Lowest noise amps

Guest
An old friend swears that vacuum tubes for audio have less noise than
the best op-amps. Is this true? This sounds completely untrue to me.
I've seen op-amps around 1nV/sqrt(Hz) in the audio range.

What type of voltage amplifier has lowest noise for audio? As far as I
know, it's an op-amps or a FET, right? OK, some op-amps have FETs, but
I've seen some 1/4nV/Sqrt(Hz) FETs.

Paul
 
pmlonline@gmail.com wrote:

An old friend swears that vacuum tubes for audio have less noise than
the best op-amps. Is this true?
Click to expand...
No - but good toobs are actually surprisingly quiet.

This sounds completely untrue to me.
I've seen op-amps around 1nV/sqrt(Hz) in the audio range.

What type of voltage amplifier has lowest noise for audio? As far as I
know, it's an op-amps or a FET, right? OK, some op-amps have FETs, but
I've seen some 1/4nV/Sqrt(Hz) FETs.
Click to expand...
Discretes will outperform even the best op-amps. Best noise I've seen is
0.3nV / rt Hz for a discrete fet.

Graham
 
On Thu, 18 Aug 2005 22:35:44 +0100, Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote:

pmlonline@gmail.com wrote:

An old friend swears that vacuum tubes for audio have less noise than
the best op-amps. Is this true?

No - but good toobs are actually surprisingly quiet.

This sounds completely untrue to me.
I've seen op-amps around 1nV/sqrt(Hz) in the audio range.

What type of voltage amplifier has lowest noise for audio? As far as I
know, it's an op-amps or a FET, right? OK, some op-amps have FETs, but
I've seen some 1/4nV/Sqrt(Hz) FETs.

Discretes will outperform even the best op-amps. Best noise I've seen is
0.3nV / rt Hz for a discrete fet.

Graham
Click to expand...
It's all about power. Goose up some monolithic designs and you can do
just as well noise-wise. But I've done as good as 1nV/rt-Hz in
squeezed current situations.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
Pooh Bear wrote:
Discretes will outperform even the best op-amps. Best noise I've seen is
0.3nV / rt Hz for a discrete fet.
Click to expand...
Check this out. I found 0.1pV/rt-hz gain of 500 transformer
preamplifier. The datasheet shows 0.1pV/rt-hz for both transformer and
buffer, it says you can bypass the buffer for a gain of only 100. I
wonder what the input noise is without the buffer! You can probably
get even better cores than that. I've heard that cobalt core materials
have nearly zero Barkhausen noise.
http://www.thinksrs.com/downloads/PDFs/Manuals/SR554m.pdf

Paul
 
Even better yet, supposedly this guy built a 65pV/rt-hz 5Hz-100kHz
amplifier:

J. Lepaisant, M. Lam Chok Sing, D. Bloyet
Low-noise preamplifier with input and feedback transformers for low
source
resistance sensors
Rev. Sci Instrum. 63(3), March 1992, p2089

Is that just a low noise transformer?

Paul
 
pmlonline@gmail.com wrote:
Even better yet, supposedly this guy built a 65pV/rt-hz 5Hz-100kHz
amplifier:

J. Lepaisant, M. Lam Chok Sing, D. Bloyet
Low-noise preamplifier with input and feedback transformers for low
source
resistance sensors
Rev. Sci Instrum. 63(3), March 1992, p2089

Is that just a low noise transformer?

Paul
Click to expand...
I've only just skimmed it, but it looks like a typical non-inverting
buffer, where some of the output is coupled back to the input in
antiphase, to give negative feedback.

Instead of setting the gain with resistors like in a normal op-amp
would, with gain.

G = 1+R1/R2

it is done with transformers. So the gain must be set by the turns ratio
of the transformers.

The actual circuit has FETs in front of the op-amp.

The Stanford data sheet someone pointed to earlier shows the noise
figure to be 1dB at optimal frequencies. As an RF engineer, I think in
noise figures and temperatures.

A noise figure of 1dB is equivalent to an internal noise generated by
the preamp equal to a resistor at 75 Kelvin.

http://www.satsig.net/noise.htm

A decent UHF (500MHz) pre-amp can achieve noise figures of around 0.3dB,
or 21K, so produce between a third and quarter of the noise of that
Stanford device.

