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
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
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?
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
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.
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
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.
You guys are getting nuts. Please look at the original capacitor value again:

0.01uF or 10 nF

Most boutique caps don’t even come that small. 0.1uF is usually the lowest they go. I can pretty much guarantee it is not in the audio path.

To put this in perspective, a tube preamp output cap is usually at least 2 to 4 uF. That’s 200 to 400 x larger than this de-coupling cap. Also, the ceramic is probably 10-20x physically smaller than any boutique cap would be.

Before doing any more arm chair upgrade discussions I would strongly suggest finding out what the cap actually does by looking at a schematic. For all I know this is part of the LED driver

Sheesh.

Best,


E
Sheesh is RIGHT E!

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.





If you absolutely can't stand the idea, get Cornel Dubelier PP.
Post removed 
@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?


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
generatorlabs
  Mark Levinson recommend replacing them with Ceramic X7R .01uF @ 200v

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.

Cheers George 
@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.

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 found some radial C0G
http://www.mouser.com/Passive-Components/Capacitors/Ceramic-Capacitors/_/N-5g8m?P=1z0wpzoZ1z0wgtrZ1z...


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


By the way, important to note that thermal stability ant temperature/longevity are not the same thing.

You can have a very stable cap that has a low thermal rating.

Also, who cares. These are RF filtering bypass caps. You could use 1 or 4 and the audible effects would be 0.  A cap with 20% variance in value is just fine here.
@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!