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#197629 - 02/23/08 10:01 AM Amplifier Capacitance ? Axiom amp versus my monos
SirQuack Offline
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Registered: 01/29/04
Posts: 13311
Loc: Iowa
In the research that I have done, it appears that capacitance is also important in an amplifier design, and in some respects more important than talking about all these "watts" available as most of us get hung up on. In addition, things like damping factor and current delivery are important.

In purchasing my Odyssey Monoblocks some time ago, one of the advantages over other amp designs I was looking at, was that each of my monoblocks has 120,000 microFarad capacitance, for a total of 240,000 uF for both of my m80's.

The new Axiom 1400-8 amplifier has 140,000 uF total for the entire 8 channel amp. Now, I am not totally up on digital versus class A A/B designs, so maybe the more efficient 1400-8 does not require as much capacitance?

I was just thinking that if it is able to route up to 1400+ watts to any given speaker, if needed but unlikely, than it would also require a lot of capacitance for reserve to be available when called upon?

This may be a question more suited for Ian, Alan, or Tom, as they are obviously more in tune with the design. I'm just trying to understand this part of the equation.

Thanks in advance, Randy
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#197630 - 02/23/08 10:10 AM Re: Amplifier Capacitance ? Axiom amp versus my monos [Re: SirQuack]
chesseroo Offline
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Registered: 05/13/02
Posts: 4733
Loc: western canada
I would like to hear a reply on this as well.
Indeed capacitance has been a major factor in how effectively some of the amps i've tested have played back speakers.
This big Coda 10.2 i tried once had enough reserve to playback for a second to two AFTER shutting the unit off (longer at lower volumes).
That's alot of reserve. I believe they had typically used about 120,000 uF in a Class A/AB amp.
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#197640 - 02/23/08 11:25 AM Re: Amplifier Capacitance ? Axiom amp versus my monos [Re: chesseroo]
Ken.C Online   content
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As a side note, my Audiobytes play for a second or so after shutting the unit off--more if it's turned up louder.
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#197691 - 02/23/08 07:31 PM Re: Amplifier Capacitance ? Axiom amp versus my monos [Re: Ken.C]
SirQuack Offline
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dang, I figured many people would have input on this one.

I'll give it a free bump. \:\)
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#197692 - 02/23/08 07:32 PM Re: Amplifier Capacitance ? Axiom amp versus my monos [Re: SirQuack]
Ken.C Online   content
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I'm curious, too, so double bump.
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#197713 - 02/23/08 11:33 PM Re: Amplifier Capacitance ? Axiom amp versus my monos [Re: SirQuack]
JohnK Offline
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Registered: 05/11/02
Posts: 10305
Well, Randy, with all this bumping going on and since I've finally read this, I'll point out that maybe some of the research that you mention has accidentally led into the land of the practitioners of voodoo audio. Watts are in fact the bottom line(often fewer of them than is sometimes imagined)and if a watt is deivered with flat response over the 20-20KHz audible range(or maybe more realistically 15KHz for most of us, as Ian among others has pointed out)and with inaudibly low noise and distortion, that's all that any amplifier can do; there ain't no more. All the other factors in amplifier design technology(damping factor, capacitance, slew rate, etc. etc.)lead to this end result.

Some of the material linked in the past here includes the capacitance discussion on Rod Elliott's Westhost technology site in the "Increasing capacitance" section and in the following "Major myth regarding capacitance" section. There are rules of thumb regarding the amount of capacitance needed(e.g., 100uF per watt)in the power supply section to smooth out the pulses of AC power enough so that the the PSRR(power supply rejection ratio)of the amplifying section can handle it without audible flaws. This and more precise calculations result in requirements in the tens of thousands, not hundreds of thousands of uF; more doesn't somehow become audibly better.

Another topic discussed here in the past was damping factor and the explanation by engineer and speaker designer Dick Pierce in Audioholics was cited. Bottom line is that anything in the double digits is fine. Just about any well-designed modern amp does this, except for some tube designs which have a factor around one or two and are obviously for this and other reasons not designed with the highest fidelity in mind. Damping factor specs in the hundreds or even thousands are seen on occasion, but are audibly meaningless; again, more doesn't mean better.
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#197740 - 02/24/08 12:34 AM Re: Amplifier Capacitance ? Axiom amp versus my mo [Re: JohnK]
pmbuko Offline
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Registered: 04/02/03
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Way to squash the capacitor party, John.



