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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?
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Showing 1 response by pragmasi

pragmasi
212 posts
 
@georgehifi
The XR7’s are great for HF filtering, I use them on dac I/V stage voltage rail decoupling.That’s why ML are recommending them.
That’s the answer, XR7 would be the first choice for this application... makes me wonder why they used PP in the first place.

C0G are great for setting time constants (e.g. RIAA equalisation) bot not necessary or most effective here. It’s actually the limitations of the dielectric that make X7R the best choice as high frequencies (such as RF) will get shunted to ground... these caps are on the DC rails where you don't want any AC.
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