Don Pearce wrote:

On Mon, 01 May 2006 10:02:35 +0100, Pooh Bear
wrote:


Don Pearce wrote:

On Mon, 01 May 2006 08:38:56 +0100, Pooh Bear
wrote:

Don Pearce wrote:

On Fri, 28 Apr 2006 15:36:00 +0100, "Dave Plowman (News)"
wrote:

In article ,
Don Pearce wrote:
But being a total of approx 13k will have little effect across 150 ohms.

True - I was just trying to correct your 1.2k, which while hardly a
typo was certainly a slip of the decimal point.

No - that's the input impedance of a Neve desk - one of the classic
designs. Others too. More modern ones may be higher.

One more thing - the Neve mic pre has a pretty poor noise performance.
At -128dBu equivalent at the input, that is about 6dB above pure
thermal noise. That is 4 or 5 dB more noise than they should be
achieving.

Please do your sums properly Don before making gaffes like that !

Graham

Thank you! I did make a gaffe.

No problem, we all goof up from time to time. ;-)

The actual figure for the Neve noise
figure is about 3dB. That is still unforgivably poor for high end kit
- it is in fact no better than my little Behringer.

Indeed

Ten years ago I
was designing satellite receivers working up at 12GHz. The noise
figure I was working to was 0.3dB.

The last audio preamp I made had a noise figure of about 0.5dB,
because I was willing to use multiple parallel discrete transistors
for the input circuitry.

Care to name which ones you were using ?


Yes - I have a box of old MAT-01s from PMI. They are strictly reserved
for such projects. I don't know if they are still available.

I recall the beast.

Somewhere I think I have some of those similar Nat Semi parts that featured multiple
devices on-die. Forget the part number now. Oh no - I *was* right - the LM394 - just
checked in case. Though that would be an IC but the M just means monolithic. They're not
even insanely expensive now !

Also took a look at some esoteric fet data a while back. Noise somewhere down in the
500pV/sqrt Hz region. Interfet is the company.

Making it any better than this would have
been possible, but unwarranted because unlike the satellite receiver,
it wasn't pointing at a cold sky, but a warm microphone.

Back in the days when I was at Neve, the then V series ( Mks 1 and 2 ) consoles (
and just about everything else except the digital console ) had a mic pre using a
step up transformer and a 5534. The quoted noise for that was a rather poor -126dBu
and it didn't actually measure any better either IIRC ! I was somewhat surprised to
say the least.

The 5534 is not bad, but I wouldn't say it is the quietest way of
doing things.

Indeed not. They could have used something from AD or PMI and instantly improved the
noise figure.

I had to make a very small preamp (just one op amp) for
a high impedance (50k) microphone. I searched for ages for quiet op
amp before I realised that an OP27 is optimized pretty well perfectly
at this impedance, with an excess noise of only about 1dB. Amazing!

They're good op-amps. Never had the budget to design them into anything though.
:-(


The recent mic pres I've done ( quite economy types ) manage about -128.5 - as long
as you factor in the extra little bit to account for the true noise equivalent
bandwidth of the measurement set : -3dB @ 22kHz 4th order is about 23kHz NEB.

Graham

I really wish noise was expressed as a noise figure, rather than a
level. That way it wouldn't matter what impedance you were using, you
would simply have a figure of merit that told you how much worse the
pre was than theoretically perfect.

Hmmm, I wonder how that would go down with those who 'cheat' by using a 150 ohm source
instead of 200. I note that Mackie ( I think ) is now quoting noise with the input
*shorted* too.

Graham

On Mon, 01 May 2006 10:36:06 +0100, Pooh Bear
wrote:


Yes - I have a box of old MAT-01s from PMI. They are strictly reserved
for such projects. I don't know if they are still available.

I recall the beast.

Somewhere I think I have some of those similar Nat Semi parts that featured multiple
devices on-die. Forget the part number now. Oh no - I *was* right - the LM394 - just
checked in case. Though that would be an IC but the M just means monolithic. They're not
even insanely expensive now !


Yes that is very similar. The great thing about using these discretes
as front ends is that the knee frequency for 1/f noise is way lower
than the transistors in the average op-amp. So not only don't they
hiss - they don't rumble either. Three of them in parallel is about
right for a mic at about 150 to 200 ohms.

Also took a look at some esoteric fet data a while back. Noise somewhere down in the
500pV/sqrt Hz region. Interfet is the company.


