D class power requirement.

[OldSolder]

I have a Kustom Sienna 30 acoustic amp and like the front end but the power amp at 30w is not enough.
I'm toying with the idea of putting a 150w @ 8 ohm D class pwr amp into it but am not sure how big a pwr trans I would need.
The amp kit requires: From the spec sheet. +/- 40 to 60VDC, 50 to 55V nominal.
What I need to know is whether a 300Va trans would be adequate or overkill.
With D class amps being very efficient do they draw less current than A or AB amps.
The trans needed is a toroidal.
Thanks in advance.

[nickb]

Considering just the power amp, at full tilt the average power out will likely be less than 50W**, especially for a clean acoustic. This average current will about double in the transformer due to the very distorted current waveform that goes through the rectifiers. Thus a transformer of 120VA plus the preamp requirement will do the job.

**PS: I wish a could find an authoritative source to back this up. This is just my best guess and probably on the high side. Upped to 120VA to allow for 85% efficiency.

[OldSolder]

To elaborate, this amp is rated at 150w RMS into 8ohms and 250w RMS into 4 ohms. I have 125w bass speaker 8ohms in the amp and plan to use it for steel guitar. Hence the need for more power.

[J M Fahey]

There´s a couple points to consider:
1) a "bass" speaker is the worst choice for an Acoustic guitar, it will have no mids to speak of, definitely not the kind of mids and mid-high definition and sharpness an Acoustic guitar needs.
Heck, it will be muddy even for an *electric* guitar !!!!
The tweeter there is not enough,it´s crossed over way too high, no big deal if complementing the current light speaker but using it with a woofer will leave a terrible hole in the middle.
Guitar will sound both muddy and "fingernails on the blackboard" brittle.

Get a *good* PA speaker with extended high frequency response, so tweeter just adds a little sizzle.
This one is very good for that task, it has a light aluminum voicecoil and reaches quite high,*flat*:
http://www.usspeaker.com/faital%20pro%2010fe200-1.htm

notice it reaches 5 kHz with very good efficiency, tweeter can take over above that with ease:


no regular woofer or bass speaker reaches that high, most roll off around 1500 to 2000 Hz, leaving a hole where an acoustic guitar shows its voice, and to boot below that they are muddy, sluggish.

2) your cabinet is very small and light, you have little space if at all for a heavy large toroid.
I suggest you search for a Class D power amp which includes its own lightweight SMPS, there are many, similar to the ICE Power modules, which are straight fed 240V , no bulky heavy iron.

3) is your customer aware that this upgrade job will cost WAY more than the $139 a Kustom Siena costs new?

Maybe he´s under the wrong impression that "a few bucks" will turn his Yugo into a Ferrari.

[R.G.]

[...]
The amp kit requires: From the spec sheet. +/- 40 to 60VDC, 50 to 55V nominal.
What I need to know is whether a 300Va trans would be adequate or overkill.
With D class amps being very efficient do they draw less current than A or AB amps.

Class D is much more efficient than AB, meaning that for the same audio power output, the watts out of the power supply tracks the audio output closely, within a few percent. For Class AB, the power out of the power supply is both larger than the audio power delivered, and very much more variable.

So for the same audio power output, Class D amplifiers use from less to much less power than Class AB. Your intuition about making the transformer just about the power output of the amp was good.

However (you knew that would be coming, didn't you?) the power you put out into your speakers both varies a lot and also is generally less than the power rating of the amplifier over long periods of time. Music is intermittent - it gets louder and quieter. A perceptible beat in the music is the most obvious illustration. The variation of the loudest parts of the audio to the average audio level for the whole musical passage is called the "crest factor" in the audio biz. Crest factor varies with the music played, obviously. For classical music, the crest factor is usually about 20db, meaning the loudest passages are ten times the power of the average power level. The crest factor is less for compressed, whole-band rock recordings or things like compressed rock on the radio. But it's generally something like 6db or so, never 0db (that is, equal loudness all the time). So the long term power used, even for a loud set of live music, is typically lower than the amplifier power rating, even if the amp starts clipping on the loudest peaks.

This matters because conventional transformers have a thermal time constant (the time it takes to heat to 63% of the final temperature caused by a power increase) that is rated in hours. It takes time to heat up a heavy lump of iron and copper, and the transformers are deliberately designed to waste as little power inside themselves as possible.

Textbooks on audio power amp design can spend whole chapters on this topic. The bottom line is - there is no definitive way to figure out the "proper" transformer rating, because there isn't one - it depends on what music you play.

