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Old 18th September 2008, 08:29 AM   #1
digi01 is offline digi01  China
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Question LM1875 as preamp

i got some lm1875 chips.i want to play different.is anyone running this poweramp as opamp?how about its sound?
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Old 18th September 2008, 08:44 AM   #2
AndrewT is offline AndrewT  Scotland
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it might make a nice headphone driving opamp.

Maybe even convert it to run in single ended ClassA for the lowish current that high impedance phones draw. Try 45 to 60mA for 64ohm phones.
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Old 18th September 2008, 09:10 AM   #3
lineup is offline lineup  Sweden
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I guess you have to use at least gain=10.

But maybe there is some TRICK to use less gain, as preamp,
and still keep LM1875 100% stable.

To have gain=10,
for LM1875 as Powerful Pre Amplifier
is maybe not any problem.
Pre Amplifiers can often have x4 - x10 voltage gain.
Same as +12 dB to +20 dB
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Old 18th September 2008, 12:55 PM   #4
digi01 is offline digi01  China
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thank u

im working on it then,will post the result soon.

zang
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Old 18th September 2008, 01:33 PM   #5
lineup is offline lineup  Sweden
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You should carefully study the datasheet
Application Hints
Eventhough LM1875 is fairly easy to use, there are some things that can go wrong, if we are unlucky.

Read this part and onwards.
Quote:
Application Hints
STABILITY
The LM1875 is designed to be stable when operated at a
closed-loop gain of 10 or greater, but, as with any other
high-current amplifier, the LM1875 can be made to oscillate
under certain conditions. These usually involve printed circuit
board layout or output/input coupling.

Proper layout of the printed circuit board is very important.

While the LM1875 will be stable when installed in a board
similar to the ones shown in this data sheet, it is sometimes
necessary to modify the layout somewhat to suit the physical
requirements of a particular application. When designing a
different layout, it is important to return the load ground, the
output compensation ground, and the low level (feedback
and input) grounds to the circuit board ground point through
separate paths. Otherwise, large currents flowing along a
ground conductor will generate voltages on the conductor
which can effectively act as signals at the input, resulting in
high frequency oscillation or excessive distortion. It is advisable
to keep the output compensation components and the
0.1 F supply decoupling capacitors as close as possible to
the LM1875 to reduce the effects of PCB trace resistance
and inductance.

For the same reason, the ground return
paths for these components should be as short as possible.
Occasionally, current in the output leads (which function as
antennas) can be coupled through the air to the amplifier
input, resulting in high-frequency oscillation. This normally
happens when the source impedance is high or the input
leads are long. The problem can be eliminated by placing a
small capacitor (on the order of 50 pF to 500 pF) across the
circuit input.
Rod Elliott of ESP has got one LM1875 application Power Amplifier.
You may read his article here:
20 Watt / Channel Stereo Power Amplifier
Rod Elliott - ESP (From Design Notes from National Semiconductor)
http://sound.westhost.com/project72.htm
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Old 18th September 2008, 01:39 PM   #6
lineup is offline lineup  Sweden
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If you use LM1875
as one very, very powerful pre amplifier,
you may use two switchable output jacks.

> One output for RCA phono stereo to connect to any Power Amplifier.
> One HeadPhones output jack

Because LM1875 will drive any HeadPhones perfectly.
Probably with True Hi-fi, High Fidelity performance.
It is one piece of cake, for one chip designed to drive 8 Ohms speakers.

Lineup audio amplifiers forum

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Old 19th September 2008, 01:43 PM   #7
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Quote:
Originally posted by digi01
thank u

im working on it then,will post the result soon.

zang
Any update yet?
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