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RE: Akai GX 255 Nuvistor preamp

Posted by Paul Joppa (P) on September 20, 2008 at 11:02:09

In Reply to: Akai GX 255 Nuvistor preamp posted by wa4aos on September 20, 2008 at 08:33:16:

Tape head preamp design depends a lot on the type of tape head in use. (In this way it is similar to phono preamps; MC and MM cartridges have very different requirements.)

Ordinary tape heads are usually specified by their inductance.

High inductance heads run 400mH to 1000mH and perform like MM cartridges in that the preamp input capacitance and resistance combine with the tape head inductance to create a second order filter which sets the high frequency performance, usually a resonance in the 12kHz to 20kHz range. These are ideally suited to tube amplification, with input impedances of 100k to 250k ohms being usual. I think this would be typical of heads in the Ampex 350s.

Low inductance heads run 30mH to 100mH generally (as I understand it). They are usually optimized for low-impedance transistor input preamps. Step-up transformers might be a good approach with tubes.

Intermediate impedances would be in the range 100mH to 400mH. The Technics and ATR heads run about 200mH; so does the special custom head developed for the Tape Project. These work like the high-impedance heads but with a resonance at 30kHz to 40kHz the tuning of that resonance is less critical and (IMHO) the treble is cleaner. More care is needed in making the preamp quiet, and more gain is needed, but otherwise they are well suited to audiophile-type tube designs.

Would you share some of your experiences with reducing microphonic problems with nuvistors?

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