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Old 10th September 2007, 07:42 AM   #11
suzyj is offline suzyj  Australia-Aboriginal
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Here's a screen-grab of the PCB, for those without the ability to read Protel stuff.
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Old 10th September 2007, 01:36 PM   #12
bob_v5 is offline bob_v5  England
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wow, cheers for the help guys.
When I said about stacking chips I did mean to wire them in parallel, but physical stack the chips, with resistors solderd directly to the pins that couldnt just be stacked. I know the assembley would be very tricky, perhaps impossible, but it also seems like the it would give the best finnished amp if I could do it.
It was the amp that suzyj was talking about that inspired me!
If I made the same board is there any reason to use surface mount stuff? (Apart from the TPA's obviously) I have limited experience with SMD, but dont have the equipment and dont like doing it!
As for the pads underneath, my plan to solder them is to drill a small hole under the chip, tin the pad before fitting the chip and solder from under the chip through the hole. Dont know if any one has any better ideas, but I have heard a lot of people having problems with the heatsink pad, and this seemed like an obvious way of doing it to me..........
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Old 10th September 2007, 02:24 PM   #13
bob_v5 is offline bob_v5  England
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or just flip the chips upside down and use a heatsink on top, as with the link jcx posted above, doh, I should read things before i start typing away......
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Old 10th September 2007, 03:31 PM   #14
jcx is offline jcx  United States
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suzyj,
by eyeball your thermal pad soldering hole looks bigger than the thermal pad - TI has poor/misleading drawingings some places but you should find a dimensioned solder pad drawing which shows a much smaller exposed pad on the bottom - I'd try 0.080" (2mm) as max thru hole that doesn't overlap thermal pad boarders

actually I'd really just put the chips on upside down again, many CPU/Memory/Graphic chip cooler options with clamps or thermal conductive glue are easily adapatble - but don't short out the pins with silver filled compounds, be very careful with them or use ceramic filled epoxy or grease

I used 1 Ohm current sharing resistors - extra hf impedance from ferrite bead core on output may be needed to isolate cable Cload which could cause high frequency instability (10's MHz)

I don't like narrow gnd fill spacing, "fringing" capacitance adds up suprizingly quickly - and CFA op amps are highly allergic to any stray C on the negative input node - see the demo board layout
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Old 10th September 2007, 05:51 PM   #15
KSTR is offline KSTR  Germany
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jcx,
nice project you did there, 4-layer PCB and all... that "current pump with offset" also is a very interesting solution for the input-CM range problem. I'd definitely be interested in the whole schematic (and I know it will not quite resemble the principle schem you shared). Did you spice-sim the stuff, with the THS6012 model?

A bunch of these TPA/THS in a bridged parallel class A config also would make a great amp for high efficiency mid/treble horns, say, with a BMS 4529 2" coax driver...

- Klaus
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Old 10th September 2007, 09:25 PM   #16
suzyj is offline suzyj  Australia-Aboriginal
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Quote:
Originally posted by jcx
suzyj,
by eyeball your thermal pad soldering hole looks bigger than the thermal pad - TI has poor/misleading drawingings some places but you should find a dimensioned solder pad drawing which shows a much smaller exposed pad on the bottom - I'd try 0.080" (2mm) as max thru hole that doesn't overlap thermal pad boarders

actually I'd really just put the chips on upside down again, many CPU/Memory/Graphic chip cooler options with clamps or thermal conductive glue are easily adapatble - but don't short out the pins with silver filled compounds, be very careful with them or use ceramic filled epoxy or grease
The holes are 0.125". They're bigger than the pads on the underside of the TPA6120. The plan was to solder the TPA6120s to the board like a normal SOIC, put short pieces of 1/8" brass rod through the holes with some thermally conductive goo, then clamp the whole board to a small heatsink on the other side. It accomplishes much the same as you've done with your dead-bugs, but without bending the pins.

Quote:
Originally posted by jcx
I used 1 Ohm current sharing resistors - extra hf impedance from ferrite bead core on output may be needed to isolate cable Cload which could cause high frequency instability (10's MHz)

I don't like narrow gnd fill spacing, "fringing" capacitance adds up suprizingly quickly - and CFA op amps are highly allergic to any stray C on the negative input node - see the demo board layout
Thanks for the tips. The fill spacing is 15 mils. I might push it away from the negative inputs a little.
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Old 13th September 2007, 06:50 PM   #17
nonoise is offline nonoise  South Africa
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Quote:
Originally posted by jcx
suzyj,
I don't like narrow gnd fill spacing, "fringing" capacitance adds up suprizingly quickly - and CFA op amps are highly allergic to any stray C on the negative input node - see the demo board layout
I agree with jcx we used these devices at work and they perform very well but they are very fast and require due care especially with regards to stray capacitance.
Watch out for stray capacitance from the ground plane below the device too.

The document he revered to is (slou 169)
from Ti. you will find that their DC offset could be a problem unless the feedback resistors are kept to small values (Ok a speaker is less of a problem than mW rated headphones).
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Old 13th September 2007, 06:59 PM   #18
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Yes, the offset is quite high but the input bias current is in the uA region and can be as much as 12 uA.
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Old 19th September 2007, 09:48 PM   #19
mzperx is offline mzperx  Hungary
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Default Bridged output amp with 20pcs TPA6120

here is my design with 20pcs TPA6120...
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Old 20th September 2007, 06:28 AM   #20
mzperx is offline mzperx  Hungary
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and now the schematic package is attached as well.

Meantime some significant improvements were performed:
The amp has got a new fully symetrical input stage (w OPA1632) and the output resistors were decreased from 10R to 2R. So the total outout resistance is already only 0.1 Ohm.
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