Micpre of Graham
in [Design]
|
From: John Larkin on 21 Mar 2007 17:26 On Wed, 21 Mar 2007 19:44:43 GMT, "Kevin Aylward" <kevin_aylward(a)ntlworld.com> wrote: >Ban wrote: >> Phil Allison wrote: >>> "Ban" >>>> >>>> I have attached a link to a commercial product, just to show that >>>> the art of making a good preamp is not *that* simple. >>>> http://rapidshare.com/files/21831341/mic_pre_02.png >>>> >>> >>> >>> ** Good pre-amp ? >>> >>> With a pair of 2.2 uF film caps ( = 1.1 uF ) in series with the mic >>> input ? >>> >>> Not too good for the noise figure at or below 1kHz. >>> >> Why do you think a reactive element increases the noise? > > >Input shot noise droped accross the capacitor. Say Ic=2ma, hfe=200, >In=sqrt(2.Ic/hfe.q) = 2pa/rthz. At 1k, Xc = 160 ohms, giving 0.3nv/rthz. Does Ib have full shot noise? If Ie is set by the voltage drop across a metal-film resistor, it has no shot noise. So I'd expect Ib, some fraction of Ie, to be shot-noise free as well. John
From: Ban on 21 Mar 2007 18:39 Eeyore wrote: >>>>> Ban wrote: >>>>> >>>>>> 1. When you switch on the phantom power the Vbe of the >>>>>> transistors gets reversed momentarily(+17V instead of -0.7V) >>>>> >>>>> Can you explain how you think that happens ? >>>> >>>> ** Of course, you know the colossal fool cannot. >>>> >>>> If the +48 volt supply is somehow snapped on, then about + 15.7 >>>> volts momentarily appears on each base. >>>> >>>> Not a reverse Vbe situation at all, as the emitters are supplied >>>> from +17. >>> >>> Exactly. >>> >>> I was hoping that Ban would see his error himself. >> >> Well, the thing is there is a divider formed by the 4k7 from 17V and >> the 6k8 from the opamp output. > > Yes, that's equivalent to 2k8 from 10.23V. > > >> In the first moment the opamps have 0V, when the >> transient arrives, one opamp goes to the pos rail and forces the >> other to the negative. > > What makes you think that happens ? The 'transient' appears in common > mode on *both* inputs. Of course there won't be a transient with a > properly ramped 48V supply of course. > > >> Now there are 4k7 || 5k(pot) with the 470u cap voltage, which >> can be as high as 4V, depending on the history. That makes 1.5V >> across the first junction and around 10V across the other one. > > You've missed something very basic. > > Read Phalluson's post and see if the number he quotes rings any bells > for you. You *are* wong about this I have to tell you. > Why do you not take your simulation, give a 48V step function with say 100ms risetime as phantom power and see what happens with the output and across the transistor. Play a bit with values, reduce the 47uF to say 2.2u, or take some higher slew rate opamp for the first OP like OP37. Check the voltage across the transistors. Maybe you check what a single pulse will do or even an adapter to an unbalanced jack, the short Phil was talking about. I could see some 300ms of saturation, rail to rail. That will pop out your speakers quite a bit, man. -- ciao Ban Apricale, Italy
From: Eeyore on 21 Mar 2007 19:47 Ban wrote: > Eeyore wrote: > > Why do you not take your simulation, give a 48V step function with say 100ms > risetime as phantom power and see what happens First off since you can't apparently see the mistake you're making, I'll point out that switching on the phantom power *does not* reverse bias any transistors. There's a 3k resistor to ground on both inputs. The largest voltage that can occur as a result of switching on the phantom is 48V * 3/(3+6.8) which is 14.69V. > with the output and across the transistor. Play a bit with values, reduce the > 47uF to say 2.2u, So you've also missed the point about compromising the input noise figure ? > or take some higher slew rate opamp for the first OP like OP37. What exactly would be the point of that ? > Check the voltage across the transistors. I can calculate that ! > Maybe you check what a single pulse will do or even an adapter to an > unbalanced jack, the short Phil was talking about. That's a pulse in the opposite direction. That one is actually the problematic one. > I could see some 300ms of saturation, rail to rail. Where did you see this ? > That will pop out your speakers quite a bit, man. It's obviously inadvisable (and unnecessary) to switch the phantom on and off when the channel is on. Graham
From: Phil Allison on 21 Mar 2007 20:30 "Ban" > Phil Allison wrote: >> "Ban" >>> >>> I have attached a link to a commercial product, just to show that >>> the art of making a good preamp is not *that* simple. >>> http://rapidshare.com/files/21831341/mic_pre_02.png >>> >> >> >> ** Good pre-amp ? >> >> With a pair of 2.2 uF film caps ( = 1.1 uF ) in series with the mic >> input ? >> >> Not too good for the noise figure at or below 1kHz. >> > Why do you think a reactive element increases the noise? > ** It raises the effective source impedance. Can't you get ANYTHING right ? Wog idiot. ....... Phil
From: Kevin Aylward on 22 Mar 2007 14:30
John Larkin wrote: > On Wed, 21 Mar 2007 19:44:43 GMT, "Kevin Aylward" > <kevin_aylward(a)ntlworld.com> wrote: > >> Ban wrote: >>> Phil Allison wrote: >>>> "Ban" >>>>> >>>>> I have attached a link to a commercial product, just to show that >>>>> the art of making a good preamp is not *that* simple. >>>>> http://rapidshare.com/files/21831341/mic_pre_02.png >>>>> >>>> >>>> >>>> ** Good pre-amp ? >>>> >>>> With a pair of 2.2 uF film caps ( = 1.1 uF ) in series with the >>>> mic input ? >>>> >>>> Not too good for the noise figure at or below 1kHz. >>>> >>> Why do you think a reactive element increases the noise? >> >> >> Input shot noise droped accross the capacitor. Say Ic=2ma, hfe=200, >> In=sqrt(2.Ic/hfe.q) = 2pa/rthz. At 1k, Xc = 160 ohms, giving >> 0.3nv/rthz. > > Does Ib have full shot noise? If Ie is set by the voltage drop across > a metal-film resistor, it has no shot noise. So I'd expect Ib, some > fraction of Ie, to be shot-noise free as well. I am not sure what you are saying here. Base current and collector current shot noise are modelled as inherent current sources across the respective junctions. External components can't change these values. However, external components, might modify the effect of the inherent shot noise. -- Kevin Aylward ka(a)anasoftEXTRACT.co.uk www.anasoft.co.uk SuperSpice |