Mircophonic SS amp?

[Axtman]

I was given a cheap nonworking Peavey Rage 158 amp. I repaired the input jack but the amp has a peculiar problem. On the high gain setting, the pcb becomes microphonic! I know of microphonic tubes but never microphonic solid state components. I looked for cold solder joints or breaks but could not find any. I am going to tap around with a chopstick and see if I can isolate the problem. Has anyone else had issues with microphonic components?

[xtian]

Yes, have seen microphonics coupling caps.

[Tom Phillips]

Yes, have seen microphonics coupling caps.

and I have also found microphonic metal case transistors and coax cable.

[drewl]

Is it microphonic at idle or with a signal?

Most of the time it is a bad/loose PC board connection which can be a pain to find.
If it is a component tracing the signal with a scope can lead to the stage and or part.

[Enzo]

Please do tap on everything looking for what is sensitive.

[guitician]

Is it doing this with the guitar plugged in? Without a shorting jack an amp will be very microphonic if the gain is high.

[J M Fahey]

Ceramic caps are so microphonic that I have succesfully made a contact bass pickup with a 0.1uF ceramic cap, go figure.

And yes, tapping with a chopstick or a plastic BIC pen is the way to find the culprit.

[gbono]

Ceramic capacitors can be microphonic (piezo electric effect anyone) but it is a function of the class (dielectric constant) of the ceramic material used to build the cap. Class I dielectric = C0G (NPO) usually are not microphonic but also don't allow for large capacity. X5R (and even X7R) will act as piezo pick ups.

We won't go into voltage coefficient due to dielectric constant of materials but just accept that class II/III dielectric change their capacitance with impressed working voltage (DC) (this effect can drop the capacitance by over 75%). Lots of good application notes on these subjects at AVX, Kemet, TDK, etc.

[Enzo]

Just think, we might be able to make a voltage variable tone circuit.

[dai h.]

Ceramic capacitors can be microphonic (piezo electric effect anyone) but it is a function of the class (dielectric constant) of the ceramic material used to build the cap. Class I dielectric = C0G (NPO) usually are not microphonic but also don't allow for large capacity.

(From what I understand as what happened with the multi-layer type) manfs. have been able to make the pieces they are made of thinner and thinner over years.



I've seen Kemet(?) ones up to .47uF and these were expensive but the smaller nFs-10s of nF range (SMT ones seems to be lower in price) seem to be reasonable in cost to the point that they could be a viable choice in place of film (say C0G/NP0 and CH (slightly different Japan Industry Standard spec.) to replace mylar--the aforementioned seem to have less distortion, though I understand that (for solid state circuitry) things around the circuitry (layout, grounding, power supply) need to be optimized to make full use of "better parts" as well).

As far as the microphonics, I've tried replacing 2n2s as part of what looked like an RF filter (10R0 series +2n2 shunt to ground on each side of the balanced line) at the front of a mediocre solid state mic pre circuit in the mixer section of a Tascam 488mkII cassette multitrack (something not particularly valuable used for occasional experimentation with the mixer circuitry). Changing the stock ones to (CH temp. characteristic--I think) 2n2 seemed to eliminate the microphonic behavior from turning up the mic pre gain up high and rapping my knuckle on the plastic housing.

For situations involving subjective sound (fuzz, gtr. amp, etc.) though, you may need to be more careful, since (I suspect) the "bad ceramics" might be more suitable than the "good ceramics" sometimes.

[gbono]

Just think, we might be able to make a voltage variable tone circuit.

If you're a RF/Uwave designer you might find these interesting - might even be able to replace those "old" varactor diodes.
Digitally Tunable Capacitors (DTC) - Peregrine Semiconductor

Piezo effect in ceramic capacitors is not new. When manufacturer's of ceramic caps (MLCCs) trying compete with tantalum and aluminum dielectrics to achieve the same "volumetric efficiency" the ceramic cap designers start to design stacks with very small spacings and class II/III materials there are going to be trade offs.