home‎ > ‎

Gaincard amp with VU Meters



This integrated amp was built using Peter Daniel's (Audiosector.com) minimized gaincard design, with premium quality parts.  I originally intended to add a preamp gain stage based on the Levinson JC-2, but have found no need for the gain, hence the input volume pot, along with the selector switch, are the only "preamp" in this unit.  I also added VU meters and a buffered subwoofer output.

This webpage describes the construction of this integrated amp back in August 2013.


Above: Top view of the amplifier internals (front is at the bottom).  Counterclockwise from the upper left: Input jacks, outputs binding posts (beneath which are the amplifier circuits & copper heatsink plus the rectifier PCB, upper right is the power transformer, volume pot on the right front, VU meters in the center (above which is the VU meter amp and subwoofer buffer circuit) and selector switch on the lower left. 



The Circuit:
The design I chose is a Peter Daniel design for a minimized gainlcone amp, published on DIYAudio.com back in 2008, shown below:

Although the circuit is quite simple, its execution calls for extremely high quality parts, some of which are virtually unobtainable today.  It is a direct coupled design (no caps in the signal path), but the power supply capacitors are all BlackGate (now long out of production).  The resistors are all Caddock MK132 except the feedback resistor which is a Riken.

The Amplifier:

Above is the amp sub-assembly.  There is a PCB (for each channel) on each side of the copper bus bar heatsink, to which the LM3875s (selected for low offset) are mounted.  The large red capacitors are Blackgate Type N (non-polar) for local power supply decoupling near the amp.  As you can see, I wired up this assembly before putting it in the chassis.


Above:  Bottom of the amp assembly.  Note the tapped holes for mounting the heatsink.  The large black wire passing thru the heatsink is the common star ground for the amp.



Above: The amp assembly in place in the chassis and wired up.  Atop it in this photo you can the the rectifier PCB, with 8 MUR860 rectifiers.  Not easily seen here, beneath that PCB, are the main power supply caps - a pair of 1,000uf 50V Blackgate caps.  The white wires all carry audio signal, and are teflon insulated shielded wire.  The transformer, just visible on the left, is an Antek AS-2222 200VA dual 22V shielded unit.

Tour of the Chassis:

The rear panel of the amp under construction.  I added 5 RCA inputs plus an additional set of RCAs for a subwoofer output.  The middle holes are for 5 way binding post speaker outputs, and on the left are the IEC AC input and fuse.  The chassis was a custom size, all aluminum chassis made by Par-metal.com.  They include steel pressed in threaded inserts for a very high quality chassis - highly recommended!

Above: Inside view of the Input RCAs, which are Vampire PCB2F/S gold/teflon jacks.  Note that the RCA closest to the center is actually an output intended for use with a separate subwoofer amp (or integrated sub plate amp).  The grey output binding posts near the amps are gold Superior BP30-2BR.

Above: Following the signal from the inputs (all by shielded coax) to the selector switch, an Electroswitch M5 type. 
The wires going out to the left of the selector switch go to the volume pot...


Above: The volume pot, mounted upside down here, is an Alps Black Beauty TK40, dual 50K.  I made a small circuit board to connect the inputs and outputs - one to the amp modules, another to the subwoofer buffer PCB.

Above: This view shows the two vertically stacked green PCBs that buffer the sub output (lower board) and provide buffer/amplification/meter illumination for the VU meters (upper PCB).  The upper PCB includes +/-15VDC voltage regulators for both of those circuits.  The blue rectifier PCB to the left is mounted on tall standoffs because the large Blackgate power supply caps are mounted beneath that board. The VU meters (one visible on the right) are Weston model 2022, the same as used in the classic Levinson LNP-2, and the meter amp circuit was reverse engineered from that design.  See my LNP-2 build project here.


Above: another view of the interior, from the left side.  


Above: view of the meters from the rear.  I made an oddly shaped PCB to fit around the meters and mount above them to hold LEDs for meter illumination, 4 LEDs per meter.  I also used Roscolux 3441 Full Straw filter film to tweak the color of the LED light to better match traditional incandescent bulbs.


Final Notes: The faceplate is from FrontPanelExpress.com, and the knobs are leftovers/spares from the afore-mentioned LNP-2 project.


Above: rear view of the completed amp.