Newbie note: In all of the following, in case you're not really conversant with the concepts, know that a higher impedance load on a power amp means the amp puts out less current and less power. Higher impedance intuitively sounds like it means more of a load, but it means less of a load because less power can come out of it (sort of -- and close enough for this discussion).
On September 26, 2007 at 08:52, Fred Harding said...
Ernie, I'm going to respectfully disagree re impedance
matching vc's vs. a selector box.
I'll try and be brief. The objective is to protect the
amplifier.
Exactly.
With impedance matching vc's, you set those to the appropriate
setting. It increases the amount of impedance per vc,
whether the speakers are on or off.
With the goal being that the overall impedance of all speakers together is not less than 8 ohms. That is, the amp is never overloaded when ALL volume controls are turned up all the way. This matching also ensures that each speaker gets its proportional share of the power.
With the selector box, you set it to increase impedance,
yes, but when a room is turned off at the volume control,
no additional impedance is on the bus.
Are you saying that when one of the selector box switches is turned off, that the impedance seen by the amp increases? That could be, but if it's one of the boxes where speaker volume goes up when fewer speakers are used, then the box is changing the loading such that the amp always sees approximately the same impedance. If all speakers stay at the same volume whether one or six switches are on, THEN the impedance seen by the amp is higher with fewer switches on.
Now about volume controls: if the volume control does not go open when all the way down, then, yes, "no additional impedance" is seen by the amp when the volume control is all the way down.
But what is the effect of this? When eight VCs feeding 8 ohm speakers are set to the X8 position, then the impedance when all are on is 8 ohms, the normal impedance you'd want the amp to see. It's not likely that raising this impedance, say to 16 ohms(as for instance if there were an 8-position switch box and half were off), would improve the sound of the amp -- first, it would simply be delivering less power, and second, if this were the case, people would be designing 16 ohm speakers to get better sound out of amplifiers! Or would be selling impedance matching devices to make 8 ohm speakers appear to be 16 ohm speakers for better sound. Nobody's doing that.
Essentially, if a person uses impedance matching vc's,
they are loading the amplifier more with speakers off
then the same situation using a selector box.
That's totally true. But they are never overloading it under any circumstance, and overloads are where performance suffers. With all the VCs presenting their load, the amp is seeing the load it was designed for, so that shouldn't give "worse" performance. The other side of this is that loading an amp with 12 ohms or 16 ohms won't give better performance than loading it with 8 ohms.
To my mind,
that means that I'll get better sound quality with a selector
box. The great Eric Johnson, now with Universal, explained
this to me years ago. If it makes sense, credit to him;
if it does not, it is my interpretation.....
This whole line of thought relies on the assumption that an amp will sound better with a load higher in impedance than 8 ohms. I think that's a false assumption. All of the things stated about how the impedances will vary are true. I just don't think they matter to the performance of the amp.
