DIY Class A Amp

[captainbeefheart]

How is level of power possible, given the limitations of a typical home outlet at 115 V and 20 amps, or max VA of 1840 VA (115 * 20 * 0.8) ? 1200 watts delivered to the speaker would require 1200/1840 = 65% efficiency.
Does this amp take a 230 V outlet or a 30-40 amp breaker?

Wattage is wattage, it always stays the same:

Watts = I * V

Since you have 20 amps at 60v rms which is 1200 watts convert to house voltage on the primary of the power transformer:

1200 = 120 * x

Solve for x

1200 / 120 = 10

Your outlet only needs 10 amps. The average house branch circuit is 15 amps so you are fine. Have a look at the fuse rating on the amp and see what it is.

 

[captainbeefheart]

The amplifier also stores energy in the power supply so when there are transient demands from music it doesn't always need to come straight from the wall, the capacitors provide the instantaneous power reserve.

 

[MRC01]

Wattage is wattage, it always stays the same:

Watts = I * V

Since you have 20 amps at 60v rms which is 1200 watts convert to house voltage on the primary of the power transformer:

1200 = 120 * x

Solve for x

1200 / 120 = 10

Your outlet only needs 10 amps. The average house branch circuit is 15 amps so you are fine. Have a look at the fuse rating on the amp and see what it is.

Wattage is the same on input & output only when efficiency is 100%. Class A amps are only 25% efficient; let's assume class AB is 50% efficient. 1200 watts to the speaker must draw 2400 from the wall, which is 20 amps at 120 volts, and that's assuming a power factor of 1.0 (ideal conditions), so it would probably have to draw more than that from the wall.

 

[captainbeefheart]

Wattage is the same on input & output only when efficiency is 100%. Class A amps are only 25% efficient; let's assume class AB is 50% efficient. 1200 watts to the speaker must draw 2400 from the wall, which is 20 amps at 120 volts, and that's assuming a power factor of 1.0 (ideal conditions), so it would probably have to draw more than that from the wall.

Yes but we are talking program material and the short lived peak power comes from the capacitance.

Have a look at the amps primary fuse/circuit breaker rating and that will tell you all you need to know. You can't really just go by an efficiency number and do conversions because it's more complex than that.

In many cases I have seen large power amps not hold up to their rating on continuous sine wave testing due to the design being for program material and only providing the power rating for short duration's (peak transients) and continuous power out is de-rated. As I mentioned the fuse/circuit breaker is a good indication of what your amp will draw from the wall.

 

[MRC01]

Yes but we are talking program material and the short lived peak power comes from the capacitance.

Have a look at the amps primary fuse/circuit breaker rating and that will tell you all you need to know. You can't really just go by an efficiency number and do conversions because it's more complex than that.

In many cases I have seen large power amps not hold up to their rating on continuous sine wave testing due to the design being for program material and only providing the power rating for short duration's (peak transients) and continuous power out is de-rated. As I mentioned the fuse/circuit breaker is a good indication of what your amp will draw from the wall.

Yes, that's my point. I doubt that any class AB amplifier can deliver clean continuous 1200 watts (say, 600 per channel) when plugged into a standard household electrical outlet. The numbers just don't add up. You'd need more than the usual 15-20 amp current capability at that outlet.

 

[captainbeefheart]

@MRC01

Here is the information you want for your amp: https://www.crownaudio.com/en/site_elements/xls-drivecore-power-draw-thermal

It states that at 1/3 power pink noise typical of program material at extreme clip with a 2 ohm load for both channels it pulls 15.64 amps from the wall outlet.

 

[egellings]

With a large class A amplifier, you will get to know your electric company. It's just too much energy wastage for what you get.

 

[captainbeefheart]

With a large class A amplifier, you will get to know your electric company. It's just too much energy wastage for what you get.

Yes which is why I always try and push people away from them if their situation requires lots of power.

In my experience since I don't require lots of power and a 20 watt amplifier will make my ears sore I do enjoy my Class A amplifiers and their not too much of a space heater, which may or may not be a good thing in the winter times

One of my favorite circuits if one can get away with 20 watts ( it actually produces 50 watts but at higher distortion) is using two of those large hockey puck Mosfets per channel, one as a current source load and the other a single ended output stage. The driver is a Jensen transformer which also processes feedback taken from the output Mosfet Drain. Gain is 20db with 18db of feedback and idle it hangs around 90 watts of dissipation per channel. That's about double the idle power of the 1200 watt Crown we were discussing so not terribly worse for the sound you get which is like a much more powerful 300b SET tube amplifier. The Drain to Gate feedback converts the Mosfet output curves to resemble triode curves and consequent similar harmonic distortion profile.

