Re: Op-Amp Mic Pre Trouble

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 wrote:
nirvanabt4@xxxxxxxxx wrote:
Hello, I have a LM4562 op amp circuit set up as shown in the
microphone preamplifier section of the data *** 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?

--
ciao Ban
Apricale, Italy

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