On 2006-04-29, Don Pearce wrote:

To get the best possible noise figure from a mic preamp, you must match
the amp input impedance to the microphone. This is 600 ohms in most
cases. but:

Microphones tend to be somewhere around 100 ohms in most cases. And to
get best noise performance, you don't match impedance. You make sure
the impedance of the microphone gives the best balance between current
noise and voltage noise from the amplifier. The amplifier will be
designed to put this point somewhere around the 100 ohms you expect
from a mic.

600 ohms is a figure used for line level connections.

I own two dynamic microphones (consumer grade ones with unbalanced outputs
an 3.5mm plugs) both are 600 ohms.

the few pro or semi-pro mics I've handled were also labeled 600 ohms.

Transformers don't have "an impedance" as such. They transform the
impedance of whatever they are connected to by the square of the turns
ratio. So what impedance is presented as a result of putting in a
transformer depends on what the impedance of the equipment is.

as long as the mutual inductance is above the impedance of the source and sink
yes - a VHF balun won't make a good audio DI unit.

Bye.
Jasen

Don Pearce wrote:

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

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.

Never knew that - thanks. I wonder why?


It is a balance between minimising the load on the the mic and keeping noise
under control. The optimum noise performance of the original descrete class
A Neve mic pres was about 4.8K. A 2:1 transformer gives you 6dB of noise
free gain and an input impedance of 1.2K plus all the other benefits of
truly balanced floating inputs.

Ian

"Ian Bell"

It is a balance between minimising the load on the the mic and keeping
noise
under control.


** Utter nonsense.

No such balancing act exists.


The optimum noise performance of the original descrete class
A Neve mic pres was about 4.8K.


** ********.


A 2:1 transformer gives you 6dB of noise
free gain ......


** Even worse ******** !!!

A 2:1 step up tranny provides no noise advantage * AT ALL * since it
increases the source impedance as seen by the pre-amp by 4 times and so
*doubles* the noise voltage along with the signal voltage.

Where the hell do folk get these WACKY ideas ???


and an input impedance of 1.2K plus all the other benefits of
truly balanced floating inputs.



** The input impedance is around 1200 ohms direct with most mic pres.

Inserting a 2:1 step up REDUCES the load seen by the mic to 300 ohms, likely
cutting its output voltage in half.

The net effect is a poorer signal to noise ratio.




........ Phil

On Sat, 29 Apr 2006 14:46:45 +0100, Ian Bell
wrote:

Don Pearce wrote:

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

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.

Never knew that - thanks. I wonder why?


It is a balance between minimising the load on the the mic and keeping noise
under control. The optimum noise performance of the original descrete class
A Neve mic pres was about 4.8K. A 2:1 transformer gives you 6dB of noise
free gain and an input impedance of 1.2K plus all the other benefits of
truly balanced floating inputs.

Ian

Thanks.

d

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

"Jasen Betts" wrote in message
...
On 2006-04-29, Don Pearce wrote:

To get the best possible noise figure from a mic preamp, you must match
the amp input impedance to the microphone. This is 600 ohms in most
cases. but:

Microphones tend to be somewhere around 100 ohms in most cases. And to
get best noise performance, you don't match impedance. You make sure
the impedance of the microphone gives the best balance between current
noise and voltage noise from the amplifier. The amplifier will be
designed to put this point somewhere around the 100 ohms you expect
from a mic.

600 ohms is a figure used for line level connections.

I own two dynamic microphones (consumer grade ones with unbalanced outputs
an 3.5mm plugs) both are 600 ohms.

the few pro or semi-pro mics I've handled were also labeled 600 ohms.

Transformers don't have "an impedance" as such. They transform the
impedance of whatever they are connected to by the square of the turns
ratio. So what impedance is presented as a result of putting in a
transformer depends on what the impedance of the equipment is.

as long as the mutual inductance is above the impedance of the source and
sink
yes - a VHF balun won't make a good audio DI unit.

Bye.
Jasen

Hello Jasen,

Excuse me for butting in, but there is a difference between the advertised
impedance and the actual impedance.
If you measure the AC microphone impedance, you will see it is usually lower
that the advertised value and also varies across frequency.
Further, unbalanced microphones are typically higher in impedance than
balanced. 600 in the norm for unbalanced and 200 for balanced.
I good rule of thumb used for years is the preamp should present a load of
at least 5x the rated microphone impedance. This rule prevents frequency
coloration of the signal due to the frequency dependence of the actual
source impedance. Dynamic microphones have the most variation over
frequency, and capacitor or electret microphones, the least.

