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.
Above: I added a recess on the rear of the front panel for the tinted plexiglass cover over the volume display.
Above: The tinted plexi cover glued into the panel recess, so it is flush with the back of the front panel and will sit against the subpanel.
Above: Top view of the preamp. Counter-clockwise from the upper left: power transformer, rectifier PCB (red), with 18V regulator PCB (for the LDR and buffer) toward the center, the headphone amp PCB on the lower left, the meters and PCBs for LED meter illumination along the bottom, the VU meter amp is on the front of the right wall, followed by the Tortuga LDR PCB and selector PCB. The board on the center rear (with the large white Sonicaps) is the Pass B1 buffer. Input and output RCAs on the rear, along with IEC, fuseholder, and 12V trigger output jack. Not readily visible, but the star ground is between the RCA connectors on the rear panel.
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.
Above: Power supplies. Dual 18V transformer at the rear, with a Brian Bell (of Gainclone fame) rectifier PCB to supply approx +/-25VDC. The squarish PCB to the right of the rectifier PCB is the 18V regulator for the LDR board and B1 buffer, based on the LT1084 regulator. The headphone amp, to the lower left, contains +/-15 regulators for the headphone amp and meter amp. Note that the 18V supply is always on then the unit is plugged in so it can be turned on via the Apple remote. The small perfboard next to the headphone amp is a relay that is powered by the 12V remote trigger on the Tortuga LDR board, so when the LDR is turned on, the headphone amp, meter amp & meter illumination is turned on. By the way, the preamp draws only 2.5 watts in standby, and about only 5.5 watts when on.
Above: The headphone amp PCB. Its actually the same PCB I designed for the VU meter amp, which is a simple IC based gain stage, but with parts selected to match the Grado RA-1 headphone amp. It contains the +/-15VDC regulators for both the headphone amp and the VU meter amp. You can also see the headphone jack in the lower left.
Above: the front of the preamp. The meters are vintage true VU meters, Weston model 2022. They were spares from my Levinson LNP-2 project,
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.
Above: One more shot from the front during final testing. The switch on the left simply controls the 12V trigger signal from the LDR board. In the lower position (labelled "Main Out"), the 12V trigger signal is fed to the rear panel jack visible in the upper left. It feeds the main amplifier which turns on automatically when the preamp is turned on. By moving the switch to the "Phones" position, the 12v signal to the trigger output is disconnected, effectively turning off the main amp when you want to use headphones. For anyone with other Apple remote controlled devices in the same room (I have an Apple TV), you'll need to pair the remote to the LDR preamp, a simple process documented on the Tortuga website, to prevent the remote from activating both devices. Hence my label on the remote.
Next Steps:
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.
