| KevinLee |
Hello All,
Is there an advantage to using one type over the other in feed back circuits or in a CD player output stage? What differences can one expect sonically?
I know that it is easier to get polystyrene in larger values and that silver mica tends to cost a bit more.
I have the choice of these two caps when doing upgrades and have next to no experience using these two types of caps.
Any suggestions or experiences appreciated.
Cheers
KevinLee
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| Romy |
I am a person who is VERY far from screwing with electronics but “accidentally” I have explored exactly this qestion within one of my phonostages with a full RIAA in feedback. I used the different types of polystyrene, different types of polysterol, the different type of micas, two type of air caps and so on, so on and so on… Ironically the cheapest 12c genetic no-name mica killed absolutely everything with VERY strong margin….
So…
The Cat |
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| mcp |
IMO polystyrene is neutral whereas silver mica is very revealing. It can be difficult to work with sometimes as it brings out the harshness in some designs. But when you get it right, the music sings. It has that sparkle, especially the highs.
In my amplifiers, I use silver micas exclusively for feedback and miller comp. For hf limit on input, polystyrene or silver mica. |
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| KevinLee |
Thanks for the input so far,
I just swapped about 8 caps in my integrated amp to mica from polystyrene and look forward to hearing the difference.
Do silver mica caps have a long break-in period?
Thanks
KevinLee:) |
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| haldor |
Silver mica caps biggest claim to fame is how stable they are (ie their characterists don't change with age/temp etc) . If anything is not going to require "break in" it will be silver mica caps.
Phil |
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| sam9 |
What exactly is the function of the cap? That makes some difference. In any case, audio signal cannot pass through any siver mica cap I'm aware of. The highest value is something like .01uF or less and that essentially blocks any audio signal. Generally to pass full audio bandwidth you need a value of 1uF or higher.
The point of all this id that siver mica or polystyrene can have an effect of the quality of the audio signal, but indirectly. Perhaps they are used as power supply bypass or possible compensation caps for an opamp.
So it's a least in part a question of "what function do they perform in the specific circuit?" and not merely "what flavor of cap do you like?" |
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| andy_c |
| quote: | Originally posted by sam9
(...) In any case, audio signal cannot pass through any siver mica cap I'm aware of. (...) |
Ahh, but you didn't consider frequency compensation and the Miller effect. It's not uncommon for a power amp VAS to have a low-frequency voltage gain of -10,000 (if it's cascoded and has a high-impedance load such as a MOSFET driver). When the mica cap is connected from input to output of such a stage in its compensation role, the capacitance seen by the input stage is multiplied by 1 minus this gain (Miller's theorem), resulting in a very large effective capacitance value. It's not uncommon for the open-loop -3 dB frequency of the amp to be not only within the audio range, but even below 20 Hz for some designs. So thanks to Mr. Miller, the mica capacitor is very much in the picture. |
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| sam9 |
True enough, I was thinking mostly about possible functions in a CD player per the orginal post. Unless it's something exotic that uses discrete opamps, I don't think Miller capacitance is likely to be of concern.
In the case you mention, it remains unclear to me if factors other than the capacitive value and the bandwidth of the cap are really that important. Despite being unclear, I use silver micas for this purpose as I don't think any of the "polys" have enough bandwidth to be sure c/l gain goes negative before the accumulated phase shift reaches 180 deg. In principle, an NPO cermaic would do as well but since there are only a couple needed per amp why not take the "Caddillac" so to speak? |
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| AKSA |
Silver mica is definitely the go.
I've tried all sorts of caps in this role, and it has profound effect.
Cheers,
Hugh |
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| Circlotron |
If the voltage across the cap *varies* as in an active filter, (with a coupling cap, the voltage across it should not vary if it is big enough, so "cap effects" are a bit suspicious :dodgy: ) then you want to use a cap that doesn't change it's value with applied voltage otherwise it will introduce nonlinearities. With ceramic caps, NPO = pretty good, X7R = awful, Z5U = tragic. I did an experiment once with a Z5U, and normally the energy in a cap increases as the square of the voltage (double the applied voltage = 4x the stored energy) but these Z5U's decreased their capacitance at such a rate that the stored energy was *directly* proportional, not a square law thing. :scratch:
Anyway, one of silvered mica's claims to fame is very low losses at high charge/discharge frequencies. e.g I have many times in the course of work seen a Cornell Dubilier cap 20x20x8mm handling 2000 VA's @ ~450kHz wile only getting slightly above skin temperature. That kind of treatment sends most other capacitors to cap heaven VERY quicky. :devilr:
Whether micas has any useful audio properties I don't know. One things for sure though, if you wanted a cap that would still be useable 10,000 years from now, mica would be a good bet. Very stable material. |
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| mcp |
Here's a simple experiment where any forum members can try. Take a power amp, replace the feedback cap with a ceramic and listen to the amp. Take note of the mids and highs. Then replace it with a silver mica.
BTW make sure the caps are the same values as the original. |
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| AKSA |
Graham (Circlotron),
Interesting info on the silver micas. I like 'em.
Very nice likeness on your avatar!
