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Differences:
| Homer Nods:
There was an error in the schematic last shown -- not on the
printed circuit board -- the schematic to the right is correct. |
| This LM3875 inverted "chipamplifier"
PCB and kit differs a bit from those seen on many websites.
We have chosen to include a low pass filter on the output which reduces
the possibility that high frequency oscillation will destroy your
speakers. In addition, we allow space for a small Bourns trimmer
potentiometer so that you can set the amplifier to match the output
level of your pre-amplifier. The 1000uF/ 50VDC electrolytic capacitors on
the V+ and V- pins of the LM3875 are bypassed with 100nF/100V ceramic
capacitors, and a 1uF/100V film capacitor bridges the V+ and
V-. Of course, you could omit these additional items for a
"minimalist" approach, but we prefer to prevent problems
before they happen, rather than reacting to them.
The version in the schematic which pops up when
you click is quite low impedance. It also has a lower
D.C. offset than the original version (and theoretically lower
noise). You can experiment with the resistor values to see
what's right for your situation. If you are not using a
preamplifier, use the higher value resistors. If your
source can't deal with a low impedance, use 10K for R3 and R6,
220K for R2.
Boards for this project are available here:
CHIPAMPS
|
| For heat sink selection you should
consult the National Semiconductor interactive design guide at http://www.national.com/appinfo/audio/files/Overture_Design_Guide15.xls
If you intend to run the amplifier to its maximum rating you will need a
heat sink with a 3.5 C/W rating for the LM3875T and 2.5 C/W for the
LM3875TF. The device must be mounted on the sink with either
silicone heat sink compound or a heat sink pad, the appropriate screw
and insulated washer. |
| You can
derive information from the Aavid Thermalloy or Wakefield websites for
matching heatsinks with the necessary C/W rating. It is
important not to under-rate the heatsink! Those little TO-220 sinks
used for your voltage regulators aren't up to the task and will result
in premature device failure. Better to select an extrusion 2 or 3
inches long, 1 inch wide with 4 or 6 fins. Aavid Thermalloy has a
web-based interactive selection tool which allows you to input
dimensions and air circulation velocity to get a handle on the
appropriate sized heat sink for their #42 material. |
| To order, or see the
varieties and permutations, just click HERE |
|
LM3875 Inverted Amplifier Schematic
(click on image for larger version)
 |
Description:
| The LM3875 printed circuit board is shown
to the right. Industry standard 0.062" FR-4 laminate, with
silkscreen to show component placement, green soldermask and plated
through-holes. It measures 3.25" x 2.50". The four
mounting centers are 0.250, 0.250; 0.250,2.200;2.950,0.250 and
2.950,2.200". I panelized the board in order to keep production
costs reasonable so the final dimensions of your board may not exactly
match the 3.25" X 2.500" dimension, but the variations are
only +/- 0.100" or thereabouts. |
| The board uses star-grounding, bringing
the signal path ground to the power ground connection. |
| The Low Pass Filter (10 ohm resistor with
1.0 μH inductor -- the resistor/inductor has no affect until about
100kHz at which point energy is rolled off. The effect of the RL
combination will depend upon the inductance and capacitance of the
cables as well as crossover. If you don't experience HF
oscillations or don't expect to have a long cable run it can be "jumpered". |
|
LM3875Kit Bareboard
|
Parts
List: Low Impedance Version
| Description |
|
Quantity |
Suggestion |
| LM3875T |
U1 |
1 |
|
| Capacitor, 1,000 uF/50V |
C3,4 |
2 |
|
| Capacitor, 1uF, 100V Film |
C7 |
2 |
Can substitute values
from 0.47uF to 1.0uF. Polypropylene types preferred |
| Capacitor, 100nF/100V Ceramic |
C5,6 |
1 |
|
