Tortuga LDR Preamp with Pass B1R2 Buffer

This project was completed 10-2018.

This webpage documents the construction of a balanced Tortuga LDR preamp with a Nelson Pass designed direct coupled buffer.

This project started in 2017, but really got going earlier this year when Tortuga Audio released their new OLED display to work with their latest (V25) LDR preamp boards. These Tortuga boards now include integral LDR input switching as well as self calibration of the LDRs (to account for aging), and I opted for a 4 input version (they support up to 6). I bought two boards to build a fully balanced version. A couple years ago I built their previous LDR preamp in a single ended configuration, with both direct outputs and thru a Pass B1 buffer (which uses Toshiba FETs and is capacitively coupled). That unit sounded great and performed well, particularly with the B1 buffer. When Tortuga announced their new V25 LDR boards, I decided to build a fully balanced preamp with the new boards, and Nelson Pass had recently published a direct coupled (no capacitors in the signal path) version of his B1 buffer, call B1R2 (Revision 2) which seemed to fit the bill as well. The great looking and very easy to use OLED display is a very nice bonus.

Links:

Tortuga Audio. By the way, Morten Sissener, the owner and chief engineer at Tortuga, is very helpful to us DIYers. Both he and his products are highly recommended.

DIYAudio thread introducing the Pass B1R2 buffer, see post #17, and a thread dedicated to the B1R2. Nelson Pass is also very active in the DIY community, also highly recommended (a bit of understatement there).

I built a stand-alone Pass B1R2 buffer last year, using the same buffer PCBs used in this project. That webpage includes more info on the Pass buffer with schematics. Link to that project.

1. The Chassis & Faceplate:

The project started with a custom chassis from Par-Metal.com. I chose a variant of their Series 12 all aluminum cabinets, 17" wide by 3.5" tall chassis that was 11.5" deep. It was a custom configuration as their standard Series 12 cabinets have vent holes (I wanted none) and usually come with a rack mount front panel ( which I did not need) in addition to the sub panel. I've has a number of custom enclosures made by Par-Metal, and I've always been happy with their work. These are very high quality chassis, they fit together well, and they use pressed-in threaded steel inserts for assembly. I don't use the black anodized screws they include, but instead substitute stainless hardware.

Next up was designing the front panel, using the software tools from FrontPanelExpress.com. I wanted to add VU meters to the preamp (on the left above), and would need to make a tinted window for the OLED display (right above). I also has one special requirement for this preamp: a mode switch to reverse the channels and to play only left or only right (which I use frequently for speaker setup and testing). As you can see above, the meter gain switch on the left has 0dB, +5 dB and +10dB settings, plus a -4dB setting (to match professional reel to reel tape recorders, which are typically setup at +4db), all referenced to 0 dBv (0.775V).

This photo shows a draft version of the faceplate, printed full size from the FrontPanelExpress application, in front of the actual panel during final assembly. This full size hardcopy is very useful verifying clearances and fit of the various component parts on the faceplate before actually ordering it.

Once the faceplate was in house, I could cut & drill the front sub-panel to match the faceplate, as you see above.

Swiss cheese bottom panel after drilling for two LDR boards, two power supply/buffer boards, one PCB for the VU meter amp, plus an additional dedicated 12V supply PCB for the LDR boards. And the power transformer, ground lug, etc., etc.

Rear panel after drilling/cutouts. I planned to have 4 XLR inputs, one of which would also have a RCA option (far left, more info below), plus 2 XLR balanced outputs and an unbalanced output (e.g. for subwoofer amps). The XLR cutouts were made with a GreenLee punch (and a bit of filing) and the cutout for the IEC connector on the right was made with a Dremel tool. I tapped (#4-40) all of the XLR and IEC mounting screw holes to make assembly easier. The faceplate along with front sub-panel and rear panel are shown below.

Due to the configuration of the XLRs, I decided to wire the back panel first, then make the connections to the PCBs. As you can see, I color coded the wiring to differentiate left and right and inverted and non-inverted. Note that I used 14 gauge solid copper wire as a ground to connect all the XLRs, and the 14 gauge vertical wire connecting the upper row of XLRs and lower row of XLRs will be used as the common star ground for the entire preamp. On the left the IEC connector with integral fuse and power line filter is a Qualtek 860-04/003, recommended by Gary Galo in audioxpress 3/2005.

