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Old 11th June 2013, 03:57 AM   #11
agdr is offline agdr  United States
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I cleaned up the layout a little bit, still just pondering if things would fit. It looks like the clipping indicator circuit will still fit if I go with the super-small PW package for the TLO64. The clipping indicator could just be left off (not populated) to avoid any super small SMD soldering. All of the rest is no smaller than SOIC-8 and 1206.

I'll eventually post a 4-layer version which will make the remaining routing pretty easy. One more routing layer and one full groundplane. I may make an exception to not having any signal currents in the ground plane on this one and instead have the required boatload of the LME49990 decoupling caps (not on there yet) connect to the ground plane.
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Last edited by agdr; 11th June 2013 at 04:01 AM.
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Old 11th June 2013, 10:17 PM   #12
agdr is offline agdr  United States
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I know there are a couple of other massively parallel op amp designs out there from the past that I've run into. I just came across a link for one of them, a NE5532 amp published in Elektor in 2010:

The 5532 OpAmplifier, part 1 - ELEKTOR.com | Electronics: Microcontrollers Embedded Audio Digital Analogue Test Measurement

The S/N improvement trick in paralleling (noise is RMS, signal is linear) apparently produced a 15dB S/N gain. At 16W they were shooting more for a power amp (speakers) than a (much lower voltage swing and distortion) headphone amp. I'll see if I can still buy that article.

Last edited by agdr; 11th June 2013 at 10:21 PM.
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Old 16th June 2013, 06:37 PM   #13
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S/N for this amp isn't that spectacular, especially considering that it only has a gain of 6 (~15.5 dB) if you do the math. Building a power amp with a dynamic range of 125-126 dB(A) isn't rocket science. Equivalent input noise is about 12.5 nV/sqrt(Hz) here.

I'm not too fond of those massively parallel opamp designs. While designing a 5532-based speaker amp was probably fun, they strike me as inefficient and too "brute force". 32 5532s can deliver what, 1.6Arms? That's a whopping 5 watts into 4 ohms. At 160 mA of idle current no less. With just about a gazillion solder joints. Methinks using a buffer involving 3 pairs of nice medium power transistors would leave plenty of room for SOA protection circuitry.
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Old 17th June 2013, 02:13 AM   #14
agdr is offline agdr  United States
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Yeah, I agree, I think those legacy parallel op amp designs intended for speakers were really just done for fun. Parallel op amps are the wrong thing for speakers. For headphones at least the load is more in the ballpark, if narrowed down to the 2/3-or-so of cans that don't have huge current or high(er) voltage requirements.

My biggest hiccup so far on this amp is that the build really isn't looking very DIY. If soldering 18 SOIC-8 packages doesn't drive someone nuts, add 32 0603-sized decoupling caps and the super small TLO64 package for the clipping indicator. I think the board would need to be sent out for assembly, making it no longer DIY. Maybe there is something in the middle like getting the SMD parts done by an assembler but leave the through-hole parts as DIY.

Last edited by agdr; 17th June 2013 at 02:17 AM.
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Old 27th June 2013, 12:12 AM   #15
agdr is offline agdr  United States
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By request, here are the Eagle schematic and PC board files for the 2 layer version of this amp if anyone would like to try messing around with it in the freeware version of Eagle. You will need to create and add some library parts that will come up missing, like the LME49990, the jacks, the rotary switch, etc.

Unzip the files and put them in a directory somewhere. In the Eagle (V6.4.0) Control Panel (first window that comes up when you start it) go to file -> open -> schematic then browse to the .sch file. Same for the board, file -> open -> board and browse to the .brd file.
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Last edited by agdr; 27th June 2013 at 12:19 AM.
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Old 31st July 2013, 03:51 PM   #16
Fenris is offline Fenris  United States
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If all the SMD parts are on one side of the board, its sooooo easy to solder:

1. Buy an electric skillet
2. Using a soldering iron, add just a touch of solder to each pad.
3. Clean board
4. Spread a thin layer of solder paste over the pads. If you're obsessive, use a toothpick to put just a little on. Personally, I just wipe it on with a finger.
5. Place the parts where they're going to go. The paste will prevent minor jostles from moving anything.
6. Put the PCB on the skillet (SMD facing up) and turn on skillet
7. When solder melts, SMDs will snap into alignment from surface tension
8. Turn off skillet, remove PCB, clean. Done.

I can do 5 small boards faster this way than with through hole components.
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Old 31st July 2013, 04:29 PM   #17
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I do something similar, but I use solder paste and a toaster oven.

I you use a skillet/toaster oven or similar, you should never use
it to cook food with again.

Last edited by Avro Arrow; 31st July 2013 at 04:31 PM. Reason: spelling
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Old 31st July 2013, 04:57 PM   #18
agdr is offline agdr  United States
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Fenris & Avro Arrow - Interesting! Well that would make it workable. I had given up at the thought of hand soldering all those chips.

Hmmm... I may mess with this one a bit more as time permits. It would be fun to give one of those soldering methods a try.

I'm really curious how this amp would sound. The design should preserve most or all of the insanely low distortion numbers in the LME49990 data sheet. They show 0.000005% THD at +/-15Vdc rails into 600R with an 8Vrms swing. That is for one chip. Paralleled like this it should drive lower impedances with the same result, but with an even better S/N (signal adds linearly while noise adds as RMS).

Last edited by agdr; 31st July 2013 at 05:09 PM.
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Old 31st July 2013, 05:00 PM   #19
qusp is offline qusp  Australia
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I hope you are using lead free paste Fenris smearing it on with your finger like that.
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Old 1st August 2013, 11:59 AM   #20
Fenris is offline Fenris  United States
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Actually, I'm talking about flux paste. Like this stuff:

2 oz. Non-Spill Rosin Soldering Paste Flux : Soldering Tools & Supplies | RadioShack.com

The SMD soldering paste with flux and solder is a pain to work with, so I don't bother.

The technique is covered here:

https://www.sparkfun.com/tutorials/59

except I use a regular iron and solder on the pads first and then add some new flux to ensure a good clean joint.

I've used this to solder several 41Hz AMP32 boards, as well as a bunch of my own design. You might get a little scorching of any silkscreen on the bottom, but otherwise it works very well.

BTW, I've also got my own design for something similar. I'm putting eight quad opamps on one board, good enough for two headphone channels. Paralleled boards can get up to eight or nine watts, bridged up to 32 watts.
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Last edited by Fenris; 1st August 2013 at 12:11 PM.
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