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autoformer 6th November 2012 06:21 AM

LM3875 Layout
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I'm relocating my Daniels LM3875 gainclone to its new home in a diyAudio compact chassis with Quasi heat sinks.

I'm no expert, but I figure it's a good idea to keep the AC lines as short as possible. So, it's just a couple of inches from the IEC switch/jack to the transformer, then on the front side of the chassis, just a couple of inches to the rectifier board, which is located right behind the front panel. Obviously, I'm going to trim the AC wires very short.

DC wires will run around along the sides. I'll use a 14ga copper wire for the star ground that will run from the rectifier, around the transformer to each board and tie in with the transformer shield and chassis ground.

Amps themselves will be positioned very close to the rear panel to keep input leads and speaker outputs as short as possible. This is a straight up power amp, so no attenuator.

This layout will allow me to keep the DC wires away from the AC wires. They will never cross. Does this matter, or am I wasting my time worrying about the wiring layout? Would love to hear any ideas you might have.

AndrewT 6th November 2012 11:51 AM

I agree with your wiring philosophy.
Keep the transformer rectifier smoothing caps as tight as posible.

Use twisted pairs everywhere, except dual polarity where twisted triplet is the equivalent.

Don't cut your transformer wires just yet.
You may find that hum can be reduced by rotating the transformer around it's fixing bolt.
Find the optimum and then cut the wires to fit.
You may have to replace the solid core transformer wires to flex to allow this to be done. Twisted solid core 1sqmm is very stiff.
Twisted solid core 1.5sqmm is even stiffer.

autoformer 6th November 2012 09:32 PM

Thank you very much for your post, Andrew. That's exactly the kind of tip I was looking for. I'll try rotating it as you say. The transformer wires are stranded, so no problem there.

Speaking of toroids, do you think higher quality custom-made toroids, such as those available from Canterbury Windings are worth considering for gain clones? The toroid I'm using now is a Chinese-made Antek, and the amp does have the slightest amount of hum, but I thought might have been due primarily to having the rectifier so close (less than 20mm) to the boards. Then again, I didn't try turning the transformer as you suggest.
Canterbury Windings Home

AndrewT 7th November 2012 08:40 AM

I got a quote from Canterbury after I saw a recommendation here.
I will use Toroidy in future for HQ toroids.

AndrewT 7th November 2012 10:56 AM


Originally Posted by AndrewT (
I agree with your wiring philosophy.
Keep the transformer rectifier smoothing caps as tight as possible.

longer wires from PSU to circuit are not a problem. I consider longer to be an asset. Longer = more wire to wire capacitance and that capacitance is very high quality, due to plastic film and air for the dielectrics. Longer also increases the r for more HF attenuation. Longer also increases inductance for more 2pole HF attenuation. Thinner also increases r for more attenuation of HF.

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