[tortugaranger]

The V2.1, our latest version of the LDR3x board is now available for pre-order purchase @ $199.
Expect to release and start shipping the V2.1 by January 5th.
You can find out more about the V2.1 and also pre-order it here: http://www.tortugaaudio.com/product/ldr3x-v2-1/

[point5028]

Hi

I would like to buy the new one for upgrade. But there is different PCB size between V2.1 and before V2.0 .
V2.0 Board:  2.5 by 4.7 inch
V2.1 Board: 2.5 by 5.0 inch
Dose the 4 corner screw location will be the same or different?
Could you check the distances of screw hole for me.

dave

[tortugaranger]

Hi

I would like to buy the new one for upgrade. But there is different PCB size between V2.1 and before V2.0 .
V2.0 Board:  2.5 by 4.7 inch
V2.1 Board: 2.5 by 5.0 inch
Dose the 4 corner screw location will be the same or different?
Could you check the distances of screw hole for me.

dave

Even though the V2.1 board is longer by 0.3 inches than the V2 board, the 4 corner screw hole dimensions are exactly the same.
It has something to do with relativity and black holes. 

[tortugaranger]

Along with the updated LDR3x.V2 board (the V2.1), we are also updating our IO3 Input Relay Board - the IO3.2  The updated IO3.2 adds output isolation relays in addition to the 3 input relays. When mated with the new V2.1 board, this allows complete input/output isolation during Auto-Calibration of the LDR3x Passive Preamp Controller Board. Without this isolation the user would have to disconnect their inputs & outputs.

The new IO3.2 board also includes a copper ground plane that can serve as the star grounding point. It includes 6 additional solder pads in the star ground plane for landing various grounds like audio as well as power/earth to a single ground point. Those 6 solder pads don't show up properly in these 3D cad pics.

We have a somewhat similar IO board that we use with out LDRx/LDRxB Passive Preamps but that version uses LDRs as audio switches rather than relays. At present we have no plans to market the LDR based IO board as that is unique to our finished preamp products.

Here are a few 3D cad renderings of both these new boards. The V2.1 is a 6 layer board, hence the exploded view.

[point5028]

I want to buy  LDR Module.  Can you add it to the store?  Thanks!!

[tortugaranger]

I want to buy  LDR Module.  Can you add it to the store?  Thanks!!

Sure. Will have that up and available within 24 hours or less. Will post an update here when it's live. 

Ok, it's live:  http://www.tortugaaudio.com/product/ldr-module/

[tortugaranger]

By now those of you who already ordered the V2.1 will have received an email update on where we came out on the V2.1 design as we begin building/shipping units.

We ran into a surprising finding with our original concept of a 6 layered board for the V2.1. Unfortunately we did not fully discover this until we'd gone through not only 1 but 2 iterations of production boards (ouch!). Simply put, the 6 layer board didn't sound good enough. In fact it was a step backwards. So after puzzling over that with extensive testing and listening I finally concluded there's no way we're going to proceed with an inferior board. 

The good news is as a result of all the development work over the past 3 months we discovered numerous ways of improving on the existing V2 platform through a combination changes in certain hardware components together with substantial changes in the software. Thus the final V2.1 has achieved our ultimate goal of an improved LDR3x, just not the way we had originally anticipated.

Changes introduced with the V2.1 include:

Improved auto-calibration accuracy:  Auto-calibration accuracy has been substantially improved approaching 0.1% matching between target and actual resistance level at each step of the attenuation range. This was largely accomplished through enhancements in the auto-cal software algorithm. Increased accuracy translates directly into improved stereo imaging and sound stage. Please note that this is also applicable to existing V2 boards via free software updates which can be download from our website.

Precision pre-calibration process: Using precision temporary plug-in 0.1% Vishay Dale resistor modules it’s now possible to pre-calibrate the auto-cal circuit thus substantially improving the overall accuracy of auto-calibration. This further improves channel balance with commensurate improved stereo imaging and sound stage. We now do this as part of commissioning each V2.1 board prior to running auto-cal. This process is only warranted with new boards or after making hardware changes such as changing opamps. This process is also applicable to existing V2 boards but will require updated software (free download) plus the resistor modules. Tortuga Audio will soon be offering a relatively inexpensive 4-pack of pre-calibration modules as an option when purchase the V2.1 or as a separate purchase (the DIY’er can also build these themselves).

