clintrubber

neve preamp and high gain transistors
« Reply #20 on: June 21, 2005, 10:35:05 AM »
Quote
anybody got the best circuit for testing this with the minimum of components

A 3Euro DMM with a transistor-socket to measure hfe did the trick here  :wink:
I don't know at which current it did it, but the indication I got was well over 600 so OK enough.
Brads method is far better though, but I don't think I cut too big a corner.

It's funny btw, that both the first transistors of the '283 pre- & out-stage have this >600 recommendation.
I figure if the first transistor of the outputstage was still able to add noticable noise there'd be something else wrong  :wink:


CJ

neve preamp and high gain transistors
« Reply #21 on: June 21, 2005, 02:56:19 PM »
Won't noise go up with the hfe?
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

bcarso

neve preamp and high gain transistors
« Reply #22 on: June 21, 2005, 03:04:05 PM »
Current noise will rise reciprocal to sq root hfe for the same collector current because the base current is rising reciprocally.  Base current has at least full shot noise.  If the circuit impedance at that point is low it won't necessarily change the noise much.

neve preamp and high gain transistors
« Reply #23 on: June 21, 2005, 03:30:49 PM »
Quote from: "bcarso,PRR"
Current noise will rise reciprocal to sq root hfe

Noise situation in BJT is not that straight.
Therory is one thing, but if someone take catalogue diagrams
of noise figure (of some transistor) and make complete two - port
noise model (not that triviality with current and voltage noise at the input like Leach ) very weird things computes.
Somewhere noise induced by base - current is not dominant.
And every transformer - coupled audio frequency circuit is that case.

Dementjev said: There are in every threepole (or twoterminal) three
statistically independent noise sources. But in some model uses three
parameters: Voltage, current and correlation quotient.

                                                             xvlk

bcarso

neve preamp and high gain transistors
« Reply #24 on: June 21, 2005, 05:22:54 PM »
Yes for a complete treatment the correlation coefficient needs to be accounted for.  A similar situation exists at moderate-to-high frequencies with induced gate noise in FETs.

But---you can get pretty close out to 20kHz treating the midband to high frequency current noise as a slightly-higher-than-shot-noise generator with a 1/f-ish rise below say about 2kHz or so.  For the next level of refinement, have the term of rbb times the current noise adding to voltage noise.  I'm mostly happy if my estimates are worst-case so that I am pleasantly surprised when I start building stuff.

Motchenbacher and Fitchin, or the later Motchenbacher and Connelly have pretty decent treatments.  Also there are the very helpful noise data in the back of the book where e sub n and i sub n are shown for a number of parts, albeit only valid for the samples they measured.

b2

Re: neve preamp and high gain transistors
« Reply #25 on: June 22, 2005, 03:25:37 AM »
Quote from: "rafafredd"
I´ve got some bc184L transistors and they measure 250-350 hfe.

I´ve read that the neve 283 preamp and output stage calls for 600Hfe or more, hence the need of bc184 "c" version.

But do I really need that high of a Hfe for acceptable noise levels? Does someone in TheLab tried lower gain transistors, like 350Hfe on this circuit?

As always, Thanks!
Hi,Rafafred

In my 1290  I've put on BA283 input stage BC109C Hfe=400 to 600,
instead of bc184l and and for this others bc107c.
It's work very well ....I've done Dual unit and i've never had any probs.

For the Input transformer,I used Sowters 9145,it' works very very great but you must inverted the wire on the output to decrease the ringing....
Thank 's Sowters .

A bientot B2

rafafredd

neve preamp and high gain transistors
« Reply #26 on: June 22, 2005, 05:23:13 PM »
I´m using BC184l with hfe aprox. 350, and I don´t have noise problems. My unit sounds pretty good and silent by the way... But, I´m using OEP transformer at the input, wired as 1:3.2, so I have a greater step-up at the input, making noise problems less prone in this case (I guess   :?  )

CJ

neve preamp and high gain transistors
« Reply #27 on: June 22, 2005, 05:33:37 PM »
I was going to say, don't obsess on the 600 hfe thing, I mean half of that will drop you how many db? The matching is probably more important.


Anybody ever hear of a 2N3799? PNP with hfe 300-900. 3 for a buck at the surplus store.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

bcarso

neve preamp and high gain transistors
« Reply #28 on: June 22, 2005, 05:50:04 PM »
Yes the 3799 is a cool part in a metal can for medium/high Z sources.  From the good old days of Motorola.

BladeSG

neve preamp and high gain transistors
« Reply #29 on: May 08, 2006, 11:31:57 PM »
I am correct then in assuming a MPSA18 will sub for the BC184C (except the pin out) in a BA283AV card?

Is it as simple as putting these MPSA18 so that they'd face 180 deg to the way a BC184C goes in?

From data sheets: pins 1/2/3

BC184C = collector/base/emitter
MPSA18 = emitter/base/collector

Thanks,

Steve


BladeSG

neve preamp and high gain transistors
« Reply #30 on: May 09, 2006, 10:24:12 PM »
I suppose I'll give it go with them oriented 180 deg from where a BC184C would go in and hope I haven't missed some small detail.....   :green:

BladeSG

neve preamp and high gain transistors
« Reply #31 on: May 10, 2006, 07:27:27 PM »
Thanks for that  :sam:


 

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