Those UHF premaps I talk of usually have less noise at lower
frequencies, but they might be next to impossible to keep stable at
audio frequencies. With gains to well over 30GHz, they tend to oscillate
somewhere I suspect.
 
pmlonline@gmail.com wrote:
An old friend swears that vacuum tubes for audio have less noise than
the best op-amps.
Click to expand...
This may be close to being true in high-Z circuits as long as there is
no high sound pressure level, shock or vibration.

But remember that tubes are microphonic and any physically induced
noise level in the audio range can be way higher than the thermal
noise, even leading to oscillation if a preamp tube is close to a
speaker.
 
<pmlonline@gmail.com> wrote in message
news:1124404946.405795.224020@g43g2000cwa.googlegroups.com...
Pooh Bear wrote:
Discretes will outperform even the best op-amps. Best noise I've seen is
0.3nV / rt Hz for a discrete fet.

Check this out. I found 0.1pV/rt-hz gain of 500 transformer
preamplifier. The datasheet shows 0.1pV/rt-hz for both transformer and
buffer, it says you can bypass the buffer for a gain of only 100. I
wonder what the input noise is without the buffer! You can probably
get even better cores than that. I've heard that cobalt core materials
have nearly zero Barkhausen noise.
http://www.thinksrs.com/downloads/PDFs/Manuals/SR554m.pdf

Paul
Click to expand...
The noise figure is 1 dB, which is not all that good.
It is only great at 10 Hz to 10 kHz
It also distorts and causes harmonics at low level signals, because of
non-linear magnetic, and they do not list a spec for it either.
 
pmlonline@gmail.com wrote:

Pooh Bear wrote:
Discretes will outperform even the best op-amps. Best noise I've seen is
0.3nV / rt Hz for a discrete fet.

Check this out. I found 0.1pV/rt-hz gain of 500 transformer
preamplifier. The datasheet shows 0.1pV/rt-hz for both transformer and
buffer, it says you can bypass the buffer for a gain of only 100. I
wonder what the input noise is without the buffer! You can probably
get even better cores than that. I've heard that cobalt core materials
have nearly zero Barkhausen noise.
http://www.thinksrs.com/downloads/PDFs/Manuals/SR554m.pdf
Click to expand...
Input impedance of 0.5 ohms won't be any use for audio ! Rubbish frequency
response. This is designed for entirely different applications.

Remember you asked about *voltage* amplifiers. These usually have high input
Z.

Graham
 
Dave wrote:

pmlonline@gmail.com wrote:
Even better yet, supposedly this guy built a 65pV/rt-hz 5Hz-100kHz
amplifier:

J. Lepaisant, M. Lam Chok Sing, D. Bloyet
Low-noise preamplifier with input and feedback transformers for low
source
resistance sensors
Rev. Sci Instrum. 63(3), March 1992, p2089

Is that just a low noise transformer?

Paul


I've only just skimmed it, but it looks like a typical non-inverting
buffer, where some of the output is coupled back to the input in
antiphase, to give negative feedback.

Instead of setting the gain with resistors like in a normal op-amp
would, with gain.

G = 1+R1/R2

it is done with transformers. So the gain must be set by the turns ratio
of the transformers.

The actual circuit has FETs in front of the op-amp.

The Stanford data sheet someone pointed to earlier shows the noise
figure to be 1dB at optimal frequencies. As an RF engineer, I think in
noise figures and temperatures.

A noise figure of 1dB is equivalent to an internal noise generated by
the preamp equal to a resistor at 75 Kelvin.

http://www.satsig.net/noise.htm

A decent UHF (500MHz) pre-amp can achieve noise figures of around 0.3dB,
or 21K, so produce between a third and quarter of the noise of that
Stanford device.

Those UHF premaps I talk of usually have less noise at lower
frequencies, but they might be next to impossible to keep stable at
audio frequencies. With gains to well over 30GHz, they tend to oscillate
somewhere I suspect.
Click to expand...
And RF practice is also entirely different to audio.

Graham
 
In article <4304FF30.E40C1540@hotmail.com>,
Pooh Bear <rabbitsfriendsandrelations@hotmail.com> wrote:
[...]
Discretes will outperform even the best op-amps. Best noise I've seen is
0.3nV / rt Hz for a discrete fet.
Click to expand...
The Interfet will do about 0.25 if you run selected ones at about 10mA.

I've often wanted to see what the noise of a great big power MOSFET would
be when it is running at a modestly high current.

