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Old 9th March 2011, 12:07 PM   #1
GregH2 is offline GregH2  Australia
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Default High voltage fet preamp - ideas or designs please?

Hi All,

I am in the process of building an F4 style current amplifier and need a preamp with about, preferably, 30V p-p output. 16V p-p would do though to start with.

The B3 sounds like it would fit nicely but I haven't heard much about it or seen any schematics as yet, anyone got any info on it?

Was also considering a BOZ but don't know how linear it would be with this kind of voltage swing. I model it with under 1% distortion with a ZVN3310A, but in real life I would expect more. Any thoughts on this? I can probably live with a few % distortion, but more than 5% would be pushing it I think.

Another possibility is a modified Rod Elliot DOZ preamp. However, my current amp uses FETs and no feedback and I would prefer to use a preamp of the same design philosophy.

Anyway, anyone got any ideas?



Last edited by GregH2; 9th March 2011 at 12:23 PM.
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Old 9th March 2011, 05:06 PM   #2
flg is offline flg  United States
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You might want to read these threads:


"Crazy" power for preamp

Zen9-like Preamp for Power Followers

"It was the perfect high end audio product: Exotic, inefficient, expensive, unavailable, and toxic." N.P.
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Old 9th March 2011, 06:42 PM   #3
juma is offline juma  Germany
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This is what I liked while playing with F4.
With 1.6V RMS at the input it will deliver clean 40V peak-to-peak at the output.
Power supply quality is essential (-3.6V especially, I recommend Salas' shunt).
Cap multiplier is OK for +90V rail.
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Old 10th March 2011, 10:03 AM   #4
AndrewT is offline AndrewT  Scotland
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I suggest a maximum output from the Pre-amp of about 10 times the sensitivity of the Power Amp for maximum power. You can settle for quite a bit less than this but anywhere in the range of +10dB to +20dB (3times to 10 times) works.

This ensures that the maximum audio signal is significantly below the maximum unclipped capability and keeps the Pre-amp distortion, of what is usually a single ended gain stage, well below the worst distortion operating regions.

There is not universal agreement on this clipping to maximum signal overhead of Pre-amp to Power Amp.
regards Andrew T.
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Old 10th March 2011, 11:53 AM   #5
GregH2 is offline GregH2  Australia
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Thanks for the comments everyone. I especially like the Zv9 style preamp.

I need to do some modelling and reading to understand exactly how cascodes work.

One thing I'm concerned about is I don't have any high quality audio JFETs, only J201 and J202s. As many of these designs rely on self biasing I'm not sure how well they will work with my devices which have highly variable IDSS. Anyway, I need to go research and learn.

Thanks again,

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Old 10th March 2011, 12:48 PM   #6
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What about something like the BOZ with a much higher voltage and a SJEP170r550 ?
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Old 11th March 2011, 02:38 AM   #7
GregH2 is offline GregH2  Australia
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Originally Posted by woody View Post
What about something like the BOZ with a much higher voltage and a SJEP170r550 ?

This is more the kind of thing I had in mind. A J201 source follower buffer stage feeding a 100V common source power JFET stage. That SJEP170r550 looks like a very cool device.
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