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Old 5th November 2015, 10:19 PM   #1
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Default LM3886 Power Output

HI ALL
I have built an ESP project 19
Single Chip 50W Stereo Amplifier
using the LM3886 PCB

Also his project 97 Hifi Preamplifier
Hi-Fi Preamplifier
From scratch.
I have to say this thing is incredible
According to him this will put out 56 watts at +/-35volts into 8 ohms
BUT I used a 2x28 volt transformer +/-42volts Per Rail
And this thing is giving my Pioneer SX950 85 watts per channel some stiff competition
What is this LM3886@+/-42 volts putting out at 8 ohms (Cooling Fan)
(The Pioneer is still a Monster)
Thanx
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Old 5th November 2015, 10:50 PM   #2
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VV/R=watts
220 watts peak or 110 watts RMS
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Old 6th November 2015, 12:04 AM   #3
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Quote:
What is this LM3886@+/-42 volts putting out at 8 ohms (Cooling Fan)
I hope you never turn it up to 100% output and the speakers are a true 8 Ohms, otherwise you will be inviting trouble.

Last edited by Tromperie; 6th November 2015 at 12:10 AM. Reason: Correct an error.
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Old 6th November 2015, 12:28 AM   #4
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Before I built the preamp I plugged it into cd player and ran it full blast for an hour.
What was supposed to have happened?
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Old 6th November 2015, 12:36 AM   #5
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Why not read the LM3886 app. notes and find why (not simple) a max +/-35V for 8R loads is recommended and more than that is risking damage that the thermal limiting circuits can't cope with. When the nominal speaker load impedance drops to 4R, the recommended supply limits are then reduced further to only +/- 28V.
http://www.ti.com/lit/ds/symlink/lm3886.pdf
http://www.ti.com/lit/an/snaa021b/snaa021b.pdf
Note comments in sections 4.1-2 in the second doc.

Just connecting the higher rail supplies would not pose much of a threat but when a speaker is also connected and you approach a higher than maximum rated output before clipping, the amplifier will be distorting badly, because thermal limiting will also be acting to keep the device within its SOA. It may eventually fail or not but it won't be doing what is considered safe or recommended operation. You don't actually hear any extra clean watts as by magic. The price is heavy distortion on broad signal peaks.

Maybe you like it that way or maybe you don't notice. The good news is that the difference between 68 and 85W is almost zero - you can't easily pick the difference directly other than by secondary vibrations like speaker rattles, or objects in the room. If you do hear that your amp is louder and much cleaner, a full service to the Pioneer amplifier is probably long overdue.
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Old 6th November 2015, 01:47 AM   #6
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by nigelwright7557 View Post
VV/R=watts
220 watts peak or 110 watts RMS
NO WAY you can get that much power out of +/-42 V rails into 8 ohms.

1) the power transistors have a saturation voltage across them, if no better data or mesurements available, 4V drop is a good (real world) estimate, so your +/- 42V now become +/- 38V peaks ... being optimistic.
2) we don't have power supply details, but no regulated one was mentioned, so I assume a regular iron transformer + diode bridge + filter capacitors raw/unregulated one is used, what 99% amps use by the way, and it will also drop, you must also substract peak to peak ripple voltage , both add to at least 20% drop, maybe higher if not generously filtered.
This would leave some 30 or 31 Vpk available, some 110/120W peak or 55/60W RMS .
In any case, the built in thermal protection will quickly turn on as soon as the case (and chip) start to heat up.

Probably no big deal in a home or Hi Fi situation, where amp may be used at , say, 10% of its power most of the time; a definite problem for DJ/PA or Musical Instrument use.

Dylon Amps, an Argentine MI amp manufacturer, had big problems because they rised supply voltages to get true 70W RMS into 8 ohms (it's datasheet rated around 50/8) and amps started muting at random times when used live, yet most worked very well on rehearsal duty.

So that's an example of what you may find.
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Old 6th November 2015, 09:11 AM   #7
AndrewT is offline AndrewT  Scotland
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stupid decisions giving rise to increased risk of damage.
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Old 6th November 2015, 09:31 PM   #8
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Quote:
Originally Posted by AndrewT View Post
stupid decisions giving rise to increased risk of damage.
This Amp is crystal clear and has plenty of Backbone.
Has no negative effects at all @ 42 volts.
The spec sheet says 84 volts max voltage.
I have 2 more of these transformers that are scheduled to feed some more of these LM3886's.
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Old 6th November 2015, 10:25 PM   #9
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Quote:
Originally Posted by CaptainCrunch View Post
The spec sheet says 84 volts max voltage.
At no load. You need to measure power supply droop at high output.

With a musical program ( transient in nature) and forgiving speakers (nice impedance curve) you can get away with stunts like this. I bet some popular speakers would give your amplifier fits.

And if you did a steady state test with sine waves, you would see your amplifier exhibit some of the characteristics you've been warned about. All the caveats you've been warned about are real.
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Old 6th November 2015, 11:27 PM   #10
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Quote:
Originally Posted by Fast Eddie D View Post
you can get away with stunts like this.
As long as this thing sounds as Great> as it does>>(Make that Excellent)
Plenty of Power and doesn't blow up or catch on Fire. I'm Happy.
I custom made a 12"x15" chassis made out of 1/8" aluminum and bolted the heat sink directly to it with holes drilled thru the chassis right under the heat sink, also Drilled holes all across the front of the Chassis for evacuation of accumulated heat from the transformers and the power supply(s). Cooling fan in the middle of the back Panel drawing out. I know what these chips do when they get HOT because I disconnected the cooling fan and let it get hot. It's protection circuit kicks in. It does not do that with the configuration I have. The Aluminum Chassis is also a giant heat sink its self. Also there is no evidence of the SPiKe Protection Circuitry kicking in because it would be very Audible.
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