Loch Eite LDR Hi-Fi Attenuator

LDR Optocoupler Hi-Fi Attenuator - Technical Data & Laboratory Tests

These tests are carried on pre-production 'Loch Eite Mk I' LDR attenuator. The tests comprehend all the device from the input to the output jacks.


We decided to publish a comprehensive test of our new LDR attenuator, the 'Loch Eite Mk I', we was surprised that none of our competitors publishes lab test of the products. Like all electronic devices, LDRs are subject to better or worse performance depend on technical solutions and implementations. We believe that LDR attenuators can bring to very good results and to true enjoyment of music

THD & Noise

We feed the device under test with a sine wave from an Audio low distortion generator, the output is either sent to 12 bits, high-resolution oscilloscope and via an AD 16 bits converter to a Mac PC. The 16 bits AD converter is calibrated for frequencies between 1 Hz to 20 kHz. A graph of the noise floor and distortion pattern of the generator + AD converter is published here, please take it into account when confronting the charts. An explanation of the charts is provided in this article. We will test the attenuator at different frequencies.

Frequency Response

We fed the device under test with a sine wave sweeping from 1 Hz to 24 kHz, the output is sent to an AD 16 bits converter to a Mac PC. The 16 bits AD converter is calibrated for frequencies between 1 Hz to 20 kHz, therefore the result after 20 kHz does not reflect the response of the device under test. 

The screen needs to be explained:

The frequency response test is carried on at different attenuation levels,

IMD Intermodulation Distortion

For this test, we send a signal composed of 32 evenly spaced frequency between 20  Hz to 20 kHz, it sounds like a pipe organ a bit out of tune, the Spectrogram will show us if there are distortions between the 32 fundamentals. This test the 'Loch Eite Mk I' in a more 'real' situation, in fact, music is composed of multiple frequencies at the same time. The test is carried on at 2 different attenuation level. 

HUM and mains power supply related noise

LDR Attenuators are, under a point of view, an active device. This means that the signal ground must connect with the driver's ground at some point. The driver is, most of the times, an MCU unit with another bunch of active electronics connected to it. In the case of the 'Loch Eite Mk I' we have an MCU, touchscreen, clock, relays and LDR drivers, voltage regulator and power supply. All these devices generate noise and the unit must be designed to limit the noise at the minimum.

All the testing programs we have seen before are of no use when it comes to analysing frequencies between 1 to a few hundred Hz.

To highlight HUM noise we connected a low distortion signal generator and with the help of a high-resolution oscilloscope, we then analysed the spectrum via FFT.  Here is the hard copy of the screen:


The cursor 'a' point at 50.00 Hz, while the cursor 'b' at 100.00 Hz, typical hum frequencies. As you can see there is no trace of Hum noise above the floor noise level. This is also confirmed by listening tests.

Empirical Listening Test

Listening tests always are very subjective. I won't use silly terms like 'liquid' sound, 3D soundstage etc. Every system is different, and every listener is different, I invite all those interested in trying the 'Loch Eite Mk I' at home if it doesn't meet your expectations you can return it for a full refund within 30 days, no postage costs, no restocking fees.

For me the 'Loch Eite Mk I' sounds great, maybe I'm biased. What I can guarantee, above personal preferences and highlights that my system may have or may have not, is that the sound definition is a step above many amplifiers I had the pleasure to listen to, the soundstage is 'extended', more substantial than I used to listen to, the sound neutral, like it should be. What else? It adds almost nothing to the music, and it takes out nothing.  But that is what an amplifier, or an attenuator, is supposed to do.