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Old 27th September 2007, 12:22 PM   #1
AndrewT is online now AndrewT  Scotland
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Question National LM3886TF or not?

I've opened up an active two way and buried in the depths but covered with a clamping bar are two chipamps.
From the little I can see they seem very similar to LM3886TF, 11pin, all black package.

Are there other chipamps that look similar, or can these only be 3886TF?

They are running on +-39.5Vdc, but the power is quoted in the literature/manual as 50W + 35W to bass/mid & treble. I wonder if the treble driver is 12ohm?
regards Andrew T.
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Old 27th September 2007, 01:12 PM   #2
BWRX is offline BWRX  United States
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They could also be LM3875TF

I don't know if the TO-220-11 amps from other manufacturers are offered in an insulated version like National's LM3875 and LM3876. It does not look like TI's OPA54X series have insulated packages available.
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Old 27th September 2007, 01:40 PM   #3
AndrewT is online now AndrewT  Scotland
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I can see the pins on one of the chips.
Pins 1&5 are commoned. This probably indicates that it is not 3875.
Similarly pins 9 and 10 have separate traces going further into the PCB. Again cannot be 3875.

LM3876 is still a possibility if this is a universal PCB.

Thanks for the input.

Just measured Re (but no access to the drivers) so assuming there are no components on the drivers, I have re= 3r8 and 4r0.
So much for the thought they may be 8ohm and 12ohm drivers.
Still can't understand why the delivered power should be different nor why the quoted power are way below what National say for +-39.5V even allowing for PSU sag under load. +-32V at bottom of ripple should still be 50W/8r0
regards Andrew T.
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Old 27th September 2007, 03:27 PM   #4
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Andrew is it anything like this?
They claim 15w for the high frequency and 40w for the low frequency but there appears to be no difference in the power the amps would give (see datasheet pdf)
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Old 28th September 2007, 08:06 AM   #5
AndrewT is online now AndrewT  Scotland
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Hi Consort,
yes similar. Tannoy Reveal 6D.
It even has balancing transformers for input and slave output on the SPdif PCB. It looks very thorough every pluggable connection is glued together, every screw fixing is glued, all the PCBs are glued to the heatsink (edge on=standing out), every cable between the three PCBs and to the mains and to the drivers are wrapped in a permanent black tape (it is Class11 double insulated).

The more recent versions have a higher power quoted but still maintain a power differential between LF & HF.
Could they have taken account of gain between the two channels and for a constant swept input signal taking the LF to maximum (distortion limited) power output, the treble produces a lower power output? That has got me wondering, could the +-39.5Vdc indicate I already have the newer 75W+50W specified power amps?

I was hoping to use new higher power amp boards to replace the dual chipamps but the packaging to fit this in the volume of the sealed box at the back of the speaker is quite well done and I may have to abandon unless I change the transformer and then I lose the double insulation standard.
regards Andrew T.
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