Above: The LDR preamp (with the Ayre QB-9 DAC). This project was built mid-2015, based on the Tortuga Audio LDR3X.V2.1 controller. It includes a Pass B1 output buffer, a headphone amp (clone of the Grado RA-1), and VU meters with switchable gain.
For background, I believe the popularity of LDR passive preamps started with the Lightspeed attenuator, here:
The most sophisticated LDR design I have seen is the Tortuga, who have lots more info on their website:
The following is a photo tour of the project. Note you can click on most photos for a larger version.
Above: The faceplate was designed and fabricated using FrontPanelExpress.com, and includes the usual cutouts and engraved lettering. The sub-panel that will go behind the outer panel is shown above as well. Note the extension to the left of the lower center cutout for the display - that was to accommodate the IR receiver to allow remote control of the preamp, using the Apple remote. The main chassis is a Par-metal.com Series 12 aluminum enclosure.
I added a recess on the rear of the front panel for the tinted plexiglass cover over the volume display.
Above: The heart of the preamp: the Tortuga LDR3X.V2.1 LDR board, connected to the Tortuga IO3 relay board for input selection both mounted vertically on the side wall. The ribbon cable is for control signals only, no audio. The ribbon cable to the right goes to the Tortuga DM-1 front panel display and encoder control which controls volume, balance, input selection, and panel display brightness. I bought the upgraded Auto-Cal Op Amp with my LDR preamp, allowing supply voltages higher than the nominal 12V for the LDR board, so that I could power that board and the output buffer (which requires at least 18VDC) from the same power supply.
Note that I added a resettable fuse (the small yellow part in the upper right above) to the 12V trigger output. I believe the 12V trigger system (not here, but in general) is very poorly designed in that the cables connecting the various components have male connectors at both ends. So at the amp end, for example, the plug is live when unplugged and the system is on, and is easily shorted out, possibly damaging or destroying the 12V trigger supply. Yes, thats the voice of experience talking. Audio Research uses a resettable fuse to help prevent any such damage, and I've copied that design here with the Littelfuse RXEF010 in series with the 12V trigger output.
here. I made two small PCBs that mount atop the meters to hold the illumination LEDs. On the right is the headphone jack, held in place with a maple wooden block. On the far left is the Tortuga encoder, and the Tortuga DM1 display module is wedged in the center between the meters, behind the tinted window. I made a pair of aluminum L brackets to hold the display PCB. In the top center is a 4P2T switch that selects the gain of the VU meter amps, allowing the meter level to be raised or lowered by 5db from their nominal setting, which in my case is 2.2 dBm.
Above: VU meter amp. This circuit is cribbed form the Levinson LNP-2, built on a board I designed at expresspcb.com, highly recommended. This board also supplies power to the illumination LEDs via the grey & white wires going off to the right. Input to the meters comes directly from the selected input, before any attenuation by the LDR board, via the red/black and blue/black twisted pairs coming in from the left.
Above: The Pass B1 buffer. Required in my setup due to long cable runs between the LDR preamp and the power amp. More info on the B1 buffer here.
As this is a work in progress, I'm planning some additional improvements:
1. Replace the stock Pass B1 buffer with a direct coupled version, like the Mezmerize.
2. Upgrade the headphone amp, possibly with the forthcoming design from PASSDIY, documented here.
3. So, everything mounted on the walls (sides, front & rear) of the preamp is permanent, everything on the bottom is subject to upgrade. Hopefully future iterations will be less cluttered with a bit neater layout.