You can be 100% sure if your transformer is rated for the maximum power output plus inefficiency continuously. So in your case, with two 150Wrms outputs, a probably efficiency of 95%, the amp could not use more than about 315W of power no matter what. A 300VA transformer would be a slam dunk. But it will run cool nearly all the time.

You could safely save some money if you decide to not run full-power sine waves through your amp all the time. If you're playing only regular music, even in a band setting, you could get by very reasonably with a transformer rated at about half that.

Probably. Depends on the music.

[R.G.]

**PS: I wish a could find an authoritative source to back this up. This is just my best guess and probably on the high side. Upped to 120VA to allow for 85% efficiency.

It's in several texts I've read on audio power amplifier design. Search for things like "power supply" + "audio amplifier" + "crest factor".

[nickb]

You can be 100% sure if your transformer is rated for the maximum power output plus inefficiency continuously. So in your case, with two 150Wrms outputs, a probably efficiency of 95%, the amp could not use more than about 315W of power no matter what. A 300VA transformer would be a slam dunk. But it will run cool nearly all the time.


Probably. Depends on the music.

As I read it there was only one 150W amp not two. So, by your reckoning 150VA would be the appropriate figure.

If you wanted a transformer rated for continuous 150W that would be an input power of 167W for 90% efficiency. With to a Irms/Idc ratio of around 1.8 for the typical bridge rectifier/big filter cap arrangement this translates to 300VA for a single amp, not two.

So since we have just the one amp I stand by the figure of 120VA as being conservative in practice.


There's a pretty big range of crest factors, the range oft quoted is 3 to 5 (which I why I choose 50W). I just never come across anything I'd consider authoritative but maybe I'm being too picky.

[R.G.]

I misread it. It does only say 150W. I was add-on remembering other Class D's I've looked at where they always had two channels.

I'd agree that 120VA would be conservative. I have seen commercial gear go lower than 50%. The funny business about how long power has to be applied for thermal and power-output tests is how we got the FTC demanding sine wave RMS tests for audio power outputs. By not following these guidelines, you get car audio amps the size of a normal hardbound book rated in kilowatts.

[nickb]

There something else I forgot to mention. In practice, because the average power out is low, so the efficiency drops off too. For example a Texas Insts 300W amp I just pulled the datasheet for is only about 75% efficient at 30W, but over 90% at full power.

The module is probably 2 x 75W bridged for 150W.

[OldSolder]

Thanks for the replies. To elaborate a little further, my idea with using this amp is first and foremost,is weight. Most gigs I do are small room where until recently 30w has been ok.
But it's way too small for outdoor gigs. I play pedal steel guitar which has a bottom string that goes down to a G#, 4th fret of the bass string of a bass guitar. If I hit a 6 note chord with strings down in that range and they are all heavy gauge strings,
I need an amp to handle that input cleanly and a speaker with a high power rating.That's why I picked a bass speaker. I also plan to make a small speaker box for another bass speaker which when connected in parallel would take the output to 250W.

Now because this amp was designed for an acoustic instrument, it doesn't have that normal guitar amp midrange scoop. According to the blurb, it has a fairly flat response so if anything I'm winding the mid range down.
I won't be running the amp at anywhere near full power but do need enough power to handle transients and big bassy chords.
So in order to keep the weight down I need to know the power rating of an appropriate toroidal power trans. Thanks for your help.

[R.G.]

Makes sense.

You're probably already at the point where the speakers are the weignt problem. It is possible to run that thing from two switching power supplies. LED lighting has made 24V high current power supplies cheap. A couple of 24V 8A power supplies might just run the thing fine. Might also need filtering and other issues, but possible.

But again, a 150VA toroid isn't all that heavy.

There is another options. A company by the name of IcePower makes Class D amps in the 50W to 500W range that contain the switching power supply internal to the amplifier module. They're not horribly expensive.

[Enzo]

15" speakers are fairly common in steel amps, are they not? Peavey made many models thus equipped.

[OldSolder]

Yes they did and you will break your back trying to lift one.
Which is why when I need the power the plan is to have a separate 10" in another box. Cheers.

[g1]

Are you able to find a sensitivity spec for this bass speaker? Keep in mind that a speaker with a 3db better sensitivity spec will do the same for loudness as doubling the amp power.

[olddawg]

There’s more than one way to Egypt... I would just find an inexpensive powered monitor and plug the preamp line out, extension amp, or effects send into it. Lots of application flexibility there.