Modified LDR Control Circuit:  Component changes were made to improve the accuracy, stability of LDR control while accommodating a wider range of LDR performance specs.

Modified Auto-Calibration Circuit:  Component changes were made to improve the accuracy and stability of auto-calibration.

Socketed Op Amps: Sockets were added rather than permanently hard-soldering of op amps into the board. This make it possible to try alternative compatible single-voltage quad op amps to optimize performance while also allowing for powering the board with higher voltage levels. Sockets allow the DIY user to experiment. Quad op amps are used in both the left/right channel LDR control circuits and also as a buffer in the auto-cal feedback circuit. To be clear, the LDR3x does not have op amps in the audio signal pathway, never has and never will. Op amps are used solely in the LDR control circuit and the auto-cal circuit.

Potential Higher Power Supply Voltage: Although the stock op-amps currently used limit the supply voltage to a maximum of 15 VDC (12 VDC regulated recommended), there is potential to user alternative op-amps that can handle upwards of 36 VDC. While this will not change the performance of the LDR3x it will provide additional flexibility in the designing your DIY preamp or retrofitting the LDR3x into an existing preamp.

Introduction of User Adjustable Input Impedance: User will be able to select up to 5 different input impedance settings between 5k and 50k and test the efficacy of each. Actual settings are user adjustable. Once selected, the LDR3x will have to be run through auto-cal at the new setting before it will be available. This has been discussed previously with respect to the V2 and indeed will be available to existing V2's via software updates. More on this will be posted here and in our documentation as soon as we can.

We are currently busier than one-armed paper hangers!

Cheers,
Morten

[tortugaranger]

Several people have asked if they can power the LDR3x.V2.1 board with higher voltage power supplies. As in higher than 12 VDC. The stock buffer op-amp we've been using is limited to 16 VDC. This is a key op amp in the auto-calibration circuit. During normal operation it's not involved in control.

We just finished testing 2 alternative op amps that are capable of operating safely up to 30 VDC - the LT1014 and the OPA4251. The OPA4251 in particular seemed to really speed up the auto-calibration which speaks to its precision and low noise (124 dB common mode rejection). At over $12 a pop wholesale it sure better do something better!

I haven't tested either in the main LDR control circuit but I've found that so called "better" opamps don't behave as well compared to the lowly LM324 we are currently using. Conversely, the LM324 is a total disaster when used in the auto-cal circuit.

So if running your V2.1 board at a higher voltage of up to say 30 VDC is a requirement for your project, I'll be adding the OPA4251 as a high-voltage-supply/auto-cal  upgrade option for around $20. You can always opt for this later since the V2.1 op amps are socketed.

If you have an outstanding order for a V2.1 and want one of these, please contact me via PM or email to morten@tortugaaudio.com



Cheers,
Morten

[tortugaranger]

The V2.1 version of the LDR3x Preamp Controller Board is now shipping with extensively updated firmware (Rev 2.1.0) that includes the ability to configure up to 5 different input impedance settings each with its own impedance level. Once you define the impedance level for a given level (from 1k to 99k) you then have to run auto-cal one time for that new setting to create an attenuation table for that setting. Once this is set up you can actually switch between impedance settings "on the fly" while listening to music. My own experience with this suggested that 99k was marginally better than the default 20k at least for my test rig. What surprised me though is I ran a 1k as well and while it was a tad less good than the 20k it wasn't at all that bad either.

When the dust settles a bit I'll post the new 2.1.0 firmware for download on the website. This firmware is fully compatible with the original V2 board. A fair bit of documentation also needs to be updated to explain how to do all this - all with that simple Apple Remote no less.

Cheers, 
Morten

[robertopisa]

Edit: OT moved elsewhere

[tortugaranger]

First  prototype of the V3 preamp slave board is finally in house. Much work left to be done before this successor to our V2.1 gets released later this year.

