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Old 14th August 2003, 10:54 PM   #21
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Hi,

I see what you mean now, that's by no means an autoxformer though.

Probably an S&B or Lundahl controlled by a rotary switch a la Shallcro or Elma 24 pos. or more, right?
IIRC, none of those are happy with DC on the input so I guess it's blocked somewhere.

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I could ask Dave Slagle about this.
No need to bother, I know these beasties...

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What's a WOT?
With Output Transformer.
Usually a small airgapped xformer to block DC and lower output Z at the same time.

Cheers,
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Old 14th August 2003, 11:02 PM   #22
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Quote:
Probably an S&B or Lundahl controlled by a rotary switch a la Shallcro or Elma 24 pos. or more, right?
Nope. S&B and Lundahl are transformers. The ones I have are autoformers, wound by Dave Slagle. That is something I am pretty sure about
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Old 14th August 2003, 11:11 PM   #23
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Frank, I've used Saurav's line stage- it is indeed an autoformer. The damndest thing. Sounds good, too.
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Old 14th August 2003, 11:16 PM   #24
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Hi,

Quote:
Frank, I've used Saurav's line stage- it is indeed an autoformer. The damndest thing. Sounds good, too.
Dave Slagle's is a wizz...anywhere I can read up on what he did?

Sure sounds very interesting.

Sorry if I caused any confusion...cheers,
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Old 14th August 2003, 11:51 PM   #25
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I've seen a few testimonials from other people who've used them. No discussions by Dave that I know of. He might have talked about it either here or on AA... I don't remember if I read anything, and it would all have been way over my head anyway. Still would be, for that matter.

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Sorry if I caused any confusion
No worries... this is a refreshing change from "Soooo why doesn't this have a power supply?"
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Old 15th August 2003, 05:15 AM   #26
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OK, and I picked this for a filament transformer:

TRANSFORMER, FILAMENT, HAMMOND, 12.6 VCT, 4 A

12.6V with a bridge rectifier shows about 10V RMS with a 2.5A constant current load. That should leave me enough for the drop across the two regulators, and I can eat up anything extra with dropper resistors.

And while I'm at it, I think I'll take all the iron off my amp (I'm changing OPTs) and try to give it a better paint job
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Old 15th August 2003, 06:44 PM   #27
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More regulator questions.

The upper diagram shows the voltage regulator. This is straight from the datasheets (I know I'm missing the caps on both sides for stability), and is fairly easy. The LT1085 will try to have 1.25V across R, so all I need is for the lower resistor to be equal to the upper resistor, and I'll have 2.5V. And if I pick resistors in the 1K range, the currents flowing through them will be in the 1-2mA range, so the 50uA that flows out of the ADJ pin won't upset calculations. Also, 1.25V * 1mA means 1/4W or 1/2W resistors will be fine.

I hope I got that correct?

The lower diagram shows the current regulator, and this is where I have questions. First, is this how I should hook it up? If it is, then for 2.5A, I need a .5 ohm resistor, or more correctly, pot, in order to adjust values accurately. And then I have 2.5^2*.5 = ~3W being dissipated through this, so I'll need a 5-10W 1 ohm pot, which sounds like a slightly unusual component.

Is that correct? Is there a better way of doing this? And finally, starting at the rectifier end, which should come first - the voltage regulator, or the current regulator?

Thanks in advance,

Saurav
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Old 15th August 2003, 06:51 PM   #28
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I don't know about the LT regulators, but the similar 3 pins from National like to have more idle current than 1 ma flowing through.
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Old 15th August 2003, 07:20 PM   #29
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You're right, I hadn't looked for that in the datasheet. It shows a minimum of 3mA, and the example schematic has around 10mA, so I'll adjust the resistor values accordingly.

How about the current regulator? Does that look right?
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Old 16th August 2003, 11:20 AM   #30
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Default If you had an ideal component supplier...

Your current regulator is fine in theory, but as you've realised, not terribly practical. However, that's easily fixed.

Change the 0.5R resistor for a 0.68R 15W metal-clad resistor bolted to your chassis. It will develop 1.7V when 2.5A flows through it. That's too much, so put at potential divider across it to divide it down to 1.25V. A 1k3 resistor from the output of the regulator to a 470R resistor to the far end of the 0.68R sense resistor would do nicely. You now have a 2.5A current regulator. The difference between this and your 0.5R solution is that it's not quite as efficient, but it uses readily available parts. This is the difference between physics and engineering.

If you wanted to be really fussy about setting 2.5A precisely, add a 50R resistor in between the 1k3 and 470R, and take the wiper to the adjust pin of the regulator.
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