Maker Pro
Maker Pro

Somebody explaining this design?

D

D from BC

Jan 1, 1970
0
Hello,

Take a look on this low noise mic preamp:
http://sound.westhost.com/project66.htm

Would anyone tell me the operation of the bipolar input transistors?
I can't figure out what is happening.

Regards
PAF

Heyy...this schematic is by Phil Allison.
I think he's on this group.. wait for answer..

It's current feedback.
Q2 is the current feedback transistor.
Basically...the more Q2 turns on the more Q1 turns off..
D from BC
 
P

Phil Allison

Jan 1, 1970
0
** Groper alert !

Take a look on this low noise mic preamp:
http://sound.westhost.com/project66.htm

Would anyone tell me the operation of the bipolar input transistors?
I can't figure out what is happening.


** Did you read the " introduction" comments ?

Got any idea what a "differential pair " is ?

The topology does look a bit strange when first encountered - but is just a
basic differential pair operated in fully differential mode with adjustable
gain supplied by varying the emitter to emitter resistance.

Forget about the BC549s, and imagine only one of the two inputs is driven.

See what it breaks down to then.



........ Phil
 
E

Eeyore

Jan 1, 1970
0
powerampfreak said:
Hello,

Take a look on this low noise mic preamp:
http://sound.westhost.com/project66.htm

Would anyone tell me the operation of the bipolar input transistors?
I can't figure out what is happening.

They're providing a variable gain true differential operation low noise
'front-end' before the op-amp.

Which bit(s) don't you understand ?

Graham
 
E

Eeyore

Jan 1, 1970
0
D said:
Heyy...this schematic is by Phil Allison.
I think he's on this group.. wait for answer..

It's current feedback.

Not really.

Q2 is the current feedback transistor.
Basically...the more Q2 turns on the more Q1 turns off..

That's a really crappy explanation.

Q1/Q2 and Q3/Q4 are 'compound pairs' that have very high linearity compared to a
single transistor.

Graham
 
E

Eeyore

Jan 1, 1970
0
Jim said:
Looks like crap to me ;-)

...Jim Thompson

Don't mind Thompson, he's a silly old right-wing fart with Alzheimers.

Graham
 
H

Homer J Simpson

Jan 1, 1970
0
Looks like crap to me ;-)

I saw one a little while ago that used 8 or 16 transistors in parallel as an
input stage. Hard to believe this is better.
 
P

Phil Allison

Jan 1, 1970
0
"Homer J Simpson"
I saw one a little while ago that used 8 or 16 transistors in parallel as
an input stage. Hard to believe this is better.


** The design performs just as advertised.

Mic pre-amps with paralleled input transistors are either:

1. Using the wrong transistor type.

2. Optimised for a source impedance much lower than 200 ohms.

3. A total wank.


You ought *post* your alleged schem, rather than just mischievously
alluding to its existence and performance.




....... Phil
 
E

Eeyore

Jan 1, 1970
0
Homer said:
I saw one a little while ago that used 8 or 16 transistors in parallel as an
input stage. Hard to believe this is better.

The use of transistors in parallel in this type of application is to reduce the
overall intrinsic device resistance that contributes to thermal noise.

Being a switching device, the 2N4403 is a reasonable choice for low intrinsic
resistance but far beter devices do exist although they're getting rare now as
the best ones were designed for low impedance low output moving coil record
pickups and there's not really much demand for them any more.

Phil's circuit is actually little different from one I designed into
Studiomaster's 8-4 rack mixer in 1979 btw. Even down to using the 2N4403. The
better input devices came along a bit later.
http://www.studiomaster.com/hp2.html

Graham
 
E

Eeyore

Jan 1, 1970
0
Phil said:
Mic pre-amps with paralleled input transistors are either:

1. Using the wrong transistor type.

The LM394 low noise super matched pair is actually lots of transistors in
parallel on a single die. Are you saying it's no good ?

Graham
 
W

Winfield Hill

Jan 1, 1970
0
Homer said:
Jim Thompson wrote


I saw one a little while ago that used 8 or 16 transistors in
parallel as an input stage. Hard to believe this is better.

That was to lower the noise, this is to lower the high-level
signal distortion, without excessive feedback. Of course
it's better. http://sound.westhost.com/project66.htm

As Eeyore said, "Q1/Q2 and Q3/Q4 are 'compound pairs' that
have very high linearity compared to a single transistor."

