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What? - A Low Power Desktop Amp

Posted 6th April 2013 at 08:52 PM by BuildMeSomething
Updated 9th April 2013 at 07:31 PM by BuildMeSomething (Updates)

2] Basically, this is me trying to do a 2-layer PCB layout based on the LPUHP circuit schematic.

I'm using FeePCB for laying out. Not hit any problems with the software yet and it produces Gerber RS247X files. The parts library is none specific, so I'm having to generate footprints from datasheets as I go.

1) Untested PCB design [should work though].
2) 2-Layer PCB.
3) 2 channels per board [16 x LME49600].
4) LME49722 with 4x gain.
5) Up to +/-18VDC rails.
6) Separate regulated power supply.
7) Utilise a 10K Alps RK27 for volume control.
8) 8 Watt into 8 Ohm [based on 2VRMS input].

Aim is to create 1 x power supply & 1 x amp board of a small enough form factor to fit in a desktop friendly case. With an output of 8 Watt into 8 Ohm load, for up close & nearfield listening in a small room.

Why the LME49722. Its drop in replacement to the nice 49720 but will run on +/-18VDC, and has slightly lower THD+N figures. Moreover, the '720 hits a performance wall at 7Vrms output and THD figures rise rapidly at this point, the '722 maintains it's low THD figures up to 10Vrms. I'm aiming at 8 to 9Vrms output... Still not as nice as the 49990.

The PSU Gerber files were sent to EzPCB last week, the 5 prototypes should arrive some time in the next week. The files for the LPDA are ready to go, these will be sent this week. Then a Digikey order once I'm back off holiday in two weeks time...

Update (13-04-07):
Mistakes so far - luckily nothing serious on the PSU boards; 1) PCB mounting holes are M2.5, M3 size would have been better. 2) Through holes for AC input measure 0.75mm, => 1mm would have been better.

Another possible problem is drill sizing for through hole parts. FreePCB uses finished sizing, so drill file either needs altering or board house checks and adjustes drill bit size... I didn't check so possible tight fitting parts ahead on the PSU boards. *** Now checked & all OK***

Checked over pad size, layout & through hole sizing on the Amp PCB and all looking ok.

Update (13-04-8):
Power supply demands. I had a naive hope that the 317/337 +/-18V regulated supply with six 4,700uf capacitors would be enough to keep the two channels happy, however it looks increasingly likely that it will fall a little short.

Solution 1) Try LT1033/LT1085 TO-220-3 devices instead of the LM317/LM337.

Solution 2) Go mono... The prototype Amp files haven't been sent yet & I have done a mono version.


Board sizing:

PSU - 64mm[W] x 66mm[L]
AMP - 70mm[W] x 130mm[L]
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