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Old 30th July 2009, 03:41 PM   #911
Salas is offline Salas  Greece
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Quote:
Originally posted by stormsonic
Iko, MPSA is 100 Mhz bandwith, gain 500 to 1500.

BC550C is 300 Mhz bandwith with gain 420 to 800. Darlington connection to increase gain.
We are near instability in that circuit with too much hfe. Watch out.


Quote:
Originally posted by col_s


Thanks, i'll give them a try. I used them for the analog supply for a multi-channel volume control using multiple pga2320's and opamps. It replaced some teddyregs that were not working very well in this circuit. I think due to their high output impedance they are better suited to supplying less components and low currents. It now sounds much cleaner. There is more and better bass, and some nasty sibilance went away.
Great. Congratulations.
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Old 30th July 2009, 03:44 PM   #912
Salas is offline Salas  Greece
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Quote:
Originally posted by ikoflexer
I would like to share a bit of good news. Just finished a high voltage version of v1.5, however, using n-channel mosfets. Although this thread is about low voltage, I think it is relevant in that this topology is the same as the one I recommended earlier to Tham. So that should give him some hope that it's not just simulated vapor ware. I used two trimmers, not set to the exact value shown as resistors in the figure (R1 and R10).

As I said, this circuit is currently running in reality, and it is stable. My intention is to use this (or some variation) for the dht headphone amp I'm developing in another thread.

That will guarantee special sound from your new headphone amp. I believe it will stay stable with that amp for load too.
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Old 30th July 2009, 03:46 PM   #913
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Quote:
Originally posted by AndrewT
20k R9 and D6 charge up the 470uF C6.
that turns on the pass FET.
Then we have to wait until the FET charges up C5.
Andrew, spot on.


Quote:

What power will be dissipated in the 20k with 160 to 180V across it?
Without the 20k and Zener the Jfet will explode.
Less than 2W. I have very little experience with high voltage and the tricks people do to protect parts from over voltage. When I don't find established methods, I just improvise. I agree, in that circuit, the jfet would not survive the turn on voltage without that protection. If you know of a better way to do it, please let me know.

Quote:

Once the caps are charged, what current will pass the 20k?
Will that be enough to keep the jFET turned on?
Yes, the current would be a bit higher but the jfet's S-G resistor/trimmer I thought of setting for about 2mA. Then the rest of the CCS mosfet biasing mechanism should work fine.
Quote:

That Darlington proposal was raised months ago.
I thought you or someone else gave an explanation of why it was not possible.
Yes, I tried darlingtons in a variety of variations on v1. If they didn't make it into v2 and v1.5, the reason must be because they didn't work well enough.

Here's a rough number of currently saved versions that I've tried; variations on this regulator: 397. And these are only the versions that I deemed worth saving, but there are others that I tried and never saved.
Code:
$ ls *salas*.asc *reg*.asc reg*.asc | wc -l
397
Yes, my wife wants to kick me in the shin knowing I don't get paid to do this and that I do it just for fun
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Old 30th July 2009, 03:49 PM   #914
iko is offline iko  Canada
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Quote:
Originally posted by Salas


We are near instability in that circuit with too much hfe. Watch out.
I agree.

BTW, salas, I kept forgetting. But if you get a chance, I would recommend that you try in your shunts the mpsa18. You have good ears, and should be able to tell the difference.
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Old 30th July 2009, 03:51 PM   #915
iko is offline iko  Canada
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Quote:
Originally posted by Salas


That will guarantee special sound from your new headphone amp. I believe it will stay stable with that amp for load too.
I hope so I would like to provide the dht with the cleanest and most dynamic supply. Also active load (gyrator) ... I hope to squeeze all that a dht has to offer Of course, it may be a royal flop in the end but if I don't try it, I won't know, right?
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Old 30th July 2009, 04:19 PM   #916
Salas is offline Salas  Greece
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Quote:
Originally posted by ikoflexer


I agree.

BTW, salas, I kept forgetting. But if you get a chance, I would recommend that you try in your shunts the mpsa18. You have good ears, and should be able to tell the difference.
Must be a better controlled for low noise part, with less GB product. Still high hfe. Possibly lends a perceptible sonic difference?
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Old 30th July 2009, 08:44 PM   #917
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Hi Andrew,
Quote:
the maximum current that a trimmer track can pass is determined by the maximum power the whole track can dissipate.
That's only at maximum resistance! You must de-rate the power by the fraction of the travel in degrees. Figure out how many mW per degree of rotation, and multiply that by the minimum rotation you see used in circuit. So, this is for the element itself...

However, the contact has not been considered at all. Multi-turn controls are not designed to pass any current through the wiper at all. Minimal at best. They are designed to operate as a potential divider, so you have current that passes through the element, but nothing to speak of through the moving contact. This is a critical error in the use of these parts. Consider a volume control that is familiar to us all. How much current is expected to pass through the element? Through the wiper contact? Consider that this is a huge control with large contact surfaces compared to trimmers. You can not use a trimmer as a rheostat!

All,
If you need to use an adjustable resistive element, they do make wire wound rheostats for this purpose. You may have seen these in old TV sets or industrial controls. A Lamda power supply comes to mind. If you only need to sample the voltage (ie: no current), then you can use a resistor as the pass element and place a higher value trimmer across this. The wiper then feeds the high impedance point that samples this voltage. jameshillj did suggest this (I think this was your intent?).

-Chris
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Old 30th July 2009, 08:47 PM   #918
anatech is offline anatech  Canada
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Hi Salas,
Remember, darlington transistors are slow. Composite darlington transistor pairs need sufficient base current in the first of the part to be fast. Therefore, unless you are careful, then a composite darlington will also be slow compared t a single transistor.

How many RF darlington transistors has anyone seen?

-Chris
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Old 30th July 2009, 10:19 PM   #919
Salas is offline Salas  Greece
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Slow they maybe, but I am afraid of extreme hfe because the loop gain with the CCS for load is high already in the reg.
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Old 31st July 2009, 02:28 AM   #920
anatech is offline anatech  Canada
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Hi Salas,
I agree. Just one more thing to consider. It also depends greatly on layout and current levels.

-Chris
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