--
--
kensmith@rahul.net forging knowledge
 
Ken Smith wrote:

In article <4304FF30.E40C1540@hotmail.com>,
Pooh Bear <rabbitsfriendsandrelations@hotmail.com> wrote:
[...]
Discretes will outperform even the best op-amps. Best noise I've seen is
0.3nV / rt Hz for a discrete fet.

The Interfet will do about 0.25 if you run selected ones at about 10mA.

I've often wanted to see what the noise of a great big power MOSFET would
be when it is running at a modestly high current.
Click to expand...
Funnily enough, the device I had in mind was from Interfet. Someone here
posted some info about them so I went and got the online data. Looks very
interesting.

As you say, you need to use some quite serious current to get those low noise
figures. Typical audio mic preamps using discretes run the input devices at
around 2-3 mA each to get better than 1dB noise figure re: 200 ohm source (
typical microphone impedance ).

Graham
 
Pooh Bear wrote:

Dave wrote:


pmlonline@gmail.com wrote:
Click to expand...
-Snip-

A decent UHF (500MHz) pre-amp can achieve noise figures of around 0.3dB,
or 21K, so produce between a third and quarter of the noise of that
Stanford device.

Those UHF premaps I talk of usually have less noise at lower
frequencies, but they might be next to impossible to keep stable at
audio frequencies. With gains to well over 30GHz, they tend to oscillate
somewhere I suspect.


And RF practice is also entirely different to audio.

Graham
Click to expand...
And the 1/f Noise of those semiconductors goes skyrocking at "near DC"
(In means of Audio Frequencies)

Jorgen

>
 
Jorgen Lund-Nielsen wrote:

Pooh Bear wrote:

Dave wrote:


pmlonline@gmail.com wrote:

-Snip-


A decent UHF (500MHz) pre-amp can achieve noise figures of around 0.3dB,
or 21K, so produce between a third and quarter of the noise of that
Stanford device.

Those UHF premaps I talk of usually have less noise at lower
frequencies, but they might be next to impossible to keep stable at
audio frequencies. With gains to well over 30GHz, they tend to oscillate
somewhere I suspect.


And RF practice is also entirely different to audio.

Graham

And the 1/f Noise of those semiconductors goes skyrocking at "near DC"
(In means of Audio Frequencies)

Jorgen
Click to expand...
Depends on the device a bit. Some are better.

I've never known the parts that make it into pro-audio to be troublesome in
practice.

Graham
 
In article <43055F51.F2FA158E@hotmail.com>,
Pooh Bear <rabbitsfriendsandrelations@hotmail.com> wrote:
[...]
As you say, you need to use some quite serious current to get those low noise
figures. Typical audio mic preamps using discretes run the input devices at
around 2-3 mA each to get better than 1dB noise figure re: 200 ohm source (
typical microphone impedance ).
Click to expand...
Watch out in the capacitive micro-pre-amp. The gate capacitance of the
very low noise JFETs is enough to effect the linearity.

At 2mA, I think I'd look at the LSK170 for my low noise FET.

--
--
kensmith@rahul.net forging knowledge
 
Ken Smith wrote...
Pooh Bear <rabbitsfriendsandrelations@hotmail.com> wrote:
[...]
As you say, you need to use some quite serious current to get those
low noise figures. Typical audio mic preamps using discretes run the
input devices at around 2-3 mA each to get better than 1dB noise
figure re: 200 ohm source (typical microphone impedance).

Watch out in the capacitive micro-pre-amp. The gate capacitance of
the very low noise JFETs is enough to effect the linearity.

At 2mA, I think I'd look at the LSK170 for my low noise FET.
Click to expand...
I just purchased 100 Toshiba 2sk170, and measured samples at 1 and
3mA: under 1nV. I like that their input capacitance is lower than
many others with similar or lower e_n. I purchased mine from MCM.

It's easy to devise circuits that eliminate any linearity effects
from the JFET's gate capacitance changing with drain voltage.


--
Thanks,
- Win
 
<pmlonline@gmail.com> wrote in message
news:1124399885.910064.266280@g43g2000cwa.googlegroups.com...
An old friend swears that vacuum tubes for audio have less noise than
the best op-amps. Is this true? This sounds completely untrue to me.
I've seen op-amps around 1nV/sqrt(Hz) in the audio range.

