Build Point-to-point LM3875 Gainclone Amplifier

In this post, I will share my building process of LM3875 Gainclone amplifier.

If I compares with LM1875 or LM3886, this LM3875 is the most simple amplifier to build with only few parts needed. This LM3875 also has the best S/N ratio compares to the others.

For this simple circuit, I prefer to do it by point-to-point connection. The benefit compare using a printed board is the purer signal transfer. I always using high quality parts that I believe the leads from those components are better in materials compares to the PCB traces.

So let's do it.

Actually this project is not new. This amplifier is come back to me again for servicing. I made this amplifier in 2008 for my friend and he asked if the capacitors inside this amplifier can be upgrade to the better grade?

The story when he was using this amplifier, this amplifier is always ON. It working for almost 3 years without switching OFF at all. So I think his request might be make sense.

So I better over-haul this whole amplifier

The process is just like building a new amplifier.

First of all, I dismantle all the components then clean the casing before start put the new components.

I put a new thermal paste and new isolator into each IC's.

Those IC are mount to the heatsink like this.

I put a cable as the common ground directly to the heatsink. The washer are put into each IC's holes to avoid any short-circuit between IC and the heatsink.

I always want to make sure there is no connection between heat sink to the IC metal body. This is very critical steps that missed to do so will cause the IC burn immediate once get the power.

I put the first resistor. It is installed at the pin 3 and pin 8 on each IC.

This is the feedback resistor using 22k ohm. The resistor that I use is the high quality PRP resistor for audio.

Next resistor is the inverting input resistor to the ground.

The resistor value is 1K ohm at the pin 8 and the other lead goes to the ground.

The power supply capacitors is now using 4 pieces of 1000uF 50v Panasonic FC gold edition. This gold edition is different with typical Panasonic FC capacitor with the purple color and gold strip.

Then each of the capacitors are installed with 100nF snubber capacitor across the leads.

The positive polarity is at pin 1, then the other lead goes to the ground.

Same with the other capacitor, the negative polarity is connect to pin 4 and the other lead goes to the ground as well.

I did for the others channel and this amplifier section is all done.

Look how simple and less components for each IC use here.

As per datasheet suggestion, I am now installing the input resistor on each IC. The value is 1K ohm at pin 7.

I also adding a resistor between pin 7 to the ground using 22K ohm. This method is quite useful to avoid any signal interference at the amplifier input lead.

The next step is preparing the power supply section.

I using a diode bridge, each of the pinout are connected with 10nF capacitors.

I also using 2 Nichicon Muse, each 1000uF 50v and I will using them at the bridge rectifier.

Those capacitors will be an initial filtering capacitors before the voltages goes to the amplifier circuit.

The secondary output from the toroidal transformer is center tap and 22 volt. They are now connected to the bridge rectifier with the center tap is goes to the ground.

To connect from the diode bridge to the amplifier circuit, I am using 6 stains of 18 AWG wires. Each amplifier circuit will have their own supply cables.

The star ground is connected at the primary capacitors board. This connection will also goes to the amplifier circuit ground.

At this point I've done all the power supply section and before I solder those output cables to each voltage input at the amplifier section, then I should check the DC output voltage first.

They read around 34.5 volt for each positive and negative rail to the ground. They are within the specification as per datasheet and can be connect to the power amplifier circuit.

This is the input-output terminals on this amplifier casing. I am using Taiwanese banana plugs and RCA which they are quite good.

When building an amplifier, I always put a volume control at the input signal.

This volume control is only using a stereo potentiometer and with this, I am more flexible to use this amplifier as an integrated amplifier or a power amplifier.

The last step is connecting the signal input cable after the potentiometer to the 1K ohm resistor after the pin7.

And it is all done.

That is how I overhaul this amplifier. This just like a new amplifier built.

Disclaimer: Any statement and photos in this article are not allowed to copy or publish without written permission from the writer. Any injury or loss from following tips in this article is not under writer responsibility.