Here are frequently asked questions relating mostly to LDRs and the nature of passive preamps.
An LDR is constructed by mating a light emitting diode (LED) together with a photoresistor in a small sealed package. The LDRs Tortuga Audio uses are about the size of a M&M candy that has 2 pairs of wires. One pair of wires is for running current through the LED. The other pair of wires is the variable resistor of the photoresistor. The resistance of an LDR is inversely proportional to the light intensity of its LED. Simply put, more light = less resistance, and vice versa.
Early preamps with only LED lights on the front panel used our original V1 preamp controller. Starting with the release of our V2 preamp controller all preamps have dual 2-digit 7 segment displays. Around May 2017 we introduced the V25 preamp controller that uses the same display. Preamps with a V2 controller briefly display a 3 digit firmware number approximately 5 seconds after power is applied. Preamps with a V25 controller will display a 4 digit firmware version. In 2018 we introduced our menu driven OLED graphical display for the V25 controller where the controller version info is available as one of the menu items.
They are really the same thing. The term “attenuator” means any device that reduces the voltage level of an electrical signal. Audio volume control (reduction) is accomplished by attenuating (reducing) the voltage level of an audio signal. The lower the voltage level of an audio signal the lower the loudness level coming out of the amplifier/speaker. Most attenuators use combinations of resistors to form a voltage divider to reduce voltage (volume). A potentiometer is the most common type of voltage divider. Tortuga Audio uses LDRs to emulate a potentiometer except that LDRs produce a more satisfying sound.
There are two broad types of preamps: passive and active. Most preamps are active. Active simply means there is direct electrical manipulation of the audio signal wherein a power supply acts upon the audio signal as part of a gain stage and/or a buffer stage. In a passive preamp there’s no direct connection between any power supply and the audio signal. One way or another a passive preamp is simply a voltage divider which passively controls volume by reducing the voltage level of the audio signal before it’s downstream to a power amplifier. Passive preamps do not provide any gain (audio signal/voltage boost) and are often referred to as “unity gain” attenuators.
Like all analog electronic devices, LDRs have a finite life and will age over time. The useful life of a typical LED is roughly 50,000 hours. If you listened to music 2 hours each day every day, 50,000 hours of listening would translate into 50,000/2/365 = 68.5 years. I’d say you’d have gotten your money’s worth. Starting with version V2 of our LDR preamp controller, auto-calibration compensates for any changes in LDR behavior as they age plus the LDR modules can be easily replaced just like a tube.
We are absolutely convinced that LDRs produce better sounding audio than other volume control technology. Better than any potentiometer. Better than any conventional resistor ladder (stepped attenuator). Better even than most transformer volume controls. Over 95% of our customers keep their LDR preamps after our 30 day in-home audition period runs out when they could have returned their purchase for a 100% refund. Most of our customers have owned, seen and heard a lot of high end audio gear. You don’t keep a new preamp if you don’t think its sound better than what you had before.
LDRs are inherently challenging to use in audio design. Their resistance curves are nonlinear (the relationship between resistance and control current is not constant. Worse, the resistance curve of each LDR is usually different from one to the next even within the same make/model of LDR. It gets even worse – their resistance curves often change as they break-in and age. Audio designer don’t like to design products using hardware with these characteristics. Can these challenges be overcome? Yes. Is it easy? No. We spent years developing and refining our LDR volume control technology. We use software driven digital processing to tame and control LDRs so they perform predictably and precisely. This isn’t typical audio design. But we made it work. Why? Because done right, LDRs sound better.
The short answer is yes. But their distortion level is still well below the ability of humans to perceive it as distortion. Distortion specs in audio are often used as a broad metric of sound quality in that manufactures claim that equipment with lower distortion sounds better than equipment with higher distortion. This is an unfortunate myth that is too easily sold to the unwitting consumer and is too often fodder for marketing departments and a source of pissing contests between engineers armed with measuring equipment.
The reality is that human beings are unable to discern the presence of distortion in normal music at levels at or below ~1%. What this means in practical terms is that someone claiming their equipment has only 0.001% total harmonic distortion will not sound any different to the listener than if the same equipment had up to 1% distortion. Why? Because human beings can’t hear it.
Look at it this way. If distortion was indeed a true metric of sound quality, all we would need to do is make a list of measured distortion from every make/model of audio equipment, sort it from lowest to highest and buy the lowest distortion audio equipment on the list that we could afford. But nobody does that. For good reason.
As far as we know, no other LDR based preamp uses updateable software based digital control of the analog LDRs together with built-in self-calibration of the replaceable plug-in LDRs so your preamp investment will last indefinitely and continue to perform optimally. Also, our LDR based preamps can all be controlled with a simple remote.
Absolutely not. Each audio signal channel in our preamps goes through a passive voltage divider composed of 2 LDRs (light dependent resistors) in a series/shunt (“LPAD”) configuration. There’s no active manipulation of the audio signal by any power supply nor is the audio signal processed by ADC/DACs. Our preamp does have and use DAC/ADC’s but they are for controlling the resistance of the LDRs and calibrating the controller but definitely not for processing the audio signal.
Yes, most definitely. LDRs have been around as long as there have been LEDs (light emitting diodes) and photoresistors which means several decades at least. Within the past 10 years the design and manufacturing of LDRs has improved to the point where certain LDRs can be used effectively in audio applications.
For the true audio Luddite, mixing anything digital with high performance analog audio is a cardinal audio sin because they assume anything digital will introduce noise that will degrade the audio quality. This is a good example of how being partially technically correct can result in misleading overall conclusions. Yes, digital circuitry creates it own world of noise. But no, it doesn’t impact the audio quality in any way that’s relevant to humans. Our latest generation LDR.V25 preamp controllers operate at a clock rate of 72 MHz which is 3,600 times higher than the highest frequency audible to the human ear.