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Old 7th December 2015, 11:07 PM   #1
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Default What transfo(s) for 4 3886?

Hi
I have already a stereo power amp for basses and I miss 4 other mono amps to acheive active tri-amping my speakers. The mediums are 8Ω nominal and tweeters are 6Ω. Some years ago I bought 16 MUR860, 4 heatsinks fisher sk92-100 (rated 1°K/W) and 4 LM3886T I'll use with thermal grease and no isolators (I know what I do).
Now I get some time to work again on the project.
I'd like to have the maximum power possible available. Living in France where mains are 230Vac guaranteed +/-10% and reading the chip and transfo datasheets I ended discarding +/-30Vac toroids I first selected unless I build a regulated supply what I'd rather not want.
Also, I would prefer the build is not dedicated to this first application but instead would be standard multipurpose amps, so I'm ready to forget energy consumption optimization related to different loads.
I hope I didn't forget something ... oh yes, all the 4 amps in a single case, and I already built 4 line receivers with THAT1206.
So what transfo(s) can I use? I read 160VA is ok for a stereo amp. So a single 300VA would provide the needed power plus a better so called "regulation", won't waste my money and make a lighter case. Or a pair of 300VA? Or 4 80VA or 160VA (in this last scenario I'll miss 16 rectifiers)?
Thank you for advises.
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Old 8th December 2015, 05:09 PM   #2
00940 is offline 00940  Belgium
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Did you read this ? LM3886 chip power amplifier power supply design.

Especially relevant data at the bottom of the page.
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Old 8th December 2015, 05:28 PM   #3
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I began with this one: Building a Gainclone chip amp power supply.
Thanks for your link man. I hope I'll find my honey.
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Old 8th December 2015, 05:37 PM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by HumNSmoke View Post
.................Living in France where mains are 230Vac guaranteed +/-10% ................
France is on the EU harmonised supply.

Your range is 230Vac +10% -6% i.e. 216Vac to 253Vac.
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Old 8th December 2015, 05:40 PM   #5
AndrewT is offline AndrewT  Scotland
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Four 60W chipamps delivering maximum power at the same time would work well with a 240VA to 480VA transformer.

But, I'll bet that all four will never be asked to deliver maximum power at the same time.

I suspect you could reduce that VA range to 160VA to 300VA and it will still work well.

You will either have all four operating from a common PSU with a common Main Audio Ground or you need to have all four isolated from each other and requiring 8 secondary windings. This last option is obviously unworkable.
That leaves the common PSU.
You now have a further choice. dual secondaries and dual rectifiers and then series connect to create a Main Audio Ground.
or
Centre Tapped secondary with a single bridge rectifier and the Centre tap becomes the Power Zero Volts that heads out to connect to the Main Audio Ground.
This last has just one bridge rectifier for the four channels.

However you need another bridge rectifier for the Disconnecting Network that connects the Main Audio Ground to Chassis. I use the double shorted version ~ shorted to ~ and + shorted to -
Then connect one corner to Chassis and connect an adjacent corner to Main Audio Ground.
You can use a ¼W 10r resistor across two adjacent corners and use a 100nF X7R across another pair of adjacent corners.
If you want to experiment with ground lifting, then add a switch across two adjacent corners.
Closing the switch defeats the Disconnecting Network. Opening the switch lifts the ground and makes the DN active.
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Last edited by AndrewT; 8th December 2015 at 05:50 PM.
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Old 9th December 2015, 04:53 PM   #6
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Hi 00940 & AndrewT
Nick Whetstone (actually Rod Elliott) advised 5W for the 10Ω in the earth/ground loop breaker.
Quote:
Originally Posted by AndrewT
Then connect one corner to Chassis and connect an adjacent corner...
I'd rather stay away this topologic way to explain how to connect this loop breaker because I'd be afraid a newbie gets hands on a 35A bridge with AC leads on the same side (like 4pins DIP6 bridges are): then he would short earth to ground for more money than the needed simple wire


