LM3875 Amplifier (Gaincard) with ARC Meters

The starting point - facepate and meters from a parted out D-110 that I came across online.

Meter cleanup.  All of the meters tested fine, but the glass faces were dirty.  I simply removed the meter backs and cleaned the glass and re-assembled.

The center meter is a line monitor, 0 to 300VAC, shown here.  The two other meters are 1mA movements, used as power monitors by ARC, NOT VU meters.  

Next step: cut the panel down to 5.25" tall using a non-ferrous table saw blade.

I had a spare chassis from Par-metal.com that would work nicely for this application.  This shows cutting the inner front panel for the meters.  7/8" Greenlee punch for the smaller holes, then a jigsaw for the larger cutouts.

This shows the back of the assembled meters, faceplate and inner front panel.

The smaller round openings are for lamps to illuminate the meters.

Test fitting the entire chassis together before any other metalwork or installation of the power supply, amplifiers and meter driver board.

There's really no room on the front panel for a power switch, and since I wanted it to turn on and off automatically with the rest of the system, I added a 12V remote trigger using a small PCB I designed, shown here.   It uses a high current relay (10A) to handle current requirements of the power amp. 

This will be mounted on the rear panel below the switch and IEC connector.

For power, I needed both a +/-30V supply for the LM3875 amplifiers as well as a 12volt supply for the meter amp PCB.  For the latter, I used an Antek (Antekinc.com) transformer, model AN-0212.  Its a dual 12volt 25VA transformer.

The meter driver PCB was found on ebay, and was listed as a BA6138 VU Meter Driver PCB Board.  It requires a simple +12VAC power supply from the transformer shown above (rectifiers and regulator are on the board), and has leads from the speaker outputs thru 301K resistors (on the left) and outputs for each meter.  Since this board came with caps of unknown origin, I replaced them all with quality Panasonic caps.  I also replaced the level adjust trimpots with fixed 10ohm 1% resistors for increased channel-to-channel accuracy.

For the LM3875 amplifiers, I used the Avel-Lindberg 130VA dual 22 volt transformer shown here for the +/-30volt supplies.

The power supply to the amplifier modules is simply a full wave bridge rectifier made up of 4 MUR860 soft recovery diodes for each leg (plus and minus voltages).  There's only a small cap on the rectifier PCB as most of the capacitance is mounted close to the LM3875s on the amp PCBs.

Closeup of one amplifier channel.  The input coupling cap is a 4.7uf Blackgate Type N, and the feedback is 47uf Blackgate (both long out of production).  The yellow wires are the +30V supply and the green are the -30V supply.

This shows the rectifier PCB on the left and the two channels of amplification on the right.  The LM3875s at the back (the bottom plate of the amplifier) bolted in place with heat sink compound to allow the entire bottom panel to serve as a heatsink.  

The two amp PCBs are joined with 1.5" standoffs for stability.

The assembled amp showing the left side.  The power switch/fuse/IEC are at the top right under the plastic cover with the 12V trigger relay PCB below that.  The smaller transformer is for the 12V power supply.  The black main power transformer is in the lower center, and you can see the back of the meters on the left.

Top view of the completed amp, front at the bottom.  Above the main power transformer is the rectifier/amplifier assembly, with the output binding posts above that on the rear panel.  The resistor and blue box cap on the binding posts form a zobel for stability.  

The RCA jacks on the top right in this photo feed the amplifier PCBs in the center.

Inside view of the front panel showing the wiring.  The meters are illuminated by 6 white LEDs, fed from the 12V power supply on the VU meter amp PCB (yellow/black wiring).

The white/black wiring is the 120V feed to the AC line level meter, routed away from other wiring. 

Note the connections to the 120V line meter at the center are fully heat shrink insulated for safety.

Completed unit from the rear.  

I added a second pair of RCAs in parallel with the first for pass-thru applications.

The un-labelled 1/8" jack on the lower right is the 12V remote trigger to start and stop the amp from a preamp.

Completed unit from the front.

In operation, turned on and off via remote trigger and audio signal fed by my preamp.