Key changes in the V3 vs. V2.1 include:

• Separate LDR (slave) board and Controller/Display (master) board
• 16 bit architecture in the LDR control and calibration circuits (DAC & ADC)
• microcontroller is still only 8 bit but has 4x as much space for code
• no more op amps in the control circuit (still have precision op amp buffers in the autocal feedback circuit).
• use of LDRs as switches for isolation of inputs & outputs during autocalibration and muting
• separate digital and analog power supply
• can accommodate up to 16 stereo channels (8 slave boards) via 2 wire serial network from single controller (master) board
• hybrid surface mount and through-hole board

Aside from these specific differences, a key objective with V3 is sonic improvements through even tighter channel balance specs and a shorter/simplified audio signal path.

The next step is to begin debugging and testing the board to find out what works and what doesn't. We'll inevitably go through a few prototype versions before the design is finalized. More on the V3 as it evolves.

[tortugaranger]

Over the past month we've been evaluating the first prototype of the V3 version of our LDR preamp controller design. As a result, we've made some significant changes as outlined and shown below.

The biggest change is the integration of input switching into a now headless (no microcontroller) LDR board (formerly called a slave board). We are convinced that integrating the input switching and thus minimizing the input/output wiring and connection points can only help to improve sound quality.

As with our V2.1 boards, the V3 will have fully integrated autocalibration of the LDRs.

All control has been moved to a single controller board (not shown, formerly called the master board) which mates up directly (or via ribbon cable) to a single graphical OLED (LCD) display module.

One very interesting option that we are continuing to explore is to adopt the Raspberry Pi computer (https://www.raspberrypi.org/help/faqs/Pi) as our core controller in lieu of a microcontroller. For those who've never heard of it, the Raspberry Pi ("RPi") is a $35 Linux computer on a single board the size of a deck of cards. Although it has far more processing power than needed to run our LDR preamps, the RPi is quickly evolving into a feature rich embedded processing device that can make the "internet of things" ("IoT") a reality far sooner than anyone expected. Initially released in 2012, the current version of the RPi has it all - USB, Bluetooth, WiFi, HDMI, video, audio, and significant memory/storage capacity - plus tons of development tools, video display drivers etc, much of which is open source. To be clear, we are NOT proposing to use any aspect of the RPi's audio features, simple it's processing, control and communications abilities.

Even if we don't use the Raspberry Pi for the V3 version of our preamp controller, we might do so later on. Equipped with a proper adapter board, the RPi would be fully compatible with the LDR Board shown board.

Last but not least, the release of the V3 remains at least 6-9 months away and that's probably optimistic.

* The controller board (now shown here):
   * Handles all control processing of the LDR board(s)
   * Connects via ribbon cables to up to 6 stereo LDR boards (12 total channels)
   * Interfaces directly with an OLED display panel to which it can mount to directly
   * Handles encoder input
   * Handles firmware updating via USB connection
   * Handles IR remote input
   * Handles Bluetooth remote input
   * Uses a single 80 pin 8 bit surface mount microcontroller
   * All electronic components are surface mount

* Each LDR board (shown below):
   * Handles a single stereo single ended input (2 channels)
   * Connects to the controller board via 10 pin ribbon cable
   * Is powered by the controller board but has its own voltage regulators separate from the controller
   * Has no digital microcontroller of its own
   * Can accommodate from 1 up to 6 inputs without an external input relay board
      * Input expansion is handled by plugging in additional LDR modules
   * All input switching done via plug in LDR modules (no relays)
   * Utilizes 5 12-bit DACs, 2 12-bit ADCs, and 3 op amps for far more precise and flexible control of the LDRs compared to the V2 board
   * All communications between the controller board and LDR board is handled via a 4 wire SPI serial data link.
   * All electronic components except for LDRs are surface mount
   * Measures 2.8 x 4.5 inch

[robertopisa]

Hello Morten,

it is good to hear that you are exploiting your knowledge and experience on LDRs to further refine your nice project! Please move my comments below where appropriate.  I am not an engineer, so take them with a grain of salt (I am a computer scientist).

