Op-Amp Mic Pre Trouble

[[email protected]]

Hello, I have a LM4562 op amp circuit set up as shown in the microphone
preamplifier section of the data sheet in the following link
http://www.national.com/ds/LM/LM4562.pdf . The performance of the pre
is very good from 10 Hz to 50 kHz on a line input. But when connected
to a balanced microphone and set to a decent amount of gain from 40 dB
to 60 dB I get a 10 kHz peak about 20 dB above the noise floor. The
microphone is an industry standard shure SM-58. The power supply is
made of batteries so I don't think the source is the power supply.
There are no capacitors or inductors in any of the signal path section
to cause oscillation besides the microphone itself. Anyone have any
clue why an op amp circuit would have a peak at 10kHz. There are 47uF
caps on the power inputs to the op amps. Any other suggestions for
capacitor values I should try here? Any help is welcome thanks.

 

[Phil Allison]

** Desperate Groper from HELL alert !

Hello, I have a LM4562 op amp circuit set up as shown in the microphone
preamplifier section of the data sheet in the following link
http://www.national.com/ds/LM/LM4562.pdf . The performance of the pre
is very good from 10 Hz to 50 kHz on a line input. But when connected
to a balanced microphone and set to a decent amount of gain from 40 dB
to 60 dB I get a 10 kHz peak about 20 dB above the noise floor.

** You get a 20 dB " peak " in WHAT exactly ??

The frequency response ??

The background noise level ??

What is you measurement method ??


BTW:

The LM4562 is a VERY NOISY op-amp.





....... Phil

 

[[email protected]]

Hey, there is a peak at 10 kHz 20 dB higher than the rest of the
signal. It is a clear 10 khz tone you can hear in headphones.

I'm using a echo layla 24/96 audio interface into FFT analyzer software
with a hanning window at a 24 bit 96 kHz sampling rate. I am comparing
a direct loop (in and out of the interface) vs a loop through the pre
amp with 40 dB attenuation before the pre. That test comes out perfect
even with the extra gain. The test material is 24bit 96 kHz white
noise.

It is only when I connect a microphone to the preamp that a 10 kHz peak
appears. To clarify the peak is 20 dB higher than the rest of the
signal. This is also very audible in headphones when listening.

Any tips if a INA217 would be a better for a mic pre, the circuit is
nearly identical in that op amp in comparison to what I am using now.

 

[John Popelish]

Hello, I have a LM4562 op amp circuit set up as shown in the microphone
preamplifier section of the data sheet in the following link
http://www.national.com/ds/LM/LM4562.pdf . The performance of the pre
is very good from 10 Hz to 50 kHz on a line input. But when connected
to a balanced microphone and set to a decent amount of gain from 40 dB
to 60 dB I get a 10 kHz peak about 20 dB above the noise floor. The
microphone is an industry standard shure SM-58. The power supply is
made of batteries so I don't think the source is the power supply.
There are no capacitors or inductors in any of the signal path section
to cause oscillation besides the microphone itself. Anyone have any
clue why an op amp circuit would have a peak at 10kHz. There are 47uF
caps on the power inputs to the op amps. Any other suggestions for
capacitor values I should try here? Any help is welcome thanks.

A gain of 1000 puts output signals that are very much larger
than input signals in close proximity with each other.

You may not have any intentional capacitors in the circuit,
but you certainly have unintentional stray capacitances. Do
you have a photo of your layout available somewhere in the
net? Or you can email it to me.

The 3 opamp differential amplifier (instrumentation
amplifier configuration) has two high gain, non inverting
amplifiers in it, each of which can produce inadvertent
positive feedback. Combine that with a 55 MHz gain
bandwidth product and you have a lot of potential for trouble.

 

[Phil Allison]

** Top Posting GROPER ALERT !!

Hey,

** Cut out the smartarse use of "hey" - OK .

there is a peak at 10 kHz 20 dB higher than the rest of the
signal. It is a clear 10 khz tone you can hear in headphones.

** Then it is an OSCILLATION - not a " peak ", you goose.

Learn what words mean before you go using them !!!!

It is only when I connect a microphone to the preamp that a 10 kHz peak
appears. To clarify the peak is 20 dB higher than the rest of the
signal. This is also very audible in headphones when listening.

** That is an OSCILLATION - not a " peak " you goose.

Any tips if a INA217 would be a better for a mic pre, the circuit is
nearly identical in that op amp in comparison to what I am using now.

** What value input resistors " R" have you used ??

Is the OSCILLATION at full output level - ie rail to rail ?

Have you used normal shielding practices ?

Is mic pin 1 grounded?


BTW:

DO NOT TOP POST AGAIN !!!




......... Phil

 

[tempus fugit]

Hello, I have a LM4562 op amp circuit set up as shown in the microphone
preamplifier section of the data sheet in the following link
http://www.national.com/ds/LM/LM4562.pdf . The performance of the pre
is very good from 10 Hz to 50 kHz on a line input. But when connected
to a balanced microphone and set to a decent amount of gain from 40 dB
to 60 dB I get a 10 kHz peak about 20 dB above the noise floor. The
microphone is an industry standard shure SM-58. The power supply is
made of batteries so I don't think the source is the power supply.
There are no capacitors or inductors in any of the signal path section
to cause oscillation besides the microphone itself. Anyone have any
clue why an op amp circuit would have a peak at 10kHz. There are 47uF
caps on the power inputs to the op amps. Any other suggestions for
capacitor values I should try here? Any help is welcome thanks.

You would almost wonder if it is picking something up somewhere when it is
oscillating like that. Is your circuit in a shielded box? If you can't get
rid of the oscillaition, maybe you could try just connecting it in a
standard inverting or noninverting configuration, or doing a 2-stage setup
using each opamp in the package for each stage, and do a gain of 20 in the
1st and 50 in the 2nd, and check the performance of it then. It may not be
much different than in the instrumentation type setup they have in the app
note.

