LM3886/Vox SS Amp Thread(w/ Samples!)

[RDV]

Thanks R.G.!

The true spirit of DIY should be to hold the cost down without sacrificing too much quality. Recycling should be done at as much as possible. This includes effects as well. YMMV.

My only regret at this point is not buying the 330VA transformer because I could've ran both the stereo LM3886 amp and the paralleled mono LM4780(for a subwoofer) I'm building now and put it all in the same enclosure. DOH!! I've even got enough heatsink in there to just put both LM3886 amps on one heatsink and the LM4780 board on the other! I'd just have to add two jacks! ...and a crossover ...and a separate PS for the crossover.

Hee hee hee.

RDV

[Joecool85]

I'm using a 330va for a single lm3886.  I like overkill 

Besides that, to get full power you really want 200va or so for a lm3886, and since partsexpress only has 160,250,and then 330...and the 330 is only $5 more than the 250...it seems a given to go with that.

[RDV]

I'm a little more prone to listen to R.G. Keen than Brian at chipamp.com.

RDV

[R.G.]

The subject of how much transformer to buy is a tricky one.

If you want full power at 100% duty cycle, it's easy. You add up the watts, and then compute average DC currents. From that you compute transformer heating current in RMS (they're different!) and pick your transformer VA as the product of its output voltage and the heating current RMS.

That always give a pessimistic answer, because music is not constant power. Only unnaturally compressed sound or constant signals can do full power all the time, and people won't listen to that. Music varies a lot over the time constant of a power transformer.

A transformer may take an hour to get to final temperature at a given power level. Anything that varies in that hour goes into the average. Typical music on radio stations and such has a crest factor of about 20 to 1. That is, the peak power is about twenty times the average. So mostly you get away with transformer VA ratings equal to or perhaps half of the rated RMS power of the amp. VA ratings of 100% of the power amp rating are quite conservative. VA ratings bigger than the RMS power of the amp are overkill by any yardstick I've ever seen. And those are the hifi yardsticks, where power supply sag is treated like an unmentionable social disease.

The thing is you almost can't prove than any transformer that doesn't go into meltdown isn't a good answer. Alternatively, there are many acceptable answers. It all depends on what you take as starting assumptions. Brian is probably as right as I am about transformer powers.

And yes, I'd be happy to give that underpowered 160VA transformer a home. Pay pal OK?

[Joecool85]

I see what you are saying RG, and agree.  My big thing though, is there is no real reason to NOT go with a bigger one, except for price.  But when a 160va is $37, a 250va $42 and a 330va $47...it makes me think the 330va is my best bang for buck, and it can't hurt.  This is kinda how Brian described it to me.

[RDV]

And yes, I'd be happy to give that underpowered 160VA transformer a home. Pay pal OK?

Whoa partner, that was this morning! I later looked to see how much I payed for that "under-powered" 160VA transformer. It was about $46.99 if you include the shipping. I do wish I'd bought a bit more powerful one but I'm gonna leave it be and use one of those big bulky multi-tapped ones that you recommended for the sub-amp(hoping that 96VA will do it).

If I change my mind(again) I'll let you know.

I just got my sub woofer from part express today and I put it in a guitar cab and I'm running it with my old PV monitor head and digging it. It's breezing my pants legs as we speak. The Darkness and Flaming Lips records are being played for sure! It's just a cheap Chinese made woofer but it keeps up with the rest of my little system.

No disrespect intended to Brian either. He thinks in audiophile terms. I don't(can't?).

RDV

[Joecool85]

I love The Darkness, the rock.

[R.G.]

Whoa partner, that was this morning! I later looked to see how much I payed for that "under-powered" 160VA transformer...
If I change my mind(again) I'll let you know.

That's OK. Actually, I suspected you might have second thoughts... 

Call it an IQ test... 

[RDV]

Call it an IQ test... 

Jeez, that's the last thing I need. I took one and didn't do too bad, then they gave my 15yr old son one and he scored about 5 points ahead of me. This wouldn't be so bad as one wants the fruit of his loins to be successful and all, but this boy pees on the toilet seat, won't bathe, and is a middle school dropout.

I couldn't be prouder.

RDV

[RDV]

Oi:

I did the obligatory "smoke test" on the parallel LM4780 amp last night and no smoke occurred.

YAY!

It seemed to sound fine and got real loud & only moderately warm with a pretty large heatsink.

Today I'm building a (ply)wooden shoebox (so to speak) to house the amp + crossover, then I'm getting going on a PS/crossover on perf.

The "shoebox" will have the 3 3/4"(H) X 6 1/8"(L) heatsink sticking out of one end(in free air) and a control plate on the other end. The controls will be: Level, a lighted on/off switch, and output terminals for the sub.

RDV

[Joecool85]

Sounds like enough heatsink for a lm4780.  Now, is it just one lm4780 or two?

[RDV]

Sounds like enough heatsink for a lm4780.  Now, is it just one lm4780 or two?

1 for a lightweight subwoofer amp.

RDV

[RDV]

Boy.
Did I fail to build the crossover?
Yes I did.
It sounds like a fartover.

Can an NE5534 be used as a buffer?
I'm wondering.
I tried the 22pF cap between pins 5 & 8.

This sucks.
I'm about at the end of my perfboard rope.

RDV

[Joecool85]

Here is a nice looking active one I came across:

http://www.snippets.org/filters/shivaeq.htm

[RDV]

Well I fixed the crossover this morning. I stayed up WAY past my bedtime last night trying to get it done and made a typical mistake that I make when tired. I used a 15v bipolar supply and at the end I wired up the power straight through .1µF caps instead of using them to bypass to ground! DOH! Instant fart machine. I adapted the design from Rod Elliot's 2way crossover project.

Here's the final schem.


I was going to build my subwoofer cabinet this weekend also but discovered that I'd cut my sides 5/8" short. Another big DOH! I only was able to do about 50%.

I'm listen testing as I type. I think the amp is going to work out fine. It gets warm, but not hot by any means, which is good because I don't want to buy any more heatsinks(the one I'm using probably weighs at least 3 lbs. . A paralleled LM4780 is the only way to go to run a 4 ohm load with a chipamp IMHO.

Cheers.

RDV

[Joecool85]

Hows it sound?

[RDV]

Hows it sound?

Like 150Hz on down. You know. Boom Boom Boom. There's almost 0 high freq content. It's a 18Db cut on everything above 150Hz.

Seriously, the LM4780 is just 2 LM3886s as you know. When I tested the amp with no x-over it just sounded like any other good straight poweramp. I think the 96VA transformer is going to work out here because I'm really not pushing it too hard, plus this puppy has multiple windings so I used the 30vct winding to build the 15v+/- PS for the X-Over.

It's all good for now.

RDV

[Joecool85]

Wow, I wouldn't run smaller than a 250va for a lm4780, but thats just me.  Cool that its working out for ya though.

[RDV]

Wow, I wouldn't run smaller than a 250va for a lm4780, but thats just me.  Cool that its working out for ya though.

That's cause yer mind's been poisoned by the audiophools.

Just wandering: Have you actually built one? I'm on my 4th.

RDV

[Joecool85]

You got me there, I haven't built one yet.  But I do like overkill