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Mark Levinson has odd choice for cap upgrades in one of their service bulletins. Why?

Pursuant to one of the Mark Levinson service bulletins, for Model 331, 332, 333 etc, they have outlined one of many things that should be performed on any amp that comes in for service.

One of the line items has me baffled. They recommend replacing four caps on each voltage gain / input board. These eight caps are PP type of .01uF @ 160v. They recommend replacing them with Ceramic X7R .01uF @ 200v. These are axial configurations.

This is not an expensive upgrade but I thought to myself that polypropylene caps had low failure rates and good longevity compared to other types especially if they were used in a proper operating envelope.

I just finished watching and reading some information on the perils of using ceramic caps in certain applications. For one, they tend to drift heavily with temperature changes. In a monster like the Model 333 there will definitely be a large temperature swing. The ceramics also tend to exhibit piezo effects with vibration. While vibration is only inducing small voltages, I can imagine the sum of many caps being subject to vibration not being a good recipe for an audio signal.

ML has stated that the ceramic replacements should be installed with spacers to keep them lifted from the circuit board. I am guessing that this could address temperature concerns, vibration or parasitic capacitance issues. They do not provide any reason.

I would really like to learn a little more behind their reasoning as it seems this particular "upgrade" is counter-intuitive. Can anyone shed some light on this?
generatorlabs
erik_squires
16,173 posts
 
The answer indeed seems to be heat, but it’s also important to understand the application.

0.01uF adds microscopic amounts of capacitance, so I assume they are merely RF filtering, power supply decoupling caps. Ceramics should be fine.

Best,

E
generatorlabs OP
25 posts
 
The answer indeed seems to be heat, but it’s also important to understand the application.

That is the part that is troubling me. Heat is a factor that is well known in class A amps. Even though we are talking small capacitance values wouldn’t we want those value to remain stable? Polypropylene capacitors are known for being stable in environments that change temperature dramatically, where ceramics start diving in the presence of heat.

And then with so much emphasis being put on esoteric electrolytic caps these days, why would one purposely introduce a type of cap that could actually introduce transient noise from vibration? Am I over-thinking this?
erik_squires
16,173 posts
 
Hi Gnerator:

Yes, you are overthinking it. :) These are probably PS de-coupling caps, and size and heat resistance are most likely the dominant factors.

The location matters. It is quite possible that those caps are exposed to unusually high heat. The temperature in an amp isn't the same at every point, it is quite possible the caps are in a particularly bad hot spot. Also possible that the manufacturer of the originals was too optimistic re: heat / failure rates. ML seems to have learned something and chosen to act proactively.

Second, it's not just about thermal stability it is about failure. Caps with higher thermal ratings last longer.

Esoteric caps are huge by comparison to their "normal" counterparts, AND honest to god, it's quite probable that in this application they make no difference at all.


Best,

E
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