Lab Amp II

Project completed 4/2017.

This webpage describes the construction of a chip-amp (aka gain-card or gain-clone amp) based around the LM3875 IC.  It is designed to be used in my electronics lab, so it has inputs and outputs on both the front and rear panels, switchable between the two.  This is actually an improved version of an amp I've used for years, and in this one I added calibrated VU meters with switchable gain.


Above: The completed amp.  From left to right: power switch, inputs (with switch above to select front or rear RCAs), VU meters, meter gain switch at the center top, volume at center bottom, and speaker output binding posts on the right side.  


Above: This is the prototype (version 1) of my lab amp, being replaced with this new build.  Some parts (e.g. the switches, binding posts and heatsinks you see above) were cannibalized for the new version, but all capacitors were replaced with new.

 
Above: New chassis after holes punched, mostly with Greenlee punches.  On the rear are holes for the RCAs and binding posts.  On the sides are 1/2" ventilation holes, also below where the heatsinks will mount over on the right.


Above: Another shot of the chassis after the metalwork was completed.  The large square cutout on the face is for the VU meters.

Above: Chassis after a coat or two of paint.


Above: Starting assembly of the components.  Clockwise from the upper left: output binding posts and switch above them, VU meters, volume pot in the center, and RCAs on the right.  Note that I changed transformers to a dual 22 volt unit, more suited to 8 ohm loads than the dual 18 volt unit shown above.  The volume pot is an Alps 50K, and the VU meters are Dixson new-old-stock (NOS) meters, with the panel mounting bezels.  The binding posts are Superior gold BP31, and the RCA inputs are Vampire PCB2F/S gold and teflon.


Above: The front panel, as delivered by FrontPanelExpress.com, highly recommended.


Above: The completed amp.  From the left: power transformer (dual 22v, 130VA), rectifier PCB with MUR860 rectifiers, meter amp PCB in the center, and then the actual amp PCBs attached to the two heatsinks.  The rectifier and amp PCBs are from Chipamp.com, and the meter amp is a board I designed using ExpressPCB.  The rectifier PCB feeds the amps directly (+/-30VDC), and also feeds the meter amp PCB which has on-board regulators to supply +/-15VDC for that circuit.  Wiring for the power supply and outputs is Kimber TCSS (teflon), and smaller silver teflon coated wire is used for most signal cables.


Above: The actual amps.  They are built using typical LM3875 circuit, with the addition of a feedback cap, input coupling cap and zobel on the output - all for protection because the amp is sometimes connected to untested sources and/or loads.  The power supply caps are Panasonic FC, but the input couplers and feedback caps are Nichicon Muse bipolar - the green ones above.


Above: The back side of the front panel.  The meter circuit is fed from the input to the volume control.  Note that the rear inputs are via the white shielded wire; most other audio signals are simply twisted pairs.  The rotary switch at the top center is the VU meter gain selector, designed to work with levels ranging from standard recording studio level, +4dBm, to -20 dBm, as selected on the front panel.


Another shot of the meter amp and switching circuitry.  I also added LED lights to the VU meter, powered by the +15V supply on the meter amp PCB.