A nit on your statement quoted above, I assume you meant 'magnetizing
inductance' rather than 'mutual inductance'..

David

In article ,
Jasen Betts wrote:
the few pro or semi-pro mics I've handled were also labeled 600 ohms.

Hmm. If you go to the AKG or Sennheiser sites and check out the specs
there most are around 150 ohms with a recommended input impedance of
greater than 1k.

--
*Some days we are the flies; some days we are the windscreen.*

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

Phil Allison wrote:
"Matti Adolfsen"

To get the best possible noise figure from a mic preamp, you must match
the amp input impedance to the microphone.


** Absolutely FALSE !!!

The best signal to noise ratio is obtained when the mic impedance is MUCH
less that the load impedance. This allows the mic to generate the greatest
signal voltage at the input while noise is a function of actual mic
impedance in the audio band.

Check your source information, there might be errors in your statement.
I'm talking about the thermal noise created in the resistive part of
the dynamic microphone. This is the metohd used by most european
broadcast companies to evaluate mic preamp performance. Best noise
figure can be obtained if the power matching is correct: source and load
impedances must be matched: Maximum power transfer from microphone to
preamp, and reduction of the noise source impedance to 50% of the
original. This theory is used in RF design too.

Btw, the best noise figure I've measured so far was from a TELEFUNKEN
V-76 tube amplifier. 0.5dB this preamp has high turns ratio input
transformer (about 1:50) feeding a pentode amplifier.

This is 600 ohms in most cases.

** Crapology.

Not so nice terminology, but this time correct information. Correct
impedance can be found from manufacturers websites and international
standards. I checked my sources:
- MIC input source impedance from 0 to 200 ohms (N10, page 6)
- MIC input source impedance for noise measrurements: 200 ohms (N10,
page 14)
- Nominal impedance of NEUMANN U-87: 200 ohms.
- Minimum load impedance for U-87 microphone: 1 k ohms (www.neumann.com).

Sources:
- Elements of infrared technology, page 236
- N10 Nordic technical recommendation 3rd edition, oct 1983

Further reading:
- Mathematical noise modeling and analysis of some popular preamplifier
circuit topologies. T.F. Darling, J.AES Vol 35, 1987
- Noise of sources, John Maxwell, National Semiconductor 1977

Matti

"Matti Adolfsen"

To get the best possible noise figure from a mic preamp, you must match
the amp input impedance to the microphone.


** Absolutely FALSE !!!

The best signal to noise ratio is obtained when the mic impedance is MUCH
less that the load impedance. This allows the mic to generate the
greatest signal voltage at the input while noise is a function of actual
mic impedance in the audio band.


Check your source information,


** Check the source of your's - ****wit.

It needs wiping immediately.



I'm talking about the thermal noise created in the resistive part of the
dynamic microphone.


** So am I.

But I actually know what I am talking about.



Best noise figure can be obtained if the power matching is correct:


** Absolute BULL**** !!


source and load impedances must be matched:


** Absolute BULL**** !!


Maximum power transfer from microphone to preamp, and reduction of the
noise source impedance to 50% of the original. This theory is used in RF
design too.


** RF theory must not be applied to audio..

The world of audio uses "max voltage transfer" when linking sources to
loads.

You are a moronic ass.

**** OFF.




.......... Phil

"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.

A typical modern console has around a 2k ohm impedance mic input.

'Rule of thumb' is to load with 10x source impedance.

Graham

"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.

A typical modern console has around a 2k ohm impedance mic input.

'Rule of thumb' is to load with 10x source impedance.

Graham

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

"Dave Plowman (News)" wrote:

In article ,
Matti Adolfsen wrote:
To get the best possible noise figure from a mic preamp, you must match
the amp input impedance to the microphone. This is 600 ohms in most
cases. but:

Could you name a modern mic with an actual 600 ohm (balanced) output
impedance?

Or indeed any?

Loads of the inexpensive but half decent Asian mics are genuinely 600 ohms.

Graham

"Poopie Bear"
Don Pearce wrote:

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 !



** How hysterical.

Dopey Drawers Pearce's worst gaffes ARE his erroneous sums.

Plus his INSANE insistence that RF and audio are the same !!!






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

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. 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. 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. 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.

d

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

"Don Pearce"
Poopie Bear

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 !


