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Chip Amps Amplifiers based on integrated circuits

Paralleled LM4780TA PC board design - How's this look?
Paralleled LM4780TA PC board design - How's this look?
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Old 25th April 2004, 05:20 PM   #11
cdoggy81 is offline cdoggy81  United States
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Sounds good & keep us posted. Have you worked with the 3875? Would be great to get a sound comparison when you are done
It's better to have loved & lost than to never have had a good pair of speakers at all.......
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Old 29th April 2004, 12:45 AM   #12
OddOne is offline OddOne
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Next pic on the hit parade... Testing the fit and finish of parts on the board.

Near as I can tell I do need to change pin spacing and orientation on a couple parts, but otherwise the circuit fits rather well. The only negative is that to get the board this compact requires that some parts "float" - a few are not pressed flush against the board. From a purely electronic standpoint this does increase the chances of radiating or picking up interference, but I'll be ecstatic if the damn thing simply works.

The electrolytics are low-ESR and gain resistors are 0.1% 50PPM precision jobs that cost way too much for a 1/4-watt resistor.

Tomorrow I will probably have the board soldered up and be resy to start testing.

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Old 29th April 2004, 02:04 AM   #13
digi01 is offline digi01  China
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i like this sample style PCB boards,it looks as though NS Eval Boards
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Old 29th April 2004, 03:21 AM   #14
OddOne is offline OddOne
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I couldn't wait.

Spotted a design flaw in the board layout. Easily corrected via workaround but it'll require slight change in a resistor location later.

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Old 1st May 2004, 02:23 AM   #15
OddOne is offline OddOne
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The two built-up boards survived my preflight testing - powering up with two 12VDC 1A switching supplies. Low enough voltage/current that problems won't be catastrophic, while allowing for obvious go/no-go testing, but sound quality's atrocious.

That part of the process completed safely, the first board got hooked into the eventual home of whatever amp I decide to stick with - a portable package with a welded square-steel chassis frame and an integrated LED power meter. Its power supply is currently only sporting 6,800uF of filter capacitance on the supply outputs, but there's plenty of room for me to add a few extra tenths of a Farad once everything else is to my liking. This same rig was going to hold a pair of TDA7293s, but they ended up being nothing but problems - thus far I'm not impressed with how much the 7293s like to explode violently over being looked at funny.

My initial impression is that in the same testbed system with LESS supply capacitance, the LM4780 in paralleled mode sounded BETTER than the TDA7293 single-mode that it will probably now be replacing. Less noise (a LOT less noise - the S/N ratio seems way better), no transients on power up/down, smooth operation to the ear when fed a sweeping sinewave, no apparent audible hum in the circuit despite my not using a true star ground. (My board design uses a lot of groundplane area on both sides of the board and a TON of vias to interconnect them for ensuring less resistance mismatch from GND terminals to the 4780's ground pins. Apparently this gameplan works!)

I'm using a higher-than-ideal supply voltage - 43VDC above and below ground at the board's power points - but aside from the expected extra heat the 4780 had no problem dealing with the slight overvoltage. (They want 84VDC max. ideally from V+ to V-.) The heatsink I used for the initial full-supply test was only 4.5" x 2.5" x 1.25" but took an hour at about a watt's output continuously to get warm enough to be of any interest. (Insulator is a single layer of Kapton MT100 film, with standard aluminum-oxide white thermal goo as the interface layer.) Heatsink is back to room temp in three minutes. I suspect that using a Xeon heatsink and fan (as in the Xeon processor) will be adequate for everything short of sustained operation at or very close to the 120-watt limit of each paralleled-4780 board.

So far I'm impressed. I just hope they keep working for more than a few hours...

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Old 1st May 2004, 03:29 AM   #16
Damon Hill is offline Damon Hill  United States
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I'm interested in the outcome of your experiment too.
Not just the sound quality, but the survivability of the
chip at those voltages. You may be pushing the SOA
limits into a given speaker load, some of which will be
a lot more demanding than others. Hopefully the
protection circuit will save them from terminal abuse.

I'm listening to my first attempt based on LM3886s,
driving Acoustic Research AR-11s (4 ohm) via Kimber
4TC cables, arguably a demanding load for chip amps.
So far, so good, but I'm running at a lower voltage
and not pushing the volume too hard. One channel
has active voltage regulation with LM317/LM337 devices;
I don't recommend this practice.

I also incorporated the suggested load protection
RL circuit and have a fairly generous heatsink. It's
all point-to-point 'haystack' wiring.

I'm more interested in the LM4780s for the 'serious'
amplifier as it should be a good bit more rugged. I
may use BrianGT's newest board and better quality
parts, though with a lower voltage toroid in the
power supply than the one I'm using now.

So far I'm reasonably pleased with the sound quality;
not quite the equal of my Leach amp, but quite
good even with a strong bass bottom. There's a
good soundfield and good imaging, and I hope the
LM4780 will equal or better it.

I'm leaning towards a compact 'integrated' amplifier
with three line level inputs from my tuner, CD and
computer sound card. Got most of the parts, but
I'm stumped on how to build a decent looking chassis
for it.
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Old 1st May 2004, 02:11 PM   #17
OddOne is offline OddOne
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Damon Hill:

Well, according to the datasheet the absolute supply limit is 94 volts and I'm feeding 86, but also according to the datasheet the overvoltage protection could kick in at anything above 84, so once I get some beefier heatsinking in place I'll hook up my homebuilt 130W RMS speakers (4 ohm impedance, which WILL stress the amps) and see if it can sustain a sine sweep at say 75 watts.

One thing I did notice is that there's NO DC on the outputs AT ALL. My calibrated Fluke DMM was reading 0.0007 VDC (yes, it's a 4-1/2 digit scale) across the outputs during one of my no-input-signal checks. This is definitely a good thing - tells me I didn't build a pooch-screw right into the circuit.

As for how to house your completed project, I've seen some incredibly sharp buildups here. Plenty of ideas around this board to imitate or be inspired by.

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Old 4th May 2004, 01:21 AM   #18
OddOne is offline OddOne
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Another pic - this one's a shot of one of the amps undergoing basic go/no-go testing as per the above description.

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