Rifa SS amp caps?

[tedmich]

I am building a large 2.1 SS amp based on the LM3886 (chipamp/gainclone) and was interested in adding some big filter caps to my linear supply for better bass transients etc. I ran across an ebay listing for big plastic screw terminal 8800uf 250v caps removed from unused equipment, they turned out to be these:


They have an ESR < 15mOhms from 100Hz-100kHz and ripple current >17A!

I got 8 for <$40 delivered, how many are needed for a 300w amp? How are the big studs on their bottoms to be mounted? (they seem common with one terminal??!)

[Jazz P Bass]

LM3886

You are going to use a 40 watt chip in a 300watt amp, how?
Throwing a cap at an amp is not how it's done.
What are the power rail voltages?
How much true current does the amp need?
Answer these and then you can decide on a capacitor.

[tedmich]

You are going to use a 40 watt chip in a 300watt amp, how?
Throwing a cap at an amp is not how it's done.
What are the power rail voltages?
How much true current does the amp need?
Answer these and then you can decide on a capacitor.

yeah I'm going to run it at 10 times the voltage....kidding!

I am doing a parallel-bridged design like this:
http://www.shine7.com/audio/bpa300.htm

Jeff Rowland Audio made +$10K amps with 6 x LM3886 per side for 275wRMS into 4 ohms
Jeff Rowland LM3886 Amplifiers

PS I'll have no more than 3 x LM38862 for ~120wRMS/4 ohms, BUT multiple added LM3886's...2.1, maybe 5.1...(7.1?)

complete (3xLM3886s) Kits are available from china on the ebay for ~$40

notice the first guy has 94,000uf of Rifa filtering (~$150)... you can over do this stuff

[Jazz P Bass]

Wow

Hell of an amp. Both of them.
Read the data sheet & keep the power rails below 40 volts.
Have fun!

[Steve Conner]

I like the Rifa PEH series electrolytics. I used the 10,000uF 63V versions in a hi-fi amp I built, and a 3300uF, 420V one in my Tesla coil.

IMG_1379.JPG

They're inverter grade and almost certainly complete overkill for audio, but who cares. Just be careful, if anything goes wrong they will launch pieces of your LM3886s clean across the room and maybe take your eye out.

The stud on the bottom is common with the negative terminal, because it's part of the case. But it's not to be used as a conductor. It should be insulated from the chassis, and you use the terminal studs on top as usual.

JMHO, but I think the LM3886 or TDA-whatever chip amps are a bit ghetto.

[J M Fahey]

Hi tedmich, nice project.
I don't get the

3 x LM38862 for ~120wRMS/4 ohms

bit; do you mean 3 chips paralelled into 4 ohms? With, say, around +/-35V supplies?
Thanks.
PS: I envy those 8800x250 beasts; I need 10 to 16 of them for my magnetizer.

[tedmich]

I like the Rifa PEH series electrolytics. I used the 10,000uF 63V versions in a hi-fi amp I built, and a 3300uF, 420V one in my Tesla coil.


JMHO, but I think the LM3886 or TDA-whatever chip amps are a bit ghetto.

THIS is ghetto?

its all about the added bling!

[Steve Conner]

The case is a thing of beauty, the guts are straight out of the ghetto.

I'm probably being irrational, the new generation of chip amps measure as well as any discrete amp, and they probably have better control over bias than a discrete amp, because the designer can put the thermal sensing diodes on the same piece of silicon as the output trannies. That means potentially lower THD at all times. I've seen the LM3886 in monitor amps intended for broadcast studios.

But somedays it seems like every other post in "Maintenance, Troubleshooting and Repair" is a Marshall Valvestate whose chip amps have exploded.

Re the original question, two caps per rail (4 total) should do it.

