New Amplifier - ULD Extreme

[Carl_Huff]

forr said, "There is something wrong with D1 and D2 across the 220 µF DC feedback capacitor."

Yea, there is. D1 is backwards! Doh!!

Changes this version:
#1) D1 made right
#2) The current mirror changed to BC550C devices.

[sandyK]

Originally posted by AndrewT
V2.0c ULD shows a string of 4diodes and a variable resistor (100r) along with 6NJLs.

Could a fifth and sixth diode be brought into the string but appearing in parallel to the VR?

The one or two extra diodes can easily be shorted out with a link if the builder does not want to use them.
The 100r can be any value from 100r to 20k.

This would allow the tempco to be dialled in by reduce the effect of the fifth diode by bypassing it with the VR.

None of this would take up any extra PCB space. It would increase enormously the options open to the first few builders, who could then advise the later builders on which options seem to work best.

I am sorry if this seems a repeat of what I said before, but it's a no cost option that has been ignored.

Andrew
Another scenario could be to leave the VR as it is, but fitting the 6 diodes, where there is then the possibility of choosing 4 diodes with the highest forward voltages and then replacing 2 with links ?
Alex.

[Carl_Huff]

Andrew T

Actually if you look closely at the PCB that I showed in post #100 in the other thread all 6 diodes are brought out in the string. However I included 'shorting traces' that could be jumpered to shunt 2 of the 6 diodes. Does that work for you? Could my scheme be improved?

Actually it would probably make sense to make the adjustment pot in the diode string be the same value as the pot that is in series with the collectors of Q2 and Q4. Is 200 ohms a good value for both?

[Carl_Huff]

Andrew T,

I just re-read your post and realized that you were suggesting that the variable resistor be in parallel (not in series) with the 5th diode. I think that could be easily accomodated in the layout that I have posted. Is there any case where we would want the 6th diode in the string? If not the 'shorting traces' for the 6th diode could be made permanent on the PCB.

[pheonix358]

Hi all,

Carl, would it be possible to put three pads on the board edge next to the output devices to enable one to hang an extra output device on the heatsink external to the board. The extra diodes would not be used and the 0R22 could act as a jumper cable.
I still need the ability to go down to 2 Ohms reactive.

Terry

[Carl_Huff]

Phoenix358,

Please excuse me for being so thick headed. Are you proposing a 4th output pair?

[pheonix358]

Yes, for those poor deluded fools like me that would like to go down to 2 Ohms reactive.

Terry

[AKSA]

Hi Carl,

Nice drafting!

Yes, Terry wants it for 2R, so four pairs are his target.

Personally, to make this universally appealing, I reckon three pairs is tops. And three pairs at 56V rails should easily cope with 2R loads at moderate volume, and if reduced to 50V rails, would cope unconditionally.

It should be understood that 50V rails with a 2R load will do no less than 529 watts, assuming four volts loss from each rail and a stiff supply, a 46V peak signal output. 529 watts should be enough, even for rock 'n roll.....

The vexed issue of the diodes comes up again and again. Since there is disbelief that they don't work as designed, best to leave them available, and bypassable if found inadequate.

Five diodes will give a reasonable bias voltage at 2.5mA. Four diodes and a pot in series will cope with the total VAS current, around 13mA, and it should be OK with 0.22R emitter resistors and generous heatsinking. Five diodes will require a bypassing resistor, around 210R, to carry most of the VAS current, leaving 2.5mA through the diode string. I have not tested this concept, so can't vouch for it. It's a risk......

Another option might be to leave the diode string, AND put in three pads strategically to fit a conventional Vbe multiplier.

Cheers,

Hugh

[Terry Demol]

Originally posted by roender




This is far more important than many people could think!
It took me a lot of time to figure out that a highly biased driver stage could change the fingerprint of a classic push-pull classB OPS towards classA sound

What do you call high bias?

T

[pheonix358]

Thanks for that info. I need 300Watts into 2R. To reduce distortion to as low as possible, is it better to further reduce the rail voltage, how low can it go and what is the relationship if any between distortion and rail voltage.

Thanks

Terry