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Author Topic: General Chipamp Info  (Read 21121 times)

RDV

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General Chipamp Info
« on: March 26, 2006, 03:37:07 AM »

As taken from my huge thread on the subject. I've extracted the juicy bits for your convienence.
HEATSINKS
"I used a small heatsink for the chip, so I had to use the fan."
RDV

"OK, special advice.
As I tell people a lot, power amps are essentially a power supply with some other junk on them that lets a little of the power out carefully. That's not a complete description. Power amps are also heat-exchange engines as well as power supplies.
Before you make yourself a sealed up amp, go to ebay and buy one of the flat-back heatsinks that come up there every now and then. You can get on for under $10 that's heavy with lots of fins. 3" along the fins, 6" wide, and maybe 1.5" to 2" fins would be good. Make that heatsink part of your amp. Ditch the fan. You have the beginnings of a very good amp."
R.G.

"I cranked back up and was blasting away and the chip shutdown on me (for a couple of seconds), the fan doesn't cut it at gig volume. But then R.G. said that didn't he?
It will however work for the low-volume stuff I do at my desk.
I'm ordering a proper heatsink very soon though, and perhaps a chassis."
RDV

"I bought one of these and I thought I'd cut it in half and make 2. There's really not too many fins now that I look at it though. What do you think?"
RDV

"I actually started to snag a couple of those myself. That'll certainly be better. Go for it!!"
R.G.
____________________________________________________________________________________________________________
Transformers
"I think this might be an ideal transformer for an LM3886, any thoughts? Perhaps the 250va one instead?"
RDV

"Actually, that's good for about two of them.
22+22 volt secondaries is about right, total of 60Vdc, +/-30. That's close to the max you'd want to use with these chips. You should get 50Wrms out of the thing with +/-30 supplies. With 50W out, efficiency of about 79%, you need to supply 50/.79 = 63W of DC from a 60V supply. That's 0.95A of DC. A full wave rectifier makes the transformer see an RMS current of about 1.6 to 1.8 times the DC current, so the transformer is seeing maybe 1.7A. The VA rating is then 44*1.7 = 75Va.
What that tells you is that a 75VA transformer will run one of these things at a full power sine wave forever without overheating.
But music is not static. It has peaks and valleys, and transformers have enough mass to "average" the power demands in terms of their internal heating. Stereo transfomers are commonly sized to supply 1/2 to 2/3 of the continuous power rating, and they don't suffer much. You could probably get by with something under 75VA if you wanted to play brinksman.
Parts express used to sell a 22+22 EI core thing at about 80VA for $15. They were the best match to the LM3886 I ever found. I don't see them there any more."
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."
R.G.
____________________________________________________________________________________________________________
LAYOUT & OSCILLATION
"The (perfed) chipamp is now getting hot at idle now, so I'm afraid I've got some oscillation going on. I'm wondering(out loud) if I need to add a Zobel to the output and what values to use. I've been reading in the DIY chipamp sites about "snubberizing", but I can't say it's making any sense to me.
This all may just mean that the heatsink(being too small and fan not able to keep up) issue is what is going on. I'm waiting for one to come in and then we'll see. Hopefully with a proper heatsink, it'll idle cool at least.
I just never had the chip getting hot at idle or low volumes before."
RDV

"Yes, it seem it might be self oscillating. Do you have the load (speaker) connected? These things don't like to be unloaded. Probably if you put a voltmeter set for AC voltage reading at the output you will be able to tell if the IC is actually oscillating.
The snubber thing is usually a small value resistor with a cap to ground loading the output, like 10 ohms 1/2W + 100nF or so. Check the datasheet to see if they suggest using something like this.
Also, the oscillation may be due to the layout and/or wiring around. Try rearranging wires, keeping input signals and control pots away from the output, etc.
Regards."
STM

"Every solid state power amp needs a Zobel network.
Every solid state power amp needs inductive catch diodes.
Every solid state power amp needs a damped series inductor.
Every solid state power amp needs power supply decoupling right at the circuit.
These are things, just like the wrapper on an effect, that are not particularly interesting or exciting - until it doesn't work.
stm's rignt - ten ohms and 0.1uF is the standard Zobel, and it seems to work for almost all speakers. And that wire arrangement is important.
However, he's incorrect about loading. Solid state amps and tube amps have opposite preferences about loading. Solid state amps work fine into arbitrarily high load resistances, including no load resistance connected at all. They tend to have trouble with too-low load resistances, including dying. Tube amps get unstable and oscillate, sometimes to death, when they are unloaded, but cope with overloads (too-low a load resistance, right down to dead shorts) just fine. They're different. However the conventional wisdom about musical instrument amps is not to unload them. It's a legacy of the tubes-only era."
R.G.

"I forgot to say that with prerecorded music(bypassing the TV Preamp/Limiter) this amp doesn't even get hot, barely warm even. With the guitar amp thing going it heats up almost instantly. I wonder if I should cap couple the input of the poweramp section?"
RDV

"It's always a good idea to cap couple if you can to prevent rogue DC from getting in.
I'm not too surprised that it doesn't warm up on prerecorded music. There's a generally accepted rule that prerecorded music has a crest factor (peak power to average power) of about 20db. The peaks are 20db larger than the average level. So if you play it cleanly, the average level barely warms the heatsinks. Not so for guitar by itself, especially with a square wave fuzz going into it. The worst thing you can to to your amp is not to play full-bore sine waves or music through it. The worst thing you can do is to play half-maximum swing square waves. That means that each output device is alternately handling half the power supply voltage and half the full current. That's the peak power dissipation point in the output transistors. Playing louder square waves actually lets them cool off some from there.
... of course, you could have some DC leaking in too..."
R.G.

