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Filter Capacitance

Hello Audiogonners! 

Perhaps this should be placed in tech talk - not sure.... 

In my own clumsy way, I'm building more understanding of amplifiers. I have no electrical training, save some very awkward swings of that hot gun that interacts with solder and a PCB, back in High School, about 400 years ago!

So, I see there are two players in the amp game that I do not understand, about which I am studying (more study of other pieces of the system, later): Filter capacitance and transformer size. Until a few days ago, I might have (entirely) mixed the two in my head... now it's only a partial mix... So, I'm interested to see, for example, that the Schiit Aegir has the following hardware, so far as is described on a Schiit-Euro site:

"The power supply is completely linear, with 600VA transformer, over 150,000uF of filter capacitance." 

That seems like A LOT of capacitance, and I'm totally confounded as to why this doesn't translate into higher output. As you know, the Aegir is described as 20 watts per channel into 8 ohms, and maybe 40 into 4 ohms. Seems like low wattage for all that transformer and capacitance weight! 

The Vidar runs the same size transformer but only 40,000uF of filter capacitance - 20K per side! AND it's a 100w into 8ohm amp - 200 into 4ohms. 

I own the STA200 by Nuforce and I cannot find anything describing capacitance, nor transformer size... I'm studying amplifier tech because I'm running into some difficulty when running the STA200 into my Moabs, at high volumes characterized by significant bass extension: think giant BOOM and you get the idea. Most of what I listen to is jazz/acoustic, so it's not a big issue until I move over to something that emphasizes boom, so I'm curious as to how the tech shows up in these applications...
listening99
12 responses Add your response
erik_squires
16,056 posts
 
The absolute maximum voltage is set by the output voltage of the transformer. We don’t actually know what this is based on the VA (Watts) of the transformer.

The absolute maximum voltage is usually a little over what the amp is rated at in watts at 8 Ohms. As impedance drops, current requirement increases until you exceed the power of the transformer.

What filter capacitance buys you is less voltage sagging while they recharge, and more current capacity, if the rest of the amp can support it.

You do quickly run into diminishing returns though, and filter capacity can become little more than bragging rights. :)

Class D amps usually run a switching power supply, which does not have the big iron needed for the transformer, so you wont' see them rated the same way.  They still need filter caps, as the class D amp needs a stable DC power supply, like the linear counter parts.
listening99 OP
172 posts
 
@erik_squires 

So, the output voltage is not directly tied to transformer size? And watts are also not directly related to voltage? 

If the amp is 80wpc, the absolute maximize voltage is about 80volts? 

The capacitance offsets limits in the ability of the transformer to recharge?

I have noticed that various companies will sell add on capacitance, boosting things up very high... I was struck at the inversion between capacitance and output (watts) between the Vidar and the Aegir, given the same power supply... 

I have appreciated your support of class D. I keep coming back to the class D option, in my mind,  but I'm just so happy with my STA200 (Nuforce). I just wish it had double the rated power...
erik_squires
16,056 posts
 
So, the output voltage is not directly tied to transformer size?


Right.  The VA gives us the power output of the transformer at maximum current draw, but any given VA rating can have different output voltages.  What matters is what is called the winding ratio.  For instance:

600 VA w/ 10:1  : Outputs 12 Vrms
600 VA w/ 5:1 : Outputs 24 Vrms

And watts are also not directly related to voltage?

For a load, they are proportional to the square of the voltage:

W = (V*V) / R

If the amp is 80wpc, the absolute maximize voltage is about 80volts?

Nope, not at all. The 80 Wpc is rated at 8 Ohms. It is really W(rms).  I probably will miss a step here:

Sqrt(80 Wrms * 8 Ohms) = 25.3 Vrms

Vrms to Vpk = 25.3 * 1.4 = 35 V

That + and -  Volts is what the power supply has to make available to the amplifier board, which is then going to swing back and forth between them to create the voltage at the speaker.

Assuming there were no losses (and there are always losses) the voltage the speaker would see would go from + 35V to -35V at maximum.

So, for an amp rated at 80 watts per channel an output winding of around 5:1 seems about right.  The VA we can estimate at a minimum should be twice this, so 160 VA. However, there are losses AND .... no speaker is exactly 8 Ohms. As the impedance drops, more current must be drawn, so at 4 Ohms we must draw at least 320 VA.


The capacitance offsets limits in the ability of the transformer to recharge?

Yes.  What comes out of the transformer and bridge rectifier is pulsing DC. From zero to some positive number on one side, and zero to some negative number on the other.  The filter caps smooth this out. More capacitance, the closer the average V is to the peak voltage, and the stiffer it will seem while playing music.
erik_squires
16,056 posts
 
One thing that is confusing is that what we call power amps are not power amps at all.

They are voltage amps. We rate them at the power they can emit without clipping at 8 Ohms, but this really is a convenience.

Truthfully most amps attempt to multiply the input voltage by 20 (~ 26 dB I believe), and so long as they do not exceed the power supply rail’s limits they will do so with a great deal of accuracy.