[J M Fahey]

Hi, I agree with you, but in the Saga

But somedays it seems like every other post in "Maintenance, Troubleshooting and Repair" is a Marshall Valvestate whose chip amps have exploded.

ahem ! cough ! cough !, maybe the fault does not lie on the poor chips themselves, rather the responsible one is (I'll just whisper) Marshall.
Just yesterday night I was reading the story of HH amplifiers.
After a long and very successful career as high end equipment makers (BBC and countless other "serious" clients) they decided to make guitar amps.
They all blew onstage sooner or later, so they redesigneed them with robust 2N3773's, Tips as drivers, and "borrowed" the Crown DC300 protection, the best state of the art could provide.
MAJ Electronic , then click HH History File.
Of course, they could do that because the boards were discrete, an impossible task with chips.
In that case, since they can not be "improved", the *obvious* solution is to use them well within their specs.
I'm using TDA7294's in my cheap 40 to 60W minicube amps (think Polytone/Roland) with *NO* external speaker connections and no problems so far.
But pulling 100W (MG100) from them and allowing musicians to plug anything there (shorted cables, "any impedance" speakers) is akin to lighting a cigarette while carrying gasoline drenched clothes.
Besides that, *personal opinion*, chips are excellent for simple, medium power amps.
Bridging them is already doubling cost complication needlessly.
Parallelling + Bridging 6 of them (and 12 for Stereo) ..... give me a break !!!!!!!!!

[tedmich]

once you go silicon its a slippery slope to these:


https://www.coldamp.com/store/bp4078.html

and
Class D Audio 600W Class D Power Amplifier and Power Supply

how can it be "heavy metal" with a <10lb class D/T amp/SMPS/neo speakers?

[Steve Conner]

I tried to make a Class-D bass amp out of those Coldamp things.

I never could get it to make its rated power without the SPS30 power supply blowing its guts out. It turned out that the designer only tested the power supply at 220V line, and our line voltage is 240+. The extra 20V were enough to make the feedback loop go unstable. I told him about this issue and hopefully he might have fixed it now, but it had me worried. Testing your design over the full range of line voltages it'll encounter in the field is SMPS 101 stuff.

Once I'd figured this out I carried on with the project using a linear power supply, but then the BP4078 blew up, too. I was partly to blame this time, since I forgot to turn on the cooling fan, but when it blew it stuck its output to the rail and would have taken out the speakers.

Here's a picture of it while it was briefly working.
scopeboy's l33t photo blog

JMF, thanks for the link to the HH stuff! I suppose I agree with you. Most of the posts we see are related to the Marshall "Mode Four" MF350, that tried to get 350W from four of these amp chips. Not a very robust or musician-proof design! Enzo and the other professional techs all hate the things.

[J M Fahey]

yea, I know.
Now on to what matters:
Steve, I *loved* your pictures, both the "cold something" and the ceramic tube glowing.
I'm a photographer and can appreciate them in their just value.
I guess that the true explanation about the wild color shifts is that your white balance was very weirdly set, because *all* colors are shifted ......... *or* you played a little with Photoshop
The warm tube picture was perfect; the spiraling red wire on the upper left lends interest to an area that otherwise would have been lacking; that there´s not symmetry to the right half makes it more interesting.
Best of all is that I guess it was there for electrical, not pictoric reasons.
Congratulations.
Both would make excellent album covers.
End of rambling.

[tedmich]

Once I'd figured this out I carried on with the project using a linear power supply, but then the BP4078 blew up, too. I was partly to blame this time, since I forgot to turn on the cooling fan

you mean the loose fan somewhere behind the module? I know its called cold etc. but I'd be paranoid about +200w of any topology without a heatsink & forced air, course I don't play with kV tesla coils either....

Since they're only $0.32/w I am tempted, especially to make the sealed sub resonant equalized subwoofer design from ESP.
Project 116 Subwoofer Amp
A lab12 in a <1ft3 box with an F3 ->20Hz would be fun!

[Steve Conner]

you mean the loose fan somewhere behind the module? I know its called cold etc. but I'd be paranoid about +200w of any topology without a heatsink

So you're saying I fried it? No comment

Thanks for the props JMF! I've been into photography since I was a kid, and I used to do my own B&W processing and printing. I promise there was no Photoshop or messing with the white balance. The colours in the scene really did look like that to the eye, except for the glowing load resistor which was a barely visible dull red.

Most digicams are sensitive to near infrared, you can test this by zapping a TV remote into the lens while watching the screen.