Amplifier Capacitance

Okay, naive question of the day: except for cost and perhaps size limitations inside the casing, why don't manufacturers go bonzo large on capacitance? For instance, I'm thinking of replacing the caps in this damaged Hafler DH220 I've got and, while I probably won't find any that fit, I started to wonder why limited myself to two 18-19k cans? Why not 50k, 80k, even 100k if I could fit it? (I doubt any of those would, but you get the drift.).

much can be written about this but I'll try to provide a compact answer ;-0
large amounts of power supply does not come for free. you'll need a high(er) current transformer to supply the current to charge up those huge caps in rapid time so that the power supply voltage does not sag when a big slug of charge is demanded by the power amp electronics. Large power supply caps are huge reservoirs of charge, true, but they take time to recharge. Often this recharge time is too slow to keep pace w/ the music hence you'll often find smaller caps (like 10,000uF) in parallel w/ the huge power supply caps. these smaller caps are there to provide the charge for sharp transients thereby taking the load off the large power supply caps. The smaller caps are more nimble in terms of recharging hence they work in complimentary w/ the larger power supply caps.
So, note the transformer current delivery capability before you go "bonzo" on increasing the caps. if you did, you'll saturate the transformer core & bring in distortion into your power supply damning your Hafler sonics. Over time you'll also damage the power transformer.
Also, the larger the power supply cap, the larger the power-on current surge. My amp has a lot of power supply capacitance & it takes 37A in-rush current upon power on!! :-o. The amp designer has to consider this when designing the power supply wiring & choosing the AC fuse(s).

Capacitance seems like once of those things that should really make a big diffence in amp performance, no? In fact, shouldn't it particularly help offset a somewhat weaker power supply as well?

yes, power supply capacitance can make a big difference in the power amp's power supply ONLY if the power supply is designed to take advantage of the larger capacitance amount.
Huge amounts of capacitance cannot hide the flaws in a weaker power supply. In fact, large capacitance will quickly bring a weaker power supply to its knees as it'll expose its weakness quickly.
hope that this sheds some light on the matter....

Thank you for a marvelous reply! This explains quite a lot to me, especially using a combo of larger and smaller caps for nimbleness as well as keeping a large reserve. I forgot that I had seen this before and wondered why.

I still have some questions though. How does the amp draw power from the PS and caps? In other words, are the caps charged then remain as a reserve while the PS powers the amplification? Then the caps are simply drawn upon during the difficult moments that exceed the PS capabilities? OR is it something more like a cycle of the PS charging the caps and the caps supplying the power for amplification. As such, as power is pulled from the caps it is replaced by the PS and during the intense moments the amp can use as much power as is available in the caps while the PS recharges the caps? Or am I completely off?

I am also wondering why my idea for "bonzo" caps would be harmful for the PS, but I have an idea or two why this might be the case. I had assumed that extra large caps would allow a modest PS to deal nicely with heavy transients. OTOH, having expended that charge, this modest PS would have to recharge my "bonzo" caps AND continue to supply amplification power. This is why it would cause damage isn't it? It would essentially push the PS to it's limits far too often?

I'm an engineering newb (actually, newb would be generous) but I've been trying to learn more lately, so I apologize if some of these questions seem remarkably stupid. Of course, it is nice being 'remarkable'. ;)

Thanks, Aaron

Aewhistory, Look at this chart: http://www.powerint.com/en/community/papers-circuit-ideas-puzzlers/circuit-ideas/careful-rectifier-diode-choice-simplifies-and-

Top graph shows amp's supply voltage. Capacitor is charged from transformer thru rectifier only in short moments of time when voltage goes up (bottom graph is charging current). Picture is greatly exaggerated - in reality line representing voltage is almost straight (very small ripple) and capacitor charging happens in very short high current "spikes". Amplifier current demand from capacitors might be constant (class A) or vary a lot with the music (class AB). Very large electrolytic capacitors are characterized by capacitance (opposition to change in voltage), inductance (opposition to change in current) and ESR (effective series resistance) that represents pure resistance. Obviously we want a lot of capacitance to store energy but we don't want inductance since it is opposing rapid current changes. Best solution to lower inductance would be to use less inductive capacitors (expensive) or to use more of small capacitors in parallel (capacitance increases, inductance decreases, ESR decreases).

