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

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?

I agree with Erik. These are small value decoupling caps. So for those unfamiliar with the amp here are some images and schematic clips:

This is my beloved 332. The caps in question lay horizontally just beneath the top cover.

https://drive.google.com/open?id=0B0yvjhlC62G7SHVJczc2ZUV2OGs

Here is a example of the schematic. The two circled caps are on the POS-VCC rail. The other two are on the NEG-VCC rail.

https://drive.google.com/open?id=0B0yvjhlC62G7RGVBdkZlcElmTGM

I borrowed the following picture from Google images. The two voltage gain boards can be seen here. The meat and potatoes, heat producing current gain boards, are sitting beneath in a vertical position. All the heat rises and makes the area around the voltage gain boards quite toasty. The caps in question are the bright yellow axial devices. They are physically large for such small value caps.

http://brainlubeonline.com/mlrepair/IMG_1020.jpg

So the real reason I started this thread is not because I was looking for an esoteric cap to put there. These caps are a focal point for ML and they are old. My concern was the temperature. A PP cap (the type originally used) should perform better in heat than an equivalent ceramic. I say "should" but I am not an engineer. I am only using information that I have seen proven in videos that compare different capacitor types.

The second reason is that if you tap on a ceramic cap it will induce a small voltage. This can be seen on an oscilloscope. These decouplers are tied to rails that small audio path transistors use. Couldn't induced spikes of noise make its way back to the audio path? Granted these tiny ceramics do not have much surface area. A larger ceramic would have higher probability of inducing vibration related noise but to me 8 tiny caps producing 8 tiny spikes adds up to 1 mess.  An amp of this size will always be on the floor, subject to whatever vibration the surrounding loudspeakers produce.

I am not saying the ceramic option is not the way to go. I am sure the ML engineers had to put some level of thought into this but in a world where folks have power cables elevated from the floor and where using cryo-frozen power outlets, wooden knobs etc is the norm in the realm of the esoteric, it seems odd that ML would take this route. I personally do not subscribe to most of the insanity but it does not mean I do not read some of the articles describing the latest "sound salvation" gadgets. I want to learn from them, as there could be some sane measurable information in there that I could use one day.

@teo_audio; Why pull your post?

I get the argument from both sides and I really appreciate all input.
Dialogue like this challenges my thought processes and possibly will answer some questions for others that casually read this thread.

Since ML has made these particular PP caps an area of concern I am going to address it. Normally I would have skipped PP caps and focused all of my energy on electrolytics. I have seen bizarre ESR readings from the electrolytic caps in this amp. Two identical caps sitting right next to each other having starkly opposite readings is a reality. I now feel that the amp has had enough of its guts replaced to live another long life.

The ML service bulletin specifically called out 6 small electrolytics, 2 op amps, and the 8 PP caps described herein, as parts that had high rates of failure. I have addressed all these items with the exception of the PP caps.

I found these Philips caps which, value for value, are direct replacements. Different construction though.

http://www.ebay.com/itm/New-20-Pcs-01uF-160V-Film-Foil-Axial-Capacitors-Metalized-Polypropylene-DC/131455088503?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D44758%26meid%3D877ec48210ec4acaa78afefdf7b41d42%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D140594885371

The very large main caps were Philips as well. Don’t get me started on the black ooze mess they made in this amp when they failed! They had a horrendous track record so Cornell Dubliers sit there now. So should I consider these ebay Philip PP’s?

I can find a suitable replacement on Mouser or Digikey but the voltage rating is more than double. I was taught that a higher voltage cap could be used but I should not go higher than 20 percent. Any thoughts on the Philips?

@imhififan Some pp cap has >50% voltage derating when operating temperature over 85 ºC , that could be the possible reason Mark Levinson recommend replacing them with Ceramic X7R .01uF @ 200v

I could see that. So you prompted me to look up derating tables for Ceramic. Got a little lesson on cap classifications as well. Here are some quotes from TDK:

A Class I capacitor* (C0G, C0H, C0K, etc.) is made from ceramic materials that are not sensitive to temperature changes, thus the capacitance value of a capacitor measured at a low temperature (example -25°C) will not significantly vary from the same capacitor measured at a higher temperature (ex. 75°C).
EIA refers to these as “temperature compensating,” and are measured in ppm/°C.
These capacitors typically have low capacitance values because of the ceramic material used to manufacture them but they exhibit nearly perfect capacitance stability regardless of their temperature, making them an excellent choice for applications in which frequency control is needed such as in radio or television tuners.
A Class II capacitor2 (X7R, Y5V, Z5U) is made from ceramic materials that are derived from a barium titanate base which is temperature sensitive. Thus the various temperature classifications stating the extent of the sensitivity over a given temperature range. These capacitors allow for larger capacitance values in the small surface mount packages.
Additionally, capacitors made from barium titanate bases (Class II, III, and IV) are ferroelectric and therefore susceptible to “aging” in which a capacitor’s ability to hold capacitance will decrease over time if left in an unheated and/or uncharged state. Class I capacitors are not ferroelectric and therefore do not age.

So I am not trying to beat a dead horse here. If we are going to dialogue about this then lets do just that. Today I learned things that I did not know yesterday and that is always a good thing.

I tried to see if the caps I need could be found with a C0G designation but caps of such small capacitance were not available with that designation.

I started to prep my monster for the eminent surgery. I still have the option of going PP or ceramic. I still also have an open mind about it. There is no real rush. So you know when you go to a fancy restaurant starving and they bring out the entree and it is a smaller than your appetite? That’s how I feel when I look at the cap comparison in this photo:

https://drive.google.com/file/d/0B0yvjhlC62G7aGVRZzYzLTVaUEU/view?usp=sharing

It just looks like something is missing :)
I bought 15 of these Vishay ceramics X7R from Mouser. They were really inexpensive.
Mouser also has a 250v PP Vishay cap that will work which is about $1.22 each.

Here is what will really get your grey matter baking....There are identical PP caps sprinkled on the current gain board yet ML did not make those cap changes mandatory. The current gain board is the heat generating furnace yet they don’t seem concerned that those PP caps are steadily being warmed. They only focused on the voltage gain board. Things that make you go hmmmmmm.

@imhififan :

Due to voltage rating of those caps? Are they working on same voltage or the caps working at lower voltage on current gain board?

I took it apart today to compare the caps on the current gain and voltage gain boards....they are the same. If I had to take a stab at why Mark Levinson chose to ignore the CG caps:
a) difficult to do it properly without desoldering all the output devices?
b) they are shielded from rising heat by the large red film caps?
c) input gain stages are more susceptible to RF?

Here is a pic:
https://drive.google.com/file/d/0B0yvjhlC62G7UUdweEVOeGtva0U/view?usp=sharing

Like I said I am no engineer and I know enough about this stuff to get it working if it is broken. I learn something new everyday and working on this type of equipment is very calming to me. I disappear in my lab trying to solve these little mysteries.

Thank you all for your responses so far!

@danpe1 

It has been so long since I have had the covers off this monster I cannot really remember the caps you are referring to. Where are the caps located? Since the original is a Sprague you could find a suitable replacement from the The Vishay/Sprague lineup. However these particular caps could be replaced with low ESR caps from Illinois Capacitors (I like IC caps and I know for certain there are a few spattered in my ML), Panasonic, Nichion etc. Admittedly the hardest part is matching terminal dimensions, not so much the value. Also you can go slightly higher with the voltage (ie 35v) with no bad side effects.