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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
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
16,258 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
bpoletti
1,553 posts
 
There are three possible reasons. 

First, because it sounds better.

Second, because it's cheap.

Third, because it sounds better and it's cheap.

Sometimes "better quality" caps actually hurt the performance.  Their resonance (physical and electrical) may not be compatible with the rest of the circuit.
erik_squires
16,258 posts
 
Not every part in an amp has to do with sound quality. For all I know this could be a bypass cap on an IC which controls relays and has nothing to do with the audio signal path.


Best,

E
audioman58's avatar
audioman58
4,198 posts
 
Personally I would think a Mundorf or VH Cu  teflon cap would be night and day better and over 600 lot D.C. .i can guarantee you better in every respect $$
is the only limiting factor.
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