Lynton mentioned 'grist for the mill', but I have one more to throw in here.
The sonic problems of potentiometers derive from the multiple contact phenomenon of the wiper, and they essentially rob resolution at very low levels. The switched attentuator replaces a continuously variable carbon or plastic track with a ladder of resistors, where the only degradation is in the solder joints and the switching points. This is known to be much 'cleaner', but there is still the problem of high cost, limited availability (or at least convenience of purchase), and highish wear rates, which knocks out the cheaper Asian imports.
The Rolls Royce (now there's a fading metaphor!) of switched attentuators is undoubtedly the Japanese TKD, which is veritably a scientific instrument, made like a Honda F1 engine cylinder head. Just to hold it in your hand is a feeling of wonder, or at least for odd characters like myself who absolutely live for good engineering!! But the cost is absurd, something you would only entertain for a $20K+ preamplifier. That my GK1 is considered in this rarified company is a great flattery, and I'm pretty happy about it!
The Welborne Labs relay system is good, but I would suggest that the relay quality is the pivotal factor here, and they may be cheapies. My favourite is the NAIS Aromat, with silver/palladium contacts, which is supplied in my Swift. I've never heard any degradation of the signal using these relays, but I don't believe they are used in the Welborne kit product.
This third relay approach, wherein the complex, 24 position switch is replaced by several relays under microprocessor control driven from a rotary selector is pretty good, but it's electrically complex, and a whole project on its own to build. I quite like it, and a Norwegian called Kolbrek (IIRC) created a marvellous design later added to by a clever young Viennese EE student, now graduated, with the interesting name of Timo Christ, which is a full on DIY project somewhere on the net.
However, since about 2002 when I designed the GK1 there have been a slowly growing list of microprocessor controlled ladder attentuators apperaing in the market with integrated opamp outputs for buffering and laser trimmed resistors. The most famous is the PGA2310, originally a Burr Brown chip, now owned by Texas Instruments. This product appeared in Oct 2001 but was revised in Sep 2004, and has stellar specification, namely -95dB to +31.5dB in 0.5dB steps (!!!), a distortion figure PLUS noise of just 0.0004%, a Zout of around 100R, a dynamic range of 120dB, and a unity gain bandwidth of 1.5MHz. With a Zin of just 10K, two independent channels on the one chip, and noiseless switching, this is surely just the ticket for a premium quality high end system.
Other chips have since appeared in the market, I believe.
More grist for the mill?
Ciao,
Hugh
