Nice work 
and good detective analysis.
A couple of things puzzle me in the scope shot showing the oscillation. Firstly, why is there apparent LF roll off, the component values we have shouldn't do that at 1kHz. And secondly the initially 'ringing' looks for to low a frequency but be normal instability.
The burst of HF that comes later can be typical of wiring and layout issues (in my experience)
and good detective analysis.A couple of things puzzle me in the scope shot showing the oscillation. Firstly, why is there apparent LF roll off, the component values we have shouldn't do that at 1kHz. And secondly the initially 'ringing' looks for to low a frequency but be normal instability.
The burst of HF that comes later can be typical of wiring and layout issues (in my experience)
sim racing!
seems a waste to have a cascode beta enhanced VAS, output triple and only 22 dB 20 kHz loop gain
I think Bob overdoes the diff pair degen, I reduced it 10x
I more than make up the audio frequency linearity with 2-pole compensation, almost 40(37) dB added 20 kHz gain
which means ~ 70x less diff input V for the same 20 kHz output
and of course much more loop gain as you go lower into conventional music 3-5 kHz power bandwidth
seems a waste to have a cascode beta enhanced VAS, output triple and only 22 dB 20 kHz loop gain
I think Bob overdoes the diff pair degen, I reduced it 10x
I more than make up the audio frequency linearity with 2-pole compensation, almost 40(37) dB added 20 kHz gain
which means ~ 70x less diff input V for the same 20 kHz output
and of course much more loop gain as you go lower into conventional music 3-5 kHz power bandwidth
Attachments
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Amazing! The snubbers work! Seems stable at the mo, can set the bias to any value and it's fine. 1kHz square wave shows ripple but no oscillation, still needs some some work and optimising but this has done the trick! Thanks!
Some further feedback.
The base dampers are usually placed in the driver bases.
On the pre-drivers, you can insert a low value resistor (3.3 to 4.7 ohms works fine, although I have used as high as 10 Ohms). You then decouple the pre-driver collectors to ground with a good quality electrolytic capacitor of 5-10uF and a parallel film of 0.1uF.
This makes the EF3 absolutely bullet proof wrt stability. See Cordell where this is discussed in some depth.
I don't use any capacitor in parallel with the driver tie resistor anymore. Base storage in modern sustained beta devices (fT ~ 30 MHz) is low enough not to be of concern - different story of course on the old 21193/21194 devices (fT ~ 4MHz).
Re the ripple on your square wave tops - it can either be due to the output inductor OR your speaker cable resonating with some sort of capacitive load. The other possibility is that your rail localized decoupling is inadequate.
The base dampers are usually placed in the driver bases.
On the pre-drivers, you can insert a low value resistor (3.3 to 4.7 ohms works fine, although I have used as high as 10 Ohms). You then decouple the pre-driver collectors to ground with a good quality electrolytic capacitor of 5-10uF and a parallel film of 0.1uF.
This makes the EF3 absolutely bullet proof wrt stability. See Cordell where this is discussed in some depth.
I don't use any capacitor in parallel with the driver tie resistor anymore. Base storage in modern sustained beta devices (fT ~ 30 MHz) is low enough not to be of concern - different story of course on the old 21193/21194 devices (fT ~ 4MHz).
Re the ripple on your square wave tops - it can either be due to the output inductor OR your speaker cable resonating with some sort of capacitive load. The other possibility is that your rail localized decoupling is inadequate.
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Not directly for this amp but interesting:
Other studies show that it's better to stay below 6v with zeners to avoid the avalanche breakdown and its noise.
See especially the summary in following article:
Noise Behaviour of Zener Diodes
More studies:
Junction Noise Measurements
Gergard tests
Morgan Jones
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Looking at your sim diagram, the ringing is the output coil and output cap combo.
If you remove the output load cap, the ringing will stop.
Square wave slopes - will need to take another look.
wind your own is the usual diy solution
plenty of inductance calculators online
solenoid or toroidial
rember that the mag field reaches out a good distance from a solenoid's open ends
this can couple to circuit wiring loops, and ferromagnetic material interacting with the external solenoid field can cause distortion
plenty of inductance calculators online
solenoid or toroidial
rember that the mag field reaches out a good distance from a solenoid's open ends
this can couple to circuit wiring loops, and ferromagnetic material interacting with the external solenoid field can cause distortion
Then either the input coupling capacitor or the unity gain nfb capacitor is undersized,
assuming your generator waveform looks normal direct.
The actual circuit built (before all this) is the attachment in post #1.
From the tilt, it seems to be around a 100Hz corner.
If the tilt is not there just after the input coupling cap, then the
problem must be the nfb cap. Maybe it's 10uF instead of 100uF.
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