"I've never given up on the original chipamp I perfed in the beginning of this. It was heating up and shutting off no matter how much heatsink or what size load I put on it.
The last couple of things I tried seemed to do the trick. I replaced the polys(.1F) on the PS bypass with monolithic ceramic and moved them right by the chip. I changed the zobel to a 2.7 ohm and another monolithic ceramic. I had removed the 10k pot early on so in case that was the problem I installed a 22k from input to ground. The feedback resistor it seems was hanging by a thread and broke off when I touched it. EEK. Replaced that also and fired it up without the preamp and it didn't get hot and sounded okay too. Cool. I've spent about 2 weeks trying to figure this out."
RDV
_________________________________________________________________________________________________________
Schems & Info Sites
"Did you build your lm3886 to match the GGG schematic or did you just do it exactly the way the chipamp.com stuff is set up?"
Joecool85

"I followed the GGG schem, then started experimenting using a bunch of various schems as guides. The way my perfed one stands right now is kind of a combo of GGG's and chipamp's along with a PS borrowed from Rod Elliott. A good place to look as they have a chipamp forum is www.DIYAudio.com"
RDV
_________________________________________________________________________________________________________
Cost of Stereo LM3886 amp
This is a stereo amp I'm using to run my small monitor speakers.

2 ea. LM3886TF - $13.00
1 ea. Dual Mono LM3886 PCB from www.chipamp.com - $26.00*w/shipping
1 ea AC Receptacle - $.50
1 ea Computer AC Cable - $0.00
1 ea. AVEL Y236503 160VA 22V+22V TOROIDAL TRANSFORMER - $46.99 w/shipping
1 ea Chassis 2 rack space aluminum enclosure - $0.00**
2 ea 10,000F 50v Electrolytic Capacitor - $7.34
6 ea 100F 50v Electrolytic Capacitor - $.60
2 ea 47F 50v Electrolytic Capacitor - $.14
1 ea 10F 50v Electrolytic Capacitor - $.07
6 ea .1F 50v Poly Capacitor -  $1.44
8 ea 8.0 Amp 600 Volt Diodes - $10.48
1 ea 1N4004 Diode - $.05
3 ea 10k Metal Film Resistor - $.30
4 ea 22k Metal Film Resistor - $.40
4 ea 1k Metal Film Resistor - $.40
2 ea 680ohm Metal Film Resistor - $.20
2 ea 2.7ohm 2watt Metal Film Resistor - $.64
2 ea 2.2ohm 2watt Metal Film Resistor - $.64
1 ea Green LED - $0.00***

Total: $109.19

*Came with 2 PS PCBs, only used 1, saved 2nd for next project.
**Broken(and unfixable IMO) Carver CD Player from the 80s had 2 space rack mount chassis. A comparable chassis from Par-Metal.com would be around $50.00
***Freebie from the wonderful Brian Marshall. LEDs are cheap.
RDV

"That's a good illustration of something I keep saying - a power amp is mostly power supply and other things that are NOT the power amp itself.
Here's what I think we can credit to the power amp circuit:

2 ea. LM3886TF - $13.00
1 ea. Dual Mono LM3886 PCB from chipamp.com - $26.00*w/shipping
6 ea 100F 50v Electrolytic Capacitor - $.60
2 ea 47F 50v Electrolytic Capacitor - $.14
1 ea 10F 50v Electrolytic Capacitor - $.07
6 ea .1F 50v Poly Capacitor -  $1.44
1 ea 1N4004 Diode - $.05
3 ea 10k Metal Film Resistor - $.30
4 ea 22k Metal Film Resistor - $.40
4 ea 1k Metal Film Resistor - $.40
2 ea 680ohm Metal Film Resistor - $.20
2 ea 2.7ohm 2watt Metal Film Resistor - $.64
2 ea 2.2ohm 2watt Metal Film Resistor - $.64
Which adds up to $43.88 if I added right.

And here's the non-power-amp-circuit stuff:

1 ea AC Receptacle - $.50
1 ea. AVEL Y236503 160VA 22V+22V TOROIDAL TRANSFORMER - $46.99 w/shipping
2 ea 10,000F 50v Electrolytic Capacitor - $7.34
8 ea 8.0 Amp 600 Volt Diodes - $10.48
1 ea Chassis 2 rack space aluminum enclosure - $0.00**
1 ea Computer AC Cable - $0.00
1 ea Green LED - $0.00***
Which adds to $65.31.

The power transformer itself cost more than all the power amp circuitry and purchased PCBs.

The imbalance gets even worse when we notice that all the free stuff was non-poweramp, and that the cabinet could have added more cost than the power amp circuitry; even worse when we note that the PCBs could have been made at home for under $10.

This is the kind of stuff that people forget when they decide to build a big power amp.

By the way - you did a great job of holding costs down. Getting to under $1 per watt is a tricky goal in any kind of home built power amp that looks presentable."
R.G.
------------------------------------------------------
I hope this is of some help to someone.

RDV
« Last Edit: March 26, 2006, 03:51:55 AM by RDV »
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J M Fahey

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Re: General Chipamp Info
« Reply #1 on: February 17, 2009, 08:55:11 AM »

Hi RDV. Impressive post, VERY useful.
Thanks.
JM
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