200VA transformer enough for stereo LM3886 setup?

[armstrom]

I've done a search and have seen some conflicting reports on what size transformer is required for one or two LM3886 chipamps. I'm hoping to get a clear answer
I want to run a pair of LM3886 chipamps in a stereo configuration. They will each be driving 8ohm loads and the secondary windings of the transformer are 22V each. So, the Transformer is 200VA with two 22V secondaries.. Is this sufficient? The two power amps will share a power supply board (all are from chipamp.com).

I've seen some posts that claim 160VA is enough to power two LM3886 amps while others have said that a single LM3886 should have a transformer between 160-200 VA.. what's the real story??

thanks in advance for clearing this up.

-Matt

[joecool85]

I like to have 120VA or better per chip for LM3886, so 240VA or bigger would be my suggestion.  A 200VA tranny might be alright, but when you are playing loud it will have a tendency to sag it's voltage - therefore keeping you from getting full output.

**edit**
Turns out I was a little bit wrong, a 200VA tranny should be fine for two of them - barely.  I still suggest 240VA or bigger.

[mensur]

Hi,
it's like this in the SS power amps:
If you have 2xLM3886 which is about 140Watts of output, you'll need 1VA per watt of output.
So, 140Watts/0.85= 167(170Watts transformer)., You have 200VA*085=170Watts, which is perfect.

Bye.

[joecool85]

Mensur is right that 200VA would be enough, but I'm of the school of "you can never have enough."

See my thread on heatsinks: http://www.ssguitar.com/index.php?topic=175.0

Also, I used a 22v+22v 330VA transformer for my LM3886 project.  It only had one chip, but was going to be two later on.

[armstrom]

Thanks guys. I'm going to go with the 200VA and see how it works. This amp will be so loud that I doubt it will ever be pushed to full volume (but it's nice to know it can!).

I'm going with a transformer from http://www.antekinc.com/  the AN-2222 to be exact. Unfortunately once you get above 100VA they only provide increments of 100VA so my next option would be the AN-3222 which is 300VA. It weighs 7.3lbs (3.3 kg) vs 5.8lbs (2.6 kg) for the 200VA model. Antek claims the transformers are good for 20% more than the rated output power so that will hopefully be fine. The price difference isn't too much of a concern though with the 200VA model costing $29 and the 300VA costing $36 (both will cost $10 to ship in the US). I'm trying to keep the weight of my amp down as much as possible though. A pair of 10 or 12" speakers, the tranny and the large heat sink I have will quickly add up to a fairly heavy amp! I doubt I can afford to drop the cash for Neo magnet speakers so keeping the transformer weight down will help

-Matt

[joecool85]

Thanks guys. I'm going to go with the 200VA and see how it works. This amp will be so loud that I doubt it will ever be pushed to full volume (but it's nice to know it can!).

I'm going with a transformer from http://www.antekinc.com/  the AN-2222 to be exact. Unfortunately once you get above 100VA they only provide increments of 100VA so my next option would be the AN-3222 which is 300VA. It weighs 7.3lbs (3.3 kg) vs 5.8lbs (2.6 kg) for the 200VA model. Antek claims the transformers are good for 20% more than the rated output power so that will hopefully be fine. The price difference isn't too much of a concern though with the 200VA model costing $29 and the 300VA costing $36 (both will cost $10 to ship in the US). I'm trying to keep the weight of my amp down as much as possible though. A pair of 10 or 12" speakers, the tranny and the large heat sink I have will quickly add up to a fairly heavy amp! I doubt I can afford to drop the cash for Neo magnet speakers so keeping the transformer weight down will help

-Matt

I like my amps to be on the heavy side :-)  j/k, but I do like to over-build my stuff.

Do you have a heatsink picked out?

[armstrom]

Yes, I picked up a surplus heat sink at my local salvage yard. It's aluminum and measures 9" long by 4" tall by 2" thick with 10 fins running along the 9" length. Hopefully it will be sufficient, if not, I will add some fans to it or just buy a bigger one

[armstrom]

Here's a pic of the heat sink...

[joecool85]

That sink is plenty large.  I can't imagine 2 x LM3886 getting hot enough for that to be a problem.

[Zappacat]

Where are you at on this project?  Just curious as I'd like to do something very similar.  I need a stereo guitar amp and I'm looking at this setup for a foundation: Non-Inverting LM3886 Stereo PCB Sethttp://www.chipamp.com/orders.shtml  Have any of you used this setup for a stereo guitar amp setup?

Which specific transformer did you end up using?  Are you implementing a tone stack into it?

[joecool85]

Where are you at on this project?  Just curious as I'd like to do something very similar.  I need a stereo guitar amp and I'm looking at this setup for a foundation: Non-Inverting LM3886 Stereo PCB Sethttp://www.chipamp.com/orders.shtml  Have any of you used this setup for a stereo guitar amp setup?