Bombaywalla mentioned two problems with a lot of capacitance - rush current and over-stressing power supply. Initial current will be higher and last longer to charge larger capacitance resulting in blown fuse or damaged rectifier. Rush current could be limited by soft start circuit - basically a temporary current limiter but amplifier has to be designed for that.
Second part is a little more difficult to explain. Imagine perfect capacitor with a lot of capacitance, no inductance and no ESR. What will be the shape of the voltage on the upper graph? - almost straight line with very, very small ripple. Charging time of capacitors will now be very short (only when voltage goes up) while charging current spikes will have higher amplitude (to deliver same average power) limited only by transformer and power line. This large current spikes might damage rectifier or overheat transformer. Again, it is a little more complicated with transformer since average amp's power is technically the same. The problem is that core of transformer will be heated with high frequency component (iron losses) of narrow spikes, while copper windings will be heated (copper losses) more since, in spite of the same average value, RMS value of current (representing heat) will be much higher.
There is also possibility that ripple current (charging current) peaks might now be too high for caps you selected. Anything can be done (carefully), but linear power supply is not that simple to design properly.

excellent reply Kijanki.

Aewhistory, if you read my reply & Kijanki's you should have all your questions answered. :-)

There is one missing element.

That is that the power supply has a timing constant. That is to say, there is a certain time period that will elapse if the power transformer is unplugged, where the voltage will sag to a certain point while the supply is under load.

Then there is a timing constant in the amplifier itself. This is the -3db point of the amplifier.

If the amplifier has a -3 db point that is a frequency lower than that of the timing constant of the power supply, then the amplifier can modulate the supply, which results in IM distortion amongst other things.

This is why an amplifier should never be direct-coupled from input to output! Otherwise, the amount of capacitance needed to get the timing constant of the supply low enough goes towards infinity.

This is why an amplifier can 'motorboat' (repeated thump) if a filter capacitor fails in the supply- the timing constant has become so high that the amplifier exhibits low frequency instability.

I apologize if I went a little too esoteric here, but there is obviously more to it than just inrush currents and the like. If something is not clear let me know.

This is more than I could have hoped for--in a good way--and I am extremely grateful! It will take me a little while to digest this, but there is a point mentioned by Atmasphere that I might be able to relate to. I recently bought an NAD 2600 with a disclosed possible flaw--a "thump" when it turns on and off. I got it for about $100, so it think it was a reasonable price. Amp plays great, but that on/off sound isn't so nice. So I had posted about this to confirm this was an issue and not just "the way this amp is" and someone confirmed this was a flaw. So my question is: is this the 'motorboat' thump to which you are referring Atmasphere, or is this a different problem?

It is nice to be able to start to diagnose these problems. This is one of the reasons I've started to try to learn about amps. I'll never be a deisgner by any stretch of the imagination, but for years I've enjoyed working with my hands building computers and doing odd repairs on things like motherboards, replacing caps, etc. So I can do it but there is a tremendous difference between "take out part, replace with same thing" and "take out part, replace with something different". At least to me it is way different. Anyway, I've got three amps that require attention that I may venture a repair on: two I bought that way for cheap, cheap (an Adcom GFA-5400 & an NAD-2600) and a Hafler DH220 that was bought working but damaged in shipping (I didn't actually want three projects, but owell, c'est la vie).

Okay, back to digesting this info.... Aaron

This is why an amplifier can 'motorboat' (repeated thump) if a filter capacitor fails in the supply- the timing constant has become so high that the amplifier exhibits low frequency instability.

I recently bought an NAD 2600 with a disclosed possible flaw--a "thump" when it turns on and off. I got it for about $100, so it think it was a reasonable price. Amp plays great, but that on/off sound isn't so nice. So I had posted about this to confirm this was an issue and not just "the way this amp is" and someone confirmed this was a flaw. So my question is: is this the 'motorboat' thump to which you are referring Atmasphere, or is this a different problem?

the turn on/off thump in your NAD amplifier is a totally different issue compared to what Atmasphere is talking about.
The thump that you are hearing during on/off is the inrush current charging the capacitors. this (huge) inrush current will create a spike on the supply rails of the power amp which in turn will create a voltage spike on the power amp output terminals (speaker binding posts). This in turn will create a thump in your speakers. The issue with the NAD is that it never had any output protection circuitry to avoid 'thumping' the speakers (if the NAD had output protection circuitry that was malfunctioning, that amp would not play any music). This is indeed a design flaw. Back in the 1980s, I & some extended family members did own integrated amps that always thumped the speakers. I guess back then output protection circuits were not always put into cheaper amplifiers - just the way it was back then!
The thump that Atmasphere is talking about is when a power supply cap fails (bad part/age/heat, etc) & the power supply is now faulty. Then, the power amp can motorboat/thump which is a sign of low freq instability.
Two different issues but quite similar symptoms.
The thing to note about the motorboating is that the amp will thump every time it's playing low freq content.
The turn on/off thump occurs just once at turn on & once at turn off - no thumping while playing program material.

