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Old 20th February 2008, 08:36 AM   #1581
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Join Date: May 2002
Location: Cambridge, UK
Quote:
Originally posted by gl
Very nice piece of work Ian.

For myself I would agree with Tazzz' take on the overall topology.

On the detail side of things VR2 needs to be 100 ohms instead of 220. It has twice the current flowing through it as the relative offset pot. I found this out the hard way when I built a similar circuit and had to figure out why I had the abs offset adjustment pot all the way over and still hadn't arrived at zero volts.

I also still believe strongly that the front end has only the JFETs for voltage gain followed by current mirrors to provide level shifting and current gain.

Other than that, it looks like time to start building. Have you found any JFETs following that issue with the GB?

Cheers,
Graeme
Thanks Graeme.

Are you referring to AC coupling of the FE to output stage when you mention Tazzz' take? I'm unconvinced of us mainly because it would be difficult to control the DC conditions in the output stage. How do you think this is achieved?

Thanks for the tip about VR2 - I would have fallen into this hole!

I'm already building but progress is slow. I have all the parts except the Dual FETs but I'm still pursuing the latter for now.

Ian.
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Old 20th February 2008, 08:55 AM   #1582
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Quote:
Originally posted by GRollins
Ian,
Just took a quick glance. Granted, the "Aleph" portion of things is open to interpretation, but the output stage looks to be straight X with no Aleph. Or did I miss a post earlier that explained things?

Grey
If Aleph is taken to mean the modulated current source then this is not an Aleph. I'm using the "A" loosely to denote an X amplifier that is operated mostly in PP class A. The Aleph is SE class A and the X is class AB so really this is an amalgam of both. Really I should start a new AX thread I suspect.

Ian.
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Old 20th February 2008, 01:05 PM   #1583
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In this context AX is not the abbreviation for an Aleph-X-amp - it stands for class A X-amp! :-)

(I will call that AX-PushPull)

Dirk
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Old 20th February 2008, 02:01 PM   #1584
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Sorry Ian,
have overlooked your answer!
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Old 20th February 2008, 04:12 PM   #1585
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Hello!

Have corrected the mistakes in my schematic!
In the heat of the moment I had forgotten to put in the right values for the input stage. Now you will get a gain of 26dB and an input impedance of 15k unbal. / 30k bal.
The relay switching circuit will follow shortly! But this will not be a problem and easy to handle.
With +/- 32V supply rails and a bias currrent of 2,5A per bridge half (5A total), it should reach 100W@8Ohm pure PP classA or 200W@4Ohm classAB.

But be warned, I still haven't build this amplifier and the bias circuit needs some attention and work.

Regards
Dirk
Attached Files
File Type: pdf ax-pp-se (v2).pdf (84.6 KB, 268 views)
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Old 20th February 2008, 08:02 PM   #1586
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Quote:
Originally posted by noisefree
Hello!

Have corrected the mistakes in my schematic!
In the heat of the moment I had forgotten to put in the right values for the input stage. Now you will get a gain of 26dB and an input impedance of 15k unbal. / 30k bal.
The relay switching circuit will follow shortly! But this will not be a problem and easy to handle.
With +/- 32V supply rails and a bias currrent of 2,5A per bridge half (5A total), it should reach 100W@8Ohm pure PP classA or 200W@4Ohm classAB.

But be warned, I still haven't build this amplifier and the bias circuit needs some attention and work.

Regards
Dirk

I think the thermistor and the resistors should only be in series with the pot for setting the bias voltage and not in series with the whole bias generator as it is now.

The additional capacitor should then be coupled across them. It is told in the digital forum that the tl431 performs its very best when decoupled both across it in its entierly and also over the feedback resistor itself.

But how do you account for those extra 20% ?
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Old 20th February 2008, 10:12 PM   #1587
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Hi Tazz,

the used thermistor as temperatur sensor is a NTC element.

When you want to put a thermistor in series with the pot (between reference pin and cathode of the TL431) you have to use a PTC.

A second possibility for the NTC (NTC network) is between reference pin and anode.

The resistor in series with the reference element is needed to get a lower voltage bias when the whole thing is only biased by the 10k resistors in stand by mode (instead of the much higher front end bias).

Can you please give me the exact thread and post for the elcap / TL431 information?
Have interest to read it...

Have not find an idea for 20% extra swing without seperat front end voltage.... :-(

Regards
Dirk
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Old 20th February 2008, 11:49 PM   #1588
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Me again....

think I need more sleep - make to much mistakes...
...have confused anode and cathode....

Here are the two corrected sentences:

When you want to put a thermistor in series with the pot (between reference pin and anode of the TL431) you have to use a PTC.

A second possibility for the NTC (NTC network) is between reference pin and cathode.



Sorry and......
.......good night!

Tzzzzzzzzzz.........
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Old 21st February 2008, 08:32 AM   #1589
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The resistor is intentionally in series with the whole bias generator for the reasons Dirk mentions. The NTC thermistor is used to thermally compensate this resistor, not the TL431 which is thermally stable anyway. You are right that this, and probably the whole bias generator, should be bypassed with a capacitor though.

Dirk's circuit has no bootstrapping and hence cannot swing close to the rail voltages. The circuit I posted earlier has some bootstrapping that increases the maximum signal swing but will still be some way off the rail voltage. If there is another 'trick' here nobody has offered any suggestions yet...

Ian.
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Old 21st February 2008, 08:52 AM   #1590
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Quote:
Originally posted by Ian Macmillan
The resistor is intentionally in series with the whole bias generator for the reasons Dirk mentions. The NTC thermistor is used to thermally compensate this resistor, not the TL431 which is thermally stable anyway. You are right that this, and probably the whole bias generator, should be bypassed with a capacitor though.
I assume the thermistor is there to thermaly compensate the amps outputstage during operation by adjusting the voltage setting on the tl431?

It seems to me anyway that this could be accomplished by mounting this net in or around the 2.5v reference internal to the tl431 instead. And choose the values so as to not to turn it on without the bias current runing through the UGS module. Then the bias servo would show a steady 100mR impedance over a large range of currents/ frequencies.

And a smaller stand by voltage could still be generated from the 10K resistors and the resistors determining the voltage across the tl431 while it is operational.
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