Precalibration overview
Precalibration (or “PreCal”) is a method of normalizing the measurement and control electronics in the LDR3x.V2 so that each board behaves the same as any other board over the full volume control range. The PreCal process uses a set of 100 ohm and 100k ohm resistors that get plugged into the V2 board to give the V2 two known resistance points that are then used to apply correction factors when performing normal LDR calibration. The resulting stored PreCal correction factors increases the accuracy of the preamp controller.
Tortuga Audio performs precalibration on each V2/V2.1 preamp controller before shipping them to customers or using them in a preamp product. The results of the PreCal procedure are permanently stored in the board’s memory memory chip. PreCal should not have to be repeated, but since the V2 was introduced as a DIY product we are making the procedure available here for our DIY customers.
Does my V2 preamp require PreCal?
PreCal is a normal part of Tortuga Audio’s commissioning process for each LDR3x preamp controller starting with the release of version V2.1 of the LDR3x preamp controller board on January 5, 2015. If you already own an LDR3x.V2.1 board or any Tortuga preamp product shipped on or after January 5, 2015 your board/preamp has already gone through PreCal and repeating the process is not necessary.
If you own a V2 version of the LDR3x or any Tortuga preamp built with the V2 board, then your preamp will likely benefit from PreCal. PreCal only needs to be performed once.
Identifying the LDRs
It’s critical that you have a very clear understanding of how to identify the LDRs on the V2/V2.1 board.
The instructions assume you are facing the front of the board looking towards the back (top in the pic below) where the LDRs are located. Going from left-to-right, the 4 LDRs are identified as follows: RV4 RV3 RV2 & RV1. These identifiers are printed on the circuit board. The PreCal sequence is #1-RV3, then #2-RV4, followed by #3-RV1 and finally #4-RV2.
Required equipment (critical!)
- Dual display modules are required. While you might get away with just a single display module it’s too easy to get confused so we don’t recommend it.
- 4 1% accuracy 100 ohm resistors connected to female square pin sockets on each end
- 4 1% accuracy 100,000 (100k) ohm resistors connected to female square pin sockets on each end
- Alternatively to 2) and 3), you an purchase a set of plug-in PreCal modules from Tortuga Audio.
- Apple Remote
If you supply your own calibration resistors you will need to solder or otherwise attach them to 0.1″ square pin female sockets. To make you own we recommend using a typical 6 inch square pin female-female jumper wire, cut it in half, strip each cut end, and solder the wire ends to each leg of a resistor. You’ll need to do this for all 8 precalibration resistors. When done, each resistor will have a square pin female socket attached to each end of the resistor. The two square pin sockets will be used to plug each resistor into the 2 rear male header pins on each of the four LDR headers.
Detailed PreCal procedure
The table below describes each step in the precalibration procedure.
| Step | Action | How |
|---|---|---|
| 1 | Disconnect audio inputs | Remove all input/output interconnects from the preamp |
| 2 | Connect preamp to power | Allow unit to boot up - wait until the display stops showing numbers |
| 3 | Confirm preamp is off | There should be no numbers showing on the displays |
| 4 | Remove the LDRs | Pull out each of the 4 LDRs and set them aside in the same order that were installed. At this point you’ll notice the 4 square male pins to which each LDR module had been attached. The 2 back pins are involved in the PreCal process while the 2 front pins are not. |
| 5 | Install the 100 ohm resistors | If you are using your own resistors, connect the 4 100 ohm resistors across the 2 back pins of each of the 4 LDR pin headers. If you are using Tortuga Audio supplied PreCal modules plug the 4 modules in such that the larger/fatter resistor is facing towards the rear of the board and the smaller/skinny resistor is facing forward. |
| 6 | Start auto-cal | Press the Center/Enter button on remote or give the Encoder a quarter to half a turn. Unit should commence auto-cal with displays showing status. |
| 7 | Switch to PreCal mode | Press the Left button on the remote deliberately 3 times in a row pausing briefly between each press (prior to rev 2.2.3 firmware press the Raise button). You can tell the unit is in PreCal mode because the display modules will show different info than is typical of auto-cal. At this point PreCal will be on LDR #1-RV3. The left digit on the left display module will show “1”. The right digit on the left display module together with both digits on the right display module form a 3 digit number indicating the calibration factor needed to read 100 ohms. By way of example, if the 4 digit display shows 1950 this means LDR #1 requires a -5% correction factor. If it shows 1132 then this means LDR #1 requires a +13.2% corrector factor. You don’t need to record these values. They are saved in the units’s memory. |
| 8 | Allow #1 LDR’s (RV3) correction factor to stabilize | Even though the 3 digit number will most likely continue to change up or down it should stabilize within a min/max range rather than continue to count up or count down endlessly. It should stabilize within just a few seconds. If the number continues to increase (or decrease) continually for more than 10 seconds without reversing the cause is most likely improper grounding of the audio signal ground from the controller board to system star ground. If this happens remove power from the unit to shut if off and diagnose/correct the problem before attempting PreCal again. While this is unlikely to happen with a finished preamp product, this type of grounding problem is not uncommon with DIY versions of the LDR3x preamp controller board due to the DIY’er not grounding the unit properly. |
| 9 | Switch to #2 LDR (RV4) | Press the Right button on the remote to switch to the next LDR. The left digit on the left display will change from #1 to #2. |
| 10 | Allow #2 LDR correction factor to stabilize | (see #8) |
| 11 | Switch to #3 LDR (RV1) | Press the Right button on the remote to switch to the next LDR. The left digit on the left display will change from #2 to #3. |
| 12 | Allow #3 LDR correction factor to stabilize | (see #8) |
| 13 | Switch to #4 LDR (RV2) | Press the Right button on the remote to switch to the next LDR. The left digit on the left display will change from #3 to #4. |
| 14 | Allow #4 LDR correction factor to stabilize | (see #8) |
| 15 | Switch to #1 LDR (RV3) | Press the Right button on the remote to switch to the next LDR. The left digit on the left display will change from #4 back to #1. At this point you have completed pre-cal with 100 ohms and need to switch the resistors from 100 ohm to 100k ohm before proceeding to next LDR. Until switchover to 100k ohm resistors is completed at least for LDRs #1 and #2, the numbers show on display for #1 LDR will continue to change. Ignore the display until resistor switchover to 100k is done on all four LDRs. |
| 16 | Switch resistors from 100 ohm to 100k ohm on all LDRs starting with LDRs #1 and #2. | Once all resistors have been switched over to 100k ohm the display values for LDR #1 will begin to stabilize. The longer you took to switch over the resistors the longer it will take for LDR #1 to stabilize. |
| 17 | Repeat steps 8 through 14 | Same procedure as before but this time with 100k resistors for each of LDRs #1 through #4. |
| 18 | Quit PreCal process | When LDR #4 has stabilized, press the right button on the remote one more time. This will save the PreCal results in memory, turn off PreCal and shut off the preamp. This completes the PreCal process. |
| 19 | Replace LDRs | Remove the four precision resistors or resistor modules and reinstall the four LDRs |
| 20 | Run AutoCal | Press the Enter button and allow the unit to go through a complete AutoCal cycle. Once AutoCal is finished, you are done. |