This is an interesting point, Andy.
A related follow up question:
For a bass player in a typical bar band scenario (too much equipment plugged into a single AC circuit in a building that has less than ideal electrical wiring to begin with) would the larger capacitance of an amplifier with an analog (line frequency) power supply perform better during musical peaks than an amp with a switching power supply, (which if I understand it correctly) draws more directly from the AC line?
Not sure if this question got lost in the shuffle but hoping someone can shed some light on it.
If @agedhorse is too busy, maybe
@rag,
@Bob Lee (QSC), or
@Passinwind could share their thoughts on this scenario?
Missed the original post so do not know the full context but for me the only way to go is that the more capacitance in the amp power supply the better the sound
Two basic reasons:
A generic explanation
First is that every amp runs on direct current (DC) whereas the mains is alternating current (AC) - very different system.
The amp power supply rectifies the AC into DC, and consists of two parts - the rectifier and the filter.
Rectified AC without filtering is useable but produces massive hum in the speaker.
The better the DC quality the better the amp performs. DC quality depends primarily on the filter effectiveness
Impurities in the filtered DC will end up in the audio output and affect the distortion component. An unstable rectifier system can also produce instability in the amp. The most common shortfall is audible hum.
Full-wave rectifiers produce better performance than half-wave
The filter capacitors in tube amps with tube rectifiers were small - maybe 50 uF at most - because the tube rectifier cannot sustain the charging or inrush current at first switch-on without reducing tube life or reliability. This is not related to the "standby" switch function, which is separate to the rectifier system.
Small capacitors result in plate voltage sag at higher power outputs and peaks, causing a reduction in power output when it is most needed. Great for lead guitar amps but not so good for bass. On this point amps with cathode bias perform better than fixed bias amps, because there is less change in plate current between minimum and maximum signals.
Quality amps use a filter choke as well, but to reduce cost and weight most guitar amps do not include a choke to the power output tubes - but most include a small choke to remove hum from the driver stage. This is a choice between cost and weight.
Tube amps operate in a high-impedance system so the smaller cap values are adequate for the purpose.
Tube amps with solid state full-wave bridge rectifiers are far superior for bass because the power supply has a lower internal impedance and results in less voltage drop at higher power outputs - ie a more linear voltage supply to the amp itself. Note the power output with no signal - ie between notes - is zero, so the amp might go from zero to full power in an instant.
The real advantage with solid state rectifiers is that filter capacitors can have high values like 1,000 uF or more without any more complexity in the rectifier system.
The result is that a solid state full-wave rectifier system costs no more to include in the design but offers much better performance - the quality of the DC is better, the hum is lower, voltage sag is negligible and reliability is excellent.
But even if 20,000 uF is included in the PS there will still be remnant ripple voltage - i e distortion and spikes - so inclusion of a choke still remains essential for high quality DC, although rarely seen in commercial amps
Solid state amps are a very different proposition
The conventional Class AB or B amp requires substantial DC current - many times more than a tube amp of similar rated power output
The current draw varies from a low value to maximum in response to the signal amplitude (volume or loudness)
So the filter capacitor, which also acts not only as a rectifier but as a power storage device, must have a relatively high value. Typical Peavey amps of the 80's and 90's used values of about 5,000 uF in the positive and negative rails.
These were engineered to deliver good quality performance - eg the 400 watt MKIV series - a fine bass amp
But amps of that era still exhibit the same voltage/current sag when driven hard
This is where the issue becomes complex
If the bass player likes a flat response then commercial amps are fine but what if you want to deliver a really solid clean bass tone in the LF range at high power ?
What if when so doing you want to have really great "sustain" or "resonance" ?
(This is dependent on the bass guitar itself because a dead bass is a dead bass. Sustain can also be achieved with a suitable pedal - ie reverb or echo).
There are two options: The first is to use a higher rated amp - maybe four times higher than that needed for a flat response sound. Of course the speakers need to be capable of handling this extra power.
The second is to add capacitance to the PS filter
My Peavey MKIV now has more than 200,000 uF in each of positive and negative rails and sounds awesome. No other mods to the PS other than a higher rated fuse and it is very reliable. This value of capacitor is way over the top but I did it because I can. 10 to 20,000 uF would be a great improvement to the average amp.
My other school of Peavey amps sound great but not quite in the same league. For example, my Peavey XRD680 300W PA amp has an extra 44,000 uF in both rails - very good on bass too.
To summarise, the filter cap acts a store of energy to the amplifier. It follows that the larger the store the more can be drawn from it.
It also follows that the higher the rated power of the amp the more energy storage is required.
But the capacitors will supply power only when the input AC voltage is less than the output DC voltage stored in the capacitors - which is only some of the time. So there is an averaging of power in and power out.
The capacitors charge when the input AC is higher and will discharge when it is less.
That means the AVERAGE power drawn from the mains DURING A NOTE will average out over the mains frequency and is directly proportional to the amp's audio power output, but when large filter caps are included they will supply extra energy for dynamic or peak signals - ie "punch" or "grunt" over and above the same rectifier's capability with small caps - but only for a brief period of time until they discharge
Sustain will last for maybe 30 seconds or more with this system.
But that's not all
The filter caps are in series with the amplifier output to the speaker. Capacitors have internal impedance which interacts with the audio signal. The smaller the cap the greater the signal attenuation.
This is true for both tube and solid state amps
Consequently for real deep bass the rule is the larger the caps in the audio power circuit the less attenuation is evident at low frequencies. Unfortunately amps with switch mode power supplies do not have this feature because the high frequency rectification system does not need large capacitors.
A practical explanation of example of the above discussion is to look at pics of the Crest Audio CA range, which show the commercial application of the concept of large filter caps. Crest obviously understand the concept.
crest amp internals - Google Search
Therefore the answer to the question is that there is no practical difference between a Class AB and Class anything else in terms of its effect on the mains other than Class D amps are claimed to draw less power overall because they are more efficient at any given power output
The important engineering principle to observe is that the total power drawn by all of the devices connected to any mains system must not exceed its rated power capacity. Therefore it is advisable for a band to add up all of its power needs and check against the venue's supply system capability.
The mains power requirement for amplifiers is usually displayed on the back panel and is expressed in VA or watts.
If exceeded, the circuit breaker is designed to automatically operate to protect the system and switch off the power - something we do not want in the midst of a performance