On a serious note, there is a common belief that a less than LARGE capacitance reserve might be completely depleted during loud and/or percussive passages, leaving the capacitors sucked dry and unable to deliver for the next passage. This isn't the case, as stated in the first article you linked, copied below:

"With large capacitors, the momentary current peaks created by the program material will not be of sufficient duration to discharge the caps to the full load voltage levels, so there is more voltage available on a more or less consistent basis. This equates to more power for transient signals, and lower ripple voltages the rest of the time."
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#197755 - 02/24/08 04:07 AM Re: Amplifier Capacitance ? Axiom amp versus my mo [Re: pmbuko]
fredk Offline
axiomite

Registered: 12/06/07
Posts: 6960
Loc: Canada
So John, to put this in laymans terms...

If the amp reliably delivers the advertised watts at a flat freqency response with acceptable THD+N then you are good to go for all that other stuff mentioned above?

Fred
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#197765 - 02/24/08 10:16 AM Re: Amplifier Capacitance ? Axiom amp versus my monos [Re: SirQuack]
jakeman Offline
aficionado

Registered: 10/03/05
Posts: 851
Loc: Toronto
 Originally Posted By: sirquack

In purchasing my Odyssey Monoblocks some time ago, one of the advantages over other amp designs I was looking at, was that each of my monoblocks has 120,000 microFarad capacitance, for a total of 240,000 uF for both of my m80's.

The new Axiom 1400-8 amplifier has 140,000 uF total for the entire 8 channel amp. Now, I am not totally up on digital versus class A A/B designs, so maybe the more efficient 1400-8 does not require as much capacitance?




Randy. Its important to keep in mind that you are talking about 2 very different amp designs with the Odyessey monos being a Class AB design and the Axiom being based on a Class D design. That presents entirely different sets of tradeoffs and design issues of which capacitor size and array are actually a small part of it.

The articles cited above are very good for describing issues involving Class A or Class AB amps but people should be cautious about making generalizations when discussing other classes of amps.

Class A and AB amps are the most common and can provide excellent audio performance but their achilles heal are that they are energy inefficient which is why they need fans or heat sinks which adds to their weight and size . Typically 40-50% of the energy gets lost as heat which puts alot of stress on internal circuits and components. That energy inefficiency also necessitates the need for somewhat larger caps to deal with energy loss. Some would suggest that larger caps provide better transient performance but I'm on the fence on that one since so much depends on how the circuits and components have been configured from amp to amp.

With Class D switching amps like the Axiom, they are very energy efficient, with the A-1400-8 being close to 95% efficient. The switching circuitry requires very fast voltage response and gate time in order to keep distortion to a minimum, so more smaller caps carefully laid out would be a better design choice. The dynamic power handling of the Axiom amp also means that the power supply rails have to turn on a dime and dump power quickly into the channels demanding more watts. Again smaller caps would be a better choice.

Finally there are size constraints. The A-1400-8 is a small technological marvel and there aren't that many places to fit large caps.
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#197769 - 02/24/08 11:14 AM Re: Amplifier Capacitance ? Axiom amp versus my monos [Re: jakeman]
SirQuack Offline
shareholder in the making

Registered: 01/29/04
Posts: 13311
Loc: Iowa
Thanks everyone for your input.

Jakeman, I figured we were not talking apples to apples comparing Class D to a Class A-A/B and that not all theories apply to all situations.

I'll read some more on this topic as well. I've noticed that most of the well known reputable amp companies out there design their amps with higher capacitance, especially if they are type A or A/B or A-A/B. So what you say makes sense about efficientcy. I'm sure they design the amps that way for a reason and are not just doing it to be snake oil salesman.

Heck, Tom C from Axiom himeself used to design huge monoblocks back in the day, and I highly doubt he would be creating amps with extra capacitance unless it was required.
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