Gaincard Amplifier with USB DAC

Integrated amp with USB DAC, completed 10/2008


This amp is built for the most part with Peter Daniel's suggested parts and USB DAC.  The PCBs, selected low offset LM3875 amp chips, and all active parts for the DAC were purchased from Peter, who is very helpful and highly recommended.  Peter's website is

The following photos show the construction process:

Above: Chassis after punching/drilling holes and painting.  I used an aluminum Bud AC-413 box, 10" by 12" by 3" (from  For the cover I used a 1/8" thick piece of aluminum.  Click to enlarge.

Above: Chassis with EAR ISODAMP SD-40 damping material applied.  Since the chassis is only 0.05" (1.2mm) thick aluminum (easy to work), I applied damping material to most of the box.  Note that I predrilled the damping material to clear standoffs.  Click to enlarge.


Above: Amp PCBs assembled and mounted on copper heatsink, 1.5 by .75 by 5".  Note that I used Blackgate 100uf/50V type N nonpolar caps on the power supply rails on each PCB, the resistors are Caddock 1% MK132 and a Riken 1% carbon film for the feedback loop.  There is a thick copper wire connecting the chassis ground pads on each PCB which serves as the star ground for the power supply.  The audio grounds (inputs, volume pot, DAC PCB) are star-grounded at the input pad on each PCB.  The chips are mounted to the heatsink with Artic Silver 5 thermal grease, and the copper heatsink was similarly mounted to the aluminum chassis with the same heatsink compound.  Note that Peter can supply LM3875 amp chips selected for low DC offset, and these have less than half the DC offset of any other gainclone amp I've built using random chips.  Click to enlarge.


The heatsink mounted on the chassis divider with the controls.  Click to enlarge.  The volume control is a Cello 59 position stepped attenuator built using 1% resistors, mostly Dale.  The selector is an ITW 2 pole 6 position switch.  The volume and selector switches were both the same type used in the Cello Etude passive preamp, shown below. 


 Above: Photo showing the interior of the Cello Etude passive preamp.  This unit was introduced in 1987, and was in production thru most of the 1990s.  I believe it sold for US$3,500.   Click to enlarge.


Above: Back to my Gaincard amp.  Divider panel, front view showing the couplers for the control shafts.  Click to enlarge.


Above: Divider/amp/control subassembly with pre-assembly wiring completed.  All wiring is Kimber TCSS teflon.  My wire color coding is: red=right, blue=left, yellow=V+, green=V-, black=ground.  Click to enlarge.


Above: Adding windings to the transformer to power the DAC.  The transformer is an Antek AN-3222 300VA dual 22V (  I added two additional windings, a 7 VAC one for the digital section of the DAC and a 9VAC one for the audio section.  Per Peter Daniel's suggestion, the windings were about 27 turns for the 9V winding and a bit more than 20 turns for the 7V winding.  I used a choke from a speaker crossover as the source for the enameled wire.


Above: Completed amp/DAC, top view.  Power transformer in the bottom (front) center in this photo, power supply PCB to its right, USB DAC in the upper right, and two amp PCBs in the top center.  Click to enlarge.


Above: Completed amp, another view showing general layout.  Click to enlarge.


Above: Completed amp, rear view showing input, output and power connectors.  The USB input is in the lower left.  The first 2 sets of RCAs are for inputs, and the third set is an output taken after the volume control, for example to feed a powered subwoofer.  Click to enlarge.


Above: Assembled amplifier PCBs.  Click to enlarge.


Above: Completed USB DAC PCB.  Note that I have simply cut the transformer portion of the PCB off and added the additional windings to the existing power transformer to supply correct voltages to this PCB - they are the white twisted pairs on the right in this photo.  Click to enlarge.


Above: another view of the amp, showing the power supply PCB on the right, built with MUR860 rectifiers and Blackgate Standard 1000uf/50v caps.  Click to enlarge.


 Above: Completed unit.  The faceplate is from  Just be sure to explicity state that you want the grain on the panel to run horizantally.  The power switch is a Bulgin MP0045/1E2BL012 pushbutton switch ( with the contacts wired in parallel.  It displays a nice blue ring of light around the button when powered on (just barely visible in this photo).  Click to enlarge.




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