Class D amplifier as a current source

Thread Starter

pleriche

Joined Oct 29, 2017
80
It's easy to configure a buck regulator as a current source, but is it possible to do the same with a commodity low to medium power Class D audio amplifier, and if so how?

My application is a neck loop for induction coupling to my hearing aids on the telecoil setting. The loop will have a resistance of a small fraction of an ohm and I need to drive 300-500mA RMS through it, powered as efficiently as possible from a battery.
 

Ian0

Joined Aug 7, 2020
9,495
Maybe you could put a resistor in the ground connection of the loop and connect that to the feedback pin.
I can't guarantee that the loop will be stable.
 

crutschow

Joined Mar 14, 2008
34,037
I think so.
Just put a small resistor in series with the loop, perhaps an ohm or so.
You will need to keep the signal level low to the amp input to avoid overdriving it.
Then the output only needs to be about 0.5Vrms (1/4 Watt) to generate 500mArms through the loop.

For 500mArms, the amp only needs to be rated for 2W into an 8 ohm speaker.
A 5W amp should be more than sufficient.
 

MisterBill2

Joined Jan 23, 2018
17,788
The neck loop needs to have quite a few turns of wire, not just a single turn. That is because usually the power available to drive such a loop is not large.
 

Thread Starter

pleriche

Joined Oct 29, 2017
80
Maybe you could put a resistor in the ground connection of the loop and connect that to the feedback pin.
I can't guarantee that the loop will be stable.
Roughly what I had in mind. But I haven't found a open loop Class D amplifier.
 
Last edited:

Thread Starter

pleriche

Joined Oct 29, 2017
80
I think so.
Just put a small resistor in series with the loop, perhaps an ohm or so.
You will need to keep the signal level low to the amp input to avoid overdriving it.
Then the output only needs to be about 0.5Vrms (1/4 Watt) to generate 500mArms through the loop.

For 500mArms, the amp only needs to be rated for 2W into an 8 ohm speaker.
A 5W amp should be more than sufficient.
That would probably work, but it's still a voltage drive, with the load impedance well below spec and the resistor will waste battery power.
 
Last edited:

Thread Starter

pleriche

Joined Oct 29, 2017
80
The neck loop needs to have quite a few turns of wire, not just a single turn. That is because usually the power available to drive such a loop is not large.
Which is why I want a Class D current source. I tried a piece of 26 way telephone cable with all the cores in series but the results were very indifferent and it was bulky and not very flexible.
 

crutschow

Joined Mar 14, 2008
34,037
it's still a voltage drive, with the load impedance well below spec and the resistor will waste battery power.
True.
But the spec is for delivering maximum power to the load, not that the amp can't work with a lower impedance.
the resistor will waste battery power.
Then use a lower resistor value (any current source still needs a resistor in series with the load to regulate the current).
Just attenuate the input to avoid overdriving the amp.
The amp should have built-in protection so it won't blow with a low impedance load.
 

Thread Starter

pleriche

Joined Oct 29, 2017
80
True.
But the spec is for delivering maximum power to the load, not that the amp can't work with a lower impedance.
Then use a lower resistor value (any current source still needs a resistor in series with the load to regulate the current).
Just attenuate the input to avoid overdriving the amp.
The amp should have built-in protection so it won't blow with a low impedance load.
Certainly no decent amp should blow, but if it shuts down instead the only practical difference is you don't get the smoke and the bad smell. And you can savour the frustration multiple times. But also consider: if it's only spec'd for a 4Ω load there's no guarantee it'll be capable of supplying the required current at a very low output voltage.
 

crutschow

Joined Mar 14, 2008
34,037
if it's only spec'd for a 4Ω load there's no guarantee it'll be capable of supplying the required current at a very low output voltage.
Since you don't need a lot of current, I doubt that would be a problem.
But if you don't think it will work, then I'm out of options. :rolleyes:
 

crutschow

Joined Mar 14, 2008
34,037
but if it shuts down instead the only practical difference is you don't get the smoke and the bad smell.
It likely shuts down from excess output current, not too low an output voltage.
And you load will not draw excess current if you properly control the input signal amplitude.
 

MisterBill2

Joined Jan 23, 2018
17,788
Multi-pair Phone cable is not even in the same state as the actual neck loop cables, neither gage nor flexibility-wise.
And why the fixation about amplifier output impedance? Matching is to maximize power transfer, but transferring power into a resistor is a waste. Always!, in this sort of application.
 
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