Micpre of Graham
in [Design]
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From: Hawker on 20 Mar 2007 15:01 On 3/20/2007 11:32 AM, The digits of Ban's hands composed the following: > I think it is from Rane. Look at the offset adjust pins used for frequency > compensation. The 5534A is IMHO better than the decompensated version(5532), > less noise, higher slew rate and GBW, for gains above 3. I think I was getting confused, you are correct, sorry. I was thinking the 5534 was the quad, not single part. I remember the compensation is in the single part if I remember correctly. I'm not doing much analog these days. Mostly doing digital design. My part number memory is getting bad. -H
From: Eeyore on 20 Mar 2007 16:09 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 ? Graham
From: John Larkin on 20 Mar 2007 16:27 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
From: cledus on 20 Mar 2007 19:08 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
From: Jim Thompson on 20 Mar 2007 19:18
On Tue, 20 Mar 2007 23:08:46 GMT, cledus <cledus(a)noemail.net> 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. I got ~960nV/rt-Hz > 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. I suspect you made a mis-entry. > >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 > From LTC ?:-) ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | America: Land of the Free, Because of the Brave |