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QED047 25th May 2009 05:45 PM

Switching current source for Class A?
Hi all. My first post here is prompted by a thought about a way to reduce the dissipation associated with Class A amplification. I have built several small ZEN-like amplifiers (another debt of gratitude owed to Nelson Pass) but I always seem to fall short of a ending up with a "daily driver" for my listening habits. What I mean by this is that in one way or another the excess power dissipation becomes a problem - I think most people know what I mean by this so I won't elaborate.

Anyhow, after searching here and there I can find nothing that directly addresses the concept of using a switch-mode constant current source in place of the linear variations (resistor/transistor/inductor/light-bulb!). On offer seems to be a way to almost halve the usual dissipation - a very significant reduction indeed!

This can't really be called Class D, although granted there may be similar issues. It's not to easy to search for either (so I apologise if I have missed it - searching for A/D isn't much help!).

Anyway, as an experiment I substituted a 2Amp switching current source into the original ZEN constant current source design and sure enough the results seem promising. The switching noise (around 500KHz in my case) would seem to be fairly simple to remove using an inductor between the current source and the lower transistor (a much smaller and more practical inductor to the sort used as the sole current source) but no doubt this is where people will be focussing their criticism of the idea.

I'm interested to hear the criticism as I can't measure (or hear!) any downside to this strategy - although my measuring (and hearing) equipment will probably not be up to the standards around here. I'm almost certainly missing something - but what is it most likely to be?

QED047 15th July 2009 06:24 PM

Still thinking about this.... Here's a photo of a breadboarded "proof of concept". The switched current source is a separate module (bottom left). Its built around a mosfet & driver with a 0.1 Ohm current sense resistor feeding a diff amp & comparator. The comparator trips as the voltage on the 0.1 Ohm ramps up and down by 10mv or so. Choosing the inductor value vs. current required I can keep it switching in the 500K~1Mhz range.

Above the current source is a separate module with a mosfet follower biased to mid rail. That's fed from a "Bride of Zen" preamp above. A 10uH inductor in series with the output sees to removing most of the HF ripple from the loudspeaker signal. Whatever remains that the loudspeaker sees - I can't hear. It sounds no different to a linear equivalent current source - but dissipates only a watt or so in the mosfet/diode/inductor.

stinius 15th July 2009 06:38 PM


Originally posted by QED047
Still Here's a photo of a breadboarded "proof of concept".
Hi QED047

I think you forgot to post the photo.


darkfenriz 15th July 2009 08:15 PM

If you want a DC current flow over given DC voltage drop you must dissipate power equal to current times voltage.

sparcnut 15th July 2009 08:26 PM


Originally posted by darkfenriz
If you want a DC current flow over given DC voltage drop you must dissipate power equal to current times voltage.

Yes, but that is not what is being done here.

darkfenriz 15th July 2009 08:30 PM


darkfenriz 15th July 2009 09:27 PM

What I wanted to say is you cannot just swap-in any switching circuit in place of a negative voltage-referenced constant current source. Just to remind bascis.
You must aim to have zero avarage voltage drop on it, so you need to use both supplies for that. This makes the constant current source design as complicated as a class D amplifier with output impedance forced to be high by clever feedback. The schematic/photo/idea picture/whatever is still missing...

QED047 15th July 2009 09:44 PM

Oops - thanks stinius!

2 Amps is all my test PSU can deliver - 4A should be a breeze with the same mosfet & diode. The efficiency is somewhere around 90% so nearly all the dissipation takes place in the follower - and this should average out to (supply/2)*I bias.

I sort of trust myself building a switching current source alot more than going the whole hog with class D (getting linear PWM ramps etc.) The goal here is *constant* current and the self-oscillating section is really simple (both to make and keep an eye on!).

QED047 15th July 2009 09:44 PM


Originally posted by darkfenriz
If you want a DC current flow over given DC voltage drop you must dissipate power equal to current times voltage.

Hello darkfenriz! Sure, but only if you're using non-reactive components (e.g. resistors, transistors) but the circuit I'm referring to uses an inductor and transistor to connect the inductor between the source of the follower stage and -v (where a constant current circuit goes in a conventional single-ended Class A topology).

The inductor takes time for the current to build when it is "in circuit" (storing power as a magnetic field). The transistor switch keeps on connecting and disconnecting the inductor to maintain a controlled current about the set-point. The inductor returns "unused power" via diode at the open switch to the +v rail. Using this arrangement there is (theoretically) no power loss in either the inductor or switching transistor, yet there is a DC current flowing across a DC voltage!

This is the basic principle behind switch mode power supplies (and Class D amplifier topologies).

QED047 15th July 2009 09:44 PM

I've just sketched it out roughly. I'm so lazy, I should have started with a diagram!

(EDIT: Due to being under moderation as a new member these latest posts of mine were delayed in appearing. They should still address the points raised subsequently though - thanks everyone for the feedback so far!)

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