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Old 9th July 2007, 02:20 PM   #1
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Lightbulb MOSKIDO - Hybrid Tube/MOSFET Amplifier

I'm warming up my soldering iron to try yet another configuration. I bought a half-dozen Tube Cad Journal (TCJ) Aikido boards from John Broskie, and they sound great. I'm currently using some in my system in combination with a source-follower Zen-like amplifier.

I saw the MOSKIDO amplifier here on DIY Audio and later on Tube Cad Journal:
Click the image to open in full size.

Now this is why I'm posting this under the Pass Labs forum. According to the Importance of Being Biased thread, multiple output devices can be desirable. So using this philosophy, how's this for a unity-gain power output stage?

It retains Broskie's local feedback feedback technique, and it's easy to adjust the bias. It may need a servo loop in implementation, although a potentiometer may suffice to maintain DC null on the output.
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Old 9th July 2007, 03:32 PM   #2
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Now with DC servo loop.

The 10uF/10k resistors around the op-amp make a 12dB filter below 1.6Hz. This keeps the DC servo from affecting the audible range. The 250k resistor buffers the op-amp output from the input signal. Those are 30V zeners (bypass and filter caps not shown, although the op-amp has good PSRR) and the op-amp is a high-voltage type (60V plus allowable rail-to-rail).

This is looking good enough to build. Now I need to dig my bipolar +/- PSU out of my junk box. I haven't used it for a while, due to these pesky single-ended Zen designs!
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Old 9th July 2007, 07:21 PM   #3
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Looks interesting. Let us know how well it works out.

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Old 10th July 2007, 06:34 PM   #4
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What about using tube frontend from Moskido and couple it with Nelson Pass' F4 power buffer?
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Old 10th July 2007, 07:23 PM   #5
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That is what I'm gearing up to build. I am in a painfully slow part aquisition phase.
I've got the Aikido boards and the F4 boards, sounds like the cool deal to me.
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Old 10th July 2007, 08:56 PM   #6
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That's a grand idea. The F4R0 appears to be made for Broskie's Aikido. The power buffer pictured above represents an alternative to the F4R0.

I'm intending to build the above referenced output stage, but probably with a single-polarity power supply (discussed
in this thread). I should also build the F4R0 and share heatsinks (put both of them on the same heatsink, but only use one at a time). I already have most of the parts (except the PCB and jFETs) in my parts bin.

I'm doing a SPICE simulation shootout between the two. Problem is, I don't have a working TL431 model, or a model for the specific jFETs (2SK370/2SJ108). I use B2Spice (Berkeley language), so if you've got a model, post it! Part of my Spice simulation will be to look at the F4R0 with a single-polarity power supply.
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Old 11th July 2007, 02:52 AM   #7
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I've also aquired the Buz900 for an original Moskido that will be in the same chassis as the F4. We are thinking down very similar paths.
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Old 22nd September 2011, 09:30 PM   #8
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Default Updated schematics

Hi,

original moskido schematics with added transistor drivers posted in the first post is not thermally stable. Attached schematics should be ok, but as far I know no one have built it yet.

I have some questions about the new schematics.
How do you adjust bias?
Is the output stage still in unity-gain, or is there additional gain? If there is additional gain can it be changed to unity-gain via adjusting feedback resistors?

Thanks.
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