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Old 14th April 2016, 07:33 AM   #1
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Default Valve Mosfet Hybrids: a solution

Hi everybody.
Some time ago I had written a thread asking for advice when interfacing the low ampage ability of valves with Mosfets. Those require quite a bit of current to load/unload their Gate capacitance.
I had tried all sorts of ways which little success. I had tried most of the designs available on the net and the result has been invariably very poor. It seems this point is ignored ...

After lots of tries and errors, I have a good solution and would like to share it here.

First, my feeling is that the simpler is the better, also for sound.
Therefore, I use only one triode for V amplification, and a pair of Mosfets for transforming the V into the amps needed for the spkrs.

I like using an integrated pair of complementary mosfets (ALF06NV8) because they are matched already and made for audio purposes.

The solution I use is: to use a Mu follower configuration for the triode, the trick is to use another Mosfet, low power high voltage (and low G capacitance) instead of a valve in the Anode circuit. I started from the paper I found on the net "the hybrid mu follower" (google will bring you to it).

Now, for the interfacing, the problem is double:
You need enough stability to have a stable adjustment of the Idle I and of course no voltage at the output (offset null).
This is done with two adjustable Rs, one between both Gates and one to the + Vcc.
You adjust both, starting with the R between Gates set to 0.
When their nominal value is too high, you do get the proper values but they are not stable: they fluctuate at +/- 100 mV, maybe up to volts. All wrong.
I got around the problem by using low values for those adj: between Gates only a 1K and to Vcc some 5 K.
Now go back to your design and just try to use low values: you will be able to get a +/- 3 mV offset on the output.

The result is the best hybrid amp I made. As it is, you can easily swap the triode for another type, to choose the best for you. Quite straightforward.

I hope this will help, it took me lots of time to arrive to a solution. My 50 watts amp has only two active parts and no feedback. Square signal up to 130 kHz, using ECC88. Wonderful sound, now pinpointing I have to work on my spkrs cabinets (back horns).
Attached Files
File Type: doc The Underappreciated Hybrid Mu Follower.doc (82.5 KB, 317 views)
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Old 14th April 2016, 01:14 PM   #2
banat is offline banat  Serbia
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schematic ?
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Old 14th April 2016, 01:30 PM   #3
RollE2k is offline RollE2k  Sweden
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banat: check the attachment :-)
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Old 14th April 2016, 02:38 PM   #4
banat is offline banat  Serbia
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Quote:
Originally Posted by RollE2k View Post
banat: check the attachment :-)
Double checked , but except textual document I can`t see any schematic , maybe my Win-XP is too old
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Old 14th April 2016, 02:53 PM   #5
carlsor is offline carlsor  United States
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Default MOSKIDO

The MOSKIDO Valve Mosfet Hybrid works great and is the best sounding amp I have ever heard.
There is a new MOSKIDO amp thread at: Moskido Hybrid Aikido-MOSFET Amplifier
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Old 14th April 2016, 03:27 PM   #6
ericj is offline ericj  United States
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Out of curiosity, have you looked at Alex Cavalli's EHHA design?

EHHA I Design – Cavalli Audio

I built a headphone amp years ago using an opamp mu-follower on top of a 12au7. worked well.
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Old 14th April 2016, 05:13 PM   #7
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Quote:
Originally Posted by banat View Post
Double checked , but except textual document I can`t see any schematic , maybe my Win-XP is too old
I had to physically click on large blank spaces to get the images to appear, so you might try that.
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Old 14th April 2016, 06:29 PM   #8
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The large, complete schematic runs off the page within the Word doc itself, and even in landscape orientation you will not get it to fit unless you reduce it.

If you save the Word Document as a normal Web page (in the save-as options), the images will be placed into their own folder.
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Old 14th April 2016, 08:03 PM   #9
banat is offline banat  Serbia
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needtubes and Jeff Y. many thanks for your help but no appearance of schematic by me ,
think that my PC is problematic , hope that thread starter will upload schematic in some other standard format .

Best Regards
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Old 14th April 2016, 10:13 PM   #10
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Quick convert to pdf:
Attached Files
File Type: pdf The Underappreciated Hybrid Mu Follower.pdf (204.2 KB, 159 views)
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