There's something that's a little interesting here....Are you using single stereo (dvc) speakers here, or just using one of a pair of 622TR's per room? If you're only delivering half of a stereo signal to a room, are you really concerned about the output quality of a speaker selector?

Plus, something tells me that the volume will never be that high in a doctor's office to notice it. Most medical offices have background music strictly for the privacy issue.

I hope I don't come across as a smartass, because that is certainly not my intent. I hope all turns out well with this job.

Thanks again guys, for all of your input.

I wanted to clarify some of the conditions.

First of all, this is not an isolated incident. I have installed hundreds of systems that are similar to this in doctors offices over the years, as I specialize in medical/dental electronics. There are no shorts or controls wired backwards. Those problems are very easy to diagnose. I have been using Dennon and Yamaha for years without a problem using impedence-matching volume controls. It is within the last year that I have been having this problem-- repeatedly. I think that "jcmitch" may have properly diagnosed the problem-- some amplifier designs are just not comfortable with impedence-matching volume controls. Could it be that Dennon and Yamaha, both being manufactured by the same company (Nippon Electric), may have changed something in their design (probably to save a few bucks) that has caused this incompatibility? I'm going to try the small NAD receiver in hopes that it is built a litter better. Unfortunately, it will cost a little more money.
If there was adequate budget, I would love to power the speakers with a Sonance 260 amplifier-- it is comfortable driving eight speakers without impedence compensation. I have even driven 16 speakers with impedence-doubling controls. Sadly, I cannot compete with the 70-Volt guys if I add in the cost of the Sonance amp.

As for using the amp to power 8 mono speakers-- I sum the left and right inputs (not outputs)for the satellite receiver or CD, and I pre-set all FM stations to a mono mode, so that I don't end up with "half-track" in each of the rooms. I use stereo controls for the VCs, but only utilize half of each control for each mono speaker. I know it's a little wasteful, but I can buy stereo controls in volume at a good price, so they really don't cost me much more. I just haven't found a mono control at a decent price that I like yet. I don't use single-point stereo speakers typically, becuase they require twice the amplification. I would only be able to use half as many to achive basically the same result. I know they sound a little better than mono, but doctors are usually cheap bastards, since they are only able to make a few hundred grand a year (they have it kinda tough you know). Also, since I get great deals on mono speakers becuase I buy a in big volume, I could actually put in two mono speakers cheaper than one single-point stereo speaker.

Something else that I am interested in trying is a filter that will roll of the bottom end at around 30Hz. I haven't seen them around for a while, but I think Sonance used to have one available. Does any one know of one that is available for a reasonable price? Or if you guys have more ideas, I'm still open.

Thanks again for all of your input.

If you are using VC's with 1x,2x,4x 8x settings, and you are also using an amp with an 8 ohm impedance and you have (8) 8ohm speakers. but you are using 4x..you have a problem, If you are only using the left or right chanel of the amp. When figuring impedance match settings on stereo VC's count your speakers per amp channel, then set the VC equal to or higher than that number.

In answer to your question about a low frequency filter, Russound offers a device called the LFP-1 which is ideal for this application. Anybody installing autoformers with a wide bandwidth amplifier should probably have one of these on every job.

How are you summing the left and right inputs at the preamp?

Sorry, I did not have time to read any but the first two or three posts...

Ditto with Yamaha and Denon working properly. Note tha the transformers in switchboxes are essentially the same as in the volume controls. Note also that the volume goes down when you impedance match because you cut the amount of power going to each speaker. Theory says the resistive approach should not have good transients, but if it is elevator music, you won't notice it over the screaming in the falling cab.

I had a similar problem lately. I wondered if there were a short somewhere.

I took an eight ohm resistor, tuned a tuner off-station for noise, then turned up the volume till the amp's output was about two volts.

This allowed me to figure out, by ohm's law, what the relative impedance, in a broadband audio way, was. I had a stable resistance, an overall voltage I could verify, and a voltage across the speakers. Math let me figure out the rest.

Turned out I had 6 ohms, so the problem was my amp. Along the way I found that the system's impedance increased as the volume controls were turned down.

Incidentally, I have heard that one particular Denon amp had a problem with this approach (685 was the number I remember). The solution, told me by Niles, was to put two 33,000 mfd capacitors back to back in series on each channel. This resulted in very low frequency impedance rising and rising below about fifteen hertz. For some reason that amp could not see a low DC load.

If you are still using the jamo volume controls I have had problems with them thumping with the bass thus causing my amp to shut down. I would change the volume controls usually I buy one and just stat going one by one till I find the faulty one. I also use the DRA-395 and have never found a problem with the amp it has always been me or the v/c's.

Hope you get it
jefflv

I finally read the other posts, except for the ones below this one, and I think this is your solution. The idea of core saturation (that is Disco DUCK, not Disco DOC) is a good one, and again it suggests the lowest frequencies as the culprits. And yes, current sensing is cutting off the amps.

As for the 30 Hz high-pass, you can figure it out yourself. Capacitive reactance, X sub c, is (forgive my ASCII algebra)

X sub C = __________1______________________
pi x frequency times capacitance

with capacitance in microfarads and frequency in Hertz.

Capacitance, C, is

C = ________________1___________
pi x frequency times ohms

so if you want the filter to have a -3 dB point at 30 Hz, assuming the speaker load is 8 ohms, then

C = ___1________ = 1326 microfarads.
pi x 30 x 8

Take two electrolytics of twice this value, wire them in series with + of one to + of the other, and you have a 30 Hz filter.

This filter will increase in impedance as you go down in frequency. If you want to limit how much its impedance increases, put a resistor of that value in parallel with this whole mess (this combination is a zobel network, and is used in crossovers to stabilize impedance seen by a crossover outside of the frequency range of the driver).

Ernie B-G,

I'm going to try building a filter like you said. I think that you are definately on the right track. Thank you.

Fred Harding:
Thanks also for your suggestion of the Russound Hi-pass filter. I found one down at Volutone for $38.00 (sorry I didn't get it at Capital). I'm going to try it the next time I go up to the job site (75 miles away). One concern I have, however, is that Russound does'nt give you any roll-off frequency. It has two 1000mf capacitors, I think non-polorized. Without getting out my calculator, it will probably cut-off below about 40Hz. I'll let you know how it works. I will also take along some capacitors as Ernie suggested. I think you guys are on the right track, just as JC-Mitch was in an earlier post.

Thanks to everyone else who has tried to help. I will post my results next week after I try it. Seriously, there really is a compatibility problem with these receivers and impedence-matching controls.