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Micpre of Graham

B

Ban

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
 
J

Jim Thompson

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

And what's so marvelous about that design?

Looks like two pounds of shit in a one pound bag to me ;-)

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

...Jim Thompson
 
B

Ban

Jim said:
And what's so marvelous about that design?

Looks like two pounds of shit in a one pound bag to me ;-)

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

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.
 
B

Ban

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.
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.
 
E

Eeyore

Ban said:
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.

The phantom power is separately switched on/off. It should as you correctly
point out be of controlled rise/fall time. The same holds true for every mic amp
I've ever seen btw.

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.

So you say. Can you check that figure ? The only issue I see is mismatch of the
2 x 4k7 1% resistors and the 2 x 1k5 1% resistors. In practice these typically
match within a batch to about 0.3%.

I did simulate it btw. The power supply for that product has about 500uV of
supply noise in the audio band and the 100Hz component is well down.

3. When saturating the opamps will return to normal operation in a staggered
way, creating spikes in the O/P signal.

I don't recall seeing that problem. Can you explain why you think that may
happen ?

4. The offset voltage varies with the gain, making it sensitive to
variations in gain setting.

No, there's no DC gain wrt the offset voltage. You can twiddle it more or less
noiselessly to your heart's content !

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

Would you like to run over the advantages of this ?

It looks unnecessarily complicated to me to be honest.

Graham
 
E

Eeyore

Ban said:
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

Where do you get the 2k5 rev log pots ? The lowest value I've ever seen is 5k.

I did a design with a dual gang gain pot btw. It worked rather well. I should
perhaps point out that this design was rejected on cost grounds ! I hate to
think what most mixer companies would say about your design.

Tell you what, you cost yours and I'll cost mine !

Graham
 
E

Eeyore

Ban said:
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.

I believe I did make a note to that effect somewhere. It really ought to be
1000uF.

Graham
 
H

Hawker

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
 
H

Hawker

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
 
E

Eeyore

Ban said:
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
 
J

John Larkin

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
 
C

cledus

Ban said:
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
 
J

Jim Thompson

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
 
C

cledus

cledus said:
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/[email protected] 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.
 
Y

YD

Late at night, by candle light, John Larkin
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.
 
F

Fred Bloggs

Ban said:
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.
 
P

Phil Allison

"Eesyore"
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
 
E

Eeyore

Phalluson said:
"Eesyore"

** 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
 

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