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