Here is a link that probably explains a great deal I’ve left out or misinformed you:

http://www.skillbank.co.uk/psu/

gs5556
1,234 posts
 
The Aegir and the Vidar both share the same transformer and other parts to keep production costs down. Not the ideal amps to make transformer vs power rating comparisons with.


millercarbon's avatar
millercarbon
12,531 posts
 
I'm curious as to how the tech shows up in these applications...

If you figure it out let me know. All the BS above falls apart (and I do mean all of it) the second you listen to a little 34 watt Raven or other good tube amp and realize it sounds way more powerful than you would ever guess based on tech specs. 

At best tech specs like capacitance correlate only vaguely and occasionally to what is actually heard. More often what happens is guys who memorized some talking points regurgitate every chance they get without regard to any actual real-world experience. Then because this happens so often it becomes common wisdom.

Well, it ain't wisdom. But it is common. 
listening99 OP
172 posts
 


 @millercarbon What then is the secret, if it's ALL BS? How does a person know if they are making a high current, high watt amp, or not? Clearly, companies are aware of their end products in some degree, comparing them to other offerings they have engineered and printing specs that correspond with relative differences connected to those specs. There is a relationship between the specs and the workings of these amps, as envisaged and contrasted in the lineup of a single amp manufacturer of numerous models, no?


millercarbon's avatar
millercarbon
12,531 posts
 
Well when you put it that way then yes there may well be "a relationship between the specs and the workings of these amps, as envisaged and contrasted in the lineup of a single amp manufacturer of numerous models". When you put it that way. Which is not the way I put it. Which was "All the BS above falls apart (and I do mean all of it) the second you listen."

Do you see the difference? You're talking measuring. I'm talking listening. That's the secret.
atmasphere's avatar
atmasphere
11,794 posts
 
How does a person know if they are making a high current, high watt amp, or not?
The first thing to understand here is that power, current and voltage have a direct relationship; 1 watt = 1 Amp x 1 Volt   That's called the Power formula, and like Ohm's Law, cannot be violated, unlike a speed limit :)

So right away, if you work the math, you can see that a 'high current' amplifier is a misnomer. You can read more about that here:
http://www.atma-sphere.com/Resources/Common_Amplifier_Myths.php
One thing about filter capacitance: the amp circuit has timing constants in it, usually a coupling capacitor somewhere, which rolls off at some low frequency that can be calculated. The power supply has timing constants too (it takes a certain amount of time to discharge the filter caps based on how much current can be drawn from them); for best results the power supply should always have a timing constant considerably lower than that of the amplifier, otherwise low frequency instability and/or intermodulations can result. You might be thinking 'what about amps that are direct-coupled input to output?' and its a good question. They will always have the ability to modulate their power supplies!

This is why you tend to see excessive amounts of filter capacitance in solid state amplifiers. Now its a pretty good bet that the amp won't see really low frequencies that will get it in trouble, due to the fact of timing constants rolling off in the sub-bass octaves upstream. But if there gets to be any noise or sag in the power supply, in a solid state amp that can be pretty audible as transistors tend to be more sensitive to voltage variations in the supply than tubes (generally speaking of course). So large filter banks are common in such amps.



millercarbon's avatar
millercarbon
12,531 posts
 
OP said:
I'm studying amplifier tech because I'm running into some difficulty when running the STA200 into my Moabs, at high volumes characterized by significant bass extension: think giant BOOM and you get the idea. Most of what I listen to is jazz/acoustic, so it's not a big issue until I move over to something that emphasizes boom, so I'm curious as to how the tech shows up in these applications..

I mock this stuff because it is so eminently mockable. But the fact of the matter is it is also nice stuff to know. You just need to keep straight that its nice stuff to know, and not drink the Kool aid and start thinking you can actually build a music system with it. You can't. You can only build a music system by listening and evaluating by ear. 

Look, some of these tech stories are really well crafted. For sure. Heck I start to believe them myself sometimes! Over the years though I noticed some things. Several tube amps would drive speakers just as loud as I wanted, and feel more powerful doing so, and have greater bass slam and extension, than some other amps that were on paper much more powerful. Smaller power supplies, less capacitance, more powerful sounding. 

Now listening99 I hope you are listening because I want you to listen to this. Someone gonna attack me for this on the basis of some technical reason or other. They will in doing so glide right past the fact it sounded better, more powerful, with greater extension. A lot of them know this to be true. Heck a lot of them have heard it, actually experienced it themselves. So they know, on some level, the pointlessness of the tech talk. Nevertheless they persist. Nevertheless they try and explain away reality. Just watch.

The technical reason for this, at least the one we are given, is bandwidth. The claim is made that transformers with greater bandwidth sound more open, dynamic, and powerful. So there is a technical explanation. Might be right. Then again, might not. 
erik_squires
16,056 posts
 
@atmasphere 

Even allegedly DC coupled amplifiers have servo controls which effectively limit the amp's low frequency bounds, no?


atmasphere's avatar
atmasphere
11,794 posts
 
Even allegedly DC coupled amplifiers have servo controls which effectively limit the amp's low frequency bounds, no?
It could be used in that way. If the amp is effectively limited from going all the way to DC then the power supply can't be modulated.
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