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Old 15th August 2005, 03:09 PM   #1
Sebba is offline Sebba  Poland
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Default Schmitt dephaser

In the description of some of Jadis amplifiers I found the following statement:

"A modified Schmitt dephaser at the input creates a crossed and self-balancing feeback fro the ECC 82 tube"

http://www.jadiselectronics.com/page...enu_amplis.htm

Could someone explain me what the Schmitt Dephaser is ?

I only found a schematic of one of those amps

http://www.pi.infn.it/~federico/Immagini/jadis.zip

, and the Schmidtt dephaser looks like a very poorly ballanced - through a 82 ohm ressistor in the "long tail" differrential amplifier, where the second tube (with the grounded grid) gets only a very smal signal from the 82 ohm ressistor and provides a small global POSITIVE feedback to the amp.
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Old 15th August 2005, 07:15 PM   #2
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Could someone explain me what the Schmitt Dephaser is ?

Real bad English. There is such a thing as a "Schmidt phase inverter", a.k.a. Long Tail Pair a.k.a. Differential amp. It's simply a common differential amp that's used as an unbalanced-to-balanced stage as an active single-to-PP xfmr. It's actually one of the better active phase splitters (the cathodyne being the other one).

the Schmidtt dephaser looks like a very poorly ballanced - through a 82 ohm ressistor in the "long tail" differrential amplifier, where the second tube (with the grounded grid) gets only a very smal signal from the 82 ohm ressistor and provides a small global POSITIVE feedback to the amp.

Good eye, there. Furthermore, it's a 12AU7A which has an amplification factor of no more than 20. Now, you might get away with using a very small resistor in the tail of a BJT LTP simply because BJTs have way more gain than a 12AU7A. Even with BJTs, 82 ohms in the tail would be problematic. He makes that bad situation even worse by including cathode degeneration resistors that are bigger than the tail resistor. This is a spectacularly bad design, and I would avoid it like the plague. Oh, and let's consider what a poor audio tube the 12AU7A is: the linearity plainly SUX, this being more of an RF and digital tube.

Oh, and the schematic given in that zip file is incomplete.

So far as this design goes: FUGGEDABOUDIT!
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Old 16th August 2005, 05:37 AM   #3
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Yes, a "dephaseur" is a phase splitter. I suspect the 82 ohms is a misprint for 82K ohms.
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Old 16th August 2005, 06:09 AM   #4
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I suspect the 82 ohms is a misprint for 82K ohms.

It isn't. The schematic gives some values for current and voltage. He's got that 12AU7A LTP connected to +375V(DC), with an I(P)= 3.0mA. Given the values: (100E3 * 3E-3) + (474 * 3E-3) + (82.5 * 6E-3)= 301.971V(DC). That agrees with the voltages given, leaving a V(pk)= 73.083V(DC). There is no way that that's a misprint.

What he does with the second stage makes no sense at all. Here, he has a 49.7K resistor in each cathode of the paralleled 12AX7As, bypassed with a capacitor, and a tail resistance of paralleled 47.9K's. WTF?! Why doesn't this stage have one tail resistor?

Once again: bad design.

Avoid.
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Old 16th August 2005, 08:02 AM   #5
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I have to admit I haven't been able to make either of the linjks work. If the cathode resistor is indeed 82 ohms, then it must be the shortest "long-tail pair" one can imagine.
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Old 16th August 2005, 10:32 AM   #6
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So the word "dephaser" must be of French origin and simply means a splitter.

Another strange thing about this amp is that it uses 2 ECC83 /12ax7 with a 330 kohm plate ressistor to drive 3 parralel kt 88 OT ! instead of cathode followers or low Ra tubes for example.

I also agree that this looks like a bery bad design but all Jadis amplifiers are generally respected and cost thousands of $
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Old 16th August 2005, 01:44 PM   #7
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Yes it's"dephaseur" with an an acute accent over the first "e". Jadis amplifiers are certainly expensive but they don't have a wonderful reputation for reliability.
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Old 16th August 2005, 06:33 PM   #8
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I also agree that this looks like a bery bad design but all Jadis amplifiers are generally respected and cost thousands of $

The audio field is just full of all sorts of con men selling some of the most unbelieveable things for big $$$$: The $500 Volume Control Knob (I'm not making this up.) There's lots more where that came from.

Then there's the psychological aspect: there are very few folks out there with the fortitude to admit that they were had. So, regardless of how bad the design of the Jadis amp is, you will find lots of folks out there praising it simply because they don't want to admit, either to themselves or others, that they wasted "thousands of $".

It's still a very bad design that looks like it was designed by someone more familiar with solid state designs. He should have either stuck to doing "sand", or done his research on hollow state before attempting this. But what the hey, if he's getting "thousands of $", then what does he care?
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Old 16th August 2005, 10:22 PM   #9
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Quote:
I also agree that this looks like a bery bad design but all Jadis amplifiers are generally respected and cost thousands of $
Perhaps, but I have also heard an undercurrent suggesting just the opposite for Jadis’ reputation in the design department. I have heard others talk about the “flaky” designs of Jadis in the past. This is one such design, unless the schematic was copied very wrongly. I have to agree with the prior posts: This design is really pretty bad on several counts. I have to wonder what these guys were thinking – were they even technically trained? It is a testament to vacuum tubes (especially with some negative feedback to tidy up the mess) that even when used sub-optimally, they can sound at least “OK”. But, some people do like the sound of lots of second harmonic distortion, which, along with lots of third harmonics and some bandwidth limitations, this amp will deliver in spades.
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Old 16th August 2005, 10:57 PM   #10
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This schematic link might be more convenient
http://www.mcmlv.org/Archive/HiFi/Jadis_JA80.pdf
Cheers,
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