Technical Information

Some background information on the whys and wherefores.

Test and Design

Test and Measurement hardware:

  • HP 8903B Audio Analyzer, 20 Hz to 100 kHz

  • HP 4274A LCR Meter, 100 Hz to 100 kHz

  • HP 3314A Function Generator, millie Hz to 20 MHz.

  • Tektronix 465 oscilloscope.

  • EMu 1212M Soundcard.

Software tools:

E-Mu 1212M Soundcard FFT Tests

Test Philosophy:

My audio web pages include a large number of FFT graphs, many of which show substantial 50 Hz mains supply and harmonics as well as pickup from the computer and monitors power supplies.

Many of the tests are conducted in less than ideal lab conditions on an open bench in front of my computer. These tests are pragmatic and reflect the reality of many DIY constructors who do not have access to the sorts of equipment or facilities of large Hi-Fi manufactures or review magazines. Where possible I include a picture of the test setup (flying wires and all) to demonstrate the basic setup and to show that I am not doing anything special that any competent DIY constructor cannot do themselves.

For the FFTs I plot the full range of the 24bit sound-card resolution to show signals down at the -140 dB range which is normally considered inaudible. I readily note that below -120 dB much of the harmonic data is as much influenced by the sound card as it is from the unit under test which is which I include test system loop-back plots as well. See my E-Mu 1212M Soundcard FFT Tests.

The basic noise floor, or the resolution of the 24 bit converters, is discernable and I differentiate between noise and interference such as the mains harmonics and the switcher pickup.

Despite these extraneous signals I believe that the low level information still has meaning and not only can give a flavour of how the unit is operating but also gives clues as to other issues which might not otherwise be apparent from refined data.

One example is the effect of poor grounding and this shows up in one series of tests on the Zeus Single Ended MOSFET Preamp / Line Driver as a large symmetrical lift around the fundament. As this peaks at -120 dB most standard measurement setups would not show it.

Another example is the inter-modulation effects where there are apparent signals created between a fundament and it's harmonics, not just between two strong tones, in the Zeus 6C33C PP Line Driver tests.

Given that long serious and often heated debate goes on about such things as capacitor distortion and cable effects (much of which is not apparently measurable with standard audio tests) these things which do show up will I believe have an effect on the sonic signature of the system.

So I present my FFT graphs "warts n' all".

Susan - November 2006.


All design and other information, drawings and images on this website are
Copyright 1992-2010 Susan Parker MIET (unless otherwise credited).

These designs and other information may be used to construct systems specifically for personal NON commercial use only.

N.B. Personal liability disclaimer applies - see T&C.