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Old 22nd January 2013, 05:35 AM   #1
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Default Question: Class AB Hybrid with small signal triode and Power Fet (or BJT) output?

I'm curious if a couple of favorite things would combine decently?
Small signal Russian triodes run on regulated/capmulti power + Power Fet (inexpensive latfet) or BJT output devices, Class AB, (without output transformers). Well, that should be remarkably efficient and cost effective for a tube hybrid.

Will these things go together as nicely as peanut butter on chocolate, or as poorly as peanut butter on hot dogs?
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Last edited by danielwritesbac; 1st February 2013 at 03:48 PM.
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Old 22nd January 2013, 06:01 AM   #2
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For inspiration, google the Luxman lv-105, or the Moscode 402A. There are probably lots more.

jeff

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Old 22nd January 2013, 07:19 AM   #3
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Thanks, yes, much like those, except I'd like to do something considerably more frugal.
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Old 22nd January 2013, 09:33 AM   #4
ilimzn is offline ilimzn  Croatia
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This certainly works but the choice of the front end tube is not at all trivial. Ig you want consistent performance of the output stage throughout the audio band, there has to be realtively a lot of current available from the tube stage - relatively to usual tube currents. Or, you need an intermediate driver/follower.
The MOSFET stage will present a rather high and, worse, nonlinear, capacitive load for the tube front end (on the order of hundreds of pF even for a single pair of lateral FETs). That being said, even a simplistic design, say ECC88 (and family) in SRPP driving a single pair of laterals can produce more than just usable results if set-up properly, but there are also better ways to do this.
BTW a single pair of lateral MOS even at hihg standing current (and 100-150mA is the usual value) has a rather high output impedance and damping factor. Unless you want to go through a mix-and-match procedure to find the best suited speakers, expect to need some NFB to sort this out, but then the choice of input tube becomes even more non-trivial, as it also has to provide excess gain for the NFB to work with.

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Old 22nd January 2013, 11:24 AM   #5
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Attached is one possible powerfet output device.
It does need added global negative feedback.
I don't know if this device is suitable.
Attached Images
File Type: gif TDA7293-Modular-Output-Device.gif (9.0 KB, 286 views)
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Old 22nd January 2013, 01:26 PM   #6
ilimzn is offline ilimzn  Croatia
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OK, I'll say this mostly as a hint - yes, it can be suitable, but it needs work. You can slo use the input stage as a DC servo. However, I'm not going to say anything more as I would be infringing on other people's IP.
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Old 23rd January 2013, 06:44 AM   #7
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Sorry, I should have known better. Any mention of that chip would usually come with the comment "needs work" except for tone amp, tv amp, and guitar amp. Please disregard the chip posting.

Next, I go to search for dmos/powerfet/latfet at Mouser. Wow. Epic challenge. They've got some, but they're tossed right in with about 3000 "ordinary" mosfets. AudioWind and ElectronicsSalon do list some (authentic) lateral and power mosfets, probably for audio use, but they are just a bit expensive.

I'm actually after a beginner's doable, low cost, high resolution tube hybrid amplifier, and if laterals are a bit difficult, then BJT would be okay.

On hand "sand"
I do have KSA1220A, 2SC5171 and MJL0302 and their compliments in addition to BC556B, BC327-25,40 their compliments and their higher voltage counterparts. So, perhaps small signal triode + low noise/high linearity BJT would be more doable?

Tubes
On-hand tubes include 6N3P-E, 6N1P-E.

Power
I've intended to use an SMPS plug pack with plenty of knots in the cord for heater supply, LM317, LM337 for front end regulators, Fairchild Stealth bridge rectifier and unreg power with transformer specs TBD but I'd like to use affordable 63v caps in split rail power.
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Old 1st February 2013, 05:38 PM   #8
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Its such a shame that this sort of thing can't be used for low cost, good performing beginner's projects.

So, I take it that a minor problem with a tube hybrid is getting enough open loop gain so there's enough margin to make negative feedback work?

And a major problem is possibly that all the tube hybrids are either well kept secrets or somebody's intellectual property because, of course, tube hybrid (small signal tubes + solid state outputs) is both low cost and extremely profitable?
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Old 1st February 2013, 08:18 PM   #9
Ketje is offline Ketje  Belgium
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Is this what you are looking for ?
I have it on my computer but forgot where I found it.
Mona
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File Type: jpg BuisMfet-Aversterker.JPG (74.7 KB, 217 views)
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Old 1st February 2013, 08:37 PM   #10
Fenris is offline Fenris  United States
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Have you looked at the MOSKIDO:
http://www.diyaudio.com/forums/tubes...amplifier.html

or my hybrid:
New amp: 13DE7 SE hybrid
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