LDR Optocoupler Hi-Fi Console
WHAT IS A LDR OPTOCOUPLER?
The LDR Optocoupler, also known as resistive optoisolator (RO), and also called photoresistive optoisolator, is a variable resistor driven by a light source, i.e. a photoresistor with an LED in a tiny enclosure. If you want to deepen the theory you can read this.
APPLICATIONS OF THE LDR OPTOCOUPLER IN HI-FI
LDRs are used in audio devices, mainly guitar reverberation devices, from decades. In the last few years, we see a proliferation of so-called "passive preamplifiers" based on LDRs for volume control. From now on we will use the correct term "console" instead of "passive preamplifier" because all these kinds of devices are not preamplifiers at all since the gain of this kind of equipment is less or equal than 0 dB.
The primary assumption is that the LDR is superior to other resistive-based volume attenuators (potentiometers and resistive ladders) for the following reasons:
Excellent sonic qualities (compared to potentiometers, au pair with the best resistive ladder attenuators)
Lack of moving parts that can deteriorate in time (wipers in potentiometers and resistive ladders)
Better channel separation (problematic with traditional stereo controls like potentiometers and multi-step selectors)
Smoother transitions while changing the sound volume (against resistive ladders that are stepped, typically by 2 dB)
Taken for granted the above-enumerated assumption, and taken into account the little cost of a quality LDR optocoupler we can believe that we reached the holy grail of Hi-Fi, but bear with me because the reality is a bit more complicated.
INTRINSIC PROBLEMS OF LDR OPTOCOUPLERS
Non-linearity and wide tolerances of LDRs make problematic the use of this components in Hi-Fi volume attenuators. For a stereo console, you need four LDRs to build an attenuator, the basic schematics is shown below.
As you can see, in theory, you need two LDRs in one channel to maintain the load constant, and this complicates the matter because:
The response of the photoresistor is neither linear or logarithmic (the latter preferred in audio equipment for volume controls)
The response varies substantially between LDRs of the same brand and batch.
Both problems are intrinsic to the technology employed in the fabrication of these devices, inside every optocoupler we have one LED and one photoresistor; both components suffer from wide tolerances that sum up to an even bigger tolerance in the compound. Having said that its' now clear that we face the following problems employing the simple schematics above.
The balance between channels. Since the volume controls for each channel are not calibrated one to another, we will have a shift of the stereo image to the left or the right at different volume settings.
Lack of logarithmic response. Our hearing is not linear, but approximately logarithmic, that's why audio potentiometers for volume control are not linear but logarithmic. This lack of logarithmic response will make the transition between different volume settings unnatural and uncomfortable.
Unpredictable resistive load. The source like the destination should "see" a constant load, as you can see this is, in theory, achieved with two LDRs one operating as input resistor and another, working oppositely, as a shunt resistor. Since the tolerance are high, and the resistor response is not linear, the load will vary significantly and unpredictably either from channel to channel and at different volume settings
If we want to put the icing on the cake, we can take into consideration the components ageing. In the above schematics, we have not four but eight components, four LEDs and four photoresistors. We can expect that after 2-3000 hours of operativity to have shifted from the original parameters between 5 to 10%, and this in an inhomogeneous manner.
LDR OPTOCOUPLERS AND THE MARKET
We have seen in the previous chapters that LDRs offers exciting features at a very low price but pose a challenge to the developer to overcome the severe drawbacks.
Nevertheless, on the market, there are quite a few LDR-based Hi-Fi consoles. We made a swift market analysis, and we couldn't find clear explanations from the different producers about HOW they overcame the severe drawbacks of LDRs.
Sure enough, for many, the temptation to build something fashionable at low cost and sellable at a relatively high price was just too strong and, anyway, a good percentage of Hi-Fi buyers buy the hype but is unable to discern if something sound right or not. Some others are, maybe, just too jealous of the technical solution adopted in their devices to give us a brief explanation of how the technology works.
OK THEN, SHALL WE DUMP ALL THE LDR HI-FI PROJECT?
Not at all, the LDR promise, and can deliver, performances in passive volume control second only to high-end transformers (you can read about our preamplifier based on Transformer Volume Control (TVC) here).
We got interested into LDR Optocouplers a few months ago, at the beginning we dismissed the technology as a no-go. We are a quality oriented small firm, and we cannot accept to embrace a technology that doesn't offer the best performance for the price, it's true that you get what you pay for but you, as a producer, must be sincere and transparent about your products.
Nevertheless, the LDR thing continued to buzz in our minds because once properly addressed all the drawbacks, can offer high performance at a more affordable price and be alternative to our TVC based preamplifier that, due to the technology employed, is quite expensive.
GENESYS AND TECHNOLOGY BEHIND THE "LIGHTSTREAM"
Our background is not only audio but, most of it, informatics and microcontrollers. We already deployed this technology for a better enjoyment and user comfort in our Reference Line Preamp, and it's working amazingly well.
We decided to invest our time in a bit of R&D on LDRs, and we arrived at the conclusion that the only sensible way to overcome all the LDRs' drawbacks is to employ IT technology applied to microcontrollers.
Our tests demonstrate that is possible to generate a profiling file for the four LDRs present in the circuit and then use it in a way to "normalise" the way the LDRs are set at any due volume setting.
Apologies for the bit of jargon but for all those interested here we explain, in general lines how it works.
The calibration process:
The calibration is carried on at the factory but can be repeated by the end user if, after a period of time, feels that the system is not performing at it's best. During the calibration, the inputs and outputs of the console are switched off, and a +5 DC voltage is channelled into the photoresistors while the outputs of the photoresistors are driven into four analogue readers of the microcontroller. In the meantime, the LED part of the optocoupler is driven by 16 bits PWMs with a resolution of 65536 steps. Following an algorithm that guarantees the respect of our targets, i.e. logarithmic volume response, balance between channels and constant load, a map of PWM values is created for each of the 100 volume settings that the system is capable (from -50 dB to 0 dB @ 0.5 dB steps).
The day-by-day operation :
The user operates the "Lightstream" like any other console or amplifier. At every one of the different 100 positions of the volume knob, that, nota bene, is connected to the microcontroller and not to the LDRs, the microcontroller read the four PWM values for that volume level and operate the LEDs, that drive the photoresistors into the optocoupler, and all with 16 bits precision.
The audio path continues to be completely "passive", no interference is made to the sound from the microcontroller or other electronics except the mechanical relays that switch the inputs and the outputs.
STATE OF THE PROJECT
On the day of this writing, 02/09/2018, we have completed 80% of the software, 100% of the electronics. The project of the case and internal engineering is at 20%. The final main board is into production and expected to be delivered to us mid-September for the first production run.
Please refer to our Blog for more updated information.
"Lightstream MK. I"
Type: Hi-Fi Passive console
Inputs: 4 stereo, relay controlled
Output: 1 stereo
Gain: 0 dB
2.8" Touch display with control of the main functions, on/off, input selection, clock, balance.
Motorised volume control 100 steps from -50 dB to 0dB @ 0.5 dB
Remote Ir control
Power supply: 12V DC (supplied external)
Power consumption: stand-by ~ 0.3W , operative ~ 10W
Written on September 2nd 2018 by Marco Mazzocchi - Chief Engineer