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Old 3rd November 2002, 07:11 PM   #1
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Default reliable tube/mosfet hybrid amp

Hi, forum.

I was wondering why it's so hard to find DIY projects of tube/mosfet hybrid amp. Is it because hybrid stuffs cannot make better sound than all-tube or all-solid-state stuffs? What I would like to build is the one using tubes at input stage and solid-state (probably mosfet) at output stage. Any idea, experience or link to a website?

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Old 3rd November 2002, 11:54 PM   #2
alfsch is offline alfsch  Germany
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i just had the same idea some month ago. I build an amp with mosfet-power out and gain 1; sound very good; and a tube stage (srpp design) also with very good sound quality (as preamp in another amp); and: the result was very bad, until now iam not shure why, made some (30...) mods, but no top sound, as exspected...
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Old 4th November 2002, 12:02 AM   #3
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Default link to...

Thanks for your input. Do you have a website on your diy stuffs? Or, a link to the schematic you're using would also be great.
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Old 4th November 2002, 01:02 AM   #4
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As far a links go, TubeCad is a great site for Valves and
Hybrid Amps is
In particular

Also some other projects on the web,

There have been a couple of threads on this forum in the past, but not many listening reviews.
The best of both worlds?
tube/solid state hybrid

When time permits I have a 6922/BJT design I would like to try myself.

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Old 7th November 2002, 10:49 PM   #5
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Default Driving Mosfets

I was considering this some time ago,
I wanted to use a 12AX7 as the srpp stage, but after enquiring on this forum, was persuaded that a tube that has more output current capability is required,
apparently MOSFET input capacitance makes them hard to drive,
particularly at higher frequencies.

I'm sure someone out there can give a more technically correct explaination

Pete McK
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Old 8th November 2002, 12:32 AM   #6
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Default how about this?

Hi, guys.
How about the following circuit and other hybrid amplifiers in the website linked below?

Click the image to open in full size.

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Old 8th November 2002, 12:33 AM   #7
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Default another try

Oops. The link to an image didn't work. Here's the full address.
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Old 8th November 2002, 01:54 AM   #8
JoeBob is offline JoeBob  Canada
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The link you posted is basically a unity gain buffer with MOSFETs with a tube voltage gain stage before it, is it not? I built a tube voltage gainstage, and am just finishing off my MOSFET power followers right now (they took me a while, spent a while not working on them), so soon I'll have some input on how it'll sound...
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Old 8th November 2002, 02:03 AM   #9
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Default sounds good

That really sounds good. I'm looking forward to seeing your result. It would be my next project.
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Old 8th November 2002, 12:45 PM   #10
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Default Hybrid amp.

I have built several valve/mosfet hybrid amplifiers. Some with ECC82/81 tubes and others with 6SN7 voltage gain stage. The MOSFET's were Hitachi types.
The sound varied from OK to very good. Without negative feedback they have a high output impedance which affects speakers with a widely varying impedance. The output impedance can be reduced by increasing the number of output Mosfet's and/or by introducing negative feedback.

I think you should build a unit without negative feedback . That sounds good. Then introduce negative feedback and see what you gain and what you loose. It might be hard for someone to tell you how it will sound in your system.

In conclusion - build it. My amp presently sounds better than an Audiolab 8000S except in the very low bass end. On most music this does not show! Midrange on the Hybrid is much better.
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