Thank you! I did make a gaffe.


** A *****ing stupid* one.

Like hundreds of others and another one, right now !


The actual figure for the Neve noise
figure is about 3dB.


** The published curve shows it is typically less than 2 dB.


That is still unforgivably poor for high end kit
- it is in fact no better than my little Behringer.


** It is SFA additional noise in practice.

PLUS none at all when a typical condenser mic is used, as is the norm.



Ten years ago I
was designing satellite receivers working up at 12GHz.


** Who gives a rat's **** ?

Go stick you irrelevant & erroneous RF ****e up you ****ing ARSE !

You are NOTHING but a trouble making, posturing, pommy

ARROGANT PIG - Don Pearce.



The last audio preamp I made had a noise figure of about 0.5dB,


** Bet the ASININE ****WIT matched the source and load impedances.

Then miscalculated the **REAL** noise figure.

ROTFLMAO !!!




.......... Phil

In article ,
Pooh Bear wrote:


"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.

A typical modern console has around a 2k ohm impedance mic input.

Thanks for that. I did have a quick glance at some specs for modern mixers
but those I looked at seemed shy of quoting the mic input impedance.

'Rule of thumb' is to load with 10x source impedance.

Yup.

Graham

--
*To be intoxicated is to feel sophisticated, but not be able to say it.

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

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 ?

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 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

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.

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. 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!

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.

d

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

"Dave Plowman (News)" wrote:

In article ,
Pooh Bear wrote:

"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.

A typical modern console has around a 2k ohm impedance mic input.

Thanks for that. I did have a quick glance at some specs for modern mixers
but those I looked at seemed shy of quoting the mic input impedance.

'Rule of thumb' is to load with 10x source impedance.

Yup.

Actually, as a follow-up, following some discussion in r.a.p , my most recent
mic pre featured a modest increase in input Z to 2.5k. I note from the links
posted here that Rupert has gone as far as going to 10k now, which did
surprise me a little. I'd like to spend some time listening to the effect of
loading on various mics actually.

Graham

"Poopie ****wit Bear"

Actually, as a follow-up, following some discussion in r.a.p , my most
recent
mic pre featured a modest increase in input Z to 2.5k.


** Wank, wank ,wank ,wank, wank ,wank .....


I note from the links
posted here that Rupert has gone as far as going to 10k now, which did
surprise me a little.


** The simple reason was given - you blind as a bat ass.

The mic input on that unit doubles as a balanced line input.




........ Phil

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

In article ,
Don Pearce wrote:
Thank you! I did make a gaffe. 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.

Heh heh. But I'll guarantee which one sounds better...

--
*OK, who stopped payment on my reality check?

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

"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

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.

On Mon, 01 May 2006 10:55:59 +0100, "Dave Plowman (News)"
wrote:

In article ,
Don Pearce wrote:
Thank you! I did make a gaffe. 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.

Heh heh. But I'll guarantee which one sounds better...

I wouldn't put money on that if I were you. I've measured the
performance of the Behringer, and I can find no fault with it. It has
problems, of course, but they are limitations imposed by cheap
implementation - certainly not in the sound department.

d

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

"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

Don Pearce wrote:

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.


Even if that were correct it is immaterial in most situations where the gain
is not set to maximum (and that is the ONLY place a mic amp achieves an
equivalent input figure this good). As gain is reduced, output noise begins
to dominate and the old Neve mixers still beat most 'modern' designs in
that respect.

Put simply, equivalent input noise is only part of the story.

ian

Don Pearce wrote:


Thank you! I did make a gaffe. 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. 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. 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.


Not that is really matters. An improvement in noise figure of 2.5dB will
only improve the signal to noise at very high gains by the same amount.
Unless you are using dynamics on very quiet sources, the self noise of your
condensor mic is the limiting factor not the noise figure of the mic pre.

Ian

On Mon, 01 May 2006 12:25:39 +0100, Ian Bell
wrote:

Don Pearce wrote:

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.


Even if that were correct it is immaterial in most situations where the gain
is not set to maximum (and that is the ONLY place a mic amp achieves an
equivalent input figure this good). As gain is reduced, output noise begins
to dominate and the old Neve mixers still beat most 'modern' designs in
that respect.

