Author Topic: Amplifier caps, what they do, and where they are...  (Read 7873 times)

Offline S Clark

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Amplifier caps, what they do, and where they are...
« on: September 16, 2019, 04:58:55 PM »
Caps in a crossover are something I'm familiar and comfortable with... but amps???  Filter caps, coupling caps, power caps... what exactly is the difference, where are they in the circuit, and where does quality come into play?
There are several of you guys that are amp guys.  A little help please. 
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Offline P.I.

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Re: Amplifier caps, what they do, and where they are...
« Reply #1 on: September 16, 2019, 11:17:40 PM »
This is a subject that is simple in concept, but a great big ball of fuzz in reality.

The nuts and bolts are these:

Filter caps/power supply caps provide ripple control after the rectification stage in an application.  This can be initial power filtration after the power transformer and rectifier or downstream for local ripple and noise rejection in whatever piece of gear.  Coupling caps do exactly tha. They couple one stage in a piece of gear to another.  This is done for several reasons.  Most common is to block any DC component from effecting the music signal (an AC signal) or to provide isolation between stages that operate at different electrical potentials.

That is the very bare bones reply.  I’m sure Jeremy (Folsom) and Steve will have a lot more to add.

It’s late and I didn’t want you to wake up in the morning with nothing to read.

Next I’ll dive into -

DA - dielectric absorption

DF - dissipation Factor

ESR - equivalent series resistance (a bit of a misnomer)

Materials

Lots more to come from people here!
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Offline S Clark

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Re: Amplifier caps, what they do, and where they are...
« Reply #2 on: September 17, 2019, 05:41:32 AM »
Thanks for a good start, Dave.  I think I understood, mostly, some, maybe.  So the initial power (in the US, 120+V) comes into the power transformer, gets bumped up to ???V, contains miscellaneous crap, and goes through a filter/power cap.  So are these usually the bigger value caps that so commonly dominate the landscape on the circuit board? 
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Offline rollo

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Re: Amplifier caps, what they do, and where they are...
« Reply #3 on: September 17, 2019, 06:49:14 AM »
  All I know is when I upgraded to Duelund CAST output coupling caps in my Consonance Mono blocks my world changed. Then power caps oh my goodness.


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Offline S Clark

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Re: Amplifier caps, what they do, and where they are...
« Reply #4 on: September 17, 2019, 07:03:49 AM »
  ... when I upgraded to Duelund CAST output coupling caps in my Consonance Mono blocks my world changed.

charles
I bet it did.  Your billfold got a lot lighter!   :rofl:

But this is one reason for the initial question.  I'm trying to have a better understanding of what the caps are doing, so that I can upgrade more intellegently. 
« Last Edit: September 17, 2019, 07:06:38 AM by S Clark »
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Offline HAL

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Re: Amplifier caps, what they do, and where they are...
« Reply #5 on: September 17, 2019, 07:46:31 AM »
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


Offline Folsom

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Re: Amplifier caps, what they do, and where they are...
« Reply #6 on: September 17, 2019, 01:33:59 PM »
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.

Offline Nick B

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Re: Amplifier caps, what they do, and where they are...
« Reply #7 on: September 17, 2019, 03:29:55 PM »
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.
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Offline S Clark

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Re: Amplifier caps, what they do, and where they are...
« Reply #8 on: September 17, 2019, 03:38:58 PM »
Thanks for that write up. Very informative, and only one question.  You mention DA.... dielectric absorption I'm guessing. 
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Offline S Clark

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Re: Amplifier caps, what they do, and where they are...
« Reply #9 on: September 17, 2019, 03:59:52 PM »
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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Offline Folsom

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Re: Amplifier caps, what they do, and where they are...
« Reply #10 on: September 17, 2019, 05:45:29 PM »
Thanks for that write up. Very informative, and only one question.  You mention DA.... dielectric absorption I'm guessing.

Correct.

Offline Folsom

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Re: Amplifier caps, what they do, and where they are...
« Reply #11 on: September 17, 2019, 05:54:08 PM »
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?

You're correct about C3. Look at what C1 is connected to, the center pin of the RCA. What's on the center pin?

C2 I'm not completely sure on because I don't know that tube. Maybe I'll look it up in a minute if someone else doesn't chime in.

Offline S Clark

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Re: Amplifier caps, what they do, and where they are...
« Reply #12 on: September 17, 2019, 06:15:19 PM »

You're correct about C3. Look at what C1 is connected to, the center pin of the RCA. What's on the center pin?

C2 I'm not completely sure on because I don't know that tube. Maybe I'll look it up in a minute if someone else doesn't chime in.

Ok, C1 is connected to the incoming + signal... but what is there to clean up at that point? I guess V2 is a rectifier tube, making V1 the power (amplifying)  tube.   

The ECL82 (V1) is some kind of triode
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Offline Folsom

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Re: Amplifier caps, what they do, and where they are...
« Reply #13 on: September 17, 2019, 08:37:32 PM »
C1 doesn't clean anything. It's the only path for signal. What it can do is block DC from other gear. The size matters because if it's too small then bass will be reduced. The combination of the impedance of the amp and the size of the capacitor determine what we call the corner frequency. For calculations and a more comprehensive explanation this site is good. For a power amplifier you want a -3db of 4hz or lower (some say 7hz, but that's proven to be too little IME).

Since the voltage is only a little bit (often under 2v at max) the capacitor voltage really doesn't matter, particularly with film capacitors because they're usually no less than 60v, if not hundreds.

C3 "cleans" when the frequencies on say the V+ rail side are attracting electrons, by getting them from ground before the amplifier's circuit (bypassing if you will). It prevents these frequencies being pulled through the amplifier.

Because the amplifier is in the path to ground from the signal (C1), the frequencies pulling electrons from the source pull them through the amplifier, hence it isn't "cleaning" them.


Offline S Clark

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Re: Amplifier caps, what they do, and where they are...
« Reply #14 on: September 18, 2019, 08:56:32 AM »
So is there usually incoming DC from the signal input that C1 needs to remove?
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