You might also want to check out:

http://www.thatcorp.com/1500desc.html

Much lower noise floor.

 

[Phil Allison]

"Phil Allison"


Correction:

The LM4562 is a VERY NOISY op-amp.

** Should be the MAX4469 .......




........ Phil

 

[[email protected]]

Yeah I just figured I'd try them out (samples) they really are noisy
though. Also, I stopped the oscillation with two 400 pf capacitors on
the inputs terminals connected together to a 47 pf capacitor to ground,
now it has a nice flat frequency response...with lots of noise, but I
am adding 60 dB of gain so it could be anything in the room making it
noisy too. Thanks for your help everyone! I'll have to try out some
other op amps and see how they change the performance.

 

[Phil Allison]

<[email protected]

Yeah I just figured I'd try them out (samples) they really are noisy
though.

** Nonsense.

At 2.7 nV/ rtHz EIN they are very low noise op-amps.

Also, I stopped the oscillation with two 400 pf capacitors on
the inputs terminals connected together to a 47 pf capacitor to ground,

** That is only RF suppression - not likely to affect 10 kHz much.

Perhaps 10 kHz was an alias frequency from a much higher oscillation
frequency.

now it has a nice flat frequency response...with lots of noise,

** Bollocks.

but I
am adding 60 dB of gain so it could be anything in the room making it
noisy too.

** You must replace that SM58 mic with a 270 ohm resistor when noise
testing.

Same thermal noise as the mic with no room noise.

Thanks for your help everyone! I'll have to try out some
other op amps and see how they change the performance.

** Waste of time

You won't beat the one you have with ANY normal op-amp.



........ Phil

 

[Ban]

Hello, I have a LM4562 op amp circuit set up as shown in the
microphone preamplifier section of the data sheet in the following
link http://www.national.com/ds/LM/LM4562.pdf . The performance of
the pre is very good from 10 Hz to 50 kHz on a line input. But when
connected to a balanced microphone and set to a decent amount of gain
from 40 dB to 60 dB I get a 10 kHz peak about 20 dB above the noise
floor. The microphone is an industry standard shure SM-58. The
power supply is made of batteries so I don't think the source is the
power supply. There are no capacitors or inductors in any of the
signal path section to cause oscillation besides the microphone
itself. Anyone have any clue why an op amp circuit would have a peak
at 10kHz. There are 47uF caps on the power inputs to the op amps.
Any other suggestions for capacitor values I should try here? Any
help is welcome thanks.

I bet you got bitten by a layout problem and this amp is oscillating. The
observed peaking is typical. Long traces to the gain setting pot will cause
this. Try replacing them with a single short resistor between the inverting
inputs for testing.
Also heavy capacitive loading might cause it. a 100R in series with the O/P
is always good.
The chosen opamp is good for filter and line-level apps, but for a pre I
would try the new AD8599 as frontend followed by a difference amp with
integrated resistors. Try to have a fixed gain of 20dB on the second stage
and use an attenuator for line level. At 60dB no single stage is performing
very well.
You are talking about battery, how are you creating a split supply at what
voltage?

 

[[email protected]]

The trace wires for the first gain stage are fairly long I'll have to
redo that at some point and see if it will fix the 10k problem. I
don't have one op amp doing 60dB, I have a 40 dB gain stage adjustable
followed by a fixed 20 dB gain stage. I do have the 100 ohm on the
output also.

And it's fairly easy to make a split supply with batteries and best
part is no power supply noise. I am using three new nine volt
batteries to get 27 volts total, then using two caps and two resistors,
one each to the positive supply and one to the negative supply. You
create a ground in the middle with +/- about 14 volts which is what I
am running the op amps at.

 

[Ban]

The trace wires for the first gain stage are fairly long I'll have to
redo that at some point and see if it will fix the 10k problem. I
don't have one op amp doing 60dB, I have a 40 dB gain stage adjustable
followed by a fixed 20 dB gain stage. I do have the 100 ohm on the
output also.
OK

And it's fairly easy to make a split supply with batteries and best
part is no power supply noise. I am using three new nine volt
batteries to get 27 volts total, then using two caps and two
resistors, one each to the positive supply and one to the negative
supply. You create a ground in the middle with +/- about 14 volts
which is what I am running the op amps at.

Hah, that really sounds for trouble! Better to have nice low impedance gnd
also at DC. Take only two batteries with the common connection soldered to
GND. Use a different pair of +/-18V batteries with the diff.amp (chosen for
+/-22V max.)
Have also this battery array soldered to gnd.

 

[Eeyore]

Hello, I have a LM4562 op amp circuit set up as shown in the microphone
preamplifier section of the data sheet in the following link
http://www.national.com/ds/LM/LM4562.pdf .

It's a fairly rubbish mic pre-amp so I wouldn't lose any sleep over it !

Graham

 

[tempus fugit]

<[email protected]



** Nonsense.

At 2.7 nV/ rtHz EIN they are very low noise op-amps.




** That is only RF suppression - not likely to affect 10 kHz much.

Perhaps 10 kHz was an alias frequency from a much higher oscillation
frequency.




** Bollocks.




** You must replace that SM58 mic with a 270 ohm resistor when noise
testing.

Phil is right in these areas. I earlier recommended the THAT1510 as a lower
noise alternative (I would think it is about as low as you're going to get),
but I didn't mean to suggest that this one is noisy. Most opamps are much
higher EIN than 2.7 nV. You are also right about the room noise - that's
probably what you're hearing. Phil's suggestion of using the 270ohm resistor
instead of the mic will give you a more accurate idea of the noise level.