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Old 20th November 2010, 09:45 PM   #11
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Quote:
Originally Posted by Bigun View Post
Nice design Wavebourn.

I'm curious (still learning, as ever), why did you opt for a tube (V2) as buffer to drive output tube if you allow sand in the game you could also use a source follower ? or even use another gyrator for the cathode load for V2 ?
Source follower would add own capacitances. I tried to minimize them.

Here is one more version, without gyrator:

http://wavebourn.com/forum/download.php?id=494&f=7

You can use 6P1P there instead of 12L6GT, with 2K7 cathode load resistor.
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Old 21st November 2010, 11:41 AM   #12
adamus is offline adamus  United Kingdom
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wavebourn, you may have to forgive my lack of knowledge here... but will something like this work

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Old 21st November 2010, 11:56 AM   #13
bigwill is offline bigwill  United Kingdom
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That will work, all I'd do is make R1 bigger and in real life use a zener from the gate to source to protect the gate from overvoltage
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Old 21st November 2010, 12:02 PM   #14
bigwill is offline bigwill  United Kingdom
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Oh, and you can take output directly from the source to get a lower output impedance, which is nice!
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Old 21st November 2010, 01:27 PM   #15
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Originally Posted by bigwill View Post
That will work, all I'd do is make R1 bigger and in real life use a zener from the gate to source to protect the gate from overvoltage
A question about the zener: it is commonly applied to protect enhancement mode mosfets, but will it also work with depletion mode mosfets, as the DN2540 discussed here?

erik
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Old 21st November 2010, 01:37 PM   #16
bigwill is offline bigwill  United Kingdom
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Quote:
Originally Posted by ErikdeBest View Post
A question about the zener: it is commonly applied to protect enhancement mode mosfets, but will it also work with depletion mode mosfets, as the DN2540 discussed here?

erik

I'm not sure! I forgot DN2540 was depletion mode. It won't hurt, but I'm not sure what protective role it will have. You might need one the other way around with depletion mode FETs, to protect the gate from excessive negative voltage with respect to the source. Someone more knowledgeable than me will be able to answer this.
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Old 21st November 2010, 02:14 PM   #17
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I put in back to back zeners as insurance in either direction, and have had good results. Some of my designs seem to place tough conditions on the FETs, during startup conditions when the tubes aren't sufficiently heated. Lost a few of them until I installed the zeners.

If I recall they were 9.1V; during normal operation most of my CCS's run with a volt or two on the gate, so there is really no effect when installed.
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Old 21st November 2010, 02:23 PM   #18
bigwill is offline bigwill  United Kingdom
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Back to back seems to be the way to go then, for either type!
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Old 21st November 2010, 02:24 PM   #19
bigwill is offline bigwill  United Kingdom
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Wait - This will limit you to 0.7v on the gates due to the forward voltage....

Edit: Ignore me, back to back is not anti-parallel
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Old 21st November 2010, 02:45 PM   #20
Bigun is offline Bigun  Canada
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How does this gyrator design compare with using a choke load ? I have recently found out that Edcor output transformers are as cheap as $17 each, which in your application would remove the need for many components and provide safety isolation for your headphones against any danger of B+ appearing at the output. Simpler to wire up, no heatsink and could allow a lower B+ depending on how much is being dropped across the gyrator. The only downside I can see is that your B+ needs to be a lot cleaner since PSRR is lower with the choke.
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