[?] Microphony in op-amplifier.

[David Chapman]

Hi,

An audio amplifier for telephone speech in the range 300Hz - 4KHz
which I've built using parts of a quad-op-amplifier (MC33179), is
suffering from microphony. When listening to the output of the amplifier
(with no signals present) and tapping or gently scraping the IC and
adjacent components with an non-metallic rod, each tap or scrape can be
clearly heard on the output line.

The amplifier circuitry I'm using is quite traditional - it's a
two-stage amplifier running from a single-supply rail of +12 volts, the
first with a gain of x10, the second with a gain of x5, each with a
series capacitor + resistor feeding the -ve input of the op-amp with a
feedback resistor to its output. The +ve input of each op-amp has a
potential divider using two equal value (10K) resistors to sit it at
half-rail voltage, with a 100nF capacitor across the lower resistor
(i.e: AC shorting the +ve input to 'ground').

I removed the input capacitor feeding into the first op-amp to
eliminate the rest of the input circuitry, but that made no difference.
I'm now convinced that the microphony is entirely due to the op-amp
and/or its associated resistive/capacitive components..

I'm using 100nF SM-1206 ceramic capacitors, both for inter-op-amp
coupling and also to decouple the resistive potential dividers on the
non-inverting inputs. I did wonder if ceramic decoupling capacitors
might be causing the problem (piezo-effect ?) so replaced them with
tantalums, but that change made no noticeable difference. Because it was
very easy to do, I also replaced the MC33179 IC but, as expected,
nothing changed.

I'm puzzled. Can anyone in this NG tell me what I may be doing wrong?

TIA - Dave

 

[Dr. Barry L. Ornitz]

I'm using 100nF SM-1206 ceramic capacitors, both for inter-op-amp
coupling and also to decouple the resistive potential dividers on the
non-inverting inputs. I did wonder if ceramic decoupling capacitors
might be causing the problem (piezo-effect ?) so replaced them with
tantalums, but that change made no noticeable difference. Because it
was very easy to do, I also replaced the MC33179 IC but, as expected,
nothing changed.

Many ceramic capacitors, especially the high capacitance ones, can be
microphonic. Barium titanate (Perovskite) blends are used for the
dielectric in many multilayer ceramic capacitors.
The dielectric constant can range from 90 to as high as 100,000 depending
on processing. It is also a piezoelectric material.

I have often heard ceramic capacitors "singing" in high voltage
applications.

 

[Andrew Holme]

Hi,

An audio amplifier for telephone speech in the range 300Hz - 4KHz which
I've built using parts of a quad-op-amplifier (MC33179), is suffering from
microphony. When listening to the output of the amplifier (with no signals
present) and tapping or gently scraping the IC and adjacent components
with an non-metallic rod, each tap or scrape can be clearly heard on the
output line.

The amplifier circuitry I'm using is quite traditional - it's a
two-stage amplifier running from a single-supply rail of +12 volts, the
first with a gain of x10, the second with a gain of x5, each with a series
capacitor + resistor feeding the -ve input of the op-amp with a feedback
resistor to its output. The +ve input of each op-amp has a potential
divider using two equal value (10K) resistors to sit it at half-rail
voltage, with a 100nF capacitor across the lower resistor (i.e: AC
shorting the +ve input to 'ground').

I removed the input capacitor feeding into the first op-amp to eliminate
the rest of the input circuitry, but that made no difference. I'm now
convinced that the microphony is entirely due to the op-amp and/or its
associated resistive/capacitive components..

I'm using 100nF SM-1206 ceramic capacitors, both for inter-op-amp
coupling and also to decouple the resistive potential dividers on the
non-inverting inputs. I did wonder if ceramic decoupling capacitors might
be causing the problem (piezo-effect ?) so replaced them with tantalums,
but that change made no noticeable difference. Because it was very easy to
do, I also replaced the MC33179 IC but, as expected, nothing changed.

I'm puzzled. Can anyone in this NG tell me what I may be doing wrong?

That's worrying. Microphony of ceramic SM caps was mentioned by Joerg in
response to my post "Choosing ceramic capacitors" on Sunday. I've only
recently made the switch from TH to SM and I'm about to solder a load of
potential microphones down myself this weekend.

100nF sounds a bit small for audio frequency coupling. What resistor do you
have in series with that? You could try 10uF tantalum coupling caps and
maybe smaller resistors.

 

[Adrian Tuddenham]

Hi,

An audio amplifier for telephone speech in the range 300Hz - 4KHz
which I've built using parts of a quad-op-amplifier (MC33179), is
suffering from microphony. When listening to the output of the amplifier
(with no signals present) and tapping or gently scraping the IC and
adjacent components with an non-metallic rod, each tap or scrape can be
clearly heard on the output line.

[...]

Is the performace of the circuit exactly to spec in all other respects?
Any slight deviation might give you a clue about the nature of the
fault.

If the capacitors don't turn out to be the cause (and they definitely
are the most likely culprits), you could be looking for dry joints,
solder whiskers, leaky PC board material or even a batch of mis-labelled
op-amps of the wrong type with some floating pin which you have not
grounded.

When conventional fault-finding doesn't give the answer, *nothing* is
above suspicion.

 

[David Chapman]

Many ceramic capacitors, especially the high capacitance ones, can be
microphonic. Barium titanate (Perovskite) blends are used for the
dielectric in many multilayer ceramic capacitors.
The dielectric constant can range from 90 to as high as 100,000 depending
on processing. It is also a piezoelectric material.

I have often heard ceramic capacitors "singing" in high voltage
applications.

Sorted! One by one I replaced the ceramic capacitors (around 6 of
them - all AVX 100nF SM1206s) around the op-amps and found that the
op-amp input and output coupling capacitors were responsible for most of
the microphony. The worst offenders were those in series with the
largest value input resistors, of course. I replaced them all by 1u0
tantalums (not that I needed more gain at lower frequencies but just
because I had the tants to hand) and the microphony virtually
disappeared.
Replacing the two ceramic 100nF SM caps that were decoupling the
resistive dividers on the +ve inputs of the op-amps, with 1u0 tants
killed it completely.

I've just learned another lesson - Don't use high-value ceramic
coupling capacitors in gain stages.
Many thanks to those who responded to my posting confirming my
original suspicions.

As a result of this experience, I've also just discovered that a 4u7
ceramic SM capacitor that was (supposed to be) decoupling the reference
voltage on a different IC amplifier was actually modulating that
reference voltage, and consequently the output signal, when subjected to
any vibration. Changing it for a 4u7 tantalum capacitor completely
cured that problem as well.

ATB - Dave