 

[dlaloum]

How is level of power possible, given the limitations of a typical home outlet at 115 V and 20 amps, or max VA of 1840 VA (115 * 20 * 0.8) ? 1200 watts delivered to the speaker would require 1200/1840 = 65% efficiency.
Does this amp take a 230 V outlet or a 30-40 amp breaker?

These amps are capable of running on a range of voltages - I shipped mine in from the USA, (to australia) - and then switched it to its 230V setting

The power supply is switched, and the amp circuits are Class D, so very high efficiency is expected - under the most extreme / loud conditions, I have NEVER had them get warm, let alone hot.

 

[dlaloum]

Just seeing this now which relates to my other response to you.

The 3 ohm load on the woofer is the tougher load even through your tweeter has a 1.6 ohm impedance. The reason why is there isn't much content in music up in the tweeter range. Look at the power rating differences between woofers and tweeters, the woofer in loudspeakers are designed to handle far more current than the tweeter due to this fact. So going back to my original statement so long as your amp can handle the 3 ohm load in the bass region you should be fine which is 20 amps of current. The rails will need to be higher at around +/-85v due to the 60v rms across the 3 ohm load to make 1200 watts. That's where your amp is going to need it's power, not up in the tweeter range even though it's a low impedance.

And yet the area where the lack of current becomes audible is always the mids and highs.... - the AVR with 100W@8ohm was a prime example... Bi-Amping might be a way of testing - by using external amps for the woofers, and the AVR amps for the mids/highs - that would soon show whether it is the 3 ohm woofer or the 1.6 ohm tweeter causing the AVR to have conniptions! (But right now I cannot be bothered as it is running so sweetly!!!... listening to Getz/Gilberto)

 

[dlaloum]

Don’t mean to rock your boat but I considered the Crown XLS Drivecore amps but the Noise floor is is in the -70db range so worse than most mid tier class ab amps. This was the only turn off. They got power to drive a small moon for sure…for PA they should be fine but def not for home listening IMHO.

Actually it is SINAD that is in the 70's, Signal/Noise is 103db (A weighted according to spec) ... that is why I considered them an experiment when I purchased them - I compared them to my Quad 606's - specced at 105db S/N - on my setup no noise is audible/noticeable. (perhaps a failure of my ears.... ) - And the sound is somewhat smoother, "easier" through the Crowns than through the Quads....

According to the Crown spec sheet, THD is rated at less than 0.5% at rated power.... that would give them a very poor SINAD of 46db. - Actual measurements by Amir put them in the mid 70's (he measured both the 1502 and 2502.... which are specified identically on their spec sheets, but were measured on the bench at 70db and 76db respectively - so there is a bit of production variation .... +/- 10% ? ) - but the issues with them such as they may be, are in the THD space rather than the noise area.

And that area is always contentious with regards to "audibility" and "preferences" - as many people prefer the sound of their far higher distortion valve setups, to the measurably lower distortion solid state amps they compete against!

So yes - I purchased them as an experiment - looking to see what the impact would be of AMPLE power (in the legendary words of Aston Martin and Bentley back in the day " More than adequate power under ANY circumstances") - but the audible results have kept them in my setup.

Running my back of the envelope (or excel!) - calculations - I could probably live with an amp that specced at around 550W@2ohm or thereabouts (less than half what these do - probably circa 150W@8ohm but with loads of current) - and would love to try such an amp, with far more refined noise/THD capabilities.... But I would have to be able to listen to it in my setup, with my speakers.... which means purchasing amps to experiment with - an expensive exercise.... and one that I am current not inclined to indulge in - my itch is currently well scratched!

 

[dlaloum]

Wattage is the same on input & output only when efficiency is 100%. Class A amps are only 25% efficient; let's assume class AB is 50% efficient. 1200 watts to the speaker must draw 2400 from the wall, which is 20 amps at 120 volts, and that's assuming a power factor of 1.0 (ideal conditions), so it would probably have to draw more than that from the wall.

