[Edwood]

Excellent.

I'm going to order some parts when I get back into town.

This will be fun either way.

-Ed

 

[gaboo]

Quote:

Originally Posted by gsferrari Shouldnt be too hard to measure outputs with a DMM and match the amps...although you can also do this by ear.

By ear has no statistical relevance...

Also, when you match the voltage w/o load, there's no guarantee that it will be matched under load.

 

[gsferrari]

Quote:

Originally Posted by gaboo By ear has no statistical relevance... Also, when you match the voltage w/o load, there's no guarantee that it will be matched under load.

hmmm...that makes sense.


Of course - the ear is probably the easiest way to go and many people fo DBT by ear...I think this is quickly getting too scientific

Let me get done with this basic switchbox first and see how it performs

 

[gsferrari]

http://headwize.com/projects/showfil...=cmoy7_prj.htm

 

[gaboo]

Quote:

Originally Posted by gsferrari http://headwize.com/projects/showfil...=cmoy7_prj.htm

"ground bus". Nuff said.

 

[Sovkiller]

What you want that pot for? I don't think it will be a good idea to introduce more parts in the signal path, even I would eliminate the jacks if possible, to be on the safe side.

IMO by simply matching the volumes by ear should be good enough, you are not comparing volume, but other attributes of the amp.....




[size=xx-small](OTOH that will make the "snobophiles"...herrr, audiophiles, to begin to worry while using the box)[/size]

 

[amb]

Joining this thread a bit late...

You do not need to shield the wires inside the switchbox, nor do you need shielding of the box itself. In fact, a plastic box is just fine, and in fact preferable because it removes the need to isolate the jacks. We're talking about the output of amps being switched here, and the output impedance is low enough (and the wiring short enough) that there is simply no issue of picking up any induced noise or crosstalk.

Use a good reliable 3PDT toggle switch and reasonable connectors and you'd be fine. There is absolutely no reason to go overboard.

 

[amb]

On the issue of level-matching, that should be done with a volume control at the input side of the amp. Putting pots in the switchbox defeats the purpose, because the pot will increase the effective output impedance of the amp and affect the sound.

If the amps being compared each have volume controls, then the proper way to level match is to play music on one of them, adjust the volume on that amp to establish the desired listening level. Then, use either a test CD with a sine wave test tone or a signal generator, feed it into that amp (leaving the volume control untouched). Measure the AC RMS output voltage of the amp with a DMM (check each channel for variances too). Feed the same signal into the other amp being compared, and adjust its volume control pot so that it outputs the same voltage. Voila.

This seems like a lot of work but precise level matching is absolutely essential for a fair A/B comparison. Human psychoacoustics is such that the louder amp would usually seem "better", "more dynamic", etc.

 

[morsel]

I built my A-B switch box using the following components:

Serpac A-21 case
Switchcraft Series 11 1/4" phone jacks
generic surplus 3pdt switch

It works just fine. Two output jacks allow two people to listen at the same time.

 

[gaboo]

morsel strikes with a secret project.

 

[gaboo]

The HDCD AES paper is a great reading. It also touches the issue of building an electrically controlled switch box: "The switch box itself is composed of sealed rhodium relays in both the active signal leads and the grounds, with careful isolation of the control signals. The relay operation sequencing is timed to minimize switching transients."