Attenuator puzzles

In Stereophile, November, 2000, page 105, reviewing the Hovland HP-100 preamplifier, Michael Fremer says: "The 31-position (2db steps) custom-made, low-mass, coin-silver-contact stereo volume control is configured as a series attenuator instead of the more conventional shunt configuration: the source sees a constant impedance and the listener is ‘...not forced to listen through high-value resistors' at certain levels." (My guess: the inner quote is from the manual.) This passage puzzles me in several ways. First on a fact: are shunt attenuators really "the more conventional" choice in active preamps? Can anyone name an active preamp that has one? Preliminary to further puzzles, some attenuator basics. For each channel, a shunt type has a fixed resistor in the signal path and multiple resistors selectable by rotary switch in the return path. (A return path potentiometer is another option.) A series type has a chain of resistors, arranged around a rotary switch, going to ground at one end, with input coming into the other end and output leaving from a switch-selectable point in the chain. In a series type using a K-position rotary switch, there are K-1 resistors in the signal and return paths combined, at any volume setting, with less in the signal path at higher settings. A ladder type uses two switch decks to switch-select a pair of resistors, one each for signal and return paths. So there are only two resistors in signal path and return path combined, for any volume setting. A shunt type is like a ladder in this regard. Back to the Stereophile passage. On the inner quote, why are high-value resistors inherently bad? And why are they put in the plural, since a shunt has only one each for signal and return paths? And whatever the badness is, how does a series type avoid it with its joined multiple resistors that add up to a "high-value resistor", in signal or return paths, "at certain levels", namely, high levels for the return path and low levels for the signal path? It is arguably bad for the signal to go through multiple solder joints, as it does at low levels in a series attenuator. This is the only badnessI can find regarding "certain levels". But despite the apparent intended thrust of the inner quote, this hardly shows the superiority of series over shunt! Another question: why didn't Hovland use a stepped attenuator? Why put serious money into a top quality switch and then use it suboptimally? The CAT preamps use a series attenuator, my old ARC SP-15 did, older Sonic Frontiers top preamps did too. Series is surely "the more conventional" choice. It takes only about half as many resistors as ladder, and half the switch decks, but their costs needn't be prohibitive. Why pass up a chance to make a product better, in one major respect, and steal a march on some very prestigious competitors? But a final question: with all the incoherence I've cited, and my view of shunt as an implausible rival to other types in active preamp design, I'm no longer sure what attenuator type the Hovland HP-100 in fact uses. Maybe the inner quote was a poorly stated argument for ladder over series. As Sherlock Holmes might say, when an impossible reading has been eliminated, an improbable one may be all that remains. Their switch itself sounds like the Shallco used in the Blue Circle Galatea's ladder, available from The Parts Connection to passive preamp enthusiasts like me.
Wouldn't it be nice if one supplier, would make one stereo 100k ladder, stepped attenuator, with 20 to 40 positions, which could be motor driven (and, since we're wishing, supply the infrared remote driver), and everyone could thereafter agree this is the good sounding unit to use, and everyone would use it, even though it might cost out at $200 or so. Every ALPS potentiometer, shunt, ladder, whatever, would be S.O.L., and every audio manufacturer and listener would thereafter have the best stuff to listen to. Reviewers could merely note, "the preamp under review contains the irreproachable and ubiquitous motor-driven 24 step ladder attenuator." And we would never have to think about this or read about this again. Except to discus the resistors used. Or the effects of circuit impedance.
Tom, this may sound like a stupid question, but when you use the phrase "return path", are you referring to resistors that are connected in parallel, or are you referring to resistors that are in series with the negative pole's path after they've passed the attenuator (or perhaps there is no difference)? I thought a "shunt" used both parallel and series resistors, an "L pad" if you will. It's the series resistor that determines how quiet the signal gets, and the parallel one is there to keep the impedance proportional to what it was before any "padding down" has taken place (i.e., the higher the series resistance, the lower the parallel resistor value will be)....Or so I gather.
Sportinlife: Keep dreamin : ) I too was looking for something like what you mentioned. I haven't found anything that comes "remotely" close : ) What i did find was a great deal on some Shalco's. Three seperate 50K stereo Ladder's for $100 total !!! It just so happens that i have three identical preamps that run 50K pots !!! Hopefully, my search is over. Sean >
Carl, I hope I can answer your question, but I may be ignorant of some of your terminology. I could have said "path to ground" instead of "return path". And by "signal path" I meant the "hot" side, which gets connected to an RCA center pin, for example. In all three attenuator types, the "hot" input wire faces a series resistor Rs and, having passed through Rs, faces a CHOICE of going to ground (1) by entering the power amp and giving it something to amplify, or (2) by going through a resistor to ground Rg. Except that in a series attenuator, Rs and Rg are composite, made up of various resistors in the chain. Rs is that part of the chain up to where the switch-selected output wire comes in, Rg is the part of the chain after that. Clear? I forget what an L pad is, but if it's the same, fine. Anyway, a constant impedance is there for the "hot" signal to see. For a shunt explanation, see "Jerry's Audio Page", at www.21stcenturyservices/audio, I believe. If that's not the address, I'll correct it if you let me know.
This may or may not help with this topic, but I know of at least one manufacturer that builds a quad deck, stepped, single resistor in path, (balanced) volume control that he is preparing to motorize. This may be adaptable to other projects, depending on how he resolves this problem. I know it does not fix anything right now, but it is at least hope on the horizon. I know that for me personally, I would LOVE to have remote capability for my stepped separate volume controls preamp.
Albert, not saying that i could afford these or anything : ( but it would be nice to know when they are available and all of the details. Please keep us posted as the situation develops. Sounds like the builder may be wealthy in a very short period of time. Sean >
Here are two sites for more information on attenuators: (1), click on "Photo Album" and go to bottom, for shunt type, and (2), for all three types. I've run into some commercial products that are probably okay, but hate the idea of 2 decibel steps when it's so easy to have smaller steps, 1 decibel, say. Big steps make for an attenuator that will work in anyone's system, but if you know the range you need for your system, why have four times as much range as you'll ever use, with the penalty of big steps? In the attenuator I've more or less built into my phono preamp, I have only a 4.6 decibel range--and it's plenty! (LP's, at least my LP's, are more alike in loudness, for a given setting, than my CD's are.)
bat sez their attenuators are: "140 step electronic shunt volume attenuator with .5dB steps." they are also remote-control, at an option. aren't these similar to the shallco's?

the best attenuator i heard was melos' photentiometer, which used the brightness of light to adjust a foto-resistor. they had problems w/these, cuz they had to be so precisely set - they'd get misaligned during shipping, & wouldn't work. one *upgrade* of their latest units was the installation of alps volume pots, which was *not* a sonic upgrade...

according to the current owner of melos, he & mark porzilli are working on a more refined version of the photentiometer which will not be so sensitive to the shipping gorillas; it should be ready late winter. i will definitely send my music director in for an upgrade if/when it's awailable...