Ultrafast NoCap-ClassAB

[MikeB]

Hi Guys !

I thougt, try something different, a ClassAB amplifier without ANY
feedbackcompensation-cap. This is my result...
I had to use tripledarlingtonoutput because of the low current (2ma)
through the folded cascodes, higher current would give too low gain.

Some specs: (out of sim)

THD:
0.003% 20khz, 1watts into 8ohm
0.006% 20khz, 9watts into 8ohm
0.05% 20khz, 64watts into 8ohm (full power)

Distortionfigures for 1khz are "identical".

Frequencyresponse:

-3db: 1.66hz to 820khz
-1db: 3.25hz to 413khz

Prototype is running without oscillation, sounding very impressive !
The sound is completely uncolorized and clean, but pleasant.
It's the most detailed amp i've ever heard, the reproduction of
voices similar to the DoZ.
The music played with this amp is somehow "live"...

What do you think ?

Mike

[traderbam]

I think you should test it thoroughly on the bench. Have you a function generator and a scope? You may find it is unstable into capacitive loads - change the input cap from 100p to 470p, then try driving it with a 1kHz squarewave into a few capacitors ranging from 1nF to 10uF. Vary the amplitude. Be prepared for oscillation!

[MikeB]

Sadly, as i am only hobbiest, i have no scope...
A functiongenerator should be no problem, i can use the computer
or just build a squarewavegenerator.
The outputcoil should protect the amp from capacitive loads,
but real world has teached me that sims sucks...
Sims showed the dominant polefreq at 63mhz, could get difficult to detect ?

Mike

[Alme]

That's an interesting schematic Mike. Maybe I'll try it if have some spare time, I can fully test it with 'Audio precision system one'. Although I doubt that there is no need in ANY frequency compensation. By the way are you sure that collector loads of differential stage are shown correctly? I may be mistaken but from the first look, with 330 Ohm there amplifier is in shutdown mode, have to be closer to 2,2kOhm

[traderbam]

Hi Mike,
I'm glad it sounds really good. The zippy response of the feedback is helping to mitigate the non-linearities I'm just warning you that, in my experience, uncompensated circuits of this topology have a tendency to oscillate at several MHz if their load becomes too reactive and this can lead to the power transistors overheating, and short circuiting and then your speakers receive dc and you have a sense of humour failure at this point (yes, I've done this myself!). Even with an output inductor I imagine the amp would become excitable or just plain oscillate into highly capacitive loads. Speakers present a variable reactive load to the amp - and the extent of this will depend upon the speaker. Just be careful not to damage anything by accident.

Without a scope it is almost impossible to see what is going on. You could try attaching an AC voltmeter across the output, with no input, and apply caps across the output and watch the reading. If it rises then there is some oscillation. Unfortunately, the slow response of the voltmeter means you may not realise it is oscillating for a few seconds by which time the output devices MAY overheat. So best when doing a capacitor test to limit the current to the output transistors in some way - like with series resitors of 10-ohm or so. You may burn some resistors this way, but not the output transistors.

If you don't have the test gear I'd just listen carefully to it and listen out for signs of instability. Sometimes you get a roughness on some sounds - usually treble instruments or strong women's voices, sometimes snare drums. This can often be a pronounced crackly sound. Sometimes you get instantaneous dips in volume. Without any music playing try sharply tapping the woofer cone of your speaker with your finger and listen for chirping. The extend of the symptoms will be slightly different in each amp you build because the transistor parameters and parasitic capacitances are different.

If all is well then don't worry about it. But be careful to reassess if you change the speakers for a different pair or change the speaker cables in case the new loading sets it off.

Happy listening :-)

[MikeB]

Hi Alme !
Yes, the 330ohms are correct, this is folded cascode, behaving very
different ! The lower these resistor, the higher the current in "vas"...
With the folded cascode i was able to speed up the whole amp
in a way that the 100pF at input seems enough compensation.
Yes, would be great if you have sophisticated equipment for testing !

Hi traderbam !
Thanks for warning, i will observe closely. I did not recognize any
symptoms yet, from other amps oscillating i collected some of the
typical symptoms.
I already had different speakers connected, no change in quiscent
current or strange noises triggered by some instruments.
I had not tested yet moving the woofer with no input, i will try now !
I expected the circuit to oscillate, but was surprised seeing it work.

Mike

[Graham Maynard]

Hi Mike,

Place an AM portable close to your construction and tune up and the MW band. Any quietening means oscillation.

Try your sims with 50% pot setting, distortion may increase. This is why we have pre-amps with buffered pots.

It could be the choke that is preventing oscillations. Chokes are normally damped with a parallel carbon resistor.

Good luck with the investigations.


Cheers .......... Graham.

[Eva]

I'm just curious :

If you haven't got an oscilloscope, how do you know that your circuit doesn't oscillate?

Only severe oscillations at zero crossing are noticed as an increase in bias current. Some oscillations are so small in amplitude that they may only be noticed measuring the VAS current

[john curl]

I would put a 5-10 ohm resistor in parallel with the output inductor (2uH). Everybody does, for good reason.

[MikeB]

Originally posted by Eva
I'm just curious :

If you haven't got an oscilloscope, how do you know that your circuit doesn't oscillate?

Only severe oscillations at zero crossing are noticed as an increase in bias current. Some oscillations are so small in amplitude that they may only be noticed measuring the VAS current

Typically, there are much more symptoms indicating oscillation, but
you are right, i can't be absolutely sure. In sims it doesn't oscillate,
but phasemargin was not very big. So i changed it to doubledarlington,
this should remove any doubt about oscillation. (in theory)
You really shouldn't underestimate the speed of folded cascode !
Shouldn't an oscillating amp sound bad ? (muffled)
The other problem is, sims showed "oscillation" with 0.5uv at 64mhz,
i doubt that a normal scope could show oscillation at this freq ?
At least i did not find any typical symptom of oscillation, but
still missing the test with AM-radio...

Originally posted by john curl
I would put a 5-10 ohm resistor in parallel with the output inductor (2uH). Everybody does, for good reason.

Yes, i will do that, but i never understood what it is really good for ?

Mike