Cheers
-Roberto

- I think it is good idea to consider the Raspberry or Arduino, but it could introduce a potential source of pollution in the audio path, so it would be better to shield it and galvanically isolate it.

- I like your LDR boards as they have the shortest path from in/out pins to LDRs. In the scheme below, IMHO, they are longer than necessary. Do we really need 6 inputs? Using 3 inputs, you could dispose the overall 8 LDRs very close to the JL1 and JR1 pins on the shorter side of the board. Actually, I personally like the shortest possible path, avoiding multiple inputs and selectors.

- Crazy idea: could shielding the analog paths of LDRs help to lower even more the noise? I mean, having completely shielded in a (sonically dumped?) box the analog path entering the pins JL1 and JR1, going through the LDRs (possibly closer as said in the previous point), and exiting to the out pints. This would further isolate it from the rest of the board below. In the rare case that one LDR should be replaced, this requires to change the entire box of LDRs, but it is not probably that large cost compared to the rest.

To elaborate the last point: currently the LDRs are detachable along the audio signal path. Instead, in the shielded box the LDRs would be soldered along the audio signal path, and the box would be the one detachable, now along the control path. This is meant to keep pristine and short as much as possible the audio path. So the in/out audio pins to solder would be directly on the box, which will communicate with the rest of the board below using the detachable module on the control path. In this way, one could also solder pin 1 of the XLR connection directly on the box, and thus use shielded balanced cable also till this box. Hope I succeeded to describe what I mean.

[craig sawyers]

Personally more than 3 inputs would be a benefit.  I have four sources, so six would be just fine for me.

Another question Morten - how does the new architecture handle balanced?

[tortugaranger]

Hi Roberto. Good to hear from you and thank you for your thoughtful input.

- I think it is good idea to consider the Raspberry or Arduino, but it could introduce a potential source of pollution in the audio path, so it would be better to shield it and galvanically isolate it.

There's little to distinguish an Arduino from our controller board aside from the fact that the Arduino's are mass produced. Both use standard available microcontrollers and are programmed at the basic hardware level. We would have to mate any Arduino with our own custom interface board same as we would for the Raspberry. I prefer the Microchip family of microcontrollers whereas the Arduino uses Atmel although I've worked with both. Our earliest prototype LDR board used an Arduino. Personally, I'm not very keen on the Arduino programming language and prefer working in either C or Pascal. I currently use MirkroElectronika's development hardware and compilers.


The Raspberry Pi is totally different animal wherein we'd be programming on top of a full Linux operating system. A lot more complexity under the hood and we'd be insulated from the microprocessor at the hardware level. Fortunately there are sufficient drivers available for everything we need - and then some - to handle IR receiver and SPI communications, USB, wireless etc. not to mention high level display functionality. Concerns over noise and shielding may be valid but then again our current boards are already running 16 MHZ microcontrollers with shared power supply driving the LDRs and no shielding all without discernible downside.

- I like your LDR boards as they have the shortest path from in/out pins to LDRs. In the scheme below, IMHO, they are longer than necessary. Do we really need 6 inputs? Using 3 inputs, you could dispose the overall 8 LDRs very close to the JL1 and JR1 pins on the shorter side of the board. Actually, I personally like the shortest possible path, avoiding multiple inputs and selectors.

I'd say it's around 50/50 in terms of comments I receive on the topic of number of inputs. Half are asking for more than 3 and the other half can't fathom why more than 2, perhaps 3 at the most...and some say why not just 1. There's no perfect one size fits all solution here. The current prototype board design comes close  insofar as it has the flexibility to only have 1 input or have up to 6. It's only a matter of adding the LDR modules which are plug in.

It would be a simple matter to minimize trace lengths by eliminating J1L and J1R as such and simply having individual solder pads as close as possible to each input LDR switch. This would have the added benefit of shortening the board up by perhaps another 0.5 inch. However, as a practical matter this doesn't really shorten the signal path. You still have to run wires from each input RCA/XLR jack to the board. The traces may be shorter but the wires to each trace/pad would have to be longer. Pick your poison.

