Re: Op-Amp Mic Pre Trouble


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
nirvanabt4@xxxxxxxxx wrote:
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.
--
ciao Ban
Apricale, Italy


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