Hifi amp and polystyrene capacitors - Page 1
Forum section: electronics / repair

author: klausman
author_rank: Regular Contributor
timestamp: May 20, 2025, 03:46:23 pm
content: So I am working on a Luxman L-215 hifi amp (service manual attached). It's not fundamentally broken besides the power indicator being gone (I'll probably ask a separate question about that as well).However when I opened the case I was greeted by quite a few caps thatto melook like they are polystyrene:And a close-up of the top right corner where there's a whole bunch of them:This is already making me a bit concerned: some parts in there have failed, especially the elec caps are all out of spec. Not enough to break anything (yet), but I'd like to refresh at least some of them, to give the amp a longer life. Problem is: as far as I know, polystyrene capsreallydon't like heat and they drift prmanently when soldering irons come near. They may even later crack due to heat stress. So I thought that since this amp is now easily 30+ years old, modern caps would be as good as these potentially-already-failed polystyrene ones. I'd probably replace them with C0G ceramic caps and be done with it.Now here's the problem: the service manual does not mention, e.g. the four caps C109a/b and C110a/b.Onepair of polystyrene caps is mentioned (C121a/b), but everything else seems to be Polypropylene, Mylar, Ceramic or Electrolytic.Any idea what's going on here? And yes, I know about the scorch mark where D112 used to be.

author: TimFox
author_rank: Super Contributor
timestamp: May 20, 2025, 04:05:33 pm
content: Yes, those caps appear to be polystyrene.The service temperature for PS is lower than other films (85oC).In general, C0G/NP0 ceramic and polypropylene/PP film are suitable replacements for now-obsolete PS.However, if not overheated, PS does not have a high failure rate, and the construction is not hygroscopic, so I would not replace them unless they show symptoms.Mylar/polyester is inferior to PS and PP.

author: klausman
author_rank: Regular Contributor
timestamp: May 20, 2025, 04:14:59 pm
content: Quote from: TimFox on May 20, 2025, 04:05:33 pmThere are three reasons why I am consdering removing them:provocative, er I mean preventative maintenanceAs is clear, D112 got quite hot (it shorted out and ran ~21V directly to ground for who knows how long. This is also the reason why the power indicator is not working: it was on that circuitI'll be soldering near those PS caps, and I fear I may either drift one or several immediately, or create a future failure (cracking).The C0G caps for replacements would probably less than a tenner total, so I am probably going to go that route.Why these caps (that are clearly in the silk screen!) are not mentioned in the service manual at all is a mystery. I just hope that whoever put on those PS either did so at the factory/manufacturers direction or really knew what they were doing. Or maybe this is a later revision of the amp, and the extra caps were added then? But then the SM has gaps in the numbering scheme. Puzzling.

author: wraper
author_rank: Unknown
timestamp: May 20, 2025, 04:18:35 pm
content: Quote from: klausman on May 20, 2025, 04:14:59 pmIf you have nothing to do, then replace all carbon film resistors first. There's a higher chance they drifted out of spec.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 20, 2025, 04:21:07 pm
content: Unless treated with chemicals PS capacitors usually last very long and are reasonable stable. So no need to replace them unless really damages. Some visible crackling has usually little effect on the capacitors.In an audio amplifier the exact capacitor is usually not critical. So PS, PP and polyster capacitors can be exchanges in most places if the capacity and voltage rating is OK.The capacitors are relatively small, likely to trim the frequency response or compensation of amplifier feedback loops. There may be different amplifier versions. It could be both an older or newer version that has the capacitors - either for cost reduction of performance improvement.p.s. : The capacitors near the inputs are likely there to keep RF interference out and limit the input BW.

author: Gyro
author_rank: Super Contributor
timestamp: May 20, 2025, 04:42:07 pm
content: Luxman wouldn't have gone to the extra expense (hand sleeving and fiddly insertion) of Polystyrene capacitors unless they had some beneficial effect on the perceived sound quality (whatever that means) of the amplifier, they would just have stuck some mylar in there instead. If they are there to trim the frequency response, they tend to have been available in closer tolerances than other film caps but it's doubtful whether that would be sufficient to have an audible effect though.Polystyrene caps are film and foil capacitors rather than metalized film, so there is no danger of having lost capacitance, the only failure mechanism is being melted and going short. Pretty unlikely even in a warm amp. They have thin leads and have been stood well off the PCB to prevent issued during wave soldering of the board. If they're not shorted then they're fine.Luxman put them there for a reason, perceived or otherwise, rather than doing it to minimise BOM and assembly cost. [Edit: It's hard to second-guess them on the reason.]

