An audio amplifier designed for zero ohm loads? - Page 1
Forum section: electronics / projects

author: cvanc
author_rank: Frequent Contributor
timestamp: September 25, 2019, 12:33:58 pm
content: Hi all-Do we know of any prior art on amps made to drive zero ohm loads? I'm toying with a ribbon design that is a very low impedance and wonder if it is possible to find an amp that will drive it directly.Is there perhaps something from another industry that might be re-purposed to do this?Thanks...

author: dzseki
author_rank: Frequent Contributor
timestamp: September 25, 2019, 12:38:39 pm
content: I think it all boils down to continuous output current capability of the amplifier. With "zero Ohm" load an infinite current would flow, that ain't gona happen, so perhaps you should refine that "zero" into some more realistic number defined in mili or micro Ohms.

author: Gyro
author_rank: Super Contributor
timestamp: September 25, 2019, 12:41:54 pm
content: I remember an elektor magazine project for a direct drive ribbon speaker amplifier in years gone past. It will take a bit of searching, but you should be able to find it in this online archive, sorry I can't tie it down to a date...https://www.americanradiohistory.com/Elektor.htm

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 25, 2019, 12:48:14 pm
content: Just add a series resistor for testing. A more efficient design can be built based on the kind of MOSFET power stages used for low voltage, high current voltage regulators.

author: OwO
author_rank: Super Contributor
timestamp: September 25, 2019, 01:07:21 pm
content: If you have to drive a low impedance load and need low distortion, current mode feedback might work better; voltage feedback will lead to low loop gain and higher distortion.

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 25, 2019, 01:57:20 pm
content: I mentioned only a couple days ago an old Elektor project dealing with exactly that: an amp to drive ribbon speakers. It was called the "current amp", and was basically a power current source.It was in one or two Elektor issues from 1992.(Obviously your title is almost clikbaity, ribbon speakers have very low impedance, but of course not zero.)

author: cvanc
author_rank: Frequent Contributor
timestamp: September 25, 2019, 07:16:39 pm
content: LOL yeah I probably should have said "zero-ish". But for sure a very low value (well under one ohm).Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: September 26, 2019, 02:22:07 pm
content: It sounds horribly inefficient. Use a transformer. I know you probably wanted to avoid it, but it's the only sane way. A current transformer can be run in reverse, so the amplifier only needs to output a tiny fraction of the load current.Constant current is the only sensible way to drive very low load resistances. A Howland current pump will do the job. Here's a design for 3A. Connect it to a 10:1 current transformer and you have an output current of 30A. It'll probably need some extra frequency compensation, to prevent oscillation when driving the transformer.https://www.eevblog.com/forum/projects/30a-precision-current-source/msg2324862/#msg2324862

author: ejeffrey
author_rank: Super Contributor
timestamp: September 26, 2019, 04:00:53 pm
content: With a 10:1 transformer you don't need anything fancy, a normal audio power amplifier will do fine. The impedance ratio is 100:1 so a .25 ohm driver will look like 25 ohms. 6:1 would be better and works give you about 8 ohms.A transformer is the easy way to go but direct drive is certainly possible if you are custom designing the amplifier. Some subwoofer amplifiers are designed to be stable down to 1 ohm loads. they are intended to drive parallel 4 ohm voice coils in bridge tied configuration. Basically you need parallel output drivers, current feedback, low supply voltage and an auxiliary supply for the gate drive.

author: Emo
author_rank: Regular Contributor
timestamp: September 26, 2019, 05:04:13 pm
content: Hi,Have a look at the induction loop amplifiers for hearing aids. Impedance 0,5 ohm and power up to several hundreds of watts.On eBay several can be foundRegards,Eric

author: fcb
author_rank: Super Contributor
timestamp: September 26, 2019, 06:46:32 pm
content: You are looking for a 'transconductance amplifier'.It converts voltage into current (a transconductance amplifier with 1S (Siemens, the unit of transconductance) will output 1A for an input of 1V).If you want to repurpose something from industry, probably look for a Fluke 5220A - 2nd hand these are quite expensive though (probably not high enough compliance voltage for ribbon speaker at 'tweeter' frequencies).