Put simply, equivalent input noise is only part of the story.

ian

It is, as you say, important where the gain is high. But the rest is
plain wrong. If you want to include the gain setting in the noise
performance it is done by R = Rs + 1/G, where Rs is the source
resistance, G is the gain and R is the resulting effective source
resistance. The result is that even at unity gain, the source
resistance is only increased by 1 ohm - barely changed from 150 ohms.
The degree to which gain can be turned down before input noise ceases
to be dominant is that point where the noise figure of the line system
is equal to the input noise times the gain of the preamp. That should
be a gain of perhaps two or three. If you are doing that with the
level control, you should be using a preamp at all.

As for the rest of the mixer, it is running at line level, and if
noise can't be kept a long, long way below that, something is
seriously wrong. Output noise should never, ever dominate a circuit.

The main point about running at less than high gain is that the sound
you are capturing is likely to be loud, so the signal to noise ratio
is high.

d

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

On Mon, 01 May 2006 12:29:45 +0100, Ian Bell
wrote:

Don Pearce wrote:


Thank you! I did make a gaffe. 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. 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. 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.


Not that is really matters. An improvement in noise figure of 2.5dB will
only improve the signal to noise at very high gains by the same amount.
Unless you are using dynamics on very quiet sources, the self noise of your
condensor mic is the limiting factor not the noise figure of the mic pre.

Ian

True for condensors, but this was for a dynamic in a very quiet
situation. Generally when I am designing gear for myself, I see no
reason to make it any worse than I actually need to. I don't object to
spending an extra couple of quid for that, especially knowing that if
professional gear were made that way they would be charging another
ten grand.

d

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

"Don Pearce"
Ian Bell

Even if that were correct it is immaterial in most situations where the
gain
is not set to maximum (and that is the ONLY place a mic amp achieves an
equivalent input figure this good). As gain is reduced, output noise
begins
to dominate and the old Neve mixers still beat most 'modern' designs in
that respect.

Put simply, equivalent input noise is only part of the story.



It is, as you say, important where the gain is high. But the rest is
plain wrong. If you want to include the gain setting in the noise
performance it is done by R = Rs + 1/G, where Rs is the source
resistance, G is the gain and R is the resulting effective source
resistance.


** A completely wrong formula.

Where ever did it come from ?

One of this fool's dusty old books on RF theory ?


The result is that even at unity gain, the source
resistance is only increased by 1 ohm - barely changed from 150 ohms.


** Good grief !!

Has this cretin no idea how a common mode gain control operates ????


Typically, residual noise at the OUTPUT goes almost in hand with gain for
settings between 60dB and 40 dB. Further gain reduction has a rapidly
diminishing effect on the residual noise, it may drop to 0.02mV in a good
design at gains of 20 dB and under.

So, the ein goes like this

- 128dBu at 60 dB gain,

- 125dBu at 40 dB gain

- 112dBu at 20 dB gain.

The equivalent output noise ratios rel 0 dBu are therefo

68 dB, 88 dB and 92 dB.

A preamp can usually output 22dBu, so the maximum possible ratios a

90 dB, 110 dB and 114 dB.



** The SSM 2017 is a typical high performance mic preamp in IC form - ie not
an op-amp.

http://ezphysics.nchu.edu.tw/prophys...et/SSM2017.pdf

Note how the ein figures vary with gain setting.




........ Phil

Don Pearce wrote:

Thank you! I did make a gaffe. 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. Ten years ago I
was designing satellite receivers working up at 12GHz. The noise
figure I was working to was 0.3dB.


Its a lot easier to reduce the noise figure at a narrow bandwidth at
microwave frequencies.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

On Mon, 01 May 2006 13:24:48 GMT, "Michael A. Terrell"
wrote:

Don Pearce wrote:

Thank you! I did make a gaffe. 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. Ten years ago I
was designing satellite receivers working up at 12GHz. The noise
figure I was working to was 0.3dB.


Its a lot easier to reduce the noise figure at a narrow bandwidth at
microwave frequencies.

Narrow bandwidth? I don't call a bandwidth of nearly 2GHz narrow. And
asitappens, I can achieve these sorts of figure at audio as well.

d

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

Don Pearce wrote:

On Mon, 01 May 2006 13:24:48 GMT, "Michael A. Terrell"
wrote:

Don Pearce wrote:

Thank you! I did make a gaffe. 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. Ten years ago I
was designing satellite receivers working up at 12GHz. The noise
figure I was working to was 0.3dB.


Its a lot easier to reduce the noise figure at a narrow bandwidth at
microwave frequencies.

