At Radial, we pride ourselves in providing plenty of detail when it comes to evaluating our product. In fact, most of our competitors provide very little in the way of documented tests. But unless you understand the tests, it may be difficult to judge and compare. Most important, low distortion is only one indicator of how well a product may actually sound. As such, please take all of this information in stride… the specs always need to be counterbalanced with the application.
Testing is performed using an Audio Precision Model ATS2 using industry recognized testing practices. We perform a number of tests, the most common are as follows:
This test shows how the circuit behaves across the 10Hz to 20kHz audio spectrum. For direct boxes and preamps, one would expect to see a linear response. For specialty items such as a speaker emulator or interface for guitar pedals, the response will of course be tailored to sound right.
This test looks at distortion and noise across the full audio listening range. Distortion is usually greater at low frequencies. A well designed circuit will usually exhibit low distortion figures way down to 20Hz.
Lowering the self noise of any device is critical given today’s ultra quiet digital recording environment. Achieving a noise floor below -110dB or better is desirable. This test shows noise across the full audio spectrum.
Distortion versus Input
This test is performed by inserting a 1kHz tone into the circuit and then measuring how the device reacts at various levels. Most devices are optimized to work within a given range. The nominal or 0dB range is most critical.
The ideal situation would be for a circuit to perfectly replicate what comes in and send it out without artifact. Most circuits retain the phase relationship in the mid range but tend to suffer in the low and high extremes, causing the sound to be slightly skewed. This effect is also known as group delay. retaining absolute phase is of course desirable.
Inter-modulation distortion (IMD)
This test causes the device to distort using two dissonant tones which cause the circuit to beat or modulate like an out of tune piano. And just like a piano, some sound a lot better than others!
Fast Fourier Transform (FFT)
This test is performed by driving a 1kHz tone into a device, causing it to distort and then looking at the harmonic structure. Audiophiles seek out warm sounding even order harmonics in a perfect ascending cascade. Odd order (zig-zaging) harmonics are the ones that tend to sound harsh to our ears.