2. PCB Subassemblies: Tortuga LDR Boards, B1R2 Buffer, and Power Supplies

The Tortuga V25 LDR board. The input selection is done by the LDRs on the left, and the 4 LDRs that actually control volume are in the center, along with a single LDR for mono mode switching. Notice there are two empty LDR spaces top (right channel) and bottom (left channel), as the board can accommodate up to 6 inputs, but I will be using only 4. These LDRs rows along the top and bottom of the board function a the selector switch for the preamp. Lots more info on this very sophisticated product on the Tortuga website, here.

I designed a new PCB (using ExressPCB.com) for the Pass B1R2 buffer (see links at the top for more details on the design) plus a dedicated regulated power supply, based on a design by Gary Galo from The Audio Amateur back in 1990, using LT1085 and LT1033 Low Drop Out (LDO) regulators. The B1R2 buffer requires matched FETs (2SK170 and 2SJ74) which are no longer in production by Toshiba, but are again available from the DIYAudio.com store made by Linear Systems

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

The schematic for my regulated power supply and B1R2 buffer. Everything shown above except the power transformer is on the PCB. There are two of these PCBs in the preamp: one for non-inverted and one for inverted signals.

I built a dedicated 12 volt regulated power supply for the LDR boards using a PCB I found on ebay along with a low drop out LT1084CT-12 (5 amp) fixed voltage regulator, from the same family as the LT1085 and LT1033 regulators used on the buffer PCBs shown above. For heat sinking this regulator, I simply mounted it to the bottom plate of the preamp. This regulator is fed from unregulated V+ just after the rectifiers on the first buffer PCB, as is the VU meter amplifier.

I used a 50 watt Plitron dual 12V transformer for the preamp.

This is the transformer wired up to the IEC/filter connector and the dual +/-12V connections to my regulator/buffer boards. The transformer sits on a 1/8" EAR damping material pad for vibration control. Probably not needed, but I built a mu-metal wall around the transformer.

3. OLED Display & USB Connections

The OLED display mounted in the front sub-panel. It will have a tinted plexiglass window in front of it when completed. The remote IR receiver is on the left (at an angle) in the opening.

The rear of the OLED display, with the Tortuga encoder module (multifunction switch) switch mounted below it and connected by an umbilical. The open connectors on the PCB will make connections to the LDR board.

For the OLED window, I bought this tinted plexiglass on ebay, a bit thicker than the 1/8" front panel. That would allow me to machine a lip on the window, shown below.

The acrylic window insert after machining. Using the a technique I learned from fellow audiophile John Mathewson, I used a carbide router bit to machine the edge to mount the acrylic in the aluminum front panel to achieve a flush look. The router cuts the acrylic very cleanly as you can see above. I cut a 1/8" deep ledge to match the front panel depth, leaving a wider 1/16" flange to mount the acrylic behind the panel.

The acrylic window mounted in the front panel, fitting flush with the face. As this window flange is pinned between the front panel and the OLED display, I only used a dab of silicone glue on the rear to hold it in place.

This shows the OLED display from the rear after being wired up. The ribbon cables carry control info to the display and back from the multi-function switch below it and from the IR receiver (to the right of the OLED display) for the remote. The audio output from the LDR boards goes to the Mode switch you see to the right, and the signal then goes to the Pass B1R2 buffers. Other than the input and output XLRs, this switch is the only non-LDR control that the audio signal passes thru. I used a very high quality (and expensive) mil-spec Electroswitch 2 deck 4 pole switch. On the left in this photo, you can also see the two black USB cables going to the left side of the chassis from the LDR boards....

This view from the outside shows the two USB connectors (one for each LDR board) used for firmware updates. I decided to mount them on the side rather than the back for easier access in my audio rack without having to completely disconnect and remove the preamp from the rack.

