Micpre of Graham

And what's so marvelous about that design?

Looks like two pounds of s*it in a one pound bag to me ;-)

(That's a "country boy" colloquialism ;-)

...Jim Thompson

Well, it avoids that big electrolytic cap in series with the gain pot. BTW in Grahams design this is too small, rolling off at 27Hz with full gain and 1.6Hz with low gain.

Hawker wrote:

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'm always interested in looking at others mic pre-schematics. Just wondering who's the one you uploaded is. The fact that they are using 5534s and not say 2114s or even 5532s (which test a bit better than a 5534) makes me wonder about it.

thanx Hawker

On 3/20/2007 10:34 AM, The digits of Ban's hands composed the following:

On 3/20/2007 11:32 AM, The digits of Ban's hands composed the following:

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

Thank you. That is a wonderfully bizarre circuit.

John

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

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

I got ~960nV/rt-Hz

I suspect you made a mis-entry.

From LTC ?:-)

...Jim Thompson

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.

Late at night, by candle light, John Larkin penned this immortal opus:

Looks like something's wrong with the feedback connections of IC23/R88 and IC30/R115. Maybe I'm just missing something.

- YD.

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.

"Ban"

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

Why do you think a reactive element increases the noise?

Bingo, it is drawn wrongly. THX for pointing that out. Should be like in the stage before.

I substituted 50 ohms for the 18k input resistors (R3 & R5). I forgot to point this out. This may, in part, account for the differences in our results. Not sure what makes up the rest. Perhaps the noise model for the 2N2907's?

You may be right. I don't trust the model I have for the LT1028. I entered a different high bandwidth op amp that seems a bit more trustworthy (MAX4106). It appears to be stable with this component. In fact the peak in freq response moves from 1 MHz using LM833 to greater than 10 MHz with the MAX4106.

When I feel ambitious, I will dowload their freebie SPICE app and give it a try. I have used CircuitMaker2K for awhile now and need to get over the inertia to learn something new.

TNX,

-c

The BJT model IS the noise model.

...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 | |

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

Can't you get ANYTHING right ?

Wog idiot.

...... Phil