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Old 22nd January 2013, 05:39 AM   #21
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Quote:
Originally Posted by Shoog View Post
It is possible to achieve vanishingly small distortion in a preamp stage without any negative feedback - so why would you use it.
Any of the more appropriate line amp tubes such as the ECC99 or 5687 can deliver orders of magnitude more current drive which will address far more problems than using a weedy EF86.

Shoog
Some reasons at the beginning.
Negative feedback is used in such cases because it makes the basic gain adjustment very simple, it reduces distortion remarkably, it gives better frequency response and gives lower output impedance.
All these takes place without any objectively observable flaws.

How would you design a simple voltage amplifying stage having gain 6 to 10 dB, distortion below 0.1 % (@ 10 Vrms) and constant output voltage with variable load between 22k to 220 k ?
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Old 22nd January 2013, 06:37 AM   #22
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I would consistently recommend either an Aikido or a FVP5 preamp, and I would adjust the overall gain by correct selection of the input valve. No need to resort to gNFB in a preamp to achieve excellent measured results.
This will forfil of the essential requirements of a preamp. I really don't want to get into another pointless long discussion about feedback other than saying - why invoke a distortion correction mechanism when a correct design has no significant distortion.

Every valve has its function, but choosing a valve which is out of its functionally best zone of operation and then compensating for its inadequacy by apply gNFB seems like a very bad design approach.

Shoog

Last edited by Shoog; 22nd January 2013 at 06:42 AM.
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Old 22nd January 2013, 06:45 AM   #23
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Distortion is not the only issue. There are some other important too.

What valve(s) would you use for a pre amplifier with 6 to 10 dB gain and without NFB ?

Last edited by artosalo; 22nd January 2013 at 06:48 AM.
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Old 22nd January 2013, 06:49 AM   #24
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I would point you to the official source on the Aikido if that gain is essential to you;

Aikido octal mono PCBs and 5687 PCBs and Aikido and 300B amplifiers

There is a nice table for selecting a wide range of gains.

I consider a low output impedance as more essential and the ability to drive high capacitance loads as critical. High current drive would be my main design choice. I personally used a 5687 into a PP transformer output with a significant step down to achieve what I need in a preamp.

Shoog

Last edited by Shoog; 22nd January 2013 at 06:55 AM.
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