4. VU Meters

I found a nice pair of vintage 2.5" Hoyt genuine VU meters in excellent condition for the preamp. The bezels are spare Dixon/MCI meters, and fit fine with a bit of trimming of the meter cases.

To provide meter illumination I made a PCB, shaped to fit around the meter and mount on the meter. It includes 7 soft white LEDs in parallel, fed from the VU meter amp power supply, completely separate from the audio power.

This shows the LED PCB being mounted atop the left VU meter. the right LED assembly is already in place in the background.

I used a PCB I had designed some time ago for the VU meter amp, shown above populated and with some temporary test connections sticking out. The meter circuit was reversed engineered from the Levinson LNP-2 preamp of long ago, and modified for switchable gain. The PCB includes +/-15v regulators on the far left for the IC.

The meter sensitivity is switchable, as described above in the front panel section, and is highly accurate, calibrated in dBv (also known as dBu, referenced to 0dBv at 0.775V).

5. Final Assembly

Internal view of the completed preamp. Clockwise from the lower left: XLR inputs and outputs, two LDR boards (non-inverted and inverted), 12V regulated power supply for the LDR boards (above them), OLED display on the front panel, Mode switch, then VU meters on the front panel (at the top), with the VU meter amp PCB below the meters. The VU meter sensitivity switch is at the far top right, with the power transformer at the right rear (bottom). Lastly, the bottom center two PCBs are the regulated power supplies for output buffers and the buffers themselves.

Rear view of the completed amp showing the input and output connections. 4 separate XLR inputs and 2 paralleled XLR outputs. Power switch (normally left on) is on the right above the IEC power connector, and the 1/8" 12V trigger jack is to the left of that switch.

This photo shows the connections to the LDR boards from the rear panel XLRs and from the regulator/buffer boards to rear panel outputs. The wiring looks a bit messy, but I wanted to route the signal wires away from each other as much as possible (avoiding bundling) to help prevent crosstalk. The LDR board on the right handles the non-inverted signals for the left and right channels, the LDR board on the left handles inverted signals. The same is true for the regulator/buffer boards to the left: one for non-inverted, the other for inverted. I wired up two XLRs in parallel for the balanced output, and also parallel-wired an RCA from the non-inverted output to drive unbalanced subwoofer amps.

The star ground for the entire preamp is on the rear panel, between to rows of XLRs, where you can see about a dozen black ground wires connected.

In addition to the unbalanced RCA output, I also added an unbalanced RCA input in parallel with XLR input #1. The inverted pin of the XLR for each channel is connected to ground via a small DPDT switch as seen above. I copied this technique from the Audio Research LS-25 preamp. This RCA input allows using an unbalanced signal from a phono preamp as the input.

This closeup shows the rear of the VU meters, along with the VU meter amp PCB, and meter sensitivity switch (which just controls the gain of the VU meter amp) to the right. The small perf-board PCB to the right contains a DPDT relay with a 12V coil which turns on the VU meter amp via the 12V trigger signal from the LDR boards when they turn on. That 12V trigger signal also goes to a rear panel 1/8" jack to control the power amp(s) in the system. You can also see the LED meter lighting installed atop the meters. Those meter lights act as an "on" indicator for the preamp, as the OLED display is set to timeout (turn itself off) after a user defined period of time.

Although the Tortuga Encoder Module / Multifunction switch (below the display above) will operate the preamp, the preamp is generally operated using a paired Apple Remote for all functions.

And finally, the preamp in operation. It was a bit difficult to find matched knobs both with and without indicators lines or arrows. I did find a nice matched set: Kilo OEJNI-90-2-7 for the Tortuga Encoder (no markings, 6mm shaft) and Kilo OEJK-90-2-5 for the mode and meter sensitivity switches (with an indicator line and 0.25" shaft).

The OLED display is very intuitive and self explanatory, easy to read from far away, and I especially like that the input labels can be customized - you can see I've labeled input #2 as DAC in this case. Brightness and timeout of the OLED are both user adjustable. Note that the display appears more blue in the photo than it actually is - its really white. The preamp operates exactly as expected, dead quiet, with easy intuitive controls.

Current draw of the completed preamp is 3.2 watts in standby, and about 5 watts running.

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