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Noise in audio + opamps

  • Thread starter Dirk Bruere at NeoPax
  • Start date
D

Dirk Bruere at NeoPax

Jan 1, 1970
0
Looking at the OP37 (for example) with a noise figure of 3nV (root
Hz).
If I were using this as an audio pre-amp with a gain of 5 how does
this translate into the kind of S/N figures usually specced for amps
eg -NN dB ?

Dirk
 
M

martin griffith

Jan 1, 1970
0
Looking at the OP37 (for example) with a noise figure of 3nV (root
Hz).
If I were using this as an audio pre-amp with a gain of 5 how does
this translate into the kind of S/N figures usually specced for amps
eg -NN dB ?

Dirk
You have to do the sums
http://focus.ti.com/lit/an/slod006b/slod006b.pdf
page 150 gives you the lowdown.
depends on source Z, bandwidth, temperature etc. +-1audiophool


martin
 
D

Dirk Bruere at NeoPax

Jan 1, 1970
0
You have to do the sumshttp://focus.ti.com/lit/an/slod006b/slod006b.pdf
page 150 gives you the lowdown.
depends on source Z, bandwidth, temperature etc. +-1audiophool

martin

Thanks
Quite interesting.

Dirk
 
E

Eeyore

Jan 1, 1970
0
Dirk said:
Looking at the OP37 (for example) with a noise figure of 3nV (root
Hz).
If I were using this as an audio pre-amp with a gain of 5 how does
this translate into the kind of S/N figures usually specced for amps
eg -NN dB ?

The first thing you need to do is specify the reference level for measurement !

National's 'Audio Handbook' deals with all of this reasonably well including a
discussion of adding the effect of source impedance and the influence of bias
currents (noise component of ) too.

Graham
 
D

Dirk Bruere at NeoPax

Jan 1, 1970
0
The first thing you need to do is specify the reference level for measurement !

National's 'Audio Handbook' deals with all of this reasonably well including a
discussion of adding the effect of source impedance and the influence of bias
currents (noise component of ) too.

Graham

I'm just looking for a noise free way to adjust non standard line-in
to non-standard line-in!
Since the gain will be very unlikely to exceed a factor of 3 maybe
just a common emitter transistor as an amp?
I really want very little noise and to run from a 5V computer PSU.
I expect this is where someone mentions noise on the 5V, rejection
ratios and all that.
Alternatively a neat 5V opamp solution would be appreciated.

Dirk
 
J

Joerg

Jan 1, 1970
0
Dirk said:
I'm just looking for a noise free way to adjust non standard line-in
to non-standard line-in!
Since the gain will be very unlikely to exceed a factor of 3 maybe
just a common emitter transistor as an amp?
I really want very little noise and to run from a 5V computer PSU.
I expect this is where someone mentions noise on the 5V, rejection
ratios and all that.
Alternatively a neat 5V opamp solution would be appreciated.

Many devices from the TI (formerly Burr-Brown) OPA series seem to be
suitable but during a recent similar search I found lots of them to be
out of stock. AD has nice low noise amps that are higher in BW but be
careful, many of them cannot tolerate more than a diode drop in
differential input. Might not be an issue for a linear amp but other
than a small hint in the text there was no mention of that.
 
E

Eeyore

Jan 1, 1970
0
Dirk said:
I'm just looking for a noise free way to adjust non standard line-in
to non-standard line-in!
Since the gain will be very unlikely to exceed a factor of 3 maybe
just a common emitter transistor as an amp?

Do you really want to crush your audio performance ?

I really want very little noise and to run from a 5V computer PSU.
I expect this is where someone mentions noise on the 5V, rejection
ratios and all that.
Alternatively a neat 5V opamp solution would be appreciated.

5V isn't enough to run pro-level audio without clipping. Heck, it's not even adequate
for CD level outputs (2V rms @ 0dBFS) !

You should use a split supply too unless you really want to make life hard for
yourself.

The good news is that supply noise isn't too much of a problem with op-amp circuitry.
You're going to have to think this one through a bit more thoroughly.

Graham
 
E

Eeyore

Jan 1, 1970
0
Joerg said:
Many devices from the TI (formerly Burr-Brown) OPA series seem to be
suitable but during a recent similar search I found lots of them to be
out of stock. AD has nice low noise amps that are higher in BW but be
careful, many of them cannot tolerate more than a diode drop in
differential input. Might not be an issue for a linear amp but other
than a small hint in the text there was no mention of that.

The standard commodity NE5532 or NJM4560/80 is way more than adequate for line level
applications.

