Micpre of Graham
in [Design]
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From: cledus on 21 Mar 2007 00:30 cledus wrote: > Ban 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 >> > > Because I had the day off and was fascinated by Graham's "improved mic > preamp", I threw it into my Spice simulator and tinkered around a bit. > Here are some of the results > > My simulator does not have built-in models for the transistor and op-amp > that he used. So I substituted the venerable 2N2907 and LM833 parts to > see what happened. If you can believe the simulator, the noise > performance is impressive at around 2.5 nV/rt-Hz referred to the input. > However, the distortion leaves a bit to be desired. At a gain of ~30 > and driving with +/-100mV pk-to-pk, the third harmonic is about -60 dBc > at 100 Hz (~.1% THD). At 1kHz it gets better at ~-90 dBc for both the > 2nd and third harmonics. If I try substituting the LT1028 model in my > simulator for the op amp, the circuit goes unstable. The circuit may > depend on a slower op amp to keep it stable. > > For fun, I attempted to simulate a plain-jane LT1028 inverter based on > the built-in model for my Spice simulator. I'm not sure that I can > trust the model. I could get no where close to the noise performance > claimed in the data sheets. And the noise was orders of magnitude worse > than Graham's circuit. Like the data sheet recommends, I used 1.8k > feedback and 60 ohms input resistors. The noise shows around 1.75 > microVolts/rt-hz referred to the input as opposed to less than 1 nV! > Maybe I am doing something wrong, or maybe the model is not trustworthy. > But the distortion looks very impressive and is similar to the data > sheet. At +/-100 mV and gain of around 30, harmonics were all > suppressed well below 100 dBc for input freqs of 10 Hz, 100 Hz, 1 kHz > and 20 kHz. Because of the noise discrepancy, I don't know how reliable > these results are. But they do seem to follow the data sheet > extrapolation at these input levels and no external load. > > Anybody know where I can get a reliable Spice model for the LT1028? > > -c > > It would appear that my LTC Spice libraries are not very accurate for noise simulation. I ran through a bunch of noise simulations for just a simple inverter circuit (Ri=30 ohms, Rf=1k). Here are some results: OP27A (AD model) 3.9nV/rt-Hz @ 1kHz (spec sheet says 3.0 nV) OP27A (LT model) 12.4uV/rt-Hz @ 1 kHz (spec sheet says 3.0 nV) MAX4106 (MAX model) 2.9nV/rt-Hz@1 kHz (spec sheet says ~.85nV) AD797 (AD model) 1.1nv/rt-Hz @ 1 kHz (spec sheet says .9 to 1.2 nV) MC4558 13.1nv/rt-Hz @ 1 kHz (spec sheet says 12 nV) UA741 21nV/rt-Hz @ 1 kHz (spec sheet says 23 nV) TL071 12.5nV/rt-Hz @ 1 kHz (spec sheet says 18 nV) LT1028 (LT model) 1.32uV/rt-Hz @ 1 kHz (spec sheet says .85 to 1.1 nV) Most models other than LTC get me into the ballpark. None of the LTC noise simulations were even in the same city. Some Op Amps have an equivalent AD part. These appear to be a much more faithful representation of the spec sheet performance than the LTC models.
From: YD on 21 Mar 2007 06:45 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. -- Remove HAT if replying by mail.
From: Fred Bloggs on 21 Mar 2007 07:32 Ban wrote: > 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 > That circuit is just /that/ simple, a simple rehash of the same tired old subcircuits, nothing original, all kinds of matching of discretes required, and the same old two channel input gain stages with those dumb inverse pot things.
From: Phil Allison on 21 Mar 2007 07:37 "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. 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 ? ........ Phil
From: Phil Allison on 21 Mar 2007 07:49
"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. ....... Phil |