But what about the current noise? You need to multiply that by the
source impedance to add in its effect. I've looked at a few FET input
op amps with amazingly low voltage noise, and this always more than
makes up the difference.


I really wish noise was expressed as a noise figure, rather than a
level. That way it wouldn't matter what impedance you were using, you
would simply have a figure of merit that told you how much worse the
pre was than theoretically perfect.

Hmmm, I wonder how that would go down with those who 'cheat' by using a 150 ohm source
instead of 200. I note that Mackie ( I think ) is now quoting noise with the input
*shorted* too.


Yup, you really do have to read specs with a cynical eye these days.

d

--
Pearce Consulting
http://www.pearce.uk.com

"Don Pearce"
Poopie Bear


Yes that is very similar. The great thing about using these discretes
as front ends is that the knee frequency for 1/f noise is way lower
than the transistors in the average op-amp. So not only don't they
hiss - they don't rumble either. Three of them in parallel is about
right for a mic at about 150 to 200 ohms.


** More totally asinine crapology from the Pommy RF Fool.

Audio band white noise is totally dominated by high frequency noise !!

Hence - there is no audible "rumble noise" from mic pres based on op-amps
or transistors.


But what about the current noise?


** With a FET ?

With a 200 ohms source ??

What drugs is this ****ING JERK taking ???

Or not taking ??



Hmmm, I wonder how that would go down with those who 'cheat' by using a
150 ohm source
instead of 200. I note that Mackie ( I think ) is now quoting noise with
the input
*shorted* too.


** Some condenser mics have very low output Zs - less than 20 ohms.

Not wrong to quite the noise for that case.




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

"Phil Allison" wrote in
:

Audio band white noise is totally dominated by high frequency noise !!


White noise has equal amounts of all frequencies. We hear the HF dominate
because higher frequencies have more energy. (And because our ears are more
sensitive to it).

In article 0, Lostgallifreyan wrote:
"Phil Allison" wrote in
:

Audio band white noise is totally dominated by high frequency noise !!


White noise has equal amounts of all frequencies. We hear the HF dominate
because higher frequencies have more energy. (And because our ears are more
sensitive to it).

The noise voltage of many op-amps is almost flat from 1K to 100K, but below 1 K
it moves upward. I'm looking at the LF353 as it has a very low level of low frequency
noise, or at least the chart shows that. The AD711 has 18 nvHz at 1 kHz but
60 nvHz at 1 Hz.


greg

"GregS"

The noise voltage of many op-amps is almost flat from 1K to 100K, but
below 1 K
it moves upward. I'm looking at the LF353 as it has a very low level of
low frequency
noise, or at least the chart shows that. The AD711 has 18 nvHz at 1 kHz
but
60 nvHz at 1 Hz.


** What a complete ******.



........ Phil

On Mon, 01 May 2006 20:38:58 GMT, Lostgallifreyan
wrote:

"Phil Allison" wrote in
:

Audio band white noise is totally dominated by high frequency noise !!


White noise has equal amounts of all frequencies. We hear the HF dominate
because higher frequencies have more energy. (And because our ears are more
sensitive to it).

Ears are most sensitive around 1k, aren't they?

Tom

Tom MacIntyre wrote in
:

On Mon, 01 May 2006 20:38:58 GMT, Lostgallifreyan
wrote:

"Phil Allison" wrote in
:

Audio band white noise is totally dominated by high frequency noise
!!


White noise has equal amounts of all frequencies. We hear the HF
dominate because higher frequencies have more energy. (And because our
ears are more sensitive to it).

Ears are most sensitive around 1k, aren't they?

Tom


Probably. I wasn't thinking hard about that, I was just caught by the
claim that HF dominates white noise in the audio band, when white noise is
defined as being made up of all frequencies present with equal energy in
each.

On Mon, 01 May 2006 21:51:05 GMT, Lostgallifreyan
wrote:

Tom MacIntyre wrote in
:

On Mon, 01 May 2006 20:38:58 GMT, Lostgallifreyan
wrote:

"Phil Allison" wrote in
:

Audio band white noise is totally dominated by high frequency noise
!!


White noise has equal amounts of all frequencies. We hear the HF
dominate because higher frequencies have more energy. (And because our
ears are more sensitive to it).

Ears are most sensitive around 1k, aren't they?

