non-switching CFP output ?

[Bigun]

Does this have potential ?

I have been looking at how to take full advantage of the linearity inherent in the CFP output stage. The problem is, CFPs are thirsty for current because they hate to switch. The methods I've explored to date for achieving non-switching behaviour have not been that simple or as effective, see: new amp... HARRY 77

So now I have a design that appears in sims to create a non-switching CFP output which is cleaner although it's rather crude. In the CFP structure the master device is configured as an emitter follower on the output. The slave device wraps around it, taking it's base input from the collector of the master device which is held off the power rail via a resistor (setting the current level in the master device). The signal at the base of the slave is inverted with respect to the output. By providing a level of ac-only negative feedback from the output to the base of the slave device it's possible to keep the slave device always fwd biassed. The current through the output slave device never drops below the idle current which is set by the Vbe multiplier.

I have compared this with a version of the amplifier that uses only EF outputs operating in ClassAB with everything else being equal. The idle current through the CFP stage is set a bit higher, as they are always happier that way but never high enough to operate in full ClassA at the signal levels I'm using.

The negative feedback from the output to the base of the slave output devices is accomplished by a resistor & capacitor in series. See attached schematic. Also attached is an FFT plot at 10kHz at two different signal levels of +/-9V and +/-28V output (into 8 Ohms). The green trace (with more spikes) is the standard EF output structure and the blue trace (predominantly H2/3) is the non-switching CFP. It seems to function just as well at higher and lower frequencies; it's particularly clean at lower fundamentals.

How would you refine this ?

[Lumba Ogir]

Gareth,
you steadily keep searching for the truth (there`s nothing wrong with that). You could also take a look at the structure of The Ultimate Suplifier (bridged output, no global feedback). Or just at this great sounding topology with switching output (lateral MOSFET required).

[Bigun]

Originally Posted by Lumba Ogir

you steadily keep searching for the truth

Well as they say "the truth is out there" and this appears to be part of the reason this hobby becomes an obsession

I think for the most part, I already realized that TGM2 amplifier offers sufficient performance in every respect and that I don't need anything else for my listening pleasure for quite a long time. The most limiting part of my system is the quality of the material I'm playing (Chesky CDs being significantly better than anything else in my collection for example).

Still, I am drawn into this hobby like a moth to the flame....

Strange isn't it, the schematic is just a human convenience but during the design phase it makes a big difference to how I feel about the design if the schematic is simple. And it doesn't show the psu or all the work that has to go into the chasis.

I am also, currently somewhat fixated on the (simulated) harmonic spectra as a key indicator, for good or bad. I don't like what I see from switching outputs (yes this is in conflict with being very happy with TGM2). With ClassD you have a big filter afterwards. With Class AB there is no filter afterwards. With Class A there is no switching. I'm drawn to non-switching outputs. Even with some H2/3 it doesn't take much to produce higher order harmonics from IM.

I like CFPs.

Where's the truth for you Lumba ?

[Bigun]

Perhaps the circuit I posted looked a little complex, but we can strip away quite a bit for a simple 50W+ amplifier.

top image - schematic
middle image - current flow through output devices - always non-zero
lower image - FFT (blue) of CFP output verses EF output (green)

[darian]

Lumba,

What about the global feed-back from the output to the base of Q1 through R7 and R14?

Regards

[anatech]

Hi Bigun,
Cyrus input stage coupled to an odd CFP output stage. Interesting.

CFP stages will happily switch on and off. Some time we really will have some things to talk about. This circuit for one, as I have done some work along these lines.

-Chris

[Bigun]

Originally Posted by anatech

Some time we really will have some things to talk about. This circuit for one, as I have done some work along these lines.

Well, you can't leave it dangling there like that now can you - I think you'd better spill the beans (email me if it's commercially sensitive)

[Symon]

Are the 2n5550's really able to handle the drive for the 2SA1943?
These high power bipolars usually require a driver with a higher power rating than the 650mW rating of this TO92 device.

[Lumba Ogir]

Gareth,
maybe the truth is written in the stars...but there´s is not only one truth I`m afraid.
I will try to give a more detailed answer successively.

Cris,

CFP stages will happily switch on and off.

In a nasty manner if bipolar. Gareth is right.

darian,

Let me say like this:
The single-ended input gives the sound a nice character.
Possible improvements:
Converting to folded cascode (omitting Q4).
Separate power supply, lowering the output section rail voltage, increasing the bias current to 250- 500mA for a friendly crossover distortion.
A better CFP driver stage.

A more linear input stage:

[anatech]

Hi Bigun,
I never tease about anything that is commercially sensitive. I don't even mention anything like that. NDAs and all that, and I like to stay out of trouble.

No, more like sitting down with you with some paper, just bouncing ideas back and forth. No way am I going to assume a higher level of knowledge, that's just rude!

Hi Lumba,
Good suggestions as always.

I'm not sure that I agree that CFP in A-B or just B is a bad thing. I can think of so many other unhappy output configurations that are much worse off. I have seen many implementations that are left in class A, so I assume there may have been problems with switch off. Still, that does not condemn every design to poor performance.

-Chris

Edit:
Lumba, is that current source at about 100 mA or so enough to deal with gate charge, or is it not such a problem with those mosfets? I see you are using a CFP input single, cascoded to directly drive the driver transistors. I'm still more comfortable with the extra linearity a diff pair brings to the table. You can still drive the drivers the same way except that we are now talking about a split supply. Any reason why you went with a single supply and output capacitor?