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Old 17th December 2001, 02:32 PM   #1
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Default tube preamp for ZEN

Has anybody tried something like this? Any referencis? Anything?

Regards.
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Old 17th December 2001, 03:01 PM   #2
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There's certainly no reason not to use a tube preamp for a Zen...I'd say go for it.
If you're building them as a dedicated pair, you could get rid of one coupling cap.

Grey
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Old 17th December 2001, 08:57 PM   #3
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Default Tube preamp for Zen amp

If you mean the original Nelson Pass Zen amp, you may have a problem with a tube preamp. As I recall in his article, the input impedence is very low, and tough to drive. That is partly why he followed up with a suitable ss preamp for it in Glass Audio. Zens input Z is Something like 4.7 kohm.
A tube preamp can swing a lot of output voltage, but current output is required for an input impedence this low. Preamp tubes can't source much current at all, and there output impedence is typically high.
2 Potential problems if used with a low z input power amps.
MG16
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Old 18th December 2001, 01:27 AM   #4
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I don't know as I'd use a 12AX7, but a 6DJ8/6922 can do this easily. There are other tubes that would probably do just as well.

Grey
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Old 18th December 2001, 01:01 PM   #5
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I used to drive such low Z loads with the old totem pole output follower stage which is now likely now known by various other names. As I recall I used either both sides of the 12AU7 or the 12BH7 depending on the application. Driving loads down to around 600 ohms was possible with a suitably sized output capacitor.

This of course was many years ago and I don't remember the exact particulars. But If I were the build something like this again it would likely be my choice of circuits.

The difficulty of these or similar circuits is that the cathode of the upper section floats well above ground. Thus cathode to heater voltage ratings have to be watched or shorts can occur between the two. One way to attack or reduce this problem is to apply a DC bias to the filament supply and float the fillament supply at level that is some where between the upper and lower section cathode voltages. The other is to use a seperate filament supply for the upper section.

John Fassotte
Alaskan Audio
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Old 4th January 2002, 02:11 AM   #6
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Default EL84 pre work in progress

Funny you should ask, I'm currently in the middle of work with a friend building a choke loaded single stage no neg FB SE preamp using a single EL84.

In theory it ought to work fine, give enough voltage gain and feed the evil 4K input load with no hassles.

As i say, work in progress but I"ll post when I have results
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Old 4th January 2002, 08:03 AM   #7
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Very hapy to hear that! I am realy looking forward to hear more from you. Good luck with the project!

Regards
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Old 4th January 2002, 12:05 PM   #8
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Quote:
I'm currently in the middle of work with a friend building a choke loaded single stage no neg FB SE preamp using a single EL84.
Sounds really interesting. That should be able to drive most things. Can't wait to hear more.

Lee
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Old 4th January 2002, 07:37 PM   #9
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Keep in mind that the Zen circuit itself has near
0 input impedance, being a virtual ground. We get
the impedance simply by inserting the input
resistor in series.

The good news is that it can be driven by any source
impedance, in fact, it performs better driven by a
1 ma or so current source. This means that any tube
output circuit that can come up with 1 ma off the plate
will drive it just fine, regardless of the plate resistance.
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Old 4th January 2002, 11:05 PM   #10
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Default My only concerns...

As Nelson has said, if we can feed upwards of 1mA we'll be fine. We're working on a 20mA bias point at present with a 5k attenuator post the coupling cap. That side of things ought to go well.

The concern (and we're being paranoid here) is that the EL84 mightn't be the best sounding tube for the stage. Having read that Tim d'ParaVicini's V20 amp using parallelled preamp tubes as output tubes sounded better than using the usual suspects
(EL34, EL84, 2A3, 300B, KT66 blah, blah, blah,) means that maybe we're going in the wrong direction. (power used for pre that is)
Parallel pre tube single stage would just give us a Conrad Johnson Art clone I supppose.

I guess it's in the application.

Initially most of the components (specifically the resistors and socketry) will be medium level carbon films and so forth. I'll put in some good wire, coupling caps and cathode bypasses but I won't be doing version 1 with vishays, Jensen copper foils and goldpoint attenuators. We'll see how it sounds before we tweak using stratospheric bits.

Time will tell, some construction work will have to wait till this months (and next months) bills are paid.

Waiting is...

Drew
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