New Amplifier - ULD Extreme

[Terry Demol]

Originally posted by AKSA
Alex,

According to the LT sims, we have around 180mV difference.

This with 56V rails; a difference of 0.3%. You could remove this by adding 13R of degeneration to the C3423 VAS.

This would also linearise the VAS more, but at the expense of Zout, but it's practical.

Cheers,

Hugh

Hugh,

Have you got an .asc file for your version?

From memory I did some LT work on the ULD some time ago just
as an exercise.

cheers

Terry

[AndrewT]

Hi,
combining some ideas I've seen in Roender, ULD, Blameless and Krill, what about doing a 3pair NJL output stage?
Incorporate all six integrated diodes into the bias string.
Add a parallel variable resistor to two of these diodes.

When VR=0r0 there are effectively four series diodes in the bias string and we know the output is tempco undercompensated.

When VR=1k0 there is ~1.3mA passing the VR and all the remainder of the bias current passing the two diodes. This will probably be overcompensated.

At some adjustment between 0r0 and 1k0 the compensation "may" be about right. Maybe the VR should be 500r or 2k or 5k, I don't know!

Does this provide room for further thoughts?

Can we save the Vbe multiplier for a "Blameless extreme" thread?
Keep the 2 or 4 or 6 NJLs for the "ULD extreme" thread.

AKSA,
once again thanks for your thoughts (close to your heart) on improving the extremes.

[Carl_Huff]

Andrew,

Your idea certainly is interesting and is definitely in the spirit of the overall design. Thoughts anyone??

[sandyK]

Originally posted by Carl_Huff
Andrew,

Your idea certainly is interesting and is definitely in the spirit of the overall design. Thoughts anyone??

It sounds like an excellent way to keep up interest in this area , as long as we don't kill off interest prematurely in this one, due to people holding off for an even more Extreme version.

[AndrewT]

Originally posted by AKSA
However, since five internal device diodes passing 2mA are required for proper biasing (read: matched tempco so as not to undercompensate thermally), I'd still go for the Vbe multiplier since the VAS current is around 14mA, and it would need to be split between a resistor and the diodes, always a bit risky.....

why is a resistor//diode string "a bit risky"?
If 300r were in parallel to the 6diodes it would pass ~12mA. The remaining 2mA would pass through the diodes. Is there some capacitance and/or inductance in these routes that creates a resonance/oscillation risk?
NB.
as the other VR across just two of these diodes is adjusted, the 300r current will move a bit away from 12mA.
At 11mA through the 300r, the diodes string will have 3.3V across it.
At 10mA through the 300r, the diodes string will have 3.0V across it.

Maybe the 300r should be a VR=500r??
The setting up procedure becomes ever more complicated as more VR are added. A 400mW 500r can pass a maximum ~28mA. 12mA is <=42% of max rating.

[jp_howard]

Regarding Hugh's item '8. Replace all output device emitter resistors with 0.22R.': I switched from 0R22's to 0R1's this morning, and couldn't hear any difference. However I did notice that the stability of the voltage across the emitter resistors was, after this change, less well matched, and less stable. I also know from my earlier mistakes (heatsink too small, and scratched heatsink) that the 0R1's are much less forgiving of thermal issues. This means that with 0R1's the builder has to be more careful and experienced, and also can't drive the bias up as high.

On the other hand, Self's treatise that led to the blameless design notes that larger emitter resistors do add distortion.

So, there may be some philosophical decisions - do we make it a little less 'blameless' if that increases robustness? In other words: Is this project about making the most blameless of blameless amps, or is it about using the blameless amp as inspiration, but making pragmatic compromises if required? I suspect the two approaches could lead to quite different designs...

[AKSA]

Jeremy,

The distortion benefits of 0.1R emitter resistors are really only evident with four ohm and less loading. The idea is to reduce waveform compression at peaks with less voltage drop on the emitter resistors. Accordingly, if you are using low impedance loads, the 0.1R is OK, though for high current loads you'd be well advised to go three pairs of output devices for 56V rails.

Another interesting fact is that the higher this resistor, the lower can be set the quiescent current. The math, based on crossover considerations, indicates that 26mV (derived from the Boltzmann constant) should be dropped across this resistor regardless, so 0.22R is passing 120mA and 0.1R SHOULD strictly be passing 260mA. It isn't in this design, so immediately there is one compromise....

The benefit of 0.22R, OTOH, is that you won't notice any degradation with an 8R load, and quiescent current will match far better, and it's great insurance for the outputs in the event of an accident, which as you realise, is inevitable.....

In my commercial designs, I go whole hog and use 0.47R. Quite ridiculous really, however, they never blow up, the bias is always super stable, and since these resistors are inside the global feedback loop I still have a damping factor around 80, indicating source impedance is actually around 0.1 ohm.

Hugh

[jp_howard]

Thanks for the explanation Hugh.

[AKSA]

Andrew,

My pleasure, enjoy the dialogue.

I agree with you about three pairs. Should people use this amp with <4R loads, three pairs is, in my opinion, almost mandatory for 56V rails. With 50V rails, two pairs would be fine, but SOAR considerations lead me to agree emphatically.

If you have the VAS current splitting into two current paths, one down five diodes and a pot at 2mA, and the other down a 210R resistor at 12mA, then since the diode voltage drop is controlling the outputs, we want that control rigid and low impedance.

2mA controlling 12mA without current gain is not a good look. It's not my preferred way of doing it. I really would like to use a Vbe multiplier, however, others may differ on this and that's cool. I hark back to Mihai's comments about the mismatched tempcos; I really think these devices need more development work, and they are a bitch to lay out on the pcb with their five leads.

Hugh

[pheonix358]

Originally posted by sandyK


Terry
Perhaps loudspeaker designers should take more notice of what kind of a load gives the best results with the majority of amplifiers, instead of trying to make it the other way around ?
It is an unfortunate fact that distortion into 4 ohm loads for example is normally much higher than into 8 ohm loads,and even worse into 2 ohm loads.
Douglas Self has also gone into this area,although later output transistors such as the MJL21193/21194, and later variants, have lower Beta droop at higher currents than earlier devices. Whatever happened to speakers the likes of those from the late 80s, where a typical speaker impedance was 8 ohms, and some speakers like the DCM QED, which I still have, had a rmaximum impedance of 12 ohms. The sensitivity however was relatively low at 87dB/W/M
SandyK

Hi Sandy,

Loudspeakers add more distortion than any modern reasonable amp. If runningmultiple drivers in isobarick reduces this distortion then I can put up with a bit more in the amp. I woul like to see this thread with a few options in power output. As to the 80s Home theartre came along and stuffed up the good years

Terry