.
Full shot noise should then develop:
en=sqrt(R)x1.27nV/rtHz

which is 40nv/rtHz for a full collector shot noise.
For base shot noise if you take beta=100, then you'll have 400nV/rtHz,
800nV for beta=400.
 
J

John Larkin

Jan 1, 1970
0
John Larkin a écrit :

Actually that's pretty easy to check with an LF SA (1M input)

15V >---.
|
.-.
Rc | |
| |1K
'-'
|
| ||
Adjust +---||--> SA
| ||
for Ve=5V |
___ |/
10.7V >----|___|--+-----|
| |>
Rb --- |
--- | ||
| +---||--> SA
SA <---' | ||
|
.-.
Re | |
| |1K
'-'
|
|
===
GND


For shot noise to dominate you need:
2q.I.R > 4kT or R.I > 52mV
That's easily satisfied.


Now your SA has about 10nv/rtHz noise floor so you want the measurement
to be higher.

Shot noise across R will be en^2=R^2.2q.I = 2q.R.RI
or en= sqrt(R) sqrt(2q.R_drop).

In the above test, we have 5V across each resistor.
Full shot noise should then develop:
en=sqrt(R)x1.27nV/rtHz

which is 40nv/rtHz for a full collector shot noise.
For base shot noise if you take beta=100, then you'll have 400nV/rtHz,
800nV for beta=400.


But metal film resistors don't have shot noise current. So, if a
transistor's bias current is mostly set by the voltage drop across an
emitter resistor, emitter current won't have much shot noise either.
So Ic certainly won't. It seems to me that base current shouldn't
either, but I'm not 100% sure about that.

John
 
E

Eeyore

Jan 1, 1970
0
John said:
But metal film resistors don't have shot noise current.

You mean they're not supposed to surely ? I thought only bulk metal types can be
considered to be truly 'noiseless' like that.

Graham
 
F

Fred Bartoli

Jan 1, 1970
0
Eeyore a écrit :
You mean they're not supposed to surely ? I thought only bulk metal types can be
considered to be truly 'noiseless' like that.

Graham

LOL!
Resistors don't have shot noise. Shot noise comes from the electrons
getting through a potential barrier, which resistors haven't.

You're probably confusing with excess noise, which shows as 1/F noise
and is proportional to the current flowing through the resistor since
it's basically resistor fluctuation.
 
E

Eeyore

Jan 1, 1970
0
Fred said:
Eeyore a écrit :


LOL!
Resistors don't have shot noise. Shot noise comes from the electrons
getting through a potential barrier, which resistors haven't.

You're probably confusing with excess noise, which shows as 1/F noise
and is proportional to the current flowing through the resistor since
it's basically resistor fluctuation.

Yes. They sound similar though.

Graham
 
J

John Larkin

Jan 1, 1970
0
You mean they're not supposed to surely ? I thought only bulk metal types can be
considered to be truly 'noiseless' like that.

Graham


Vile Vishay propaganda. They publish some criminally silly nonsense.

I tested an assortment of resistors for shot noise and couldn't see
any, at least not as much as 20% of full shot statistics, which was
about my noise floor. And that included the cheapest surface mount and
axial carbon film 5%'s I could find.

Some day I want to do a more sensitive test, with a mess of low-noise
jfets or something. I still think carbons may have some shotty
behavior.

John
 
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