combining class A and class D

[rcownie]

I had a probably-crazy idea which kept me awake last night.

The idea is to combine the efficiency of class D with the purity of class A
and get almost the best of both.

Roughly, you build two class D output stages which generate roughly
Vout-2V and Vout+2V. Then you use these as the negative and positive
supply for a floating class A output stage.

Now, the class D stages can be pretty rough because they only have
to be close enough for the class A stage to remain properly biased.
And the class A stage can be simple, and can use a transistor with a
low Vds spec. and really fast response.
And most importantly, while the class A stage is passing
all the current, it only has a small voltage drop (e.g. 4V) and thus only
dissipates relatively little power. For example, to get 200Wrms into 8ohm
we need 5Arms, or a peak 7.1A., and a supply voltage of about +/-58V,
so we would have cut the class-A dissipation by about a factor of
(2*58)/4 = 29x.

I'm hoping the very different requirements for the class D stages would
allow the use of much smaller inductors (and perhaps higher frequencies -
obviously the class D stage has to respond quickly enough to maintain
slew rate) thus reducing the EMI problems. But I haven't worked through
that yet.

Apologies if this is an idea that's already out there - I did a search
but didn't find anything quite the same.

[lumanauw]

Hi, rcownie

It's really a nice idea
The idea of classD amp being made PS is already there as a product, LabGruppen amps. (ClassTD)
I imagine your idea equals biasing LabGruppen amps to classA (originally classB).
Problems will rise when the input signal is quite high frequency. LabGruppen amps seems to limit their response to 15khz or so, while their TD supply is clocked at 600khz. There's something about switching frequency and highest reproducible frequency.

[djQUAN]

it's a class H amp. (or is that class G? still confused as to which is which. )

edit: class G and H

[rcownie]

"It's really a nice idea"

Thanks!

I'm going to play around with some simulations and see if it there's some
gotcha ...

I'm probably most nervous about the need to be really careful that the
two class-D outputs stay in the right range (and roughly the right
relationship).

It seems as though the easiest way to guarantee that would be to make
it self-oscillating, with comparators to check the various conditions
and then some (fairly simple) logic to get each supply going in the
right direction to make things better. But as that involves some delay
(in the comparators, the logic, the transistor switching, and then the LC
response) and a feedback loop there's a danger of instability ...

And your comment about dealing with high frequencies is certainly on
target: if the signal has a high slew rate then the supply amps have
to be able to respond quickly enough before the class-A stage hits
the edge of the operating window. The faster they can respond, the
narrower the voltage window can be (and thus less dissipation in the
class-A and higher efficiency).

I guess I'm starting to understand why class-D has a few pitfalls along the
way.

One especially nice feature - if I can make the whole thing work - is that
in principle the class-A stage, if it's fast enough, should keep all the
high-frequency class-D artefacts completely off f the speaker cable.
That must be a good win from an EMI point of view.

Of course the same topology would work using two class-B amps for the
varying supplies (and that would be asy). But then that seems fairly similar
to class-AB in both quality and efficiency, which is perhaps not so interesting.

[rcownie]

Aha, followed those links and found chocoholic's thread discussing
*precisely* the same idea, clalling it class DA. Nothing new under the
sun, I guess.

[lumanauw]

I remember there's a member Skychu that has the classAD idea in his website http://home.kimo.com.tw/skychutw/

If you want to develop this idea, you can simulate this first, Mackie SRS1500. It has tracking power supply, but this is for subwoofer.
If the PC923 mechanism is fast enough maybe it can work for full range.

[Eva]

You are overestimating the performance of a linear output stage operating on only +/-2V. Also you are neglecting the contribution to distortion and powrer losses of the error and voltage amplifiers and driver stage required by such a linear output stage.

Actually a 300Khz self oscillating system can exhibit almost no phase shift at 20Khz.

[BWRX]

Class AB+D schematics

The idea certainly is not new (what is nowadays...). You may try doing a search on class a amplifier with tracking supply rails or something of that nature. I know there were a couple good threads on that topic in this forum but I can't find them at the moment.

[rcownie]

Originally posted by Eva
You are overestimating the performance of a linear output stage operating on only +/-2V. Also you are neglecting the contribution to distortion and powrer losses of the error and voltage amplifiers and driver stage required by such a linear output stage.

Thanks for your comments

The +/-2V figure was just a guess. And I'm mostly thinking MOSFETs (N-chan)
so the gate drive could be well above the +2V allowing low Rds, e.g, 0.2ohma
and thus sufficient current. But if it turned out to need 10V instead of 4V that
would still be a big win.
MOSFETs also ease the driver problem. And as for the voltage gain, the LME49810
seems a very adequate solution, though doubtless an expert - which I'm
definitely not - could suggest alternative approaches.

[GK]

Originally posted by rcownie


Thanks for your comments

The +/-2V figure was just a guess. And I'm mostly thinking MOSFETs (N-chan)
so the gate drive could be well above the +2V allowing low Rds, e.g, 0.2ohma
and thus sufficient current. But if it turned out to need 10V instead of 4V that
would still be a big win.
MOSFETs also ease the driver problem. And as for the voltage gain, the LME49810
seems a very adequate solution, though doubtless an expert - which I'm
definitely not - could suggest alternative approaches.

I'm actually building a high performance ~1ppm THD-20 1kW per channel class A amp using tracking rails. I started the thread on this project on the SS forum, but I haven't been updating it with progress reports due to the fact that the thread was a magnet for keyboard warriors and BS artists.
I'm using a class AB amp to drive the tracking supply however, but some points of relevance:
A class A linear output stage (either biploar or MOSFET) with enough output devices will have better open loop linearity than just about any class D output stage (<0.1% THD easy).
Tracking rails further enhances the linearity of the output stage due to both the lack of non linear base current (gate voltage) modulation via the collector-base (drain-gate) capacitance [which varies in a nonlinear fashion with Vce (Vds)] and slope distortion/transconductance variation.
However, you still need a reasonable supply voltage for the class A output stage. Below about 5V Vce (Vds) the collector-base (drain-gate) capacitance of power BJT's (MOSFETs) goes through the roof and device fT drops dramatically. I'm running +/-7.5V as a compromise between fT and power dissipation.
Also, the above mentioned capacitance is a real bother when it comes to the class D switching component (and its harmonics) on the supply rails modualting the base current (gate voltage) and appearing on the output.
A pair of modern RET BJT's will have a Cob of about 500pF for the PNP and 250pF for the NPN. Perforated emitter technology BJT's such as the MJL21193/mjl21194 pairs have double the Cob (1nF/500pF). MOSFET's have similar capacitances. This makes the output stage far from a pefect shield against class D switching artifacts on the rails.
A driver stage designed to have a very low high fequency output impedance (powered from clean, fixed high voltage rails) is mandatory here to address the problem.