Amp that can drive low impedance ribbon?

[bjorn.lindberg]

Hello everyone. I am looking for semantics for an amp that could drive low an low impedance ribbon speaker, say less than one ohm. I don't know if this is practical. The alternative is to build an ribbon with higher resistance using multiple strips of aluminium foil, which also has the benefit of higher efficience. The multiple stirips variant is however somewhat more complicated to produce.

I am curious if someone have tried both approaches and could brief me on the details, maybe one of them is supperior.

Regards, Björn

[amplifierguru]

Hi bjorn,

You have not indicated if this is a mostly full range ribbon or just for highs or something in between.

If for highs I would keep the single strip for better imaging and add 3ohms of quality series resistance (losing a further 12dB efficiency) so you can drive it from any 4 ohm amplifier, a nice easy load/phase. A medium size of amplifier should suffice.

If for wide range, I'd add strips to bring it up to 3 ohms - say 2 more and add a smaller resistor or experiment with none.

Personally I like higher impedances than this and would probably use a large amp with 3ohm resistor in series and live with a 6dB efficiency loss.

Don't expect 120dB. But for 90dB levels...

Cheers,
Greg

[quasi]

A lot of the mosfet designs posted in this forum will work with additional output devices.

Note though that this amp will typically have much lower supply rails than your 8 / 4 ohm version.

If you know the minimum impedance of the speaker you can work out the maximum current and configure your output stage to suit.


Cheers

[bjorn.lindberg]

It's a fullrange ribbon, i.e. from 300 Hz and up.

If I get you right it is possible to build an amp which is more or less like any other amp, just that you use lower supply voltage and for that reason the amp can handle the low impedance load?

Regards, Björn

[amplifierguru]

Hi Bjorn,

That's a fair take - delivering serious amps and low volts. Trouble is, optimised, it's not the same amp! It's not a typical design scenerio for a designer trying to offer value for 8 ohms! But we could go close.

Take myhigh power ampo, which has 4 pairs of high current MOSFETs in the output. Under normal circumstances they're unlikely to be requiredcto deliver more than 4A each. The cct is designed around that with limits here and there.

Remove those and drop the supply and they can deliver more but there are new limitations that emerge because the load is seriously different from the usual/expected.

Easy to adapt to however.

Cheers,
greg

[zinsula]

best topo for 25W class A into 1 ohm resistive load?

[peranders]

The amp doesn't have to be anything special, just choose the right power supply voltage (lower than usual) and have a sufficient amount of output devices and be very carefull where the currents go and where you apply the feedback, physically.

Elektor has published a design rather long time ago. I'm sure someone here can dig it up for you as inspiration.

It might be an idea to let the amp be of the type "voltage in <-> current out"

[lgreen]

I've made a Krell KSA 50 clone (see www link in sig line) with 4 pairs of output devices rather than the standard 2 pairs, and others have calculated that this will do very well down to 1 ohm and, while struggling below 1 ohm able (in theory) to put out over 41A continuous into 0.6 ohms resistive....though I ain't gonna try it and don't know if I would actually achieve this in practice.

As I omitted the output protection circuitry from my build I won't be testing this into less than 2 ohms, but it does seem very powerful and was a rather "less" complictated build for something of this power level.

You may want to post in the krell thread and see if a configuration of the Krell ksa50 amp would be suitable; maybe with certain bias settings you can get what you are looking for.

[bjorn.lindberg]

Ok, thanks everyone, the Krell KSA 50 clone could maybe be something. I guess I need something straight forward since my experience with amps building is quiet limited. One question though, what exactly is an 'output device'?

[jcx]

maybe you should consider chip amps, completely integrated audio power amps, paralleling 2-3 LM4780 dual channel amps running from +/- 10-12 V requires only minor changes to the data sheet schematic

in fact a quick start approach would be to find a hackable ht amp that already has these wired up in a nice chassis and ps (ps V will be too high though)

you'll need low value power resistors to parallel the outputs and possibly a noise gain R-C network across each amp to allow you to reduce the gain below the min stable value for the chip amps