Pass B1R2 balanced preamp   with vU meters

The unpopulated PCBs for the power supply and buffer circuit, bottom side on left, top on the right.

The assembled power supply/buffer board.  The rectifiers are visible on the left, followed by filter caps, then the heatsink-mounted regulators in the center, and the actual buffer circuit on the right. The trimpots are to zero DC offset. The heatsinks are overkill, as the circuit draws little current and the don't even get warm.

The matched SK170 and SJ74 FETS are on the right, behind the 15 turn Vishay bulk metal foil precision 26 turn trimpots (used to zero out any DC offset).  The resistors used here (in the audio path) are Takman REY25 1% metal films.  The buffer itself is just a handful of parts, much smaller than the associated power supply.

For power, I used a Plitron 50VA dual 12volt transformer, ideal for the dual 12V DC supplies I used for the Pass buffers.

These are the regulated power supply/buffer boards (plus and minus on each PCB) wired up in the chassis.  

The board labelled Right has the inverted and non-inverted signals and the same is true for the Left channel on the other PCB.

I used an updated version of a PCB I had designed some time ago for the VU meter buffer/amp, shown here. The meter circuit was reverse engineered from the Levinson LNP-2 preamp of long ago, and modified for switchable gain. The PCB includes +/-15v regulators on the top edge for the IC.

The meter sensitivity is switchable, and is shown here pre-assembled for testing.  It is highly accurate, calibrated in dBu (referenced to 0dBv at 0.775V). Note that dBV is also referenced in volts, but uses 1.0v as the 0dB level. 

This circuit will drive Dixson vintage VU meters.

The chassis itself was sourced from Par-Metal.com, highly recommended. In this case, it actually started life as another project but was re-purposed for this preamp.  Shown here as received from Par-Metal.

The faceplate layout for this project was done online in FrontPanelExpress; the printed copy of the design is at the bottom in this photo.  

The controls include (L to R): a gain switch for the calibrated VU meters, a mode switch (stereo, L only , R only, reverse), an input selector, and lastly, the volume control.  The front panel Mode switch labelling includes a position for a Mono mode, but that was not implemented in this build.

Above the paper copy is the actual aluminum panel fabricated from my design, and above that is the inner panel of the chassis.   The inner panel doesn't match the face exactly as this chassis was re-purposed. 

The rear panel as drilled and tapped for the connectors.

The rear panel with the connectors being wired up.

I wanted the power switch to be mounted without a nut showing on the front of the faceplate, so I made a little sub-panel where the switch actually mounts, shown here.  That allows for a cleaner look on the finished front panel.

I added a 12V trigger output to remotely turn on/off an amplifier, so I build a small 12V regulator shown here, with a resettable fuse (Littelfuse RXEF010)  the left.

This assembly was mounted on the bottom of the chassis and the yellow/black wires go to the 1/8" trigger jacks on the rear panel, below.

This photo shows the inside of the rear panel, including the IEC connector (with integral fuse and line filter), along with the afore-mentioned 12V trigger jacks at the top.  The two jacks are simply wired in parallel to feed multiple amps or other components.

The IEC is a Qualtek 860-04/003.

Completed preamp internals, view from the front. Starting in the upper left: power supply transformer, remote trigger jacks and IEC, XLR inputs and outputs along the back panel.  On the right are the selector and 4 gang volume control (an Alps 10K pot), center rear is the power supply/B1R2 buffer PCBs, with the meter buffer/amplifier in front of them.

The star ground is on the heavy wires running between the XLR jacks (where all the black wires converge).

This photo , from the rear, shows the front panel wiring.  From left to right: the volume pot, input selector switch, and mode control (the latter mounted on the front panel)  

To the right of the VU meters is the gain control for the meter amplifier circuit, which is on the PCB directly behind the meters.

The yellow box encircles the buffer circuits, the only active circuits in the audio path.

The transformer is shielded by walls of mu-metal shielding.

This shows selector switch on the left, wired up to the various XLR input jacks, along with the 4 gang Alps volume pot on the right.  Both are mounted closer to the rear panel to keep cable lengths short, with brass shafts thru the front panel to the control knobs.

On the far right, you can see the inside of a pair of RCA jacks and a switch, allowing the first input to be used either balanced or unbalanced (e.g. for a phono preamp), with the switch grounding the inverting input on the XLR in unbalanced mode. 

There's also one pair of RCA jacks connected to the non-inverting outputs for connection to a sub amp or other single ended component.

Closeup of the meter setup.  Meter buffer amp is at the bottom, wired to the gain switch on the right.

The meters are vintage Dixson true VU meters.

Rear view of the preamp.  The far left input is for phono, and is switchable between single ended/RCA and balanced.  All other inputs are XLR only.  

There are 2 balanced outputs in parallel, along with a single ended/RCA output on the non-inverting output.

The XLRs are all Neutrik gold  and the RCAs are Vampire gold/teflon.

The completed preamp, front view.

The same view, powered on, with dimmed room lights to better see the meter lighting, which also serves as an on/off indicator.