Cheers,
Hugh |
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| sam9 |
| I think I recall from somewhere that the principle application of silver mica caps is in thr RF industry. That would seem consistent with Circlotron's post. |
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| richwalters |
Nice to hear Silvered Mica still in active audio despite being far more expensive than polystyrene. Despite most of the "plusses" going to both types, I've repaired too many crowded amp chassis where users have complained of flagging power output leading to tube meltdown. PLEA.... don't solder polystyrene direct to the tube sockets i.e a-g2 via damper resistor for stability where a KT88 is blazing above in UL class A........the heat will eventually lead to cap failure and tube meltdown.
Check the upper temp ratings of polystyrene.......some types are quite restrictive,whereas mica excels.
Now do the sums...we've had 35°C ambient this summer, some polystyrenes upper limit is only 70°C, that allows 35°C rise on a tube pin quite likely.att'n:
:hot: |
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| haldor |
Might also have something to do with the fact that silver mica caps with larger than 1 nF are just about impossible to find. That would tend to limit their use in audio.
Phil |
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| jean-paul |
Strange enough I can get Richard Jahre 32.4 nF 125 V silver mica caps more easily than I can find polystyrene caps ...
This reminds me of someone on this forum that claimed all low voltage silver mica's have to be tested before being used as reliability seems to be an issue. When I don't forget I'll search tomorrow for that.
IIRC it had something to do with the way the leads are connected to the mica ( mechanical stability ). When sealed with resin the caps should be better in that aspect. |
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| SY |
I've used both silver mica and polystyrene caps in tube preamps (for RIAA eq and RF suppression) and power amps (compensation cap). If there was any sonic difference, it escaped me. Reliability points strongly in the direction of silver micas. Never lost one, whereas I've replaced bunches of polystyrenes that went to join Jesus.
If you want to be ultra-cool, use the old flat-pack silver micas with the six dot color code. |
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| jean-paul |
| quote: | | I've replaced bunches of polystyrenes that went to join Jesus. |
Didn't know He was into caps too.
Did the polystyrenes fail from overheating or what ? I never had one that failed till now ( knock knock on table ). I used NSF of german origin mostly. Never liked the english ( forgot the brand, sorry ) polystyrenes with the ultrathin wires as they suffered from overheating quite fast.
To make things even more unclear here were polypropylene caps from Siemens sold as polystyrene caps. Even a trained eye couldn't see the difference. Fortunately they were OK both soundwise and technically. |
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| SY |
| It's a combination of heat and voltage, I'd suspect. I'll bet the rated WVDC is degraded quite a bit in a 50-75° C environment. |
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| jean-paul |
| I only used them in relative cool ( both meanings ) solid state equipment, maybe that's why they didn't fail more than other caps. I had my share with electrolytics in Quad amps, but that's another story. |
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| KevinLee |
Hello,
I just replaced a few ceramic feedback caps in my integrated amp with silver mica caps.
The difference was not small.
I used to play this greatest hits album by Lenny Kravitz and its sound was very fatiguing on my system. The sound is now very articulate and the album is easy to listen to .
For me this is good.
Cheers
KevinLee:) |
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| mcp |
Hi Kevin
Glad to hear you like it.:nod:
You may want to have a look at the input. There should be a cap of small value to ground (ranging from 100p to 470p, sometimes even to 1n). If it is a ceramic, replaced it with a silver mica or polystyrene.
| quote: | Originally posted by SY
Never lost one, whereas I've replaced bunches of polystyrenes |
I had polystyrenes failed on me before. That could be due to using it at the limits of WDC.
| quote: | | If there was any sonic difference, it escaped me. |
It could be on the speaker side. Often speakers are calibrated at 1m/1w for flat response. So, most tweeters are padded down resistively. I find padding doesn't just attenuates, but dulls the music. Like losing harmonics.
My approach is not to pad the tweeter and build the speaker from there. I find it so much more "lively". The only component I pad is super tweeters, and ever so slightly. As for 1w/1m flat response, I abandoned that long time ago. It should be restricted to near field monitors in recording studios only. |
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| SimontY |
Hi,
Is it easy to locate feedback caps in an amp - would one need a service manual? Are they typically ceramic types?
mcp,
We seem to have strayed slightly off-topic here, but what resistors do you reccommend to minimise quality loss from a tweeter? (I only need 1ohm in my current ones)
Cheers,
-Simon |
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| SimontY |
Oops, kinda forgot my main point!
I replaced two mylar caps in my cd player output with silvered mica, actually it was a slightly different value to 'tailor' the treble roll-off. The sound was only slightly changed, it got more relaxed, but that could be more treble roll-off than cap type...
After reading this thread I think I will replace the rest with silvered mica. I was going to use polystyrene as I thought these might be better, but as I've started with mica ones, I may aswell finish with them...
-Simon |
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| mcp |
NFB Caps: If you are not familiar with amplifier designs, I'm afraid it can be quite hard to tell.
Ceramic nfb caps: Normally in lower cost amplifiers.
Tweeter Resistors: Normal 5W ceramic if I need to pad and less than 1 ohm. |
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