Capacitance?

[Deleted]

What is capacitance and how is it's rating on an amp interpreted? I am considering powering my surrounds with either a used Adcom that has a rating of 120,000 uf or a new UPA-200 that has a rating of 40,000 uf. Both have similar power, but have way different capacitance. What difference does it make if any? I'm trying to decide on an amp before Emo's sale ends. Below is a partial reprint of an earlier post.

Adcom GFA-545 Specifications:
Power Output: 100W/channel into 8 Ω, driven channels, with < 0.04% THD
Frequency Response: 10 Hz - 20 kHz (-0.25 dB)
S/N Ratio: > 110 dB
Input Impedance: 100 kΩ
Capacitance: 120,000 uf
Auxiliary AC outlets: 2 for other peripherals

[garbulky]

Uhm Adcom. 120,000 capacitance is nice I'm not a 100% certain what capacitance does, but I believe it translates to fast power delivery.

[knucklehead]

Think of capacitors as a short term electricity storage device. For short term high current flow to the speakers where the transformer cannot keep up with demand the capacitors are utilized to prevent the amp from clipping.

Here's everything you ever wanted to know about condensers/capacitors - and probably more:

en.wikipedia.org/wiki/Capacitor

[bobbyt]

Capacitor = reserve charge that can be dumped very quickly as high current. More of it means more buffer to draw down during heavy bass hits that exceed your amp's continuous current ability.

They're sometimes called "stiffening capacitors", because they serve to stabilize voltage during heavy current demand instead of letting it sag.

I do find the microfarad ratings silly. In car audio, you can have .5, 1, 2 farads (at 12 volts), but in home audio they brag about how many millionths of a farad. 120,000 microfarads = 0.12 F.

[wizardofoz]

They also act as ripple reduction (to smooth out the rectified AC) and are often bypassed with much smaller values to address different frequencies. Over capacitance can be just as bad as under. Not all capacitors are the same either. Internal construction and materials will also make a difference.

[KeithL]

Everything that's been said here is true - but also somewhat irrelevant. Capacitors do indeed provide filtering (to remove ripple) and energy storage (to prevent the power rails from sagging under heavy loads). The important thing that everybody misses, though, is that all of this depends on the other circuitry in the amplifier.

In other words, a given amplifier must have enough capacitance TO WORK PROPERLY WITH THE REST OF THE CIRCUITRY IN THE AMPLIFIER; and whether this is true or not will be obvious by the measured performance of the amplifier. For a given amplifier design, there will be a minimum amount of capacitance required for it to perform at its best. Having less than that will hurt performance, but adding extra beyond that point won't help it, and may indeed even degrade performance. While amplifiers of higher power in general require more capacitance to perform well, that is indeed a generalization.

Unless you're designing an amplifier from scratch, there's not much point in second-guessing the engineers who did. (Capacitors aren't especially expensive; only a poor designer would fail to use the proper amount to get the job done.)

Likewise, bypass capacitors are used to aid in the suppression of high-frequency noise, and are only useful with certain other types of capacitors, and in certain circumstances. Adding them when they aren't indicated serves no useful purpose.

Jan 10, 2013 1:42:49 GMT -5 wizardofoz said:

They also act as ripple reduction (to smooth out the rectified AC) and are often bypassed with much smaller values to address different frequencies. Over capacitance can be just as bad as under. Not all capacitors are the same either. Internal construction and materials will also make a difference.

[audiohead]

Adcom makes nice amps 120,000 ufs is nice.

[tme110]

While it's could be nice to have some extra 'capacitance' in an amp's power supply, I'm not sure I'd make a purchasing decision based on it. It's just not something you'd normally be comparing when shopping for an amp (assuming the amps in question are similarly competent designs).

[Jim]

Jan 10, 2013 3:03:25 GMT -5 KeithL said:

Unless you're designing an amplifier from scratch, there's not much point in second-guessing the engineers who did. (Capacitors aren't especially expensive; only a poor designer would fail to use the proper amount to get the job done.)

I really like your statements. Many people shop by comparing capacitance, thinking that more is always better.

I think you covered some really good misconceptions about capacitance.

Thanks!

[donh50]

Basic electronics: I = C * dV/dt or current equals capacitance times the change in voltage divided by the change in time. Now, most basic power supplies use a full-wave bridge that recharges the capacitors to their peak value 120 times a second. Between those peaks, the capacitors must supply enough current to keep the voltage from changing "too much". How much is "too much" is related to a lot of variables, including the voltage headroom of the amplifier's power (voltage supply) rails and the output power. 100 W average into 8 ohms requires 28.28 Vrms or 80 Vpp and 3.54 Arms or 10 App. Chances are the amplifier uses rails with at least 5 to 10 V margin, maybe more. If the amplifier's rails were +/-50 V (100 Vpp, 10 V/side over 80 Vpp) and we allow say 2 V drop on each side for the output transistors and biasing that means the supply rails must not drop below 80 + 2*2 = 84 Vpp, a drop of 16 V. Plugging that drop into the first equation with 16 V drop and 10 A current recharged at 120 Hz (dt = 1/120 = 8.33 ms) then C ~ 5200 uF. Probably a lot less than people might expect. Lower voltage rails means more capacitance is required, natch.

The UPA-200 is actually 125 W/ch or 250 W total into 8 ohms (rated average power). That is 3.95 Arms = 11.18 App at an output voltage of 31.62 Vrms = 89.44 Vpp. Assuming a 100 Vpp rail and again 4 V needed across the outputs now yields C ~ 14,200 uF. There are two channels so you'd need at least 28,400 uF. Still quite a bit less than the 40,000 stated, though of course I do not know what the actual supply rails are.

This does not take frequency of the output into account; it only shows how long the power supply can put out the required current before the output begins to drop.

No promises I did the math right; it's late for me! - Don