Query About Terror Bass 1000 Impedance

[CPL]

I know the TB 1000 has a switchable impedance from either 4 ohms to 8 ohms, depending on your cabinet set up. I was wondering what would happen if you ran an 8 ohm cab while still switched for 4 ohms? Typically, on amps that don't have this feature, it would just run on a lower power output (i.e. like an amp 500 watts @ 4 ohms/ 350 watts @ 8 ohms, that type of thing) and that would be that.

Would the TB 1000 act the same way, or is it a different situation because of the impedance switch? Would it harm the amp if I tried that, even though the cab is a higher impedance? I wouldn't think it would, because it wouldn't with other amps as long as you don't go lower in impedance with it than you should, but I want to make sure.

This is really out of curiosity, too. Not necessarily planning on this, but if it does act in the same fashion and lowers the power output, that wouldn't actually be a bad thing for my current situation. I practice in a tiny room and I have to keep the volume barely on 1 with the gain almost to 0. I am running it through a 4 ohm 18" speaker cab, but I've order an 8 ohm 2x10" cab for it to use in the room, since the 18" is a bit boomy and lacking in some high end definition, especially at the low volume it's on. My initial plan afterwards is replace the 18" speaker with an 8 ohm version and I'll run both cabs when ever I play live and able to give it some more volume. That should be a nice set up.

Thanks for any info about this.

Cheers,
Paul

[Gladmarr]

I'm guessing here, but it seems like the impedance switch is there because the speaker impedance is part of the output filter for the Class D amp. I spoke to a guy who designed Class D amps for Fender and he said this is a poor design, as it should work like other solid state amps (basically the way you described - lower speaker impedance means more power output from the amp). I think switching to the wrong impedance for your speaker might make the output unstable or change the tone in an undesirable way.

[a.hun]

I'm also guessing, my knowledge of class D amps is fairly limited, but from what I do know I think that Gladmarr is probably right.

1. Class D amps use pulse-width modulation, where the width of fixed (very high) frequency pulses is varied depending on the level of the input signal. An output filter is then used to remove the high frequencies, leaving the modulating waveform to drive the load.
2. The filters must be carefully tailored to give a fairly flat final frequency response. The filtering (and frequency response) will be affected by the output impedance.
3. The power output is limited by the voltage of the power supply rails. Two things here:
If designing for a given output at say 8 ohms (eg 500w), using a lower impedance load would produce more power so running at 4 ohms would fry things. The output would then need to be limited somehow which I think is exactly what the impedance switch in the TB is doing.
Using higher voltage output transistors limits the bandwidth (frequency response), so there are limits on how high you can go there to handle increased voltage (and power) levels.
(How I know all this: http://www.thegearpage.net/board/index. ... st-1984594" onclick="window.open(this.href);return false;)

Things have moved pretty fast in class D technology, but one of the old problems was indeed that they were more impedance sensitive, which meant that different impedances could shift the tonality as well as power output pretty dramatically.

Of my own class D amps, (I have 3, none from Orange), two can happily run into different impedances (either 4 or 8 ohms) without switching but with different power outputs. The third (Crate Powerblock) is stereo and does specify either 4 ohms per side for stereo operation (2 x 75W) or 8 ohms mono in bridged mode (150W). It is also the oldest design. Perhaps the Fender guy is right and the TB output section is a bit old fashioned like that. Having said that it was certainly a powerful and effective design. See this in depth review (p.70 onwards HERE) of the TB500, especially the tech review after the main one which mentions the extra power potential at the 4 ohms setting.

Anyway if I were you I'd just follow Oranges instructions and always set the correct impedance for the load you are actually running.


Andy.

[CPL]

Yeah, I'm not going to push my luck and just use the 8 ohm switch when running an 8 ohm cab. I was just curious to see what the effect might be, but I certainly don't want to find something out the hard way, either. Thanks for the info about the Class D stuff. Even though I do own a couple of Class D amps, I really didn't know much about their workings in comparison to the other designs.