Tom


Probably. I wasn't thinking hard about that, I was just caught by the
claim that HF dominates white noise in the audio band, when white noise is
defined as being made up of all frequencies present with equal energy in
each.

Hmmm...equal energy means that it would be more energy per octave at
higher frequencies, right?

Tom

"Lostgallifreyan"

Probably. I wasn't thinking hard about that, I was just caught by the
claim that HF dominates white noise in the audio band, when white noise is
defined as being made up of all frequencies present with equal energy in
each.


** WRONG.

Pink noise has equal energy in each octave band or fraction thereof.

However, white noise has about 30 dB more energy at the high end of the
audio band compared to the low end.




........ Phil

On 2006-05-01, Lostgallifreyan wrote:
Tom MacIntyre wrote in
:

Probably. I wasn't thinking hard about that, I was just caught by the
claim that HF dominates white noise in the audio band, when white noise is
defined as being made up of all frequencies present with equal energy in
each.

white noise is evenly distributed by frequency. (per Hz)

But frequency perception is logarythmic,
each octave has twice as many Hz from end to end as the one below it
therefore, with white noise, twice as much energy as the one below it.

Bye.
Jasen

"Lostgallifreyan" wrote in message
9.130...
"Phil Allison" wrote in
:

Audio band white noise is totally dominated by high frequency noise !!


White noise has equal amounts of all frequencies. We hear the HF dominate
because higher frequencies have more energy. (And because our ears are
more
sensitive to it).

Aren't all Gallifreyans bar The Doctor now lost, since the Dalek war
eradicated them ?

Arfa

"Arfa Daily" wrote in
:


"Lostgallifreyan" wrote in message
9.130...
"Phil Allison" wrote in
:

Audio band white noise is totally dominated by high frequency noise
!!


White noise has equal amounts of all frequencies. We hear the HF
dominate because higher frequencies have more energy. (And because
our ears are more
sensitive to it).

Aren't all Gallifreyans bar The Doctor now lost, since the Dalek war
eradicated them ?

Arfa




Precisely. You won't know which one I am. I have a lot of fun with this
name, I can tell you. Besides, time is Strange, too strange for Daleks.

"Lostgallifreyan"
"Phil Allison"

Audio band white noise is totally dominated by high frequency noise !!


White noise has equal amounts of all frequencies.


** Wrong.

It has equal amounts of noise energy in equal amounts of bandwidth.

So, 50% of the noise energy is in the band from 10 kHz to 20 kHz.

90 % is in the band from 2kHz to 20 kHz.

95 % is in the band from 1 kHz to 20 kHz.

99% is in the band from 200Hz to 20 kHz.

Get it ?



We hear the HF dominate
because higher frequencies have more energy.


** No - because it does utterly dominates the energy spectrum.


........ Phil

In article ,
Don Pearce wrote:
Yes that is very similar. The great thing about using these discretes
as front ends is that the knee frequency for 1/f noise is way lower
than the transistors in the average op-amp. So not only don't they
hiss - they don't rumble either. Three of them in parallel is about
right for a mic at about 150 to 200 ohms.

Last time I played with this idea I found it very sensitive to RF
interference.

--
*Always borrow money from pessimists - they don't expect it back *

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (No-News)"
Dumb Pommy Prick

Yes that is very similar. The great thing about using these discretes
as front ends is that the knee frequency for 1/f noise is way lower
than the transistors in the average op-amp. So not only don't they
hiss - they don't rumble either. Three of them in parallel is about
right for a mic at about 150 to 200 ohms.

Last time I played with this idea I found it very sensitive to RF
interference.


** Matching the source and load impedances RF style does that !!

You Bloody IDIOT !!




.......... Phil

On Mon, 01 May 2006 11:39:50 +0100, "Dave Plowman (News)"
wrote:

In article ,
Don Pearce wrote:
Yes that is very similar. The great thing about using these discretes
as front ends is that the knee frequency for 1/f noise is way lower
than the transistors in the average op-amp. So not only don't they
hiss - they don't rumble either. Three of them in parallel is about
right for a mic at about 150 to 200 ohms.

Last time I played with this idea I found it very sensitive to RF
interference.

Well, there should have been nothing inherently RF sensitive about it.
Like any other sensitive system, it needs all the usual RF-proofing
tweaks to keep it sane.

d

--
Pearce Consulting
http://www.pearce.uk.com