Following yesterday's post about output impedance I was reminded of a day in Sea Cliff New York where HP (Harry Pearson) the founder of TAS lives.
This was many years ago and Arnie Nudell and I were running Genesis Technologies, a high-end loudspeaker company based here in Colorado. We had arrived at HP's and setup a new pair of loudspeakers we wanted him to review.
All Genesis high-end loudspeakers had built in subwoofers, a trait started by Arnie while at Infinity. Arnie believed (as do I) that loudspeakers should be full range to the best of their abilities - adding extra boxes to make them reproduce music properly was a wrong headed idea that placed additional burdens on customers they didn't need.
The built in subs had two means of sending the audio signal to them: an internal feed from the main speakers or an external feed from the preamplifier. The external feed from the preamplifier was the preferred method since the first method took the output of your power amplifier and necked it down to get the gains correct: the preamp feed therefore being the cleaner of the two.
When we go to visit a reviewer we rely on their existing setup: in the case of a speaker manufacturer the reviewer's electronic chain is used and, in the case of an electronics manufacturer, the reviewer's loudspeakers are used. We, being a loudspeaker manufacturer, relied on HP's system of the day which, if I remember correctly, was some sort of tube preamplifier and power amplifier: perhaps Conrad Johnson. The preamp and sources were located well away from the loudspeakers, the power amp near them.
Arnie's routine for speaker setup always begins with the subs off. This is a good idea because if you methodically setup the midrange and tweeters sections for best tonal balance and imaging without the subs on, then you can easily focus on just the mids and tweets contribution without also wrangling the woofers around.
He spent the better part of the morning tuning and tweaking and when he was happy, asked me to connect the subs and then we would go to lunch - dialing in the rest of the system upon our return.
Following our return from a delightful meal at one of the many great restaurants the town has to offer, we returned and started listening again, this time with the subs active. It sounded awful, very much unlike when we left for lunch. Arnie glared at HP's setup people and asked if they had "messed" with anything. "Nope".
Retracing our steps I turned off the subwoofers via their power switch - no dice, still sounded like crap. Then one of us got the bright idea to start unraveling our steps, one at a time. So the first thing I did was disconnect the long XLR cables from the preamp that was feeding our subs to complete the reverse process. Bingo! The system jumped back to life.
Puzzled, I repeated the steps and added back the XLR cables from the preamp to the subwoofer input and the same collapse of the musical life took place. The preamp, a tube, simply did not have the current capabilities to drive two long cables - one feeding the main power amp, the other feeding our subwoofer. Our subwoofer input was something I designed and I knew it well - it's input impedance a mere 100k Ohm so it wasn't dragging down the preamp - it was the cable itself.
Without saying anything to HP we simply switched over to the internal subwoofer connection, finished setting up and at the end of the day went to dinner and then home. We got an excellent review.
The point of this story is to show just how critical output impedance and drive capability is on a piece of equipment. Tube preamps are by far the worst for driving long runs of cables if they are not designed with that in mind - but all products can have the same problem.
Bottom line: test your assumptions. Don't just assume the designer has done his or her work in a way that suits your particular needs. Try it with and without, in and out, and make your own determination.