Also I spoke too quick when I said our mains are +/-10% waranteed. I found it there https://particuliers.edf.com/fichier...V_PART_MCE.pdf but an other one says the standard is
http://fr.leonardo-energy.org/wp-con..._2_en50160.pdf so the % depends on whether we are customer or supplier in this matter. These parasite guys in european administration make me vomit with their tons of useless standards (but that's another story). Whatever all this makes transformer selection even hardest. I'll try to stick to advised if I find.
BTW, I didn't say 480VA, but instead 4*80 or 160.
Having yet the 10cm/4in tall heatsinks I imagine I'll put all the stuff in a 3U case I'll split in 2 floors. It is better to screw the 3886 to the low side of the heatsinks instead of centered or on the top, isn't it? If yes the audio part will live in the lower floor and the supply in the upper one. As toroids with lower VA are usually flatter I feel I'll use 2 or 4. Regards to your link to neurochrome I discovered the Antek brand which is really a great company with huge choice opened to us DIYers, light years in front of radiospares and their electrostatic/magnetic unshielded/chinacheapo. Antek even gives us an additional measurement for output voltage under mid load! <sad>Why don't have no more such a factory/shop in Europe, toroids weight will ruin our bucks in shipping cost</sad>?
neurochrome is really another great place. I even discovered the guy Tom Christiansen cooked a 3886 and even uses the THAT12xx A chance I didn't yet solder mines on my own pcbs fully balanced line with THAT 1206 plus 1646 .
Are its schematic/pcb drawing freely available somewhere?
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Old 9th December 2015, 05:50 PM   #7
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by AndrewT View Post
Four 60W chipamps delivering maximum power at the same time would work well with a 240VA to 480VA transformer.
That matches my math (linked to in Post #2) for sine wave operation. 63 VA per channel (8 Ω) and 121 VA per channel (4 Ω).
However, few people listen to pure sine waves at full blast, so unless you intend your amp to operate that way, you can reduce the transformer requirements quite a bit.

For a music signal with 14 dB crest factor, you'll need 27 VA per channel (8 Ω) and 51 VA per channel (4 Ω).

Above numbers assume ±28 V rails.

Quote:
Originally Posted by AndrewT View Post
But, I'll bet that all four will never be asked to deliver maximum power at the same time.
One could make some assumptions about the frequency distributions of various kinds of music, crossover frequencies of the speakers involved, and calculate the amount of power delivered per channel. You may very well find that all channels deliver considerable power more or less continuously.

Quote:
Originally Posted by AndrewT View Post
However you need another bridge rectifier for the Disconnecting Network that connects the Main Audio Ground to Chassis.
Considering such a network would violate the electrical code in most countries, I'd shy away from making such a recommendation.
There's nothing mystical about the grounding of a chip amp. Some like to think it's some sort of black art, but the art is really only black until someone turns on the light. It's perfectly possible to design an LM3886-based amplifier which doesn't suffer from hum issues.

Tom
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Old 9th December 2015, 09:08 PM   #8
00940 is offline 00940  Belgium
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For good toroids in Europe, check Transformatory toroidalne - Producent transformatorów Toroidy.pl

You can also get custom transformers from audiophonics.fr . I don't know what their big ones are worth but I've no issues with those, up to 120VA.
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Old 11th December 2015, 07:58 AM   #9
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Quote:
Originally Posted by HumNSmoke View Post
I have already a stereo power amp for basses and I miss 4 other mono amps to acheive active tri-amping my speakers.. . .
I couldn't answer the transformer question without more information.
What is the wattage rating of the bass amplifiers?
What is the crossover frequency between the bass and midrange amplifiers?
What is the load and efficiency of the bass drivers?
What is the efficiency of the 8 ohm midrange drivers?
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Old 13th December 2015, 09:47 AM   #10
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Hi Daniel
Sorry I didn't reply sooner, I was (and I'm still) writing a reply post to 00940 & tomchr about the transfo voltage sizing...
The bass amp is the home made early 80's Crescendo (1kVA supply), 2*140Wrms @ 8Ω
The cabs are home made too, in the same old days.
L 8Ω 80W 90dB@1W@1m) > LP 1to2kHz
M 8Ω 80W 92dB@1W@1m) same good flat from 1k to 4kHz (0 to 30° off axis)
H 6Ω 100W 90dB@1W@1m) > HP 2to4kHz
I want to start this stuff again, but I removed the passive cross-over (weird, I think because of inductors ferrites I randomly choose in these years).
I'll start with a behringer dcx2496 I bought some years ago when I got some parts for the 3886.
Whatever I'd prefer the 3886 build limitations/tradeoffs would not take into account an eventual undersizing of the bass amp, so that the build could take place in any other larger association, or even [a lower?] as standalone with basses handled.
These are info you missed I hope. IIRC there was some info in a THAT datasheet or application note which talked about the real music power spread along the audio frequency range.

Last edited by HumNSmoke; 13th December 2015 at 09:49 AM.
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