If the amplifier has a -3 db point that is a frequency lower than that of the timing constant of the power supply, then the amplifier can modulate the supply, which results in IM distortion amongst other things.

Ralph, let me see if understand what you wrote above. I'm getting confused because w.r.t. the amplifier you are using frequency & w.r.t. the power supply you are using timing constant. Freq & timing constant are reciprocals of each other.

So, what you are saying is that for motorboating to occur the time constant of the amplifier has to be higher ("If the amplifier has a -3 db point that is a frequency lower than...") than the time constant of the power supply. Correct?
So, basically, the bandwidth of the power supply should always be lower than the bandwidth of the power amp. Correct?

Class AB - there is not fixed "time constant" since discharge time depends on loudness/load. We could take discharge time at max volume and compare it to reciprocal of -3dB bottom frequency of the amplifier but it wouldn't make difference to me since I don't listen at full volume. We could argue that smaller modulation at lower volumes will be "proportional" to level of the signal but once we draw less than max current, power supply voltage modulation at low frequencies becomes compensated by amplifier (since it is regulated).

Class A - does not apply since amplifier draws the same current even at DC output.

I'm still working on some of this conversation, but could someone explain "time constant" to me? I'm assuming this is something like what it sounds, but I'm not sure. It sounds almost as if amplifier power supplies work like CPUs in the sense that they have clear cycles and clocks that designers must take into account to keep everything running. I don't know if the analogy holds, but it sounds similar to me.

I'm starting to realize just how much I DON'T know about power supplies. Wow. And I am the knowledgable one among my friends. Eeeeek!

Aewhistory, Time constant is amount of time it takes to bring voltage to 63.2% of desired value. For instance applying 10 volts to 1000uF capacitor thru 10ohm resistor will result in 6.32V on capacitor after time equal 10ohm x 1000uF = 10ms. Same would apply to discharging from 10V to 3.68V (10V-6.32V). This time is called time constant RC.

Instead of saying "Time necessary to charge capacitor" we say "time constant". It is shorter and more precise. In our case it just means amount of time to have significantly lower supply voltage (discharge supply capacitors) when playing very low frequencies very loud. We want this voltage steady since any variations might compromise amp's operation (output affected by supply voltage changes).
We are looking at small changes in supply voltage (not 63.2%) but using terms like "time constant" or "-3dB frequency" just to have some reference point. From that we can, if necessary, recalculate exact percentage changes at particular frequency.

Kijanki, thanks for that explanation, it was excellent. Do you teach electrical engineering by any chance? You, bombaywalla, and Atmasphere have done a magnificent job explaining very esoteric (to me) subjects. This is difficult to do, especially when speaking to a layman. I've taught college history now for 12 years and it can be daunting getting people to relate to human history, so I've always wondered how people in the sciences make their subjects more approachable and relatable. Discussing matters here certainly gives me an idea how this can be done as I've never encountered a group with the combination of technical savvy and possessing the ability/willingness to explain as here at Audiogon.

Teaching? No, Almarg would be much better at this. He explains things with much more clarity. History was always difficult for me.

but once we draw less than max current, power supply voltage modulation at low frequencies becomes compensated by amplifier (since it is regulated).

Kijanki, I don't understand this statement. Power amp power supplies are not regulated (I don't see a feedback loop around the power supply). The power amp itself has minimal feedback around itself for sonic purposes (as you already know) & this feedback is for the music signal & not for any vairations of the power supply. The amp is counting on the power supply to be essentially DC. So, any power supply modulation will modulate the output voltage signal (AM-AM modulation/distortion).
I'm afraid that I'm not seeing how the power supply modulation at lower/bass freq is compensated by the power amp. Thanks.