- Crazy idea: could shielding the analog paths of LDRs help to lower even more the noise? I mean, having completely shielded in a (sonically dumped?) box the analog path entering the pins JL1 and JR1, going through the LDRs (possibly closer as said in the previous point), and exiting to the out pints. This would further isolate it from the rest of the board below. In the rare case that one LDR should be replaced, this requires to change the entire box of LDRs, but it is not probably that large cost compared to the rest.

To elaborate the last point: currently the LDRs are detachable along the audio signal path. Instead, in the shielded box the LDRs would be soldered along the audio signal path, and the box would be the one detachable, now along the control path. This is meant to keep pristine and short as much as possible the audio path. So the in/out audio pins to solder would be directly on the box, which will communicate with the rest of the board below using the detachable module on the control path. In this way, one could also solder pin 1 of the XLR connection directly on the box, and thus use shielded balanced cable also till this box. Hope I succeeded to describe what I mean.

One of the more interesting philosophical issues around design is whether one is designing around a real or imaginary problem. Is noise really a dominant problem here that needs a solution? In my view that remains an interesting question as it relates to the bigger topic of perceived benefit of balanced audio within short run stereo audio...to which I say... I don't know. I can say that with some conviction that noise has not been an issue with any of our LDR preamps aside from the your run-of-the-mill ground loop hum issues which are really a separate noise category best solved through good grounding practices. RF/EM noise has not been a problem as far as I'm aware. Therefore taking special precautions such as shielded boxes and/or cables internally within the preamp do not seem warranted.

Also I doubt anything is gained by having to replace a whole set of LDRs within a removeable shielded box vs. individual LDRs modules. If it's removeable it's still plugged in and not soldered in. Tubes live quite effectively within a plugged in reality, so my thought is LDRs should be able to as well. We use gold plated pins and sockets so corrosion shouldn't be an issue.

One design architecture that we've explored quite a lot is to move the input switching out to the individual RCA/XLR input jack. While this would indeed minimize the signal path of each individual input, you still end up with common signal(s) that have to run back to the LDR board. Plus you'd end up with numerous switching control signal cables running from the LDR board to each input jack which frankly would end up looking really awful.

After going around the track numerous times with various alternatives we arrived at the design you see below in the recent post. By locating the LDR board as close to the rear panel as practical you minimize the signal wire runs to/from the LDR board and ultimately the total signal run distance between the rear panel inputs/outputs and the LDRs. At least that's the thought behind it.   

There's one additional step we could take but have resisted. The LDR board could be custom designed to mate up directly with the RCA or XLR jacks. This eliminates any additional wiring between the jacks and the board. The issue  with that is you end up with a different custom board for each and every model of preamp. Many designs use this approach since it has the added benefit of reducing the manhours needed to build a preamp.

[tortugaranger]

Personally more than 3 inputs would be a benefit.  I have four sources, so six would be just fine for me.

Another question Morten - how does the new architecture handle balanced?

I get a LOT of inquiries asking for more than 3 inputs.

Balanced audio is done by using 2 LDR boards and designating one as the right board and the other as the left board. Within each board, the + and - balanced audio signals are terminated to the right/left channel respectively.

[DaveC113]

There's one additional step we could take but have resisted. The LDR board could be custom designed to mate up directly with the RCA or XLR jacks. This eliminates any additional wiring between the jacks and the board. The issue  with that is you end up with a different custom board for each and every model of preamp. Many designs use this approach since it has the added benefit of reducing the manhours needed to build a preamp.

PCB mounted jacks often don't have pin1 / ground going to chassis right next to the jack, which can cause the "pin 1 problem".

[craig sawyers]

Will the OLED module allow the naming of inputs (CD, Deck, Streamer etc)?  Numbered inputs are fine, provided you can remember what you have connected to each number.

[tortugaranger]

Will the OLED module allow the naming of inputs (CD, Deck, Streamer etc)?  Numbered inputs are fine, provided you can remember what you have connected to each number.

Yes.

[33na3rd]

Perhaps there could be an optional external OLED housing for folks installing a LDR board into an existing preamp?