author: TimFox
author_rank: Super Contributor
timestamp: May 20, 2025, 05:18:12 pm
content: In an audio amplifier, one location where capacitor value/tolerance may be critical is the RIAA equalization network. Otherwise, there can be a measurable distortion from some capacitor dielectric types such as X7R or polyester when in the signal path, especially when the voltage dependence of the capacitance is significant.Polystyrene, polypropylene, and C0G/NP0 are preferred for both signal path and equalization purposes.

author: David Hess
author_rank: Super Contributor
timestamp: May 20, 2025, 05:53:02 pm
content: Quote from: Gyro on May 20, 2025, 04:42:07 pmPolystyrene might have been cheaper than polypropylene at the time, but see below.QuoteI think metalized polystyrene capacitors exist, but foil capacitors have a big advantage in audio applications. The self healing mechanism of metalized capacitors makes them noisy in audio application; you can hear the self healing as hisses and pops.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 20, 2025, 06:14:02 pm
content: PS capacitors were available with relatively low tolerances ( like 1 or 2 %). At the time when they were popular, the PC capacitors were not significant more expensive then other film capacitors, especially for small values. SMD was not yet a thing and pic an place machines still a dream.

author: klausman
author_rank: Regular Contributor
timestamp: May 20, 2025, 06:19:58 pm
content: Quote from: Gyro on May 20, 2025, 04:42:07 pmHere's a realization: maybe my amp is old and the service manual is new. Maybe Luxman realized some components were not strictly needed, and I have a new-enough service manual that they are not mentioned anymore (hence the gaps in component numbering schemes). Obviously, that doesnotmean I can just willy-nilly remove components, but at least it explains the service manual oddity.

author: Gyro
author_rank: Super Contributor
timestamp: May 20, 2025, 06:23:42 pm
content: Quote from: Kleinstein on May 20, 2025, 06:14:02 pmYes, I was talking more about assembly cost than component cost. Just look at the PS caps with the need to thread on the black sleeving and hand insert into the PCB (with those skinny leads) compared to the Mylar and ceramics on the board. It's hard to know off hand whether the board was partially machine stuffed or completely hand stuffed but the PS caps were definitely done by hand.

author: Gyro
author_rank: Super Contributor
timestamp: May 20, 2025, 06:39:06 pm
content: Quote from: klausman on May 20, 2025, 06:19:58 pmQuite possibly so. Polypropylene would certainly be easier to handle and not significantly inferior to Polystyrene in a signal or RF circuit [Edit: Film and foil (FKP) anyway].

author: Kleinstein
author_rank: Super Contributor
timestamp: May 21, 2025, 08:33:06 am
content: The back shlieves on the PS caps could be part of the capacitors. So added by the capacitor manufacturer.There are other capacitors near the OP-amp that are also not suitable for automated insertation. So I thing the board was manually populated.The capacitors look like they are 100 pF, a value that would more naturally be a ceramic type.

author: ondraN
author_rank: Regular Contributor
timestamp: May 21, 2025, 11:58:00 am
content: I've been doing repairs for quite a while, but I've never seen this type of capacitor fail. They are stable and moisture resistant and don't have the microphone effect like ceramic capacitors. They were mainly used in preamplifiers and RIAA corrections.

author: mawyatt
author_rank: Super Contributor
timestamp: May 21, 2025, 03:24:31 pm
content: Can't recall we've ever seen a Polystyrene or other "pure film" cap fail either (at least within the past 40 years), except for the infamous RIFA types across the mains, and obvious when they fail! The older (~70 years) oil-paper types used in tube amp days often failed tho, remember replacing many of those as a kid working my way thru school.OP might better be served by replacing the Electrolytics and maybe some of the resistors (especially if they are the old "Carbon Comp" types.Best

author: Gyro
author_rank: Super Contributor
timestamp: May 21, 2025, 04:38:59 pm
content: Quote from: Kleinstein on May 21, 2025, 08:33:06 amI wondered that too (whether the sleeves were pre-applied) but there are a couple of caps on and below the little sub board where bare lead is visible above the sleeving.There are a couple of larger PS caps at the right edge of the board, I suspect that they might be part of the RIAA equalization.

author: David Hess
author_rank: Super Contributor
timestamp: May 21, 2025, 08:55:42 pm
content: I have seen a few different film capacitor failures.In power applications, a film capacitor can short, melt inside, and "extrude" itself, sort of like an epoxy dipped solid tantalum capacitor spitting out molten metal.In audio applications, metalized film capacitors can become noisy from self clearing, which is why film-foil capacitors may be preferred. Today C0G and NPO capacitors are excellent, but maybe they can become noisy also?Cheap metalized film capacitors sometimes self heal losing a large part or all of their capacitance, or maybe something attacks the metalization.