Rivera buffs out there: what is "focus"?

[haslar]

I 've owned a Rivera M60 for over a year, and am still wondering what exactly the "focus" knob does.

The manual mentions "speaker damping"... what exactly is speaker damping?

Thanks !

[pureoldsound]

I thought this was used to make the bottom end tight or loose.

[aleclee]

Most people think that the single most important specification for a speaker is flat frequency response. The hold over from this belief is when you see a guitar speaker manufacturer tout “frequency peaks at all the right places.” Actually, the edge of the sound wave transmits the most information about what we are listening to. Remove the percussive envelope from the sound of a piano, and you’ll probably hear a flute. The idea is to get the speaker to move forward instantaneously, stop dead in its tracks, and then accelerate at full warp speed. This is called speaker “dampening.” It has do with how well the amplifier has control over the movements of the voice coil. You might think of it as a lack of “ringing.” The specification of speaker damping is the ratio of the amplifier’s output impedance to the speaker’s impedance. The lower the output impedance, the better.

[riverastoasters]

Originally posted by haslar
I 've owned a Rivera M60 for over a year, and am still wondering what exactly the "focus" knob does.

The manual mentions "speaker damping"... what exactly is speaker damping?

Thanks !

It's supposed to clean up (i.e. remove distortion) from some frequencies. It does this by compensating for resonances.

It will depend on speaker cab you are using whether you hear much difference. Also I think it doesn't do much at low volumes so you hear more when you crank it. Also if you tune down you will probably get more difference.

I haven't found it to be essential - my favorite Rivera doesn't have it.

[Gauge613]

from my reading of i think it was Rick Johnson? - he's a heavy hitter on the Rivera forum, the focus adjusts the tightness of the bass (and 10 is the most focused and this is the setting for rivera amps without the focus control)
from my playing a quiana yesterday, that seems right
but i dont' actually own a rivera so this is as much help as i can be

[Scott Peterson]

What they said.

It controls the speaker "looseness" or "tightness" and I found it to impact the lower end of the sonic frequency picture most.

I used it on my Rake to dial in the amp for the given room. It was a very important feature on that amp for me. I owned a at least one Rake (I had three over the years) from 1996 to 2004.

[csanford]

Originally posted by haslar
I 've owned a Rivera M60 for over a year, and am still wondering what exactly the "focus" knob does.

The manual mentions "speaker damping"... what exactly is speaker damping?

Thanks !

The focus control reduces low-frequency feedback in the output stage. This is also sometimes called a 'resonance' control. Reduced LF NFB gives the amp a looser feel, because the amp has less control of the movement of the speaker cone (damping).

Chris

[riverastoasters]

Originally posted by splatt
that may be one further *effect* of speaker damping, but it's not its primary raison d'etre.

It's pretty much what it can do. If speaker damping has a different "raison d'etre" then the designer is hoping for things that do not happen.

If for example, you want to believe that the designer is actually controlling the speaker to move more synchronously, etc., which might be inferred from what people originally decided speaker damping was supposed to do, then guess what. It's not happening unless you know the speaker is pretty unreal - such as having frequency dependent impedance and no voice coil resistance.

What simple speaker damping controls can do in practice is control one resonance or anti-resonance in the coupling between the amp and cab, and to an extent limited by the voice coil resistance. If you attenuate a little in the neighborhood of DC, then you will, at high volumes, get rid of some of the low end distortion that can make the sound flubby. Lots of speaker cabs have impedance features in the bass that would seem to be the primary target of this control.

But once the voice coil resistance is big compared to the internal impedance of the amp, then not much on the amp side of this picture matters. In those cases, then you aren't going to hear much difference with the Focus control.

[riverastoasters]

Originally posted by riverastoasters
It's not happening unless you know the speaker is pretty unreal - such as having frequency dependent impedance and no voice coil resistance.

That should read "frequency INdependent impedance". The whole problem is that actual speakers have frequency dependent impedance.

[aleclee]

Originally posted by Scott Peterson
I owned a at least one Rake (I had three over the years) from 1996 to 2004.

2005, actually.

[gregc]

Supposed to simulate a closed back (loo se)or open back (tite)sound....
it does this by varying the 'damping factor'.
gregc