Specialists > Audiologists
Amplifier caps, what they do, and where they are...
HAL:
Yes, the power supply filter caps are usually the large can ones in the chassis. Usually physically bigger in SS vs tube gear.
Dave gave a great intro and cap materials for construction vary greatly from film to electrolytic styles. Large values 25uF+ are usually electrolytics. The ESR, DF and DA are very different for different styles.
There was a very good set of 3 articles in Audio magazine in Feb,Mar, Apr 1980 done by Jung and Marsh that describes caps well and what the types are for. Been a long time, but a great intro to understanding.
Cover of the first article in Audio:
https://americanradiohistory.com/Archive-Audio/80s/Audio-1980-02.pdf
Folsom:
To call PSU caps "filters" is kind of silly. The basic meaning of "filter" is that it "filters" 60hz out. It has nothing to do with all the other harmonic crap that they "filter" to some degree or another.
The rectifiers give you the newish type waveform, and the black lines from peak to almost peak are the capacitors discharging (the device uses that power being discharged). The capacitors charge to essentially the peak voltage output of the rectifiers (which is whatever your transformers outputs). Between the peaks the capacitors then release their charge, as they're trying to maintain the voltage that they charged to. Because you aren't seeing voltage drops at a given cycle, you're effectively seeing DC - hence "60hz" has been filtered out.
Today I feel no need to call them filter capacitors. For me they are power capacitors because they maintain power. When they discharge they let out voltage and current. The bigger they are, the slower their voltage drops (the slower they discharge relative to the total amount of energy they hold) and the more current they can provide (the more used the faster the voltage drops). One giant one is usually not possible. The more capacitors you have to get the desired amount so your voltage doesn't plummet, but multiplying capacitors also increases some high frequencies bypassing being attenuated by the capacitors.
Proximity matters, too, because the ESR/ESL changes. So sometimes you have parts that make noise in a circuit and you have to put capacitors near them to help filter that. The farther away the more ESL (inductance) and ESR (resistance) so the less effective they will be. You can buy capacitors that are low ESR, effective around the audio band for example, or low ESL that is more effective in the RF range. Proximity is more important for inductance, as the resistance/inductance is still typically low in the lower frequencies than RF.
When it comes to quality there's subjective and objective differences. Technically many attributes Dave mentioned are desirable but sometimes have consequences. For example ringing can happen when a cap is very stiff in measurements (low everything but capacity), or rather it may not be any good to dampen existing ringing. Capacitors are subject to vibrations, and voltages above 12v may excite them to vibrate - the measured result isn't typically easy to see in audio gear but in other equipment sometimes is very obvious and some manufacturers sell low vibration caps.
Choosing the right capacitor is related to what you need it to do. For example before an active filter like in my 7297, I use low impedance (ESL) capacitors because then the filter is more successful in eliminating HF garbage. But I don't necessarily like the way they sound when directly feeding the amplifier circuit . They aren't doing any frequency passing duties so if they absorb some (DA) it really isn't critical. The problem with a lot of discussions on capacitors is people will say something like "DA is what matters" but that's because they expect electrolytics (which have more DA than film) in the signal path. Basically any wise audio manufacturer has mostly eliminated lytics in the small level signal path (stuff your IC's carry). And we don't have to worry about how much total capacitance we need these days, it's cheap and small till you start getting into 100w classA type stuff.
I expect everyone to have slightly different preferences on what their gear sounds like with different capacitors, all of good quality. Unless we're talking about Chinese off-brands (not all Chinese) lytic capacitors, most are of good quality. But some manufacturers have their own secrets for making audio specific capacitors that measure like, well, pretty much all capacitors. When it comes to film they're almost all really high quality and exceed electrolytics in all ways except capacity to size ratio. But sometimes film capacitor sizes mixed with lytics don't do so well and cause ringing - luckily basically no one does that anymore.
When it comes to carrying signal (coupling) you can measure distortion of a film capacitor but it's so low that there isn't really any indicator for why they sound so different. It's just kind of up to you to decide. But I can say my favorite measures the lowest... but it may not sound right in some stereos that I haven't totally gone through and modified everything :rofl: . Certain capacitors measure higher 3rd harmonics, but what's funny is it still shouldn't be audible. However, despite that these measurements are barely exist... I can tell you polyster (measures highest in 3rd order harmonics of film caps) masks a lot of information if it's in my phono preamp. They can however sound exciting sometimes, too. If your stereo is sounding kinda dull the extra "too low to hear" distortion can bump up some excitement. And if your stereo isn't really revealing, you may not notice a non-loss of information. Different people listen for different things... And high resolution is a double edged sword because if it doesn't sound just right it's distracting so getting it isn't always what you want unless everything is kinda perfect.
I know a lot of audiophiles are obsessed with higher quality capacitors for speakers, but I'd say in general a lot of the cheaper capacitors that aren't repackaged lytics, will measure about the same as most expensive ones. That or some expensive ones might measure bad. You're getting *different* sound not necessarily better. For example I think the Jantzen's sound pretty good in some stereos because they make a lot of information readily available. But in my stereo they sound wrong. I think metalized have their own character that tends to do what I described, and it's useful in many stereos; but if you put it in one that has much high levels of resolution then it sounds weird. It's really all personal preference, and you have to pay more for different sound since most manufacturers of general electronic capacitors make pretty much the same capacitors for general use. So I'll say when you open a speaker up and look at a crossover and see some inexpensive capacitors I don't immediately think it's a bad thing, I think either they wanted to save money by not marketing expensive parts that may not be an improvement or it was the best voiced capacitor to the designer. Now what gear they used to design the speaker with will probably reflect that choice. Here's an example, Magico uses a lot of Mundorfs cheaper capacitors despite the fact that they don't have budget constraints - they are the sound they want.
It's hard to have right and wrong answers with capacitors when they aren't too low in capacitance for their use, when you're talking about something that is subjectively appreciated. Even if they can make better measurements, the reality is unless it stops some kind of error in digital equipment or run-away oscillations in analog gear, and appear to be working then, right & wrong is based on what you want to hear.
Nick B:
Interesting and differing points of view as to description and beyond my ability to get into such specifics :roll:
I think over the years since I got into audio...late 80s. probably the most expensive caps I’ve had are the Black Gates which were an upgrade to my custom Audience preamp. That’s my guess and I’m also curious what the most expensive caps are nowadays. Dueland comes to mind.
Nick
S Clark:
Thanks for that write up. Very informative, and only one question. You mention DA.... dielectric absorption I'm guessing.
S Clark:
Ok, let's see if we can go through a simple tube amp schematic. Cleverly, I typed in "Simple tube amp schematic" into Google and picked something that might work. I'm seeing 3 caps. I assume the 3300 uF is the power cap (filter cap). What about the other two? If there's a better schematic, post it and I'll delete this one. Well, crap. It's bigger than I thought. Can we shrink it?
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