Bombaywalla - Amplifiers are line regulated. It means that amplifier supplied from 40V and set to produce 5V output voltage will still produce 5V output with supply lowered to 35V or increased to 45V. There will be small error because regulation is not perfect but it is in order of one percent or less. There is an easy way to test it - just set your amp at moderate listening level and then reduce line voltage from 110V to 90V. Your amp will play at the same level. You could measure it with test tone and voltmeter.

Amplifiers are line regulated. It means that amplifier supplied from 40V and set to produce 5V output voltage will still produce 5V output with supply lowered to 35V or increased to 45V.

Kijanki, I can believe this BUT I do not think that this is due to "line regulation".
If the power supply rail is 50V (or 45V) & the power amp is outputting a 5V signal, there is plenty of headroom for the output transistors (& the rest of the power amp stages) such that if the power supply rail goes up/down by 5V, the output voltage will not change. I believe that this is the reason that the power amp will continue to output the voltage its being asked to.
If one keeps on lowering the power supply rail, one will come to a voltage where the BJTs will saturate/the MOSFETs will triode & the output voltage will drop as the devices go into the linear region.
I still believe that all power amp stages are very sensitive to the cleanliness of the power amp power supply rail.
thanks.

Speaking to the original poster's question . . . yes, there are side effects to unduly increasing the filter capacitance, even though there are many "high end" amplifiers that are designed without regard to them.

Keeping focused on conventional solid-state amps like your Hafler . . . these are indeed simple unregulated supplies, and (ignoring startup conditions) the main filter capacitors have three functions: the first and most obvious, is to smooth AC ripple voltage on the supply rails. The basic equations for this aspect of their design are of course widely documented.

The second role (and what most of the discussion seems to be about) is to dampen short-term changes in supply voltage, as caused by variations in amplifier current draw with the level of the music, and periodic fluctuation of the mains voltage. The key word here is "dampen", because when combined with the losses upstream of the capacitors (mainly the transformer), a resonant circuit can form with a peaked response, causing such variations to get larger.

The only way to avoid this situation is to model the transformer losses with reasonable accuracy, and calculate or simulate the power-supply response to very low frequencies . . . as it is very difficult to sweep an amplifier with a stimulus through the milli-Hertz region and measure the power-supply's reaction. Atmasphere mentions the possibility of motorboating, and this is indeed a concern with piling on the capacitance on a traditional C-L-C-filtered tube amp. But it's not nearly as much of an issue on your Hafler, as it affects both the positive and negative rails symmetrically (common-mode), to which most solid-state amps have excellent very-low-frequency noise rejection. Also, the capacitors are effectively acting in series, so the change in capacitance is somewhat less than it seems like it "should" be when you're dealing with the volumetric constraints of squeezing bigger cans in an existing chassis.

But the third role of the main filter capacitors is what's usually overlooked: they are in series with the output ground, and return the speaker current back to the supply. This is true even in a fully DC-coupled amplifier - although it may appear that the transformer center-tap is the supply return, in practice the transition from the filter caps to the center-tap occurs at a frequency several octaves below the audioband. Indeed, many "DC coupled" amplifiers will operate quite happily with their center-tap disconnected, so long as there's something in place to keep them from actually having to amplify anything approaching DC.

So what does this mean? Electrolytics of course have well-documented distortion mechanisms as they approach low-frequency rolloff, but since they're very effectively enclosed within the global feedback loop (at the frequencies where feedback is highest) this is unlikely to manifest itself in the output to virtually any degree. But they also have significant inductance, which has the effect of reducing the effectiveness of global feedback as frequency increases. Put another way . . . as frequency rises the main filter caps get lossier, thus a signal voltage starts to appear across them . . . and the amount of error that the feedback must compensate for increases.

This can be especially injurious as the amount of available global feedback is also usually falling at 6dB/octave as frequency increases, due to the amplifier's frequency compensation scheme. If the amplifier is operating in class B (including the class B region of a "class AB" amplifier) the speaker current manifests itself as a pair of half-waves (i.e. the voice of a DALEK) across each respective filter capacitor. And while these half-waves are symmetrical and supposed to cancel each other out, big electrolytics have the widest tolerances of any electrical component . . . so they won't be matched anything close to perfectly . . . the remaining difference is one more little mess that increases as the amount of global feedback to clean it up decreases.