Esoteric stuff isn't needed.

Graham
 
F

Fred Bartoli

Jan 1, 1970
0
Eeyore a écrit :
The standard commodity NE5532 or NJM4560/80 is way more than adequate for line level
applications.

Esoteric stuff isn't needed.

With 0-5V supply it might be so :)
 
J

Joerg

Jan 1, 1970
0
Eeyore said:
Joerg wrote:




5 volt supplies are a non-starter for serious audio.

It'll be tough. I am currently doing one and it will have +/-12V.
 
E

Eeyore

Jan 1, 1970
0
Joerg said:
It'll be tough. I am currently doing one and it will have +/-12V.

That's reasonably OK as long as you're not doing any serious summing typically.
+/- 15V is more the norm but I run my own supplies at +/-17 V to get that
elusive last dB or so of headroom. Even so, the internal signal levels I usually
normalise to -6dB wrt external levels.

Graham
 
C

Chris Jones

Jan 1, 1970
0
Dirk said:
Looking at the OP37 (for example) with a noise figure of 3nV (root
Hz).
If I were using this as an audio pre-amp with a gain of 5 how does
this translate into the kind of S/N figures usually specced for amps
eg -NN dB ?

Dirk

You need to think about the impedance of the source before choosing the
op-amp, in case you have not already.

You would probably benefit from reading Douglas Self's books, in case you
don't already have them. On page 83 of "Self on Audio" he mentions that in
one particular circuit, the 5532 was measured to be quieter than an OP-27,
even though the datasheets would lead one to expect the opposite. I think
it is something to do with the bias current cancellation scheme in the
OP-27/OP-37 which introduces common-mode noise current, though that only
matters when the impedance presented to the two input pins is not equal.

Here is some stuff about op-amps and audio
http://www.dself.dsl.pipex.com/ampins/webbop/opamp.htm

Chris
 
J

Joerg

Jan 1, 1970
0
Eeyore said:
Joerg wrote:




That's reasonably OK as long as you're not doing any serious summing typically.
+/- 15V is more the norm but I run my own supplies at +/-17 V to get that
elusive last dB or so of headroom. Even so, the internal signal levels I usually
normalise to -6dB wrt external levels.

So what's your opamp du jour for this stuff? AFAIK the NE5532 doesn't
come in TSSOP and not in quad pack. If something doesn't migrate to
TSSOP these days I become a bit concerned about remaining production life.
 
D

Dirk Bruere at NeoPax

Jan 1, 1970
0
Eeyore said:
That's reasonably OK as long as you're not doing any serious summing typically.
+/- 15V is more the norm but I run my own supplies at +/-17 V to get that
elusive last dB or so of headroom. Even so, the internal signal levels I usually
normalise to -6dB wrt external levels.

Graham

I'm feeding into a soundcard on a PC, so it's not rocket science.
The best I've found is this:
http://www.analog.com/UploadedFiles/Data_Sheets/738738312AD8099_pra.pdf
1nV/rtHz isn't bad.

--
Dirk

http://www.onetribe.me.uk - The UK's only occult talk show
Presented by Dirk Bruere and Marc Power on ResonanceFM 104.4
http://www.resonancefm.com
 
J

john jardine

Jan 1, 1970
0
Dirk Bruere at NeoPax said:
I'm just looking for a noise free way to adjust non standard line-in
to non-standard line-in!
Since the gain will be very unlikely to exceed a factor of 3 maybe
just a common emitter transistor as an amp?
I really want very little noise and to run from a 5V computer PSU.
I expect this is where someone mentions noise on the 5V, rejection
ratios and all that.
Alternatively a neat 5V opamp solution would be appreciated.

Dirk

The cheap TS974/2/1/ (Farnells) does a nice 4nV-root hz.
Wide bandwidth, low distortion and runs down to +/- 1.35V.
Used 8 in series for a particular x100,000 bandpass filter (30kHz bandwidth)
and was seeing 100mVpkpk noise at the output. (as should be).
john
 
E

Eeyore

Jan 1, 1970
0
Joerg said:
So what's your opamp du jour for this stuff? AFAIK the NE5532 doesn't
come in TSSOP and not in quad pack. If something doesn't migrate to
TSSOP these days I become a bit concerned about remaining production life.

The 5532 wasn't originally available in SMT because AIUI the die used to be too large.
It is now however available in SOIC and has been for some time.

The other regular candidates are the NJM 4560 and 4580 along with the venerable TL07x.

I rarely use any of the esoteric stuff since I'm normally designing for low cost and
the above do a fine job.

Graham
 
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