| Capacitor, 1uF PE/PP |
C1 |
1 |
4.7uF shown in schematic. This value will
determine the Lower F3 of the amplifier.. |
| Capacitor 1uF |
C2 |
1 |
1uF Film |
| Resistor, 10 ohm, 2 watt |
R4 |
1 |
|
| Resistor 20k Ohm 0.25 Watt |
R2 |
1 |
Yaego Metal Film |
| Resistor 1k Ohm 0.25 Watt |
R3,6 |
2 |
Yaego Metal Film |
| Trimpot 20k Ohm |
R1 |
1 |
Bourns |
| Input Connector |
|
1 |
Molex 0.100" Male Header |
| Output Jack |
|
1 |
Onshore Tech ED1601 |
| Power Jack |
|
1 |
Onshore Tech ED1602 |
| Heat Sink |
|
1 |
Aavid Thermalloy with thermal
impedance of 3.5 or better |
| Circuit Board |
|
1 |
Click
for LINK to TECHDIY Store |
|
Completed LM3875Kit
|
Parts
List: High Input Impedance Version
| Description |
|
Quantity |
Suggestion |
| LM3875T |
U1 |
1 |
|
| Capacitor, 1,000 uF/50V |
C3,4 |
2 |
|
| Capacitor, 1uF, 100V Film |
C7 |
2 |
Can substitute values
from 0.47uF to 1.0uF. Polypropylene types preferred |
| Capacitor, 100nF/100V Ceramic |
C5,6 |
2 |
|
| Capacitor, 1uF PE/PP |
C1 |
1 |
4.7uF shown in schematic. This value will
determine the lower F3 of the amplifier.. |
| Capacitor 100nF Film |
C2 |
1 |
|
| Resistor, 10 ohm, 2 watt |
R4 |
1 |
|
| Resistor 220k Ohm 0.25 Watt |
R2 |
1 |
Yaego Metal Film |
| Resistor 10k Ohm 0.25 Watt |
R3,6 |
2 |
Yaego Metal Film |
| Trimpot 100k Ohm |
R1 |
1 |
Bourns |
| Input Connector |
|
1 |
Molex 0.100" Male Header |
| Output Jack |
|
1 |
Onshore Tech ED1601 |
| Power Jack |
|
1 |
Onshore Tech ED1602 |
| Heat Sink |
|
1 |
Aavid Thermalloy with thermal
impedance of 3.5 or better |
| Circuit Board |
|
1 |
Click
for LINK to TECHDIY Store |
|
|
| Assembly: |
| The smallest parts
should be plugged into and soldered onto the circuit board first,
starting with the 100nF ceramic capacitors, resistors, film
capacitors, potentiometer. |
| Suggestions:
|
| I have mounted
the resistors on the top of the board, and on the bottom. It
doesn't seem to make any difference in the performance. If
you use more exotic resistors you may have to mount them on the
bottom of the board IN WHICH CASE YOU SHOULD SLIP A PIECE OF HEAT
SHRINK TUBING OVER THE LM3875 LEADS so that they don't make
inadvertent contact with the resistors. I have left enough
space for the input coupling capacitor so that a variety of
devices can be employed. Personally I use the Siemens/Epcos
1uF/100v "silvercap" which is included with the kit. |
| You can omit the trimpot on the input of
the amplifier and hardwire a resistive voltage divider such that the
maximum output of your preamplifier does not result in overloading the
amplifier. Once again, consult the National Semiconductor "Overture
Design Guide". For those of you who are doing this for
the first time the trimpot is a good idea -- if the amplifier is even
slightly overdriven it will distort badly. |
| I have found that a
polypropylene capacitor of 0.47uF to 1.0 uF works best at C7.
You need a device rated for about 150% of the combined supply
voltages at this location. That is, if you have +/- 28V
rails a 100VDC rated capacitor is probably about right. |
| I use 0.100" Molex connectors for
the input, but you can hard-wire the input. The OnShore Tech
connectors for the power supply connections and speakers will
accept up to 14 AWG wire, but I would deem this "overkill" in
this application. I am always plugging in and disconnecting so the
connectors are a handy feature. |
| Have
an oscillation problem? If the chipamp oscillates
(if you don't have a scope you will note that the heat sink may
get very hot, very quickly) the first thing to do is eliminate the
potentiometer. I have found that some multi-turn
potentiometers caused oscillation at 1.2 MHz. Remove the
potentiometer and jumper the input directly to the input
capacitor. If you are careful with the wiring and include
the 0.7 to 1.0 microHenry choke the amplifier is not likely to
oscillate. |
| Last Modified
December 4, 2006 |
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