Potting the input stage changes tone?


2011-11-25 9:16 pm
I was just wondering if anyone has had any experience in potting an amplifier's, or preamplifier's circuit, or partial circuit?

I've heard these pros and cons:

Pros: Thermal stability. Shock resistance. Good insulation from moisture.

Cons: Capacitance. Detrimental to sound quality. The harder the potting epoxy, the more sterile and bright the sound. Diminished bass.

Is there any truth that potting with a hard epoxy resin will ruin the tone? Make it sound bright and harsh?


2003-02-17 7:38 pm
circuit impedance determines the amount of influence of added parasitic/fringing C from potting dielectric properties

for dynamic mic pre should be no problem, for guitar pickup even 10 pF can make audio frequency response difference

high V physics people have used paraffin wax for electronics potting - easily removed - lower melting point types may even be OK with polystyrene Caps

potting is mostly used for environmental protection in industrial applications where chemical corrosives may be an issue

"audio" circuit potting is more likely just an attempt to keep the circuit secret than for any real performance advantage

an iteresting factoid about conformal coating/potting - most polar compounds (basically all casting/potting compounds that "set" by chemical reaction) won't block moisture - they can have few % equilibrium moisture content - so moisture will eventually reach the potted board surface - to keep leakage current low, achive ultra hi Ohm you have to use good PCB material, and completely clean the finished board of any ionic contaminants before potting/coating
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Potting makes servicing difficult. You have to remove all that gunk before replacing any of the parts that will inevitably fail. If electrolytic caps get fully potted, it can have a detrimental effect on their lifespan because they cannot adequately out-gas as they were designed. And they will need replacing eventually anyway. For these reasons, in the case of usual audio electronics in a home environment, I am absolutely against potting in all situations. Only exception might be to insulate very high voltage supplies, like you may see in some tube amps, but I don't consider that "usual audio electronics". The cons FAR outweigh the pros, in my opinion. I've had to repair potted audio electronics before, and it was a major headache for no obvious benefit.

First post, Greetings! :)

How does it change the capacitance? I realise that it must have a dielectric constant higher than air but the distances are huge.

@jcx v. interesting points and better than a factoid I would say.

It seems to me to be a fundamentally good idea, particularly from the pov of air flow - if not actual vibration. Since I think one can hear the sound of different circuit board materials and/or mounting (though that could be just suggestibility as its brand of pinginess shows up in the sound) I wouldn't be at all surprised if it improves things.

Not great for repairs, though.