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BrianGT 9th May 2008 05:12 PM

Finally made a LM1875 amp
2 Attachment(s)
After having a stack of boards sitting around for a year, my friend Allan convinced me to help him make an amp with them. We put a stereo amp together with them last night. I am quite pleased at how the boards work together.

Now it is time to put together a nice compact chassis for it. We used a small Avel Lindberg 2x18vac transformer.

Here is a pic:

BrianGT 9th May 2008 05:13 PM

2 Attachment(s)
another pic:

(this is just the testing setup. we will put rewire it up in a nicer chassis soon)

rickmcinnis 9th May 2008 05:49 PM

Stuffing guide?

I have been in the same mode with the boards I got from you.

Would you mind posting a stuffing guide?

Everytime I looked at them I got myself confused!


Rick McInnis

zymurgn 9th May 2008 08:27 PM


I(Allan) am currently working on that supply list for you, as well as a manual that is thorough yet not too confusing to a novice DIYer such as myself.

In the meantime, Brian and I can answer questions you might have to help you put those boards to use.

Bluto 10th May 2008 12:45 AM

Happy to see this. More efforts to be newbee friendly.

You reap what you sow - it will come back to you in spades.


rickmcinnis 12th May 2008 09:44 PM


Thanks in advance!

I have built numerous of Peter Daniel's 3875 amps, which are self explanatory.

There seem to be places for additional components that confuse me as to where to put the "important" ones.

My intention was to uset the same values/components as recommended by Mr. Daniel so I can get a true idea of the differences between the chips.

I look forward to seeing the "stuffing guides" becoming availabe. Thanks again,

Rick McInnis

digi01 13th May 2008 12:58 AM

cute unit!

danielwritesbac 13th May 2008 04:29 AM

An observation:

I've noticed something about the LM1875 and its power supply.

Mine likes several of 2200uF per rail much better than any amount of larger caps. There's a difference to the speed and impact of the bass notes. This "impact" is helpful.

At onboard with the amplifier, it doesn't matter so much, except that the sound is clearest with one of these: 220uF, 330uF, 470uF. And, when the sound was clearest by this selection, heat is lowest.

This arrangement allowed for 30v rails and running cool; however, it was past the amperage limiter when used along with 4 ohm speakers. Generally, the 26v rails worked for most speakers. On the 30v rails, use with 8 ohm speakers is quite exciting.

For the rectifier, it matters not whether its botique or a simple arrangement. A nice, trouble-free KCPU1004 (KBU 10a 400w) one-piece rectifier is brilliantly arranged so that its + and - can correspond to the traces on a PCB, lining up nicely with the capacitors. This performer also makes hookup to be nicely simplified.

A little helper: At the power supply board, one of the power caps (per each rail) can be bypassed with 47nF or 68nF. This has been observed to lend a bit more speed to the bass notes.

Well, that was my observation for today. ;)

BrianGT 13th May 2008 04:47 AM

I haven't had a chance to work on the manual yet, but here are some board pictures, and info:

Amp board:
R1 - not used (optional resistor to ground before C1)
R2 - 22k (input to ground resistor)
R3 - 1k (gain resistor , gain = 1 + R4/R
R4 - 22k (nfb resistor - on bottom of pcb)
R5 - 2.7 2w (optional - zobel network)
R6 - jumper (optional resistor between input ground and output ground)
C1 - 2.2µF (optional input capacitor)
C2 - 22µF bipolar (optional)
C3 - 0.1µF (power supply decoupling)
C4 - 0.1µF (power supply decoupling)
C5 - 0.1µF (power supply decoupling)
C6 - 47µF (power supply decoupling)
C7 - 0.1µF (optional - zobel network)

PSU board:
R0 - 10k (series resistor for LED)
R1,R2 - 1 2w (optional "snubber" network capacitors)
C1,C2 - 1500µF (main power supply caps)
C3,C4 - 0.1µf (power supply decoupling)
C5,C6 - 0.1µf (optional "snubber" network capacitors)
D0 - blue LED
D1-D4 - MUR860

Transformer wiring:
For the PSU transformer connections, AC1 denotes the first secondary winding of the transformer, and AC2 denotes the second secondary winding of the transformer. It is also possible to use a center tapped transformer with this PSU board by wiring the center-tap to either AC1(bar over top) or AC2.

As for size, the LM1875 amp boards are almost half the size of the original LM3875/LM3886 amp boards.

Basic Assembly Info:
Solder components on board in order from lowest profile to highest profile, with the PSU diodes and LM1875 chip being soldered last.

As the pics show, the boards were made on the 11th week of 2007... over a year ago. I believe I mailed 10-15 boards out to various people, so let me know if you have any additional questions,

Thanks to Allan for transcribing his component notes and taking pictures!


BrianGT 13th May 2008 05:01 AM

As for the wiring connections:

Amp board:
V+ = goes to the V+ connection on PSU board
V- = goes to the V- connection on PSU board
GND = goes to GND connection on PSU board
IN = goes to positive input signal
SG = goes to input ground (signal ground)
OUT = goes to positive output terminal
OG = goes to output ground

PSU board:
AC1 connections = goes to first secondary winding on transformer
AC2 connections = goes to second secondary winding on transformer
(for center-tapped tranformer, connect center-tap to either AC1(bar over top) or AC2 (no bar)
V+ = goes to V+ connections on amp boards
V- = goes to V- connections on amp boards
GND = goes to GND connections on amp boards, and also a connection to the central chassis ground

Other Connections:
For the input AC connections, put the AC H connection in series with a fuse (I used 2A slo blow), and in series with a power switch if desired. This will then wire into the first primary. Wire the AC N connection to the other primary wire. Wire the AC earth ground connection to the main chassis ground.

-One could decide to remotely wire the D0 power supply LED to the front panel for indicating the the amp is on.
-One could decide to route the output ground to the central ground, instead of the OG connection on the PCB.
-If PSU cap bleeder resistors are desired, solder them to the underside of the pcb on the 1500uF capacitor terminals. I used 2.2k 2w resistors for this purpose on the initial test amp.

Transformer selection:
I used a 160va 2x18vac Avel Lindberg transformer from Parts Express, and anything similar should work. The acceptable power supply range from V+ to V- is 16-60v. See the datasheet for power output levels with various supply voltage

Datasheet from National:
(the schematic in manual is very close to the actual circuit used on the amplifier PCB)


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