# Power supply requirements for audio amplifier

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#### iceblue

##### Member
I am currently designing an audio amplifier and am uncertain about the power supply requirements. Could somebody please give me some feedback on my supply requirement calculations/logic.

The amplifier will be 5 channels. The amplifier IC's I will be using (LM4780) can supply around 20W rms per channel into an 8 ohm load if I power it off +/-20V. So lets say I use a 15-0-15 transformer rectified to around +/-20V, how do I determine the current drawn from the transformer?

I've been working it out as follows:
5 channels at 20W each = 100W total
I = P/V = 100/15 = 6.7amps
So I believe I need a 15-0-15 transformer capable of at least 7A.

Now bearing in mind that the amplifier will be used for a home theatre setup and thus its not likely all 5 speakers will be drawing the full 20W simultaneously, do I still need to use a 7A supply or can I relax this requirement (and by how much).

Thanks.

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#### KeepItSimpleStupid

##### Well-Known Member
Well, having messed this up a little bit, you do need to know whether it's 20W into 8 ohms or 4 ohms or whatever. I do reccomend starting from the speaker side and for example (3 A)^2 * 8 = would be 72 Watts.

Agreed, to that you can't be supplying both the positive and the negative half cycle at the same time, so if you have +- rails, both cannot be used at full power at the same time.

The other problem is that speaker impeadence is not constant and you could have a Z of even 2 ohms at resonance.

I built an amp with +-50 V rails and 4 x 35 VAC at 3A each windings. I'm current limited in terms of power. I do use a 500 VA AC voltage regulator which improves the amp, but the AMP definately sounded better when I used a 20 A secondary Constant Voltage Trasformer. I had to abandon the latter because of transformer hum.

#### audioguru

##### Well-Known Member
speaker impeadence is not constant and you could have a Z of even 2 ohms at resonance.
Absolutely not!
A speaker is a high impedance at resonance where it draws a very low amount of power. Its rated impedance is its minimum impedance where it draws the most power over a range of frequencies.

The OP completely forgot to add the power dissipated by the amplifiers (heat) that is actually more power than the amplifiers' outputs.
So the amplifier's power supply must provide more than double the output power of the amplifier.

Sometimes all 5 channels must produce full power simultaneously so if you cheat on the available power supply power then the amplifier will sound awful.

#### iceblue

##### Member
Well I didn't really forget about the power dissipated by the amplifiers, just never included it in my post.

Ok so essentially you are saying that I need to use a power supply capable of supplying sufficient power for all components and cannot take any shortcuts.

Out of interest, does anybody know how those "home theatre in a box" kits (DVD/BD player with built in amp and speakers) are powered? Some of them have pretty decent power ratings (50+W rms per channel) yet they are pretty light so obviously don't have very big transformers in them.

I was originally aiming at building a 60W per channel system but the PSU requirements are now creating a problem as its pretty hard to get something capable of more than around 70W without having to dig deep into my wallet.

#### MikeMl

##### Well-Known Member
Most of the ratings on those boxes are pure crap. Some older 5.1 amplifiers weigh a ton. Possibly, the new ones are using switching power supplies.

#### #12

##### New Member
I haven't actually seen one, but I can surmize that they are using fast switching power supplies and fast switching power amplifiers. Now that these have been invented, the use of major sized transformers is disappearing in a lot of retail products.

#### KeepItSimpleStupid

##### Well-Known Member
When I built my amp some 30 years ago, I used a 4 windings x 35 VAC @ 3 A RMS each. That gave me 2 supplies at +-50 V each. Filtering was 40,000 uf total or 10,000 uF per rail.

The transformer (custom) cost me $130. The caps were computer grade surplus at$3.00 ea. Those caps ar \$20 each now. Add two bridge rectifiers, fuses etc and a thump/inrush limiter circuit.

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#### carmusic

##### New Member
5..1 setupor 7.1 setup claims often 100w x7 but they are roughly able to deliver about 25-30W max per channel when they are all driven simultaneously (that nevers happen in a movie or music for long periods) ive saw pionner amplifier like that that consumes 245W but they amps are rated at 80w rms (20hz-20khz) x 7!

#### iceblue

##### Member
Thanks for all the feedback. I've managed to find a 15-0-15 3A transformer at a good price so I will just use a couple of these and power the speakers in pairs or something (need to do some more calculations still to decide on final config).

@Carmusic: what you mentioned about all channels seldom drawing full power simultaneously during movies is exactly what I was talking about when asked if I may be able to relax my power supply ratings. But I'm rather going to just build it with a large enough power supply for my needs.

Two last questions:
1) If I use multiple transformers would it be better to connect them all in parallel and use it as a single power source for all speakers, or to power two speakers off one transformer (e.g. left front and right rear off the same transformer, since for the most part of a movie those two speakers wouldn't be playing at full power simultaneously)?
2) If a transformer is rated (for example) 15-0-15 3A, does that mean each coil can supply 3A amps simultaneously?

#### MikeMl

##### Well-Known Member
5..1 setupor 7.1 setup claims often 100w x7 but they are roughly able to deliver about 25-30W max per channel when they are all driven simultaneously (that nevers happen in a movie or music for long periods) ive saw pionner amplifier like that that consumes 245W but they amps are rated at 80w rms (20hz-20khz) x 7!

Like I said: the specs on these is pure crap!

#### carmusic

##### New Member
not they are not crap each channel is able to deliver 100w but not all channel on the same time. If they advertised it as 700w rms then that is crap, but they often advertise it at 7 x 100w rms which is true

#### 4pyros

##### Well-Known Member
often advertise it at 7 x 100w rms which is true

No that is not true 7 X 100 is 700 watts. Thay should use the total spec witch may only be 100 watts total.

Andy

#### KeepItSimpleStupid

##### Well-Known Member
Specs. on car stuff has been crap for a while.

1. There is always interaction when the system draws off the same power supply.
2. 15-0-15 3A means that 3A rms is available at 30 V. (3A)*(15) + (3)(15) = (3)(30) = 90 VA

#### iceblue

##### Member
Ok, Thanks all.

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