What type of voltage amplifier has lowest noise for audio? As far as I
know, it's an op-amps or a FET, right? OK, some op-amps have FETs, but
I've seen some 1/4nV/Sqrt(Hz) FETs.
Click to expand...
Vacuum tubes have more noise because they're hotter. kT/q is twice what it
is for solid state.

Norm Strong
 
In article <de4qhj01mnq@drn.newsguy.com>,
Winfield Hill <Winfield_member@newsguy.com> wrote:
Ken Smith wrote...
[...]
Watch out in the capacitive micro-pre-amp. The gate capacitance of
the very low noise JFETs is enough to effect the linearity.

At 2mA, I think I'd look at the LSK170 for my low noise FET.

I just purchased 100 Toshiba 2sk170, and measured samples at 1 and
3mA: under 1nV. I like that their input capacitance is lower than
many others with similar or lower e_n. I purchased mine from MCM.

It's easy to devise circuits that eliminate any linearity effects
from the JFET's gate capacitance changing with drain voltage.
Click to expand...
This is not the distortion I was refering to. In a capacitive microphone,
the sound moves one plate of a capacitor by the amount I'll call X. For
small spacings between plates, the capacitance of the capacitor varies as
1/X. If the charge on the capacitor is fixed, the voltage varies as 1/C.

You end up with 1/(1/X) or simply a voltage that varies with X.

If the gate capacitance is high, you end up with 1/(Cgs + 1/X) and hence
distortion.

--
--
kensmith@rahul.net forging knowledge
 
Ken Smith <kensmith@green.rahul.net> wrote:

In article <de4qhj01mnq@drn.newsguy.com>,
Winfield Hill <Winfield_member@newsguy.com> wrote:
Ken Smith wrote...
[...]
Watch out in the capacitive micro-pre-amp. The gate capacitance of
the very low noise JFETs is enough to effect the linearity.

At 2mA, I think I'd look at the LSK170 for my low noise FET.

I just purchased 100 Toshiba 2sk170, and measured samples at 1 and
3mA: under 1nV. I like that their input capacitance is lower than
many others with similar or lower e_n. I purchased mine from MCM.

It's easy to devise circuits that eliminate any linearity effects
from the JFET's gate capacitance changing with drain voltage.

This is not the distortion I was refering to. In a capacitive microphone,
the sound moves one plate of a capacitor by the amount I'll call X. For
small spacings between plates, the capacitance of the capacitor varies as
1/X. If the charge on the capacitor is fixed, the voltage varies as 1/C.

You end up with 1/(1/X) or simply a voltage that varies with X.

If the gate capacitance is high, you end up with 1/(Cgs + 1/X) and hence
distortion.
Click to expand...
Although in a 'follower' circuit, the effect of the gate-to-source
capacitance is considerably reduced. (Gate-to-ground capacitance is not
affected unless you bootstrap the surrounding metalwork)

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
 
Adrian Tuddenham wrote...
Ken Smith <kensmith@green.rahul.net> wrote:

If the gate capacitance is high, you end up with 1/(Cgs + 1/X)
and hence distortion.

Although in a 'follower' circuit, the effect of the gate-to-source
capacitance is considerably reduced. (Gate-to-ground capacitance
is not affected unless you bootstrap the surrounding metalwork)
Click to expand...
Using a high-gain source-feedback circuit effectively bootstraps
the JFET's source, eliminating most of its high Ciss gate-source
capacitance, with an effectiveness approaching a follower circuit
if enough loop gain is present. A cascode configuration reduces
the Crss gate-drain capacitance, and bootstrapping the cascode off
the source drives that already small capacitance toward zero. All
that's left from the JFET's capacitance is high-frequency e_n-Cin
noise, which is not canceled out by configuration or the feedback
circuits (this means one musn't get carried away in choosing too
large a JFET, just because it has a nice low voltage-noise rating).

The 2sk170 presents a nice compromise for many applications. For
example, I'm using them in a sensitive capacitance position gauge.

The 2sk170 with its under-1nV spot voltage-noise level isn't best
employed in a follower circuit, because this low noise level would
force one to follow the JFET follower with an unusually-low-noise
BJT amplifier, such as one using Rohm's 0.55nV 2sd786 transistors.
That's not appealing, compared to a properly-designed common-source
JFET amplifier that provides all the advantages without the pain.


--
Thanks,
- Win
 
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