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-   -   LM3876 for small sub amp? (http://www.diyaudio.com/forums/chip-amps/82305-lm3876-small-sub-amp.html)

phreeky82 29th June 2006 05:01 AM

LM3876 for small sub amp?
 
How does this chip perform at low frequencies, any issues with using it for a small powered sub (i.e. with computer speakers)?

The driver wont be capable of incredible ultra-low frequency performance due to its compact size so I don't need something outstanding, but still want it acceptable etc.

RDV 1st July 2006 02:29 AM

I'm using an LM3886 for a 12"sub which shakes the walls. With a decent PS it will do fine.

RDV

tiltedhalo 1st July 2006 04:03 AM

lm for sub
 
With a proper circuit the LM38- series can extend close to DC;)
low enough for you?:bigeyes:

Nuuk 1st July 2006 08:43 AM

I'm using an LM3875 for each of my woofer amps that power TL woofers flat to 20 Hz and they sound very good. :att'n:

luka 1st July 2006 08:56 AM

I have it to. LM3886
http://www.diyaudio.com/forums/showt...0&pagenumber=4
I'm using it for 12" sub for car, with this smps, and I am very happy with LM
Post no.190
http://www.diyaudio.com/forums/showt...&pagenumber=19

FastEddy 1st July 2006 10:37 PM

LM3876, 1 possible setup
 
Suggested circuit: http://www.national.com/images/pf/LM3876/01183201.pdf ... pretty straight forward. Pin out is at: http://www.national.com/images/pf/LM3876/01183202.pdf . Physical plan at: http://www.national.com/packaging/mkt/tf11b.pdf

" 56W continuous average output power into 8 ohms
100W instantaneous peak output power capability
Signal-to-Noise Ratio >= 95 dB(min)
An input mute function
Output protection from a short to ground or to the supplies via internal current limiting circuitry
Output over-voltage protection against transients from inductive loads
Supply under-voltage protection, not allowing internal biasing to occur when |VEE| + |VCC| <= 12V, thus eliminating turn-on and turn-off transients
11-lead TO-220 package
Wide supply range 20V - 94V ..."
From the National specs at: http://www.national.com/pf/LM/LM3876.html

So, one could drive the bass low band pass filter / crossover directly from the output. (I would also suggest putting a 3 db/octive low pass filter with, say, a rolloff around 200 to 500 Hertz in or very close to the input on the circuit board (pin 10) ... to insure that nothing above 500 or so got near the input, thus reducing a lot of headaches with the sub woofer box and/or crossover and cables trying to "resonate" or pass signals above about 300 to 500 Hertz.

National indicates that there may be greater distortion when driving this chip above 40 watts (note the distortion graph with test parameters @ 40 watts into 8 ohms. Chip makers know when their chips look good and when they don't ... so the best graph was done at 40 watts ... you figure it out.) ... and I would make sure that my sub woofer had at least 8 ohms from DC to 500 or more Hertz (installed in the box with crossover) as any less might result in some interesting distortion not otherwise expected.

I would also be very, very generous with the heat sink, say, at least 2 pounds of aluminum. I would also want that 10K variable resistor at input to be the best quality / reliability I could fine. (Cheap heat sink for inside a sub woofer box: 1/4" thick x 1 foot x 1 foot or larger aluminum plate ... mounting hole / connection hole right in the middle. Mount this aluminum plate inside the sub woofer box on the roof ... install your favorite conductive stuffing for static removal as well as dampining ... )

Personally, I would have the power supply outside of the sub woofer box. That way I could add extra power supply filtering as required and be assured that there was little fire danger in side of the sub woofer box. Power supply should have lots of "headroom" too, designing for 100 watts power in and 75+ watts power out. Upper voltage output limit is that 35 VDC (VCC) indicated in the graph ... 35 VDC at VCC (pins 1 & 5) is what results 40 watts audio power into 8 ohms (nominal). I would not include the "mute" feature, leaving that part of the circuit out entirely. I would also make a provision to connect the (internal) 3876 (3886) amp to some external binding posts ... so in the case of a chip or circuit failure I could still drive the sub woofer box from another amp.

..........
FYI: http://www.national.com/pf/LM/LM3886.html is also a good candidate for this = a little more power rating, but I would not loosen up the PS voltage or anything else above as this chip would simply give you a little more "headroom" and possibly a little less distortion than the LM3876.
" 68W cont. avg. output power into 4 at VCC = 28V
38W cont. avg. output power into 8 at VCC = 28V
50W cont. avg. output power into 8 at VCC = 35V
135W instantaneous peak output power capability
Signal-to-Noise Ratio >= 92dB
An input mute function
Output protection from a short to ground or to the supplies via internal current limiting circuitry
Output over-voltage protection against transients from inductive loads
Supply under-voltage protection, not allowing internal biasing to occur when |VEE| + |VCC| <= 12V, thus eliminating turn-on and turn-off transients
11-lead TO-220 package
Wide supply range 20V - 94V ..."

phreeky82 1st July 2006 11:52 PM

wow awesome replies, thanks

so the 3886 can drop in in place of the 3876?

to be honest it's going to be under-used to some extent, for a fairly hardcore computer speaker setup. not under-used in that it wont be playing good quality music at good volume levels (of course it will! :)) but i will be building the amp along with two other chip amps inside the sub box, not yet decided on a built-in or separate power supply but would prefer built-in.

if i go that way the amps + power supply will be enclosed in a metal case with the heatsinks protruding outside of the sub box, so no fire hazard or anything really. i will also go fan forced cooling simply due to the fact that generally my computing environment is not silent as it is (computer fans audible anyway).

i'm tempted to simply buy kits for the amp modules, mainly due to the neatness of properly designed PCBs.

FastEddy 2nd July 2006 12:01 AM

LM3876 v. LM3886
 
" ... so the 3886 can drop in in place of the 3876? ..."

It appears so. The pinout is the same.

As long as the power supply is not driven too hard, either has pretty much the same specs too. It looks like the only difference is the output ... the 3886 is able to drive either 4 ohms or 8 ohms, while the 3876 is speced for 8 ohms only. 4 ohm loads would probably bullox up the distortion ratings of the 3876.

FastEddy 2nd July 2006 12:02 AM

inside v. outside the box
 
" ... if i go that way the amps + power supply will be enclosed in a metal case with the heatsinks protruding outside of the sub box ..."

No worry mate. Go for it.

FastEddy 2nd July 2006 12:05 AM

kit v. roll your own
 
Checkout this: http://myweb.tiscali.co.uk/nuukspot/...loneindex.html


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