Which specific transformer did you end up using?  Are you implementing a tone stack into it?

I've used the mono version of that kit, I ran mine with a 330VA toroidal transformer from partsexpress.com - I only went so big because I was planning on running it dual mono later.  It was the loudest "50watt" amp I've ever heard in my life.  I'm not joking either.  It doesn't color the sound at all (some consider this good, others consider it bad - I like it, that way whatever comes out the preamp comes out the speaker).

[ApexJr.]

Looks like I missed this post, but just for the record
I have a dual 21.5V 2.7A toroid transformer for $19.95
also a 24V-0-24V 200Va toroid for $22.00
Others listed on the website  www.apexjr.com
click on the toroid on the home page

Steve @ Apex Jr.

[Brymus]

I wish I had seen that 200VA before you went on vacation Steve.
As is I am very happy with the heat sink and chips ,ect I purchased from you.

[Zappacat]

Where are you at on this project?  Just curious as I'd like to do something very similar.  I need a stereo guitar amp and I'm looking at this setup for a foundation: Non-Inverting LM3886 Stereo PCB Sethttp://www.chipamp.com/orders.shtml  Have any of you used this setup for a stereo guitar amp setup?

Which specific transformer did you end up using?  Are you implementing a tone stack into it?

I've used the mono version of that kit, I ran mine with a 330VA toroidal transformer from partsexpress.com - I only went so big because I was planning on running it dual mono later.  It was the loudest "50watt" amp I've ever heard in my life.  I'm not joking either.  It doesn't color the sound at all (some consider this good, others consider it bad - I like it, that way whatever comes out the preamp comes out the speaker).

What speaker did you end up using?

[R.G.]

This is one of those things which seems like it should be simple, but it's not.

The correct answer to "how big a transformer do I need for a stereo LM3886 amp?" is "It depends."

It depends on the signal, of course. These amps, being Class AB, pull very little power when there is zero signal. Trivially, if you only drive it to 1W of output, you only need, say, 5W of power transformer to supply the 1W of output and maybe 4W of standing losses.

But we aren't interested in 1W outputs, are we? We want the full magilla. We want the LM3886 to put out its full 68W into 4 ohms, right?

For some background read http://www.national.com/an/AN/AN-1192.pdf, which leads you through some calculations for the single-IC case early. Putting out 60W into 4 ohms makes for 39W of power dissipation, so the total power coming out of the power supply is about 100W. That's all you need.

But as the app note explains, that's if the power supply is regulated. If it's not - and guitar, even hifi amp power supplies almost never are - then the power supply has ripple on it. (Yes, it does, with any filter capacitors of less than infinite size.) So you have to dink with the power supply to keep it above the minimum the power amp needs to avoid clipping and to avoid overtemping. You also have to worry about that max voltage rating on the chip and the fact that the AC power line rises and falls. Sometimes a lot. So we have two degrees of fuzziness thrown in with (1) no regulation and (2) AC line voltage variations.

Even assuming that we got the power supply perfect, the "60W" output is a rating based on a sine wave that's just before clipping. Not many people listen to sine waves. I have, for short periods, and they're BO-ring. We all listen to music and music does not have just-before-clipping levels all the time. Even if we have heavily fuzzed the signal. Even if we overdrive the amp like crazy. In fact, if we so overdrive the amp that it's just banging the output from rail to rail, power supply dissipation goes *down* because the amp is now dissipating much less than it would with a sine wave. The dissipation is all in the load.

Music has peaks and valleys, even heavily compressed musical instrument signals. There's a beat, or otherwise why are we doing this? That beat introduces what's called a crest factor in the music biz. The crest factor describes the peak loudness to average loudness of the signal. That's commonly 20db or more in the final music we listen to on stereos, and usually not less than 10db even in full-warp guitar or bass signals.

That being the case, the power supply is only putting out 10db below max power on average. The issue then is how long is average, and do the peaks cause problems? Power peaks do two things. (a) they cause the power supply to sag because the higher current drain causes drops through the series resistances of the power supply and (b) ripple voltage goes up because the caps drain further between recharge pulses. It is a rare power supply with so much internal resistance that it sags more than 10% from non-ripple voltage issues. So adding more *capacitors* has a much bigger band-aid effect than adding a bigger transformer.

It is possible to make a case for power transformers rated from 1/10 the nominal output power rating up to about 2X for super-duper, belt-and-suspenders, solid gold wiring kinds of amps. But the returns you get by increasing the available transformer over maybe 75% to 100% of rated power are very, very much diminishing.

Remember that a transformer's power rating is NOT the amount of power it can flow through. It's power rating is how much power it can pass through without itself overheating. And that's yet another layer of fuzziness in the power transformer sizing puzzle.

You have to decide whether you want to listen to music, buy affordable parts, or build a platinum plated, NASA space-shot amplifier. Chances are, placebo effect is going to make you happy with whatever path you choose.