Field hardening for speaker impedance

[raintalk]

(Somehow this seemed like a good idea in my dream last night, and I've given up coffee so I'm not sure if I'm awake yet )

A common issue for field use of a amplifier, like for pro-sound and musical instruments. is that any combination of speakers may be attached to fit a venue. There's the proper way to do this, and then there's the quick and dirty. Quick and dirty is going to be used 80% of the time.

So the idea is to add a sensing coil to the output filter, and set up a little widget to do an impedance sweep that based on the results sets the power supply voltage to better match the load. Is there just not going to be a reading at the audio frequencies that you'd run a sweep at?

Obviously there's going to be some practical limit on maximum and minimum impedance, but I imagine a red light could go on.

On the other hand, people wanting max wattage to an 8ohm load would get it. If they plugged in 2 ohms, they'd still max watts.

[pinkmouse]

Sorry to torpedo your idea, but the only people that do that sort of thing are, to be blunt, amateurs, and they just want the cheapest possible kit. Whilst your idea is good in principle, pro's don't need it, as most systems are now configured in a modular manner, and so wouldn't pay for it, and the lower end of the market couldn't afford it...

Sorry :-)

[Nigel Goodwin]

I agree, silly idea! - it's simple enough to work it out in your head and any sound engineer should be able to do so with out even really thinking about it.

As a sound engineer for my daughters band I'd be embarrassed (if not annoyed?) at such a suggestion

[raintalk]

Let me give another example
Take a look at the bottom of page:

http://www.lcaudio.com/index.php?page=70

Note there's separate voltages and power output listed.

An amp is only going to get wired for a single voltage.

But say you had variable voltage, and plugged in 8 ohms - you could raise the voltage to +-80v and get around 350W.
Plug in 4 ohms - lower voltage to +-65v and get 400W
So you keep the same wattage with different loads.

[Nigel Goodwin]

It's just more complications, more to go wrong, and for no good reason - I don't recall ever seeing this done?, and I don't see any need or use for it?.

[raintalk]

Well Crown has amps like this
http://www.crownaudio.com/amp_htm/am...s_vi_plane.htm

But I'm thinking something much cheaper that can be adapted to class-d at a hobby level.

And for DIY - sometimes you do stuff to be cooler than you can buy.

[Nigel Goodwin]

Originally posted by raintalk
Well Crown has amps like this
http://www.crownaudio.com/amp_htm/am...s_vi_plane.htm

Interesting read, but what has it got to do with the thread?.

But I'm thinking something much cheaper that can be adapted to class-d at a hobby level.

And for DIY - sometimes you do stuff to be cooler than you can buy.

Sorry, but I just don't see any application or use for it?, perhaps as a gimmick?.

[phase_accurate]

One reason might be to adapt the output filter to the load - my preferred method for achieving flat FR is still post-filter feedback though.

AFAIK not only Crown but also LAB Gruppen built amps that could be "matched" to different load impedances.

Regards

Charles

[sx881663]

Having an amplifier optimized for one load means a more cost effective and possibly a better sounding one too boot. An example is a purist design A/B amp that will do 150 watt into 8 ohms with a single set of output transistors. This amp would blow up if you connect 4 ohms and try to do 300 watts. So how do you handle this? If you are real cheap you don’t, you just warn, 8 ohm only and let the customer suffer the consequences if he doesn’t head the warning. A more elegant solution is like McIntosh did for years and put on a load matching autoformer and get the same power at various loads.
All this is a moot point for a UcD type design as it can handle the various loads with no problem.
Roger

[sreten]

Hi,

not many people know this.........

The cheapo NAD amps had a 4/8 ohm switch.

It changed the taps on the transformer for lower voltage and more
current into 4 ohm loads. Problem was 8 ohm gave more swing
without clipping, and given the dynamic nature of music (not sine
waves), worked perfectly well in to 4 ohm also.

The issue for a "field" amplifier is now essentially irrelevant with
switch mode supplies and modern class g / class d amp designs.
They moreorless automatically give maximum output into loads.

/sreten.