Micpre of Graham
in [Design]
|
From: Eeyore on 21 Mar 2007 09:54 Fred Bloggs wrote: > the same old two channel input gain stages with those dumb > inverse pot things. What exactly is dumb about the pot ? Graham
From: Eeyore on 21 Mar 2007 09:57 Phalluson wrote: > "Eesyore" > > 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. > However, if the +48 is already on and a short is applied to the XLR input > ( 1 to 2 or 1 to 3) the 47 uF electro cap (charged to 48 volts) discharges > via the 4.7 ohm and 1N4148 diode into the -17 volt supply. > > Means a peak current about 6 amps ( approx 30 /4.7). > > Are those parts up to it ? It seems that a 1/4W 4R7 does in fact survive such torture but an 0805 won't (at least with repeated abuse) although for good measure I upped it to 1/2W on my latest design. I also put a 1N4004 in there. Graham
From: Ban on 21 Mar 2007 10:14 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? -- ciao Ban Apricale, Italy
From: Ban on 21 Mar 2007 10:16 YD wrote: > Late at night, by candle light, John Larkin > <jjlarkin(a)highNOTlandTHIStechnologyPART.com> penned this immortal > opus: > >> On Tue, 20 Mar 2007 16:34:17 +0100, "Ban" <bansuri(a)masterweb.it> >> wrote: >> >>> The schematic >>> http://rapidshare.com/files/21272377/mic_amp_2.jpg >>> looks pretty simple, but it still needs work. I found a couple of >>> gotchas: >>> 1. When you switch on the phantom power the Vbe of the transistors >>> gets reversed momentarily(+17V instead of -0.7V), degrading beta >>> and Vos. This will slowly destroy the input devices. This happens >>> always in normal operation with or without a mike. >>> 2. The power supply rejection is very poor(-20dB) especially at >>> higher frequencies. Here current sources might improve the >>> situation. A lot of additional filtering is also needed. >>> 3. When saturating the opamps will return to normal operation in a >>> staggered way, creating spikes in the O/P signal. >>> 4. The offset voltage varies with the gain, making it sensitive to >>> variations in gain setting. >>> 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 >> >> >> Thank you. That is a wonderfully bizarre circuit. >> >> John > > Looks like something's wrong with the feedback connections of IC23/R88 > and IC30/R115. Maybe I'm just missing something. > > - YD. Bingo, it is drawn wrongly. THX for pointing that out. Should be like in the stage before. -- ciao Ban Apricale, Italy
From: Eeyore on 21 Mar 2007 10:27
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? Come on, what predominantly determines the noise of that stage at high gain ? Hint: it's not those transistors. Graham |