by DougH » 12 Aug 2009, 14:25
Looks like modman merged the threads, thanks! I didn't see the original thread or I would have just used it.
Here's a few comments re. earlier comments in the original thread-
The pdf schem someone posted (attenuator1_a1) is wrong, according to what they reversed on the amp garage. The input is connected to one of the switch poles. The throws for that pole are connected to the hot side of the 25 ohm resistor. But beware, even the schematic posted at amp garage is wrong for a similar reason. It's buried in one of the threads. I used the layout instead, it was created first, from the actual unit.
Yes, since this works for 4/8/16 ohm impedances, the load it presents to the amp will be a compromise to some extent. However, resistive and reactive impedance are two separate things. Reactive loads in speakers will have many multiples of its "rated" impedance at certain frequencies. Look at a speaker impedance curve for example. The average impedance of a reactive load over the useful frequency range is higher than the "rated" load. So you can't make a direct comparison between the two but you can say that is probably safer to run an amp with a higher resistive load than it is to run with a higher reactive load.
But in the end, this one sounds pretty good. There is a subtle high freq rolloff (which I also hear in my MASS-based attenuator) but otherwise it is very transparent. Even the "bedroom level" sounds good to me, unless you turn the rheostat down very low. There will be sound degradation at that point, and Fletcher-Munson comes into play anyway. But I use the higher rheostat settings of the "bedroom level" for my 100 watter and it really sounds good. I have used it both with the 16 ohm 4x12 with the 100 watter and with the 8 ohm 1x12 with my 10 watter just dialed down a click or two and it sounds pretty "transparent" for both of them.
I've been reading up on attenuators a lot lately, and I believe part of the reason that some people have problems with them has more to do with the speakers than the attenuator. If you are using low wattage speakers a la greenbacks or blues, part of the sound you get when the amp is dimed is from the speaker changing in some way dynamically and coloring the sound that it doesn't do when it's not pushed. Using an attenuator in this instance will not replicate what the speaker does, so it will sound "different" even though the amp is turned up. Higher-wattage speakers that produce a consistent sound no matter what volume the amp is will work better, since the way they color the sound is unrelated to the volume of the amp. My 4x12 has 50W speakers and it works pretty well with this attenuator.
Attenuators are not perfect, but they can be a real useful tool.
"You have just tubescreamered or fuzzfaced yourself " -polarbearfx