Narrow bandwidth? I don't call a bandwidth of nearly 2GHz narrow. And
asitappens, I can achieve these sorts of figure at audio as well.

d

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

2 GHz at 12 GHz is 16.67% which is narrow. Don't tell me that there
is no filtering at all. A circulator or isolator has a usable bandwidth
which keeps out of band noise out of the amp. One of my KU band
receivers is aboard the ISS.

Audio is true broadband, from DC to whatever the upper limit is set
at by the design and limitations of the components if it is DC coupled,
and from less than 100 Hz if its AC coupled.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

In article ,
Don Pearce wrote:
Narrow bandwidth? I don't call a bandwidth of nearly 2GHz narrow.

Don't these things tend to be measured in octaves, as it were?

--
*A snooze button is a poor substitute for no alarm clock at all *

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

In article , wrote:
Don Pearce wrote:

On Mon, 01 May 2006 13:24:48 GMT, "Michael A. Terrell"
wrote:

Don Pearce wrote:

Thank you! I did make a gaffe. 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. Ten years ago I
was designing satellite receivers working up at 12GHz. The noise
figure I was working to was 0.3dB.


Its a lot easier to reduce the noise figure at a narrow bandwidth at
microwave frequencies.

Narrow bandwidth? I don't call a bandwidth of nearly 2GHz narrow. And
asitappens, I can achieve these sorts of figure at audio as well.

d

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

2 GHz at 12 GHz is 16.67% which is narrow. Don't tell me that there
is no filtering at all. A circulator or isolator has a usable bandwidth
which keeps out of band noise out of the amp. One of my KU band
receivers is aboard the ISS.

Audio is true broadband, from DC to whatever the upper limit is set
at by the design and limitations of the components if it is DC coupled,
and from less than 100 Hz if its AC coupled.

Not only that, but at low to DC freqs., the noise increases dramatically
of most devices. When looking at op-amps I have to often look at DC drifts,
which is the largest noise.

greg

On Mon, 01 May 2006 13:50:43 GMT, "Michael A. Terrell"
wrote:

Don Pearce wrote:

On Mon, 01 May 2006 13:24:48 GMT, "Michael A. Terrell"
wrote:

Don Pearce wrote:

Thank you! I did make a gaffe. 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. Ten years ago I
was designing satellite receivers working up at 12GHz. The noise
figure I was working to was 0.3dB.


Its a lot easier to reduce the noise figure at a narrow bandwidth at
microwave frequencies.

Narrow bandwidth? I don't call a bandwidth of nearly 2GHz narrow. And
asitappens, I can achieve these sorts of figure at audio as well.

d

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

2 GHz at 12 GHz is 16.67% which is narrow. Don't tell me that there
is no filtering at all. A circulator or isolator has a usable bandwidth
which keeps out of band noise out of the amp. One of my KU band
receivers is aboard the ISS.

Of course there is filtering. The waveguide coupler as a bandwidth, as
does the mixer. The image filter then has a bandwidth, and finally the
IF amplifier. Despite all this filtering - none of which is lossless -
the noise figure still happens. It really isn't easy even compared to
audio. Well done for having a receiver aboard ISS, but ground
transmitters tend not to be particularly power-limited, so the noise
figure probably isn't quite the issue it is with a terrestrial
receiver. In fact there would be no point because it is pointing at
the earth, which is hot.

Audio is true broadband, from DC to whatever the upper limit is set
at by the design and limitations of the components if it is DC coupled,
and from less than 100 Hz if its AC coupled.

But achieving low noise at audio - however you want to describe its
bandwidth - is actually pretty trivial. The fact is that manufacturers
choose not to implement it because they can make something that is
"reasonable" for a lot less money. The business of achieving low noise
at audio doesn't rely on being able to juggle S parameters, physical
layout, board materials and mechanical design like microwave.

d

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

On Mon, 01 May 2006 14:57:42 +0100, "Dave Plowman (News)"
wrote:

In article ,
Don Pearce wrote:
Narrow bandwidth? I don't call a bandwidth of nearly 2GHz narrow.

Don't these things tend to be measured in octaves, as it were?

Yes, but remember we are talking about scraping away at the last
vestiges of the state of the art here. Optimum noise match varies over
that range, and designing matching structures that keep the complex
impedance of the waveguide transition at the exact conjugate of the
input transistor noise match over such a bandwidth really is far from
simple. By comparison audio is a breeze - I promise.

d

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