Actually, this, and a few other tricks, are widely used in
discrete-circuit designs with good results. For example,
early oscilloscope stages usually did not have any negative
feedback, and relied on highly-linear circuits to begin with.
I've also seen this in some IC designs, although it could be
that the commonly-available IC PNP process capabilities don't
well justify this scheme. JT will have to fill us in on that.
 
P

Phil Allison

Jan 1, 1970
0
"Winfield Hill"
That was to lower the noise, this is to lower the high-level
signal distortion, without excessive feedback. Of course
it's better. http://sound.westhost.com/project66.htm

As Eeyore said, "Q1/Q2 and Q3/Q4 are 'compound pairs' that
have very high linearity compared to a single transistor."


** Funny how virtually the same words appear in the second para of MY
article:

" The compound pairs of 2N4403 and BC549s are far more linear than any
single transistor. "

Forget the read the article, did you Win ?



........ Phil
 
P

Phil Allison

Jan 1, 1970
0
"Eeysore the Imbecile "
The LM394 low noise super matched pair is actually lots of transistors in
parallel on a single die. Are you saying it's no good ?



** That example nicely confirms the correctness of my comment.




....... Phil
 
D

D from BC

Jan 1, 1970
0
Not really.



That's a really crappy explanation.

Q1/Q2 and Q3/Q4 are 'compound pairs' that have very high linearity compared to a
single transistor.

Graham

That was my lame attempt at a super basic explanation.
I guess I should have mentioned that added feedback is from more
transistors for more linearity.
D from BC
 
E

Eeyore

Jan 1, 1970
0
D said:
That was my lame attempt at a super basic explanation.
I guess I should have mentioned that added feedback is from more
transistors for more linearity.

A nice way of looking at it IMHO is that as a result of the configuration Q1 and Q3
work almost at constant current in normal operation.

Graham
 
W

Winfield Hill

Jan 1, 1970
0
Phil said:
Winfield Hill wrote,


** Funny how virtually the same words appear in the second para
of MY article:

" The compound pairs of 2N4403 and BC549s are far more linear
than any single transistor. "

Forget the read the article, did you Win ?

Sorry, Phil, I didn't see it. Rereading all 16 articles in
this thread, I still don't see it, where was your article?

Anyway, Jim's comment, "Looks like crap to me ;-)", implies
an incorrect analysis, as several people correctly saw. In
a differential amplifier the addition of two transistors and
resistors in this way makes a dramatic improvement. If the
amplifier doesn't have feedback, as in this case, some trick
like this is essential. Graham says he designed this scheme
into Studiomaster's 8-4 rack mixer in 1979, but it's an old
trick, going back to the 60's at least, SFAIK.
 
W

Winfield Hill

Jan 1, 1970
0
Sorry, Phil, I didn't see it. Rereading all 16 articles in
this thread, I still don't see it, where was your article?

Oh, wait, sorry, now I understand: you wrote the article in
question in this thread, http://sound.westhost.com/project66.htm
Sorry, I didn't pay attention to the author's name, assuming it
was Rod Elliott at ESP. When did Elliott Sound Products take
your article and design, and how did that come about? When did
you first start using the compound-transistor trick, and had
you seen it elsewhere before that? What's the date of your ESP
design? Aren't most of the ESP designs done inhouse?
 
J

Jim Thompson

Jan 1, 1970
0
Sorry, Phil, I didn't see it. Rereading all 16 articles in
this thread, I still don't see it, where was your article?

Anyway, Jim's comment, "Looks like crap to me ;-)", implies
an incorrect analysis, as several people correctly saw. In
a differential amplifier the addition of two transistors and
resistors in this way makes a dramatic improvement. If the
amplifier doesn't have feedback, as in this case, some trick
like this is essential. Graham says he designed this scheme
into Studiomaster's 8-4 rack mixer in 1979, but it's an old
trick, going back to the 60's at least, SFAIK.

The scheme correctly "stiffens" the emitters of Q1,Q3, but blows it by
taking the outputs from the emitters of Q2,Q4.

The proper way is left as an exercise for the student ;-)

...Jim Thompson
 
E

Eeyore

Jan 1, 1970
0
Jim said:
The scheme correctly "stiffens" the emitters of Q1,Q3, but blows it by
taking the outputs from the emitters of Q2,Q4.

The proper way is left as an exercise for the student ;-)

...Jim Thompson

Utter nonsense.

The configuration works so well that in around 30 years of use for high
performance microphone amplifiers it's barely changed much at all.

More recent versions do 'close the loop' with feedback to Q1 and Q3's emitters
from a differential output stage and that's probably the current 'state of the
art' for this type of circuit block.

Graham
 
Top