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[?] Microphony in op-amplifier.

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

--
David C.Chapman - (dcchapman@minda.co.uk)
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Reply to
David Chapman
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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.

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73, Dr. Barry L. Ornitz  WA4VZQ
BLOrnitz48@charter.net
Reply to
Dr. Barry L. Ornitz

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.
Reply to
Andrew Holme

[...]

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.

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~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
Reply to
Adrian Tuddenham

Suspect the caps first. But if that's not it, you might have a loose connection (cold solder joint or a crack in a resistor) somewhere.

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Paul Hovnanian  paul@hovnanian.com
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Have gnu, will travel.
Reply to
Paul Hovnanian P.E.

In article , Dr. Barry L. Ornitz writes

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

--
David C.Chapman - (dcchapman@minda.co.uk)
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Reply to
David Chapman

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