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Old 16th December 2011, 04:10 PM   #1431
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Andy,
I will draw a schematic and email it; it seems good to also "consult" Susan (I am in touch with her for quite a while) for her opinion.
I know that you and the 26 are a close couple; see my above post with a D3a-less approach.
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Old 16th December 2011, 05:47 PM   #1432
iko is offline iko  Canada
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Hi Andy, I'll take a good look at Susan's output stage. BTW, are you talking about the two paralleled 220uF? I don't have a definite plan with the F4. Just that I'm getting a couple of boards from the forum, and thought I may as well build it.
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Old 16th December 2011, 10:36 PM   #1433
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Is it possible to rework the F4 so as to have a PP transformer input instead of the existing devices, 220uF caps etc? I'd probably only need single output devices. Or is the input section needed for biasing the outputs?
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Old 17th December 2011, 01:06 AM   #1434
iko is offline iko  Canada
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This is something Nelson could answer best.
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Old 17th December 2011, 08:00 AM   #1435
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I'm sure Nelson could answer anything about solid state!! But judging from his posts, he works out his designs very carefully, makes them available, and then only really adds short comments in threads. He's a busy man.

andy
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Old 17th December 2011, 09:36 AM   #1436
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Hi Andy,
Risk of getting too OT, but I can say that the 220uFs are required in all case for bias stability & noise suppression.
The multiple device pairs are used to reduce output impedance (gm increases!), which in turn reduces distortion. The nature of the amp would be very different for single pairs.

And before setting out to build, please buy the 2SK170 and J74 first - they are hurtling toward the land of unobtanium.

For my part, I pay out the money for an DHT-SE output transformer gladly - because they do not wear out! They give a lifetime of pleasure, and I need them every day - which makes them a bargain! I would buy again if someone stole my Sowter SA08s, and I would pay the highest price for best quality OT. Even speaking as a solid state design pro, I believe that it is unlikely that I would move away from DHT-SE power amps!

This is so, because even with the cheap ElectroHarmonix 300B I can compare listening to live orchestras - without feeling I am missing anything important, and the stereo presentation of the parts is better than most seating positions! Sure, this is only possible when you have everything right - power supply, filament supply (fully solid state), driver circuit (partly solid state) and speakers suitable and equal to the amps.

At the device level, the output stage is the one place where DHTs are unmatchable. At lower levels, the JFETs 2SK170, J74 and (for SE) the K369 can get very close, and even do things that valves cannot do (MC first stage, for sure). But Nelson's power amp with zillions of JFETs, or the Power JFET version, points to the shortcomings of ordinary power MOS in this position.

OK, I'm saying - Have fun with the F-4, but please compare the finished article to top-rank 300B-SE ! I would not like to see you miss out.
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Old 17th December 2011, 10:18 AM   #1437
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Hi Rod - please remain off topic! I think for a few of us it's worth going through options of what we connect our 26 preamps onwards to.

I can see the LSK170, 2SK369 and the 2SK170 - are they all still obtainable? Do you see these as good SE choices directly after the 26 stage to use into a transformer to do the phase splitting? I'm also thinking PP2a3 here.

Presumably a 2SK170 stage would give low output impedence and allow a transformer volume control. This is getting into Thomas Mayer's territory.

andy
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Old 17th December 2011, 10:24 AM   #1438
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Hey guys, I like this OT
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Old 17th December 2011, 10:41 AM   #1439
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Quote:
Originally Posted by andyjevans View Post
Hi Rod - please remain off topic! I think for a few of us it's worth going through options of what we connect our 26 preamps onwards to.

I can see the LSK170, 2SK369 and the 2SK170 - are they all still obtainable? Do you see these as good SE choices directly after the 26 stage to use into a transformer to do the phase splitting? I'm also thinking PP2a3 here.

Presumably a 2SK170 stage would give low output impedence and allow a transformer volume control. This is getting into Thomas Mayer's territory.

andy
I agree, many are adding headphone outs to the pre which the hybrid stuff applies too as well (historically a TOL pre has a headphone out, and trust me adding a 120ohm tap isn't the best solution.).

But speaking of unobtainable, I thought the zeux output was no longer an option as the mosfets are specific and obsolete, I did search but couldn't find that thread (not on SS much.)
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Old 17th December 2011, 11:19 AM   #1440
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Quote:
Originally Posted by regal View Post
But speaking of unobtainable, I thought the zeux output was no longer an option as the mosfets are specific and obsolete, I did search but couldn't find that thread (not on SS much.)
You mean the Zeus output stage?
It is part of Susan Parker's design; you will find a long thread at this forum.
The output stage is a transformer coupled design, actually nothing special topology wise; it does not depend on one particular type of power fet.
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