So getting back around to increasing filter capacitance . . . the other thing that comes with increased capacitance is increased inductance. My question would be instead: if there's room and budget and the capacitance is already more than sufficient . . . why not increase the voltage rating, to improve the longevity, ESR, and inductance characteristics instead? A good designer chooses these components just like they should choose all the others . . . by finding the BEST ones to perform all aspects of their role in the circuit - not simply by grabbing the biggest/fanciest/prettiest/costliest thing they see, and sticking it in place.

Nice job by all in this thread!

Bombaywalla, Yes, power supply should be clean but I was talking about modulation of power supply voltage by varying load that amplifier presents. Low frequency will definitely do it (biggest current) and high frequency will do it as well (inductance of the caps). It will be reduced by amplifier's PSRR (power supply rejection ratio) but will still affect the sound. Atmasphere was talking about very low frequency signals causing big sags of supply voltage that bounces back (motorboating). Limiting low frequency response of the amplifier, as he suggested, will help but I can imagine scenarios where it will still happen. Let's play "Kodo Drums" (Shefield) - enormous amplitude of low frequencies repated once a second. That will do it as well. It becomes obvious why good amps have so many caps in the power supply.

As for power supply being clean - the biggest offender there is 120Hz ripple proportional to load. At low sound level we cannot hear it because ripple is very low (light load) but at high sound levels when ripple is strong we cannot hear it either because sound is too loud. It is almost like jitter that is undetectable unless you play louder. There is also high frequency component related to charging current spikes and also limited "softenss" of rectifier diodes (late switch off, fast snap back).

That's why many designers started using switching power supplies instead. Modern SMPS switch at zero voltage/zero current, produce high frequency noise that is easier to clean than 120Hz, have line and load regulation plus protection against overcurrent or overtemperature. Jeff Rowland uses 1MHz SMPS in his newest creation model 625 (class AB) amplifier. There are some other benefits size being perhaps the least important. One of them I can appreciate in my Rowland 102 amp. It works from 85-265VAC or DC voltage to almost 400V making it less susceptible to overvoltage and completely immune to DC on the power line.

Also many Rowland amps have active power factor correction that makes amplifier "look" like resistive load loading power line evenly during sinewave instead current spikes near the peak: http://jeffrowlandgroup.com/kb/questions.php?questionid=144

why not increase the voltage rating, to improve the longevity, ESR, and inductance characteristics instead?

The price of higher voltage, low ESR, low inductance caps is perhaps too high. I checked once site that sells Hypex class D kits. Power supply module was by far the most expensive because of BHC slit foil low inductance electrolytic caps. People try to remedy inductance of large caps by placing small non-inductive caps in parallel. It lowers caps reactance at high frequencies but also creates parallel resonance circuit that will ring under rapid current draw. There is a reason design engineer avoided it.

Bombaywalla, sorry for my tardy response; obviously you got it right.

Kijanki, I thought you might find this interesting. Years ago I worked on a popular tube preamp. I found that it was a copy of the Marantz 7 circuit, sans tone controls.

This preamp tended to have a problem- which was if you played a bassy track too loud, it could thump and do some weird stuff- especially if you were able to watch the woofer cones.

Anyway, it turned out that the timing constants of the preamp section went lower than that of its power supply. I spoke to the designer (a Brit) who was not at all happy to hear of my diagnosis, despite admitting to me seconds before that the bass instability was a problem.

The take-away is this is not an issue limited to amplifiers...

Ralph, that's very interesting. I just looked at Jeff Rowland Capri preamp - the upper limit is 350kHz but bottom is only 10Hz. It could be direct coupled with servo (integrator) in the feedback set to 10Hz. In any case, he did it for a reason. My small Rowland 102, as well as big 625, have both 5Hz bottom -3dB but all his preamps have bottom at 10Hz. He also uses SMPS in preamps. I know he does in Capri, not sure about others. He might feel that limiting rumble or power supply effects should be done as early as possible. Additional 5Hz bandwidth in the amp improves slope, adding second pole, without affecting much overall 10Hz bottom limit.

That's interesting. I would have thought the Roland stuff to go lower. We set our poles at 2 Hz so there would be no phase shift at 20Hz. We use servos too- in fact its part of our patent. It was a trick making them be stable with cutoff frequencies that low. One thing is sure- you can't use a servo to define the LF -3db point (the result is that every component in the servo circuit will have an audible artifact, and that is something a servo should never do).

If the rumble is going to be there, whether you have 2 Hz or 5 Hz cutoff is not a lot of difference as long as you have good LF stability. If there is instability rumble will drive you crazy!