The 1200W@2ohm amp I was talking about is Class D with a switching power supply - so power supply efficiency is probably well over 90%, and amp efficiency is in the same sort of area.... - not Class AB

 

[egellings]

Class A is okay for low power amps, up to maybe 10 watts or so, but I think it is unnecessary to go that route due to the availability of low distortion amplifiers with other biasing schemes & output stages, and these use a fraction of the electricity of the class A design. Crossover distortion in output stages has been a solved problem for some time now.

 

[LTig]

Crossover distortion in output stages has been a solved problem for some time now.

Actually for a very long time.

 

[Nobleman]

Does the amps wattage matter if I only listen at moderate levels? I keep hearing that my Magnepans need a high power amp to open up. What does this mean? The speakers have a 3amp fuse and my Audiolab 6000a puts out 75watts per channel and 7amps. It is true that I have to turn up the volume about 80% sometimes to feel some spl but it gives me that. So would I benefit from buying a high power, high current amp when my speakers are fused at 3amperes?

 

[dlaloum]

Does the amps wattage matter if I only listen at moderate levels? I keep hearing that my Magnepans need a high power amp to open up. What does this mean? The speakers have a 3amp fuse and my Audiolab 6000a puts out 75watts per channel and 7amps. It is true that I have to turn up the volume about 80% sometimes to feel some spl but it gives me that. So would I benefit from buying a high power, high current amp when my speakers are fused at 3amperes?

The fuse is not going to tell you much, as a lot of the effort will be for microdynamic peaks - which can go 15db or more above the base average volume level... - 15db requires 32x the power... so if your average listening level requires circa 1W - the peaks require 32W.... if your average level requires 10W, your peaks require 320W.... you start seeing big numbers fairly quickly!

The other thing is current - the amp has to have the reserves required, to put out the momentary peaks into the impedance the speaker provides at the required frequency.... Lots of very capable amps at 8ohm and 4ohm get into trouble when it drops below 3ohm.... and when they hit those issues (often in the bass around 100Hz.... where the peak mid-bass material sits) - it can completely mess up the sound in the midrange - some speakers do well with "excess".

 

[LTig]

Does the amps wattage matter if I only listen at moderate levels? I keep hearing that my Magnepans need a high power amp to open up. What does this mean? The speakers have a 3amp fuse and my Audiolab 6000a puts out 75watts per channel and 7amps.

The fuse is for the tweeter only.

 

[egellings]

If you have, say, a 10W amp and never play it louder than a coupla watts' worth, acquiring a larger amp will not get you anything useful, given that the ten watter is a good quality design.

 

[Wolf]

These amps are capable of running on a range of voltages - I shipped mine in from the USA, (to australia) - and then switched it to its 230V setting

The power supply is switched, and the amp circuits are Class D, so very high efficiency is expected - under the most extreme / loud conditions, I have NEVER had them get warm, let alone hot.

I've seen the question asked before, and it appears you know the answer. How did you switch the supply voltage input? Is it internal? What did you have to do?

 

[MRC01]

The fuse is not going to tell you much, as a lot of the effort will be for microdynamic peaks - which can go 15db or more above the base average volume level... - 15db requires 32x the power... so if your average listening level requires circa 1W - the peaks require 32W.... if your average level requires 10W, your peaks require 320W.... you start seeing big numbers fairly quickly!

Yep. If you listen at low to moderate volumes, you don't need big power, even with inefficient speakers. But if you listen to large ensemble classical works, or jazz or rock, at realistic levels, you're getting peaks around 100 dB SPL which requires exponentially more power. Here I mean exponentially literally, not figuratively.

Here's more detail on power/SPL measured in my room with my speakers. It may be helpful at least to see how quickly power requirements get into the neighborhood of 400 watts per channel with inefficient speakers.

The fuse is for the tweeter only.

I don't know which model the OP has, but the Magnepan 3.6/R have 2 fuses - one for the tweeter (2.5 amp), one for the midrange (4 amp).

Of course, most musical energy/amplitude is in the low frequencies which can draw much more than the mids or treble. Typically, 6 dB per octave. The low-mid crossover is around 250 Hz, so with full spectrum music, 62 Hz (2 octaves lower) will be about 12 dB louder which requires 16x the power, at the